high compression vs. low compression on turbos

[rice(er)]

why is it so bad to turbo high boost on a high compression vehicle? if you have a high compression car and running low psi, would you be safe? as far as i know from reading around the forums is that....low comp = higher boost .......and high comp = low boost.....<---this correct?

[flip888]

Id also like to learn more about this but from what i know, the most important difference between having lower or higher compression is what grade of fuel you need to prevent detonation since when you compress air, it heats up so much. So i think your right about high comp means you can run less boost, unless you run high octane with the high compression engine.

[CBFryman]

think about it
when you compress a gast say 11:1 you are multiplying pressure 11 times. atmosphereic pressure is about 15psi (a little less) so when you compess it 11:1 you have 161psi pushing out on the gasket and heads and only 15psi pushing back in...
now add some moderately high boost, say another 15psi, now you have 322psi pushing out and still only 15 pushing back in...too much and a blown head gasket is a guarontee.
on top of this all of that air being compessed causes alot of kenetic heat to be consentrated in one area making the temperature peak quickly (which is how diesles run, but we are speaking of a 4strok gasoline engine) this temperature peak causes detonation which we all know is bad unless you plan on running 112 leaded fules.
when you lover that compession to say 8:1, naturally asperated you have 130psi pushing out and 15 pushing in and with that same 15psi you have 260pushing out and 15 pusing in, there is also less of a temperature peak and lower octane can be used or a more advanced timing can be ran.
now with a higher compression you have a greater thermal efficency but every engine has its limit and every fule has its resistance to detonation.

[nissanfanatic]

When using lower compression pistons, you have more room to fit air in the combustion chamber as opposed to fitting it with metal. So more airflow=more horsepower. Less peak cylinder pressure=less torque.

[CBFryman]

or you can get a super strong bottom end and head gasket and run high compression with high boost creating a flatter torque curve with the same top end power

[nissanfanatic]

Not ultimately.

[drdisque]

Originally Posted by CBFryman

think about it

that's a tall order for this guy

[Schister66]

also on high comp high boost engines, you have to have the car tuned nearly perfect to avoid detonation.....

[curtis73]

Here's a website that shows a chart of final compression ratio. Its a chart plotted with initial compression ratio on one side and psi on the other. Since street engