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Compression Ratio - Boost 101

Is a low compression engine better for forced induction than high compression?

Depends on how much boost you're putting into the engine.

The big issue here is managing the amount of internal pressure within the cylinders; by making sure it's not going to damage the engine whilst still making the most power possible. Too much pressure can cause catastrophic failure where you literally blow the head off the engine. Hence why Top Fuel drag cars have big straps to hold the superchargers down in case they get blown off.

The higher the compression ratio, the more natural torque an engine produces. Adding forced induction increases the effective compression of an engine, because although you have the same compression ratio, air and fuel are entering the cylinder already at a higher pressure. This increase in pressure translates into a bigger bang at ignition, and a larger pressure from the expanding exhaust gases - resulting in more power.

Dropping the compression ratio allows a higher amount of induction pressure to be used, meaning a greater volume of fuel and air can be squeezed into the cylinder. This results in a big increase in torque and power - as long as that volume is being delivered.

When the turbocharger or supercharger is not delivering the full volume - when it's 'off boost' then the engine is relying on a lower amount (and pressure) of air coming in, which results in less power. This breathless lack of power is often mistakenly referred to as lag.

A low compression engine with big induction pressure will perform very poorly 'off boost' (i.e. when the turbo/supercharger is not delivering), and will very rapidly build power as it comes 'on boost'. In extreme cases this can literally be like flicking a switch from no power to instant full power - and a car that will be quite a handful to drive hard. Depending on the induction device, this 'boost threshold' can be quite high in the engine rev range.

A higher compression engine with low induction pressure will perform much better 'off boost' because it still has its own natural compression to generate power; it will generally not have a big jump in power, and as the induction device is generally smaller, its boost threshold will be much lower.

A low compression, big boost engine will make an insane amount of top end power, but be very wheezy and powerless down low, whereas the same sized engine with higher compression and lower boost will be very torquey low down, but won't make as much top end power.

"What's better, low compression and more boost or high compression and less boost?"

There are certainly reasons to try to raise compression ratio, namely when off-boost performance matters, like on a stree tcar, or when using a very small displacement motor. But when talking purely about on-boost power potential, compression just doesn't make any sense.

People have tested the power effects of raising compression for decades, and the most optimistic results are about 3% more power with an additional point of compression (going from 9:1 to 10:1, for example). All combinations will be limited by detonation at some boost and timing threshold, regardless of the fuel used. The decrease in compression allows you to run more boost, which introduces more oxygen into the cylinder. Raising the boost from 14psi to 15psi (just a 1psi increase) adds an additional 3.4% of oxygen. So right there, you are already past the break-even mark of losing a point of compression. And obviously, lowering the compression a full point allows you to run much more than 1 additional psi of boost. In other words, you always pick up more power by adding boost and lowering compression, because power potential is based primarily on your ability to burn fuel, and that is directly proportional to the amount of oxygen that you have in the cylinder. Raising compression doesn't change the amount of oxygen/fuel in the cylinder; it just squeezes it a bit more.

So the big question becomes, how much boost do we gain for X amount of compression? The best method we have found is to calculate the effective compression ratio (ECR) with boost. The problem is that most people use an incorrect formula that says that 14.7psi of boost on a 8.5:1 motor is a 17:1 ECR. So how in the world do people get away with this combination on pump gas? You can't even idle down the street on pump gas on a true 17:1 compression motor. Here's the real formula to use:

sqrt((boost+14.7)/14.7) * CR = ECR

sqrt = square root

boost = psi of boost

CR = static compression ratio of the motor

ECR = effective compression ratio

So our above example gives an ECR of 12.0:1. This makes perfect sense, because 12:1 is considered to be the max safe limit with aluminum heads on pump gas, and 15psi is about as much boost as you can safely run before you at least start losing a significant amount of timing to knock. Of course every motor is different, and no formula is going to be perfect for all combinations, but this one is vastly better than the standard formula (which leaves out the square root).

So now we can target a certain ECR, say 12.0:1. We see that at 8.5:1 CR we can run 14.7psi of boost. But at 7.5:1 we can run 23psi of boost (and still maintain the 12.0:1 ECR). We only gave up 1 point of compression (3% max power) and yet we gained 28% more oxygen (28% more power potential). Suddenly it's quite obvious why top fuel is running 5:1 compression, that's where all the power is!!

