compression+boost?

[subatoy]

I've been wondering this for a while now so I would like to hear an answer from the experts.
It has been said many times that high compression ratios are not good for boost but when you lower the compression then you have to run more boost to achieve the same power so whats the advantage of a lower compression?

Lets say you have the a 2.0 engine with 12.5:1 compression ratio and you run 5 psi and you achieve 250whp.
Then you take the same engine an lower the compression ratio to lets say 9.5:1 but you now have to run 10-12 psi to achiave the same 250 whp.

It seems like the strain you are causing to the engine is still the same so why not just stay with the high compression with lower boost?

all the numbers I used ARE NOT accurate i'm just using an example.

[FreshFRS]

To go go along with what you said volumetric efficiency is also higher with the high compression. with a turbo it means your going to get better power and torque when you are not in boost as well or if you wanna put it this way, much better off boost response as well as slightly better spool.

[Tansey86]

High boost + High Compression = Needs high octane gas.(e85, Race gas)
High boost + low compression = Needs not so high octane gas(93)

Its all about detonation. You have to know what kind of build you are going after to determine the compression you need. If you only want 250wheel, keep the compression and run 5 psi. If you want 600wheel, you're gonna need to lower the compression and run higher boost, if you want to keep using 93.

[SpeedR]

SupraTT has 8.5:1 if it had 10:1 it would have never made the power its known for.

[FreshFRS]

Originally Posted by Tansey86

High boost + High Compression = Needs high octane gas.(e85, Race gas)
High boost + low compression = Needs not so high octane gas(93)

Its all about detonation. You have to know what kind of build you are going after to determine the compression you need. If you only want 250wheel, keep the compression and run 5 psi. If you want 600wheel, you're gonna need to lower the compression and run higher boost, if you want to keep using 93.

good point about fuels. tuning window becomes very small at high compression + higher boost levels when using pump gas . i only use 94 from chevron up here seems to be the most consistent fuel i can find with no ethanol. i'm planning hopefully to run 12psi on pump gas on a Twin-screw supercharger and i think i will be okay from what i've seen.

[sw20kosh]

High compression simply increases likelyhood of detonation much more than boost. So it is better to have lower compression so you can run much much higher boost. If you are going for big hp numbers...

Typically a 12.5:1 motor will need to be run on low boost (5-7) otherwise you could detonate.

A 8:1 motor will swallow 17-20 psi all day long and make more peak hp than the above motor. (notice you are TRIPLING the boost pressure)

This is on pump gas. On race gas/e85, the limits of detonation are raised and you can get away with high boost on high compression.

[FreshFRS]

Originally Posted by sw20kosh

Typically a 12.5:1 motor will need to be run on low boost (5-7) otherwise you could detonate.
.

this may be true, but with DI you can stretch 93/94octa a lot more. most STG 1 turbos for this car are 8psi tunes and it seems like there is still a bit left on the table. i would start lower and work up till i find the knock sensor, that would then be my limit i'm gunna go out on a limb and guess 12psi before knock becomes a problem.

[Tansey86]

Originally Posted by FreshFRS

good point about fuels. tuning window becomes very small at high compression + higher boost levels when using pump gas . i only use 94 from chevron up here seems to be the most consistent fuel i can find with no ethanol. i'm planning hopefully to run 12psi on pump gas on a Twin-screw supercharger and i think i will be okay from what i've seen.

12 psi on pump will be tough, you realistically will not want to run more than 8ish psi.

[FreshFRS]

Originally Posted by Tansey86

12 psi on pump will be tough, you realistically will not want to run more than 8ish psi.

12psi will be tough but i think it might be doable on the decent 94 octane i get here, if i have to drop down that isn't the end of the world either, i want 300+whp and i plan on running a Positive displacement supercharger. innovate intake with Eaton TVS R1050 hopefully if it fits. start with a 6psi pulley and work up from there.

[DrifterX]

I read a great explanation of this just a few days ago.

Basically, Boost + Compression = Effective compression ratio.

When lowering or raising compression, you will see around a 3% loss or gain in power for about 1 compression ratio. This isn't perfect math, but it's simply.

When raising boost, you tend to see about a 3% per 2 psi.

If you take a motor and lower its compression from say, 12.5:1 to 10.5:1 giving it approximately a 6% loss in power. Now let's say we add 10 psi, giving it an approximate 15% increase. We've netted an overall change of 9%.

Yet our effective compression ratio is actually lower. (The math for effective compression ratio is... (boost/14.7) *2) + Actual compression ratio

So 10/14.7 = .68 *2 = 1.36 + 10.5 = 11.86. Which means you can actually boost more.

The reason for all this is, even though you are compressing it, N/A does not allow for more air than atmospheric, (Also why cars have less power at higher altitudes) whereas a turbo will cram air in there causing it to have more oxygen per square inch, or more fuel to burn with the gasoline.

I hope this makes sense.

Oh and as for why to not lower your compression ratio. If you have a low compression ratio, your non boosted power is substantially less. (Like you have lost that 6% earlier, but with no gain) so you tend to notice lag more and it is less effective for racing which requires you dip lower into RPM ranges. (I.E. road courses etc.)

[FreshFRS]

so realistically at 12psi my effective compression ratio is going to be about 14.2:1 i know of a few honda civics (street cars) that run 14:1 on 93octane with no knock. i'm thinking that With DI i shouldn't have a huge problem with that boost level then?

[DrifterX]

Originally Posted by FreshFRS

so realistically at 12psi my effective compression ratio is going to be about 14.2:1 i know of a few honda civics (street cars) that run 14:1 on 93octane with no knock. i'm thinking that With DI i shouldn't have a huge problem with that boost level then?

Well, no probably not. It seems like this engine is doing really well going over the standard 12.5:1 compression ratio. So yeah, you are probably going to be fine.

That said, I am probably not going to be getting FI for my car for at least two or three more years. (I got a warranty, and I want to make sure I get the most out of it. If a gremlin is in this car that we don't know about, I am sure as hell making toyota fix it.) I want to see exactly how far people have to push it to get it to blow up, and when the low boosters (5-8 lbs) end up blowing theirs. Once I've read about a few blow ups and I start to get an idea of how much the car can take, should be good.

Also, you can get a knock sensor (Or maybe it already has one?) to keep an eye on it.

[SkullWorks]

also compression helps to spool a turbo,

I have read the articles about effective compression ratio...I disagree with the math because it tends to make people feel good about bad things, unless you are running a chemical intercooling process (ice or co2 packed intercoolers) you have an increase in air temp that both lowers knock threshold and artificially increases the manifold pressure (Boyle's law)

the idea they are going for with the effective compression ratio also misses the fact that you are "effectively" increasing displacement, not just compression ratio.
a 75:1 motor wont ever exceed 100% VE but a turbo motor that is **:1 compression will achieve greater than 100% VE...that's why we all like Forced Induction more power same displacement.

[FreshFRS]

Originally Posted by DrifterX

Well, no probably not. It seems like this engine is doing really well going over the standard 12.5:1 compression ratio. So yeah, you are probably going to be fine.

That said, I am probably not going to be getting FI for my car for at least two or three more years. (I got a warranty, and I want to make sure I get the most out of it. If a gremlin is in this car that we don't know about, I am sure as hell making toyota fix it.) I want to see exactly how far people have to push it to get it to blow up, and when the low boosters (5-8 lbs) end up blowing theirs. Once I've read about a few blow ups and I start to get an idea of how much the car can take, should be good.

Also, you can get a knock sensor (Or maybe it already has one?) to keep an eye on it.

im pretty certain that AVCS has a knock sensor internally to help adjust timing.