Question about boost

[Kwando]

I need help understanding why it is safer to boost with lower compression. This applies to the S2000. These numbers were pulled from a thread on www.s2ki.com

Secanrio #1 - 10 psi on 11:1 CR is 18.5:1 total compression under boost.

Scenario #2 - 15 psi on 9:1 CR with the 3mm HG is 18.2:1 total compression under boost.

Why is it not safe to boost on a high compression engine and what's the difference if the end result in total boosted CR is the same between the two?

Thx

[Weston]

It's more efficient to have the turbo doing the compression than the pistons, plus you can also cool it down after it has been compressed by a turbo. So, when you're under boost, you make more power and stay cooler with the 15 psi on 9:1 compression than you do with 10 psi on 11:1. However, the 11:1 motor will be stronger before boost and spool the turbo sooner.

[boostedEG]

i am kinda guessing at this, so if i am wrong please correct me.

Why is it not safe to boost on a high compression engine
because of the really high "effective compression ratios". detonation/knocking becomes a problem.

what's the difference if the end result in total boosted CR is the same between the two?
well in reference to how much it knocks/doesnt knock i dont think there would be much a difference between the 2.... but if you have a lower CR, you would have to run more boost to reach the same effective CR, just like in scenario 2, it is raised to 15psi, which yields more HP than scenario 1.

so in short the point of lowering your CR is to run more boost to make more power.

if you already have it tuned on a specific setup @ a specific boost level and it runs great with no knocking issues, and you dont plan on raising the boost then i dont think lowering the CR will help you.

[HondaPower]

Excellent question.

[ComputerJLT]

it's mostly about knock threshold; you're far more likely to knock with the higher compression pistons than the lower compression even when you factor in compression with boost.

Though at low boost levels it's easier to make more power with higher compression and you have a more effecient burn. 11:1 + 10psi + e85 = happy motor. Unfortunately you're close to the ceiling of what you can do there but if thats all your shooting for then hell yea, good gas mileage + a decent amount of power and quick spool

[nicklk]

it's mostly about knock threshold; you're far more likely to knock with the higher compression pistons than the lower compression even when you factor in compression with boost.


Somewhat....heat is also another factor. Its pretty difficult and time consuming to tune a high c/r + boost motor, but I feel its more efficient....dependent on your turbo and motor set up....

Here's my real world scenerio to compare it to....

I was running 10.8:1 c/r and ~9psi of boost with my Integra, and it was an easy 11 second car if I had the right size slicks (I was running 22" and ran a 12.2 with a 1.9 60' and had a LSD)....

Out of boost driving was more pleasurable because it wasnt a dog, and spool was instant on a t3/t4e turbo......

I also built a 84mm GSR motor, that ran 8.5:1 c/r and 24psi of boost....it was a dog out of the hole, and I could easily pull him on the free way and from a dig on the streets (he never ran his setup at the track)...

I honestly think 10.5:1 c/r is the BEST streetable boosted motor...

[ComputerJLT]

Also as boost pressure rises on a high compression motor the overall comp ratio rasies more sharply; ie: 11:1+10psi = 18 and 9:1+15 = 18; but at 20 & 30psi the low compression motor has way less overall compression

[Skaterkid]

I'd just like to point out that high compression does not help spool, just helps off boost power. Since high compression allows you to get more energy per mass out of a given air fuel mix, the exiting gas has less energy per mass, so your turbo has less energy to draw from to spool.

[ghost_masterCL]

I'd just like to point out that high compression does not help spool, just helps off boost power. Since high compression allows you to get more energy per mass out of a given air fuel mix, the exiting gas has less energy per mass, so your turbo has less energy to draw from to spool.

but wont it increase the exhuast gas velocity?

[ryanman]

I had 10:1 pistons in my last 2.3 4g63, I had full boost (16psi) on a 14b at 1800 rpms, it was a very fun street car.