8.5:1 turns out to be a real good all around number for on and off boost performance. Many "performance" NA motors are only 9.0:1 so we're not far off of that, and yet we're low enough to run 30+ psi without problems (provided that a proper fuel is used).

Example: "I've got a 500+ CID motor and I'm looking to make 900hp. Can I use a GT42, I've heard they can make 900hp?"

Nope! There's nothing wrong with the GT42, it will definitely make 900hp, just not in this scenario. Here's why: 900hp represents a fairly constant amount of air/fuel mixture, regardless of whether it's being made by a small motor at high boost (e.g. 183ci at 32psi) or a large motor at low boost (e.g. 502ci at 10psi).

The first problem is that most compressors are only able to reach their maximum airflow when they are running at high boost levels. For example, a GT42 is able to flow about 94lbs/min of air at 32psi of boost, but it can only flow around 64lbs/min of air at 10psi. Often people are quick to assume that high boost means high heat and therefore decreased efficiency, but in reality, it takes higher boost levels to put most turbos into their "sweet spot". In this particular example, the turbo is capable of almost 50% more HP at high boost levels than it is at low boost levels.

The other problem is related to backpressure. If the exhaust system (headers, turbine, downpipe, etc.) is the same between both motors, the backpressure will be roughly the same. Let's say the backpressure measures at 48psi between the motor and turbine. The big motor will run into a bottleneck because there is 48psi in the exhaust and only 10psi in the intake (a 4.8:1 ratio). This keeps the cylinder from scavenging/filling fully and therefore limits power. The small motor, on the other hand, has 32psi of boost (only a 1.5:1 ratio) to push against the backpressure. Therefore it is able to be much more efficient under these conditions.

The bottom line is, as your motor size increases, your boost level will go down (in order to achieve the same power level). In such a case you will need to maximize the flow potential of your compressor and minimize the restriction of your exhaust system (including the turbine) in order to reach your power goals.


This chart shows the final compression ratio in your engine by combining the static compression ratio read down the left side and the amount of boost applied to the engine across the top. Use this chart shown below as a guideline to determine the proper amount of maximum boost level for a specific application.

Final compression ratios above 12.4 to 1 are not recommended for use with "premium pump gasoline." The higher the final compression ratio the higher the octane rating of the gasoline must be in order to help prevent detonation and serious engine damage.

Final Compression Ratio (FCR) = [(Boost/ 14.7) +1] x CR

boost = psi of boost

CR = static compression ratio of the motor

FCR = final compression ratio

Altitude plays an important role in determining compression ratios. If the altitude in the area where the vehicle is driven is significantly higher than sea level then the compression ratios will vary. To determine the effects of the altitude on a calculated compression ratio use the following formula:

Correct Compression Ratio = FCR minus [(altitude/1000) x 0.2]



Boost (in pounds per square inch)


6.5:1 7.4 8.3 9.2 10.0 10.9 11.8 12.7 13.6 14.5 15.3 16.2 17.0
7.0:1 8.0 8.9 9.9 10.8 11.8 12.7 13.6 14.5 15.3 16.2 17.0 17.9
7.5:1 8.5 9.5 10.6 11.6 12.6 13.6 14.6 15.7 16.7 17.8 18.6 19.5
8.0:1 9.1 10.2 11.3 12.4 13.4 14.5 15.6 16.7 17.8 18.9 19.8 20.9
8.5:1 9.7 10.8 12.0 13.1 14.3 15.4 16.6 17.8 18.9 19.8 20.9 21.9
9.0:1 10.2 11.4 12.7 13.9 15.1 16.3 17.6 18.8 20.0 21.2 22.4 23.6
9.5:1 10.8 12.1 13.4 14.7 16.0 17.3 18.5 19.8 21.1 22.4 23.6 24.8
10.0:1 11.4 12.7 14.1 15.4 16.8 18.2 19.5 20.9 22.2 23.6 24.8 26.0
10.5:1 11.9 13.4 14.8 16.2 17.6 19.1 20.5 21.9 23.4 24.8 26.2 27.6
11.0:1 12.5 14.0 15.5 17.0 18.5 20.0 21.5 22.9 24.5 26.0 27.5 28.9

Engine compression ratio - how does it affect performance and economy?

In an internal combustion engine, a piston compresses a large volume of a mixture of fuel and air into a very small space. The ratio of the maximum piston volume to the minimum compressed volume is called the "compression ratio."

