It has been coming up a lot lately. What is the difference between the compression created by a large amount of boost on lower compression pistons, and that created by a smaller amount of boost with higher compression pistons.is it equal?
Is the power created by boosting solely in the compression that it increases. or are there other factors working here?
CAN you get the same increases in POWER with higher compression and lower boost with the same ease of tuning, or is the difficulty in tuning because you will be getting larger gains in horsepower, and all the problems that come with it sooner?
Or did I just drink WAY too much coffee today?
I'm really interested in this. it will help me decide where I go from here.
#33851, "RE: lets answer this once and for all" In response to Reply # 0
This was gone over before, but it was quite a while ago. I believe the conclusion was that you can get equal amounts of power from both scenarios, but the tuning on the HC route was much more finicky. If you are prepared to deal with the headache of constant tuning, go HC. If you want to mess with the tuning minimally once it's set, go LC.
Correct me if I'm wrong here and fill in the gaps if I left anything out.
Matt
PS..I'm not saying that LC/High boost is easy to tune, just theoretically it's easier than the HC route
#33852, "RE: lets answer this once and for all" In response to Reply # 1
ok 2 cars one with 10.5 compression one with 8.6 compression both running 10lbs of boost
the car with the 10.5 will be approx 8% faster/more powerful but like you said its harder to tune/prevent detonation etc.. generally its 4% increase for every point of compression
#33853, "RE: lets answer this once and for all" In response to Reply # 2
So basicly unless your going to be a pro racer (or at least are able to tune like one) go low comp. Let's put it this way too in the ^^^ senario it tells you that the higher comp car will be roughly 8% faster than the low comp car. However if you just turn the boost up to 13psi then your even again or faster. now if he turns the boost up to 13psi he has to do some tuning before he can one up on you. (unless he has a pre set pramater for that boost level or something)If you like to live on the edge buy a diff. daily driver and learn to tune it, IMO.
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#33854, "RE: lets answer this once and for all" In response to Reply # 2
>ok 2 cars >one with 10.5 compression >one with 8.6 compression >both running 10lbs of boost > >the car with the 10.5 will be approx 8% faster/more powerful >but like you said its harder to tune/prevent detonation >etc.. >generally its 4% increase for every point of compression
Well....yes, but the point of 8.6 compression is to run higher boost safely. So, if you are running the same amt of boost, this is true. But, the point is to turn the boost up so that you get the same amt of power and it's not as difficult to tune. Of course you will get less low end power with the lower comp.
#33855, "RE: lets answer this once and for all" In response to Reply # 4
>>ok 2 cars >>one with 10.5 compression >>one with 8.6 compression >>both running 10lbs of boost >> >>the car with the 10.5 will be approx 8% faster/more powerful >>but like you said its harder to tune/prevent detonation >>etc.. >>generally its 4% increase for every point of compression > >Well....yes, but the point of 8.6 compression is to run >higher boost safely. So, if you are running the same amt of >boost, this is true. But, the point is to turn the boost up >so that you get the same amt of power and it's not as >difficult to tune. Of course you will get less low end >power with the lower comp.
