compression ratio question for FI

[Kozworth]

Hey guys, I just have a quick question about CR that google hasnt helped me answer.

I understand that 8:1 basically means that the area when piston is at bdc to the combustion chamber is 8 times larger then the piston at tdc just measuring the combustion chamber.

The two questions I am just confused with:

1. would an 8:1 be considered lower or higher compression compared to an 11:1? I keep thinking lower since the number is lower, but I then wonder if its a higher compression ratio because the piston space from bdc+combustion chamber to just combustion chamber is smaller.

2. This will probably make sense after #1 is answered, but which compression ratios do you run w/ a turbo setup? I browsed and saw some people say low boost with high compression, or high boost with low compression? Is there really a pro/con to each?

Sorry if this has been posted, I looked under the e30tech stickies too and this was not covered. thank you

[acolella76]

Lower or higher compression is in regards to the number values. 11:1 is higher than 8:1, for example.

A lot of people like to run 8.5:1-8:1 CR so they can 'crank up the boost' but it's kind of unnecessary. This isn't 1987 and we have the tuning capabilities to run boost on high compression motors.

[Kozworth]

but with a lower compression engine, isnt there less room to boost higher? I feel like low compression would lead to blowing a head gasket or detonation issues since there is less room and it would heat up much quicker.

Wouldnt a higher compression engine be better for boost since naturally it would allow much more room for air since it would be 11:1 as opposed to 8:1?

ps: love your avatar picture acoletta76

[downforce22]

but with a lower compression engine, isnt there less room to boost higher? I feel like low compression would lead to blowing a head gasket or detonation issues since there is less room and it would heat up much quicker.

Wouldnt a higher compression engine be better for boost since naturally it would allow much more room for air since it would be 11:1 as opposed to 8:1?

ps: love your avatar picture acoletta76

youve got it backwards

[euro2fast4u]

also with a lower compression ratio when you boost it has a lower chance of knocking. which in the end you will be able to run more boost.

[acolella76]

just while we are on the subject, what is the most boost you can put through a 9.5:1 M20? is 1 bar pushing it?

[The Humjet]

Nutzy on e30tech with his m20 2.7+731 could only run 12psi before he started seeing detonation. He still made 300+whp which is pretty good for that level of boost and lesss cylinder head flow.
I say run the highest cr you can without seeing detonation. If you could run 10.5.1 on e85 with 1 bar of boost your gonna be putting down much better numbers on the dyno than the next guy running a lower 8.5.1 cr run the same boost levels and same turbo set up..

[euro2fast4u]

just while we are on the subject, what is the most boost you can put through a 9.5:1 M20? is 1 bar pushing it?

to much of an open ended question. i would say 18psi then again if you have a pro tuner he should be able to run what ever amount of boost you want. also depends on the turbo size.

Nutzy on e30tech with his m20 2.7+731 could only run 12psi before he started seeing detonation. He still made 300+whp which is pretty good for that level of boost and lesss cylinder head flow.
I say run the highest cr you can without seeing detonation. If you could run 10.5.1 on e85 with 1 bar of boost your gonna be putting down much better numbers on the dyno than the next guy running a lower 8.5.1 cr run the same boost levels and same turbo set up..

when the compression ratio is increase it decresease your ability to remove timing inorder to reduce knock. it nutzy's case and i dont remember his threads but he probably came to a point where adding any more boost would actually start to hurt performace because they were knocking too much, and one way to remove the knock is to retard the timing. theres a point where you cant reduce the timing any more.

your right about how the guy running the higher compression ratio will be faster, but he has a lower boost limit the guy running 10.5 will say only be able to make 400 whp on 12 psi. but he cant increase it any more. the other guy will be making 400 whp at 16 psi. well which one is better.

i say the lower compression ratio bc that guy has the ability to up the boost. so he would be able to run 20 psi on that motor. while the other guy is still stuck at 12 psi.

make sense?

[Kozworth]

I think i get it now.. but just to check up.

If for an example, a piston at bdc was 1000cc (900 on piston at tdc, and 100 in combustion chamber). So at tdc, it would be 100cc in combustion chamber, resulting in a 10:1 ratio.

If one put a thicker head gasket, which would INCREASE the combustion chamber size, say to 120cc, the ratio would be 1000cc:120cc, or 8.3:1 ratio. Resulting in a lower number/ratio, but more room in the combustion chamber.

With more room, you have more room to compress more a/f mixture, and with more room there is less a chance for detonation?

ps: off topic, but all motor builds just basically want high compression, or a smaller combustion chamber, essentially the opposite of a boosted compression ratio correct?