Compressing the fuel and air will make them burn faster, which (though I'm not sure directly how) makes the engine run better. Due to the high compression ratio (12.51) of the 11,000 RPM Hayabusa engine and the low compression ratio (9.8:1) of the 6500rpm Mustang V8, I'm guessing that this allows for a much higher redline - the faster burn speed of the compressed fuel-air mix in the Hayabusa engine would allow it to complete burning before the piston had completed its stroke at high RPMs.

There are secondary benefits to high compression ratios, too. High compression ratio engines burn both much more cleanly and much more efficiently than lower-compression engines. For example, a diesel engine, which burns fuel very differently to a gasoline engine, will often give fuel economy 60% greater than its gas equivalent, even though diesel only has about 10% more energy per gallon.

According to Wikipedia, the increase in efficiency is due to the additional heat and brownian motion caused by compression fully vaporizing the fuel, which I think sounds a little fishy considering how much work is put into cooling the fuel-air mix in turbocharged cars. Most other websites say that it's due to the Carnot cycle, which I honestly do not understand - could someone explain it?

Another issue is engine efficiency as a function of RPMs. An engine limits power by reducing the intake of fuel and air to an engine; if only half the fuel and air is entering a piston, the compression ratio is effectively halved as well.

Considering all the advantages of high compression, one might wonder why anyone would not use a high compression ratio. The answer is simple: The increased heat density of the compressed gas will cause the fuel to begin combustion without ignition by the spark plug, resulting in an undesirable burn pattern. This detonation, or "knock", is often heard as a pinging noise and can cause severe damage to your engine.

The measure of a fuel's minimum ignition temperature and resistance to detonation is its octane, which is not, as is commonly understood, a measure of it's energy per liter. Before the advent of the catalytic cracker, fuel was often below seventy octane, and engine compression ratios were low - a Model T had a compression ratio of 4.5 to 1. However, by splitting large molecules into smaller ones (cracking), modern engines are both more efficient and better performing than their older brethren.

Modern gas has an octane of about 93 for premium in the US, and about 97 in the rest of the world. 100+ octane gas can be had, but it's very expensive (well over $5/gallon) and often has octane-boosting additives which contain lead. However, all of these are dwarfed by ethanol, which as an octane rating of a whopping 129. As a result, it can easily be used in engines with a compression ratio of 15:1 or greater, and despite having an energy density only 2/3 that of gasoline, it should produce similar fuel economy in such an ethanol-optimized engines along with very, very high redlines.

Running higher compression ratios is a very hotly debated subject at the moment and the two camps are extremely divided. As a lot of you probably know, the two “camps” consist of the old school Cosworth era tuners who have been building engines with lowered ratio’s for years (and have worked well) and the newer breed of tuners (mainly stemming from USDM Honda scene) who have had great practical and proven success building higher compression turbo engines.

Let’s start with the correct turbo choice for your application.

People run smaller turbo’s because they want them to spool early and to deliver a wider range of power. Turbo A (smaller turbo) running 1.0bar will spool faster than Turbo B (slightly larger) running the same boost pressure, but the amount of air moved by Turbo B (slightly larger) will be greater at the given boost pressure as it’s moving more air? Thus creating more power.

The problem is that the more boost pressure you run, the more the charge is heated by the turbo. This results in the temperature of the air/fuel mix entering the combustion chamber to be significantly higher. Once the piston is on it’s up stroke, this air and fuel mix is compressed and something called adiabatic heating occurs. (*which is the increase in temperature of a fluid when under pressure) if this air fuel mix reaches the auto-ignition point of gasoline you get premature detonation or det (The air/fuel mix auto igniting before the spark plug fires – not like premature ejaculation).

By upping the compression ratio in an engine, your increasing the heat that is generated when the gas is compressed on the upstroke (but also increasing the density of the gas/air mix and extracting more mechanical energy) this means that your inlet temperatures and fuel octane are significantly more important. Higher octane fuel has a higher auto ignition temperature and is more difficult to burn (the opposite to common belief that high octane fuel is actually more easily ignited) thus making your combustion process more det resistant.

The type of piston and shape of the combustion chamber, determines the speed of the flame front that travels across the compressed mixture.

In simple terms a higher compression ratio DOES create more power off boost which gives you extra torque down the arse end. It’s really down to static vs. effective compression.