Yep the low end power loss is the main reason why im going with the higher comp or stock comp pistons, these cars are already dogs off the line and the low comp. would be even worse until you hit 3K anyway and the turbo kicks in
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#33857, "RE: lets answer this once and for all" In response to Reply # 6
having higher compression can benefit a couple things, low end power without boost ie untill spool time, it will decrease spoool time so the turbo kicks in faster and it will create mroe power per pound of boost if tuned right... the fine line of lean and rich is what u have to watch
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#33858, "RE: lets answer this once and for all" In response to Reply # 4
okay, i think the answer that is being looked for has not been addressed. compression is the ratio that the piston can compress the air fuel mixture in the cylinder during its stroke from bottom to top. On say a 10.5:1 compression you would be able to compress tha AF mixture to 10.5 times its original volume. So you would have say a compression pressure of say 210 psi during a compression test. On an 8.8:1 compression you are only compressing the AF mixture 8.8 times its original volume therefore only creating a compression pressure of say 160 psi. so taking these numbers into account with regards to power delivery. the 10.5 pistons will make more power while not in boost and require less boost to make a certain amount of power say 200HP. the higher compression ratio can compress more AF into one stroke than a LC can, but you can compensate for this by increasing boost. Say a HC cylinder can compress 10 parts of AF mixture at 0 psi boost. and the LC piston can only compress 8 parts of AF mixture at 0 psi boost. you can see how the LC will make less power because it is actually compressing less AF. now if you put some boost in say like 3 psi or something, it will raise the 8 parts AF to like 10 parts. then the 8.8 will be making the same power as the HC pistons at 0 psi boost. so you could ultimately make the same power on LC pistons as you do with HC pistons, but you would have to use more boost. that is why HC piston cars seem much faster, they have more low end torque due to the fact that they don't need as much boost to create a given amount of power, while a LC car would have to wait for the turbo to spool and build boost before it can reach the same given amount of power. now the reason it is much easier and safer to tune a LC car is because of the boyle's ideal gas law the more you compress the mixture the hotter you heat the air. so on a HC car you are producing intense cylinder temperatures just from compressing the air, while on a LC car you aren't quite heating the air as much from the compression. that's basically all my ramblings for now. it may or may not make sense. lates James
#33859, "RE: lets answer this once and for all" In response to Reply # 8
Okay. I think that what I most would like to know is.... The power that you gain from boosting. Is it created by increasing the compression in your cylinders by forcing the air in? or, are there other factors at work here that i havent picked up on?
#33860, "RE: lets answer this once and for all" In response to Reply # 9
The power from boosting comes solely from forcing the air in (forced induction).
At the same boost levels, a higher compression car will have more power (assuming everything else to be equal). The "effective" CR can be calculated with your given CR plus boost. It really means nothing, but you could then see that at a set level of boost the HC car will have a higher CR, thus more power. There are other things at play here (such as machine work done to intake, head, etc), so that's why I say assuming everything else to be equal. Does that answer the Q?
#33861, "RE: lets answer this once and for all" In response to Reply # 10
I started this thread to understand the choice of higher or lower compression pistons on a boosted car, and the tuning issues that go along with that. Here's my theory (correct me if I am wrong). The tuning issues that come from a higher compression engine, come not from the fact that it has higher compression (that you would create anyway with more boost), but from the fact that you are starting with a smaller space, and increase the compression more rapidly (it is faster to increase pressure in a small space than it is in a larger one). Therefore with lower compression you are easier able to increase boost in smaller increments, creating an environment that is less sensitive to slight fluctuations, (a small spike will increase the compression a smaller percentage with a lower compression piston) making it more forgiving of your tuning. RIGHT?
#33862, "RE: lets answer this once and for all" In response to Reply # 11
>The tuning issues that come from a higher compression >engine, come not from the fact that it has higher >compression (that you would create anyway with more boost), >but from the fact that you are starting with a smaller >space, and increase the compression more rapidly (it is >faster to increase pressure in a small space than it is in a >larger one).
Well, technically, saying higher compression and "starting with a smaller space" are the same thing in this case. Regardless, the tuning difficulties come with the higher compression.
#33863, "RE: lets answer this once and for all" In response to Reply # 12
I don't want to reiterate what everyone else has said, but I believe the biggest difference between boosting with a lower or higher compression engine is in the amount of boost you can run--higher compression=less boost, lower compression=higher boost. At some point, the power output of one of these setups will be about equal. However, if both setups are tuned correctly and efficiently (fuel/timing), I believe the setup with lower compression will be able to boost much higher then the higher compression engine with limited boost, thus probably resulting in a higher horsepower engine.
I guess what I'm trying to say is, if you plan to run high boost for maximum horsepower, then go with lower compression. If you like the benefit of more lower end torque, along with boost, then by all means go with the higher compression setup. Keep in mind that there's less room for error when you run a lot of boost with higher compression engines. Just my 2 cents
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