[Sagaris]

Look at compression ratios, combustion, and boost this way:

The vacuum created by the piston drawing downward on the intake stroke (the same vacuum that is measured at the intake manifold) creates a pressure differential which causes atmospheric air @ 14.7 psi to take its place. This is since pressure, like electricity, tends to move from higher potential to lower potential. When you have a turbocharged car running at 6 psi, the pressure differential increases even more and what was once a driving force caused by atmospheric air at 14.7 psi, is now a greater driving force of 20.7 psi. Look at the ideal gas equation: PV=mRT by increasing the pressure on the left side of the equation, you must balance it by increasing mass of air on the right side of the equation. This is how you gain performance with a turbo, by an increase in air+fuel mass in the engine.

Now think of the ideal gas law again. PV=mRT

A higher compression ratio compacts the air/fuel mixture into a smaller volume which creates higher pressure in the cylinder as opposed to a lower compression ratio. So with an increase in pressure (left side of the equation) we have to increase something on the right side of the equation to balance it out. Only this time the intake and exhaust valves are closed so no mass is entering or leaving the cylinder. This means that the mass doesnt increase in order to compensate for the increased pressure, R is a gas constant and isn't going to increase, therefore it is the temperature inside the cylinder which increases.

So the greater the compression ratio, the greater the temperature increase.

The greater the temperature increase, the closer the air+fuel mixture is to reaching its activation energy (the energy required to begin flame propagation /combustion).
Increased pressure and temperature means there is more energy in the cylinder.
Increasing boost and increasing the amount that the air is compressed (increasing pressure and temperature) too much will reach that activation energy and cause autoignition/knock/ping. It is the spark plugs job to provide the extra energy to reach the activation energy at the correct time. So keeping boost, ignition timing, and compression ratios in harmony will ensure proper combustion.

In a N/A engine, you want to increase the compression ratio for greater efficiency/performance, but you also have to retard the ignition timing due to the added energy which contributes to the mixture reaching its activation energy. The increase in mass flow on a N/A engine with mods is much less than a turbo since modding an N/A engine is a matter of reducing losses with atmospheric pressure being the driving force while turbo applications supplement the driving force with additional air pressure and thus more air mass into the cylinder.

[Kozworth]

essentially, if 6 lbs of boost was put into a high and low compression engine, the high compression would reach detonation much quicker since there is less room in the combustion chamber thus reaching its activation temp easier.

So with low compression, since there is more room you can get a lower compression ratio, stuff more air/fuel mixture in while still maintaining a safe temperature (avoiding ping). Where as, a high compression engine has a smaller area so it takes less air to heat up and reach detonation?

I think i got it now, if im wrong let me know. Your post, after reading it over a couple times, seemed to be very informative.

[shEck3lls0]

Very good information. I like this thread.

[Sagaris]

essentially, if 6 lbs of boost was put into a high and low compression engine, the high compression would reach detonation much quicker since there is less room in the combustion chamber thus reaching its activation temp easier.

So with low compression, since there is more room you can get a lower compression ratio, stuff more air/fuel mixture in while still maintaining a safe temperature (avoiding ping). Where as, a high compression engine has a smaller area so it takes less air to heat up and reach detonation?
I think i got it now, if im wrong let me know. Your post, after reading it over a couple times, seemed to be very informative.

Yep, sounds like you've got it. The bolded part is not so much to do with a smaller space holding less air which is in-turn easier to warm up, rather it has to do with the greater energy present in the air/fuel mix when it is compressed that far. Think of a bicycle air pump. If you plug the tip with your thumb and try to pump, it takes a lot of effort (im not sure how healthy of a habit forcing air into your thumb is btw :D) and if the pump has no leaks, you will have a very hard time getting the pump to compress all the way. It takes work and energy on your behalf to compress that air since inside the sealed pump (think of your sealed combustion chamber) there is nowhere for that air to escape. Mechanical work is required to compress the air into a smaller space than it naturally wants to exist in. That mechanical work is energy and some of that energy turns to heat (think of how hot an air compressor, or even a bicycle pump gets after use). The mechanical energy that has converted to thermal energy (heat) increases the energy/temperature of the air/fuel mixture.

So you can look at a low compression ratio engine as compressing the bicycle pump half way and a high compression ratio engine as compressing the pump 3/4 of the way. Since more work is required to crank a high compression ratio engine, a more robust starter motor and battery may be required (in extreme cases such as the built hotrod V8's running 13:1+ compression ratios).

Low CR engines run more smoothly than high CR engines

[The Humjet]

This is a good thread I've certainly learnt a lot from this thread I'm sure the OP has leant heaps aswell.