Effective compression is the sum of the static compression, plus the additional compression added to the cylinder by a turbo or super charger, or any other forced induction tool for that matter. Effective compression is defined by the following formula:

E = C((B / 14.7) + 1)

Where E= Effective Compression, B= boost psi, and C= Static compression. Also remember that 14.7 is equal to 1 bar of boost.
Let’s do an example. Let’s say we have a SR20DE bone stock with 10.4:1 static compression and slapped on a turbo kit. It’s now boosting 7psi. That takes care of our variables. Let’s do the math.

E = C((B / 14.7) + 1)
E = 10.4((7 / 14.7) + 1)
E = 10.4((.476xx) + 1)
E = 10.4(1.476xx)
E = 15.35xxx

As you can see, we come up with an effective compression ratio of 15.3 or so. A motor in this effective compression range is easily daily driven with proper fuel and timing adjustments/upgrades.

(Running on the same turbo back to back)

If you’re running a 9.0:1 compression ratio @ 1 bar you’ll achieve an effective compression ratio of 18:1

If you’re running a 7.5:1 static compression ratio and 1 bar of boost you achieve an effective compression ratio of 15:1.

For the 7.5:1 to reach the same effective compression ratio you need to run .4bar or 5psi more boost. Combined with the fact that you have less “grunt” outside the boost threshold. Less grunt/torque means your engine produces LESS of a bang when the combustion mixture ignites, along with a slower burn due to a less compressed cooler mixture.

So in a nutshell – if you’re running 2.0 bar of boost on a 7.5:1 static ratio your achieving an effective compression ratio of 22.5:1 if you run the same setup with 9.0:1 static ratio you get the same effective compression ratio but with only 1.5 bar of boost and much better drivability outside of the boost threshold and a better spool due as a result.

Of Course, if the compression ratio is too high then the adiabatic effect will cause the mixture to auto ignite – so there is a line to be drawn obviously.

When building turbo engines static compression ratio is actually a bit of a clumsy way of measuring what’s going on because your measuring the C/R at atmospheric pressure not the desired. With that said, a higher C/R engine will produce more power off boost and subsequently spool your turbo slightly faster. You need to aim for a power goal (whatever that is) and spec your turbo setup accordingly to produce the required air at a reasonable pressure & temperature.


RIGHT at this point it might be as well to point out to readers that the handling of alcohol fuel, even in small quantities, is dangerous since poisonous Methyl Alcohol is the basis of most of these fuels.

In some cases to prevent it being used for drinking an additive is used, called Pyridine, about one half per cent being the amount.

This gives it a nasty smell and a vile taste, but pure fuel is, of course, without this deterrent.

The problem still remains, however, since it can get into the system by absorption through the skin or cuts, and can be inhaled from exhaust gases.

The effects are cumulative and if enough build up is allowed it oxidizes forming Formaldehyde causing blindness and insanity.

The use of rubber gloves, avoiding splashing and handling in confined space and in general treating with commonsense, however, reduces the risks to acceptable proportions.

Should, however, any get in the eyes immediate medical attention is necessary.

For those who have not handled alcohol fuel it might be as well to say it is a clear, colorless liquid, cool in touch, with an odor different from petrol, and has an attraction to moisture in the atmosphere.


Let us now investigate the advantages and disadvantages of going over to this fuel, and at all times taking petrol as our reference level, having in mind the basic requirements of fuel in the heat engine.

The first question must be is it easy to obtain and the answer is there are a number of garages retailing the fuel, in certain cases with other fuels added in specified quantities.

Having obtained the fuel, as already explained, it must be handled with care and commonsense.

There is no real problem in keeping in store any quantity left over from one meeting to another, provided it is kept in a can, or tank for that matter, with the cap kept on during the store period, which can extend into years, contrary to popular belief.


Cost of the alcohol depends on what other fuels have been incorporated, but as guide pure alcohol is, in small quantities, about just over half as much again as the cost of top grade petrol. You must bear in mind at this point, however, you will require double the amount of alcohol as compared to petrol for reasons which will be explained later.

Another point to consider is that alcohol is a solvent and so far as certain paints are concerned it acts as a perfect paint stripper. Alcohol also has a very thorough scouring effect on tanks, pipe lines and so on, not forgetting it can on certain

types of fiberglass tanks cause them to disintegrate into a rather nasty sticky mess.


Consumption of alcohol will be, in rough figures, double that of petrol, due to the calorific value being about half that of petrol.

The correct air-fuel ratio for petrol is 14.1 to 15.1, but for alcohol it is 7.1 to 9.1 so that means we must pass at least twice the weight of fuel, in the case of alcohol, to heat the same amount of air to the same temperature as we need for petrol.

Since the specific gravity of the two fuels is near enough the same it means in effect we have to pass through the jets double the quantity of the fuel.

Apart from doubling up the flow capacity of the jets, and we would add here that this does not mean doubling up the diameter of
the jet hole as many people think, but, in fact, increasing the diameter by 1.4 times or if you like by 40 per cent since a little thought will remind you of the fact you are dealing with the area of the hole in the jet and not the diameter.

It is of little use increasing the capacity of the jet to pass double the amount of fuel unless steps have been taken to establish that the fuel lines, taps, float chambers and so on are also capable of passing double the fuel and the actual flow should be measured.


Now unlike petrol you will find alcohol fuel will continue to provide increased power for a mixture well above the ideal mixture strength and you can always tend, therefore, to jet up on the rich side, and so avoid any possible chance of running into troubles through weak mixture causing burnt valves and holed pistons.

This larger amount of fuel compared to petrol and especially as it is a fuel with much higher latent heat value tends to do two things. The density of the charge entering the engine is higher than petrol and a greater weight of mixture is therefore being exploded.

This is a fuel with a large cooling effect provided by part of it evaporating after it has reached the combustion chamber and so tending to cool the valves, piston and so on.

Some may well get into the combustion chamber as liquid, due to the reduction in temperature of the induction system, pipes, carburetor, etc., and so extending the cooling effect, in the process counteracting the effect of the high internal temperature.

In view of this amount of fuel entering the chamber, with possibly some of it in liquid form, the ignition system must be beyond reproach since if the spark is weak the mass of fuel will just soak the plug and then at once ignition troubles arise affecting starting in particular.

Owing to the use of alcohol a higher compression ratio can be used with this fuel as compared with petrol, another consideration is the type of plug used which will be a hotter type than used before with petrol.


We have just mentioned the higher possible compression ratio used with alcohol and the limit that can be used with any particular fuel depends on the tendency of the fuel to detonate.

As a rough guide the ratio for petrol is limited to about ten to one, or with certain additives to as much as 12 to one. With alcohol, however, you can go up to 19 to one or higher in certain cases. (For all practical purposes however, 14 to one should be considered the maximum usable ratio in modern short stroke automotive engines.)

The possible use of a much higher ratio, of course, means we get a higher power output from the engine, and this, in fact, is almost the main advantage of alcohol fuel.


Detonation with alcohol fuel is really not a problem, but pre-ignition is, or could be unless the mixture is kept well on the rich side.

The reason for this is that if the mixture is on the weak side it burns slowly and can still be so doing when the exhaust valve has opened which then becomes overheated. This in turn ignites the next charge before the correct time, the whole process becoming a chain reaction causing even more rise in temperature and so it goes on until the piston holes and other damage then follows.

The first signs of this process taking place are a loss of power, a general rise quite quickly of overall temperature, the head in particular.

To avoid this, run on the rich side always and use plugs with a good heat capacity.

It might be worth mentioning at this point that an engine set up correctly for running on alcohol, even though on a rich mixture, will be found to be (compared to petrol), a much cleaner running engine inside the cylinder head, and provided the ignition side is up to its job there will be less fouling of plugs than on petrol.


Due to the different rate of burning of alcohol compared to petrol the ignition setting will have to be changed.

It will have to be advanced and the amount necessary will depend on the shape of the cylinder head and general design.

For example, on a well designed hemi-head an extra five to six degrees might well be enough, whereas on a poor designed head it might be something like 15 degrees.

Optimum ignition setting is tied up with the air-fuel ratio and it will be found that with alcohol it is not so critical as with petrol, that is to say the drop off of power is not so progressive as will be seen later.


Provided the engine is set up for running on alcohol correctly there should be little trouble in starting except perhaps on a very cold day and it should be possible to start up on the fuel mix used for the actual racing.

The main problem, due to the cooling effect of the fuel, is to get the engine to operating temperature in the short time available from fire-up to staging.

For this reason so far as motor cycle type engines are concerned, you will note,

in many cases, the finning on the cylinder barrels and heads is almost eliminated. This, by the way, also helps to increase the power to weight ratio, or if you like tends to counteract the weight of the extra amount of fuel carried as compared to petrol.


From reading this far, you should have come to the conclusion that if your engine is now on its limit running on petrol, while large increases of power are obtainable by the use of higher compression ratios it is possible to get a reasonable increase in power output, ten per cent or so, with the existing ratio, provided you make quite certain you get enough fuel through to the engine and, in fact, that you tend to run on the rich side.

Once you have gone over to alcohol and obtained satisfactory running, you have commenced an extension of your power output by anything up to 25 per cent as you adapt the engine to run with the new fuel.

The rather attractive feature that you tend, even with the increase of power to stand less chance of doing damage to the engine than when on petrol should also be considered.


One final point to consider. If you change over to alcohol from petrol where you were using a mineral oil, it is not necessary to change over to a castor based oil. For modern engines, the present type additive mineral oils offer a higher performance level than the additive castor based oils, and under controlled conditions the light viscosity oils have an advantage where the warm up time is limited.



METHANOL (Methyl Alcohol) CH30H is a volatile, highly inflammable, water-clear liquid with a mildly spirituous odor Miscible with water or nitromethane in all proportions and almost all with petrol.


Flash Point

Boiling Point

Freezing Point

Specific Gravity

Lbs/Gall approx




  61 16  148 64 -144 -97  0.796    8
is an inflammable water-clear liquid with a mild odor, containing approximately 53% by weight of oxygen. Water will mix with nitromethane to the extent of 2.5% only, by volume.
 110 43  214 101 -20 -29   1.13  11.25
ACETONE (Dimethyl Ketone) CH3COCH3
is a highly volatile, highly inflammable, water-clear liquid with a strong, sharp, characteristic odor. Miscible with all the chemicals listed here, and water.
 0 -18   133 56  -138 -94  0.791   8
ETHER (Diethyl Ether) C2H5OC2H5
is an extremely volatile, highly inflammable, water clear liquid with a strong, lingering, characteristic odor. Miscible with all the chemicals listed here but not with water.
-40 -40  95 35   -183 -116  0.714   7
BENZOLE, (Benzene) C6H6
is an inflammable water-clear liquid with a dull sweet odor Miscible in most proportions with all the chemicals listed here but not with water.
 12 -11  176 80   41  5    0.879 8.75
is an inflammable, yellow, oily liquid with a strong odor of almonds. Miscible in most proportions with all the chemicals listed here but not with water.
 190 88  412 211  41  5    1.20    12
PROPYLENE OXIDE (1 :2. Epoxypropane) CH3-CH-CH2
is an extremely volatile, very reactive, highly inflammable, water-clear liquid with a light gaseous odor. Miscible with all the chemicals listed here but only partially with water.
 32  0   93 34  -155 -104  0.83   8.25
UCON LB625 (Polyalkalene glycol)
A water-clear synthetic lubricating oil with exceptionally high film strength properties. Miscible with all the chemicals listed here but not with water.
 430 221 -  -   -25 - 32    1.0    10

Conservative Maximum Compression Ratio

Air/Fuel Ratio for Max Power lbs/lbs

Energy from Combustion B.T.U/lb

Cooling Effect (Latent heat of Vaporization) B.T.U./lb

Use in Internal Combustion Engines


17 : 1       4.5 : 1     9770         472
Methanol permits the use of very high compression ratios when unsupercharged or high boost pressures when supercharged. The large cooling effect increases volumetric efficiency and is of particular use in the supercharged engine reducing charge temperature after compression. A tendency to pre-ignition is most noticeable at weak mixture levels.

6.5 : 1

(10 : 1 with rich mixtures)

2.5 : 1 to 0.5:1 at least



Nitromethane enables considerable power increases to be obtained (70 percent minimum with proper use). Most often used blended with methanol, in various propor ,tions to provide power increases consistent with engine strength, etc. A tendency to detonation is reduced by an increase in mixture strength, reduction in engine temperature, reduction in compression ratio or the addition of methanol.


   17 : 1     9.4 : 1    12,500        225

As a basic fuel acetone appears to have all the required characteristics, these in general Iying midway between methanol and petroleum. An exception is its very high anti-knock rating which approaches that of methanol. Other uses are as an additive to other fuels, notably to methanol to reduce pre-ignition sensitivity and promote easier starting under low temperature conditions, up to 10 percent for this purpose.


   4 : 1      9.8 : 1    15,000        153
Not used as a basic fuel in spark ignition engines due to its very low knock-rating, but this characteristic is desirable in the small high-speed diesel engine where it is used in relatively large percentages (approx. 15 percent to 35 percent by volume) as an additive. Its volatile nature and low flash point make it useful as an additive tuP to 5 percent) to improve starting and give a rapid throttle response.


  15 : 1     10.8 : 1    17,300        153
Most often used blended with methanol to give a greater energy per unit volume with reduction in the latent heat vaporization, this being a compromise often used for long distance racing where fuels other than petrol are allowed.


 not known   8 : 1       10,800        143
Blended in very small proportions with other fuels it is thought to act as an ignition accelerator. As this material has a strong odor even after combustion it is sometimes used as an additive in other fuels (approx. 0.5 percent) to mask the normal exhaust odor making it difficult to detect the base fuel type.

Propylene Oxide

 not known   9.6 : 1     14,000        220
Used as an ignition accelerator additive particularly with nitromethane (up to 20 percent by volume with pure nitromethane) where noticeable increases in power are possible. Easier starting and smoother running are other benefits when blended with most other fuels (up to 5 percent)


At 0 F this oil compares to SAE 20 at the same temperature, and at 210 F it compares to SAE 50 at the same temperature Used to advantage in all two stroke engines operating on fuel/oil mixtures. The unusually high him strength properties allowing a reduction in the amount of oil in the fuel by as much as 55 percent. Of particular use in very small high speed two stroke engines where the normal oil content can be up to 30 percent of the total volume, with the attendant restriction on the amount oF fuel that can be burnt.




  • Magnesium: Attacked.
  • Tin: White deposit (long term).
  • Polythene: Cracks (long term).
  • Paints: Most attacked severely.
  • Perspex: Attacked.


Poisonous; do not allow to come into contact with skin as repeated absorption may have long term effects on health.

  • Copper/Alloys: May be attacked.
  • Polythene: Generally resistant.
  • Paints: Most attacked severely.
  • Perspex: Attacked.
Do not allow to come into contact with caustic soda, amines or hydrazine. Pipeline pressures must be kept below 100-lb/sqlin.


  • Metals: Resistant.
  • Polythene: Cracks (long term).
  • Paints: Most attacked severely.
  • Perspex: Attacked.
  • Neoprene: Some attack.
Low flash point presents considerable fire risk. Extinguish with dry powder or CO2.


  • Metals: Resistant.
  • Polythene: Cracks (long term).
  • Paints: Most attacked severely.
  • Perspex: Attacked.
  • Neoprene: Some attack.
Very low flash point presents serious fire and explosion risks. Vapor is heavier. than air and anesthetic.


  • Metals: Resistant.
  • Polythene: Generally resistant.
  • Paints: Some attacked.
  • Perspex: Some attack.
Poisonous; strong vapors must not be inhaled, may affect blood tissues permanently.


As for benzole

Very poisonous; do not allow to come into contact with skin or inhale vapors.

Propylene Oxide

  • Metals: Most resistant.
  • Polythene: Cracks (long term).
  • Paints: Most attacked severely.
  • Perspex: Attacked.
  • Neoprene: Some attack.
A very reactive chemical, must not be allowed to come into contact with copper/alloys or rust, reaction may be violent.


No problems

No problems

Alcohol Problems

For the racer there seem to be many positives for using alcohol as a fuel; are there any downsides? Yes, there are a number of issues that alcohol brings to the party that are not even considerations with gasoline fuels. The first is that alcohol is hygroscopic. It will absorb water out of the air if it’s exposed to the environment. This little feature can make a perfectly acceptable jug of fuel not worth using if the water content gets too high. This feature of alky fuels is, and has been, the bane of many tuners as they make changes to the fuel system only to find that the fuel was contaminated with water.

This is also a real problem in areas that have a good bit of humidity in the air. In the Southwest it’s not a big issue but it still means that any alcohol that is stored needs to be in containers that are not vented and that the fuel should not be exposed to the environment any longer than possible.

Another downside is that many of the rubber type seals that are used in gasoline fueled cars don’t hold up when the fuel is changed to alcohol. They don’t react well with alcohol fuels, often degrading and no longer offering an acceptable seal, or even worse they degrade and contaminate the fuel downstream of their location. While this seems like a real issue, it’s simply rectified, by using seal materials that are resistant to alcohols, from the tank to the end of the fuel delivery system.

Chemistry 101

The chemical makeup of alcohol is very corrosive to many of the coatings that are typically used on metals in the fuel system. It’s not uncommon for metal components to get surface oxidization and pitting as a result of alcohol fuels. This becomes a real issue if the alcohol is allowed to sit in the fuel system between races. The fuel system should be maintained between races to prevent the alcohol in the system from turning into what is a very strong corrosive agent.

If the fuel system isn’t cleaned frequently, preferably after each race day, the corrosive nature of alcohol will play havoc with the metal and rubber components in the fuel system, especially those components not designed for this type of fuel. This isn’t a real issue as most racers who are using alcohol fuels are already familiar with the required maintenance. For those not familiar with the maintenance rigors required when using alcohol fuels; education comes quickly and with a vengeance.

Butanol has some unique characteristics; it’s the one alcohol that most closely mimics gasoline from an energy density perspective. Its stochiometric air/fuel ratio is the closest to gasoline. Due to its chemical makeup, butanol isn’t as corrosive as methanol or ethanol. While all of this sounds great, there are some issues that prevent butanol from being a viable racing fuel at this point in time. First, is that it has a fairly high melting point and at cooler room temperatures more closely resembles Vaseline than a liquid fuel. However, it does mix well with gasoline and that has some real positives for the passenger car world; however it’s not a real boon to the racing world, yet. At this time we will still focus our attention on methanol, while ethanol is gaining more acceptance.

Failure to properly maintain an alcohol fuel system will result in, aside from the corrosion, a grit like substance, almost a fine sand type of residue, in the lines and around aluminum parts. This grit is the result of an increased electrical conductivity that alcohol has over gasoline fuels. The grit is from the galvanic corrosion caused by the greater electrical conductivity from the fuel as it interacts with the various different metals in the fuel system. This contamination will migrate throughout the system clogging fuel filters, fuel jets, and generally cause havoc within the fuel system.

It’s often thought that alcohol makes power because it has a greater amount of energy. This isn’t exactly true; in fact, the type of alcohols that are commonly used in racing have less heat energy than gasoline based on the volume. There are, in fact, four types of alcohols of which only methanol and ethanol are currently used as fuels in the racing world. The other two types of alcohols, propanol and butanol, aren’t used commonly used. Propanol has more uses as an industrial solvent than as a fuel while Butanol is an interesting chemical.

More Power

So, just why does alcohol make more power than gasoline if it has less energy per pound than gasoline? Good question! Obviously, you will have to run more of the alcohol-based fuels to get the same power, how much more will depend on the type of alcohol you’re running. With methanol and ethanol it’s about 40 percent more than gasoline. Let me espouse some of the good characteristics that alcohol brings to the table.

First, when you burn alcohol one of the byproducts of combustion is oxygen. This helps enhance the combustion process. Another is the cooling effect of alcohol as it “vaporizes” in the inlet track. This helps create denser air as the air/fuel charge enters the engine, another positive. The cooling effect also helps to cool the engine, at least on the inlet side of the equation. Remember, producing horsepower is all about creating and controlling heat.

Another positive feature about alcohol that is seldom discussed is that the incoming fuel charge, the mixture of air and alcohol, is easier to compress than a mixture of gasoline and air. The alcohol doesn’t vaporize as well or as completely as gasoline as it comes out of the carburetor or the injector. While gasoline forms a more complete vapor, alcohol forms a “vapor” made up of many very small droplets of fuel suspended in the incoming air/fuel stream entering the engine. Then during the compression stroke, the heat of simply compressing the incoming air/fuel mixture completes the vaporization process.

So, from a mechanical perspective, your engine uses a smaller percentage of the power it’s making to sustain continued operation. Long story short, an alcohol mixture takes less energy to compress than a gasoline mixture. And, as an added bonus the last vaporization step also helps to further cool the mixture. Remember, cool, in this case, is a relative term as compared to a gasoline mixture.

Additionally, an engine that is burning methanol or ethanol can support a much higher compression ratio. It’s not uncommon to see alcohol engines using as high as 13:1 or 14:1 compression rations with little fuel-related problems. Of course, high compression engines have other mechanical issues that aren’t related to fuel. That said, alcohol can support some very high compression engines without the fuel causing detonation issues which can occur if the wrong grade of gasoline is used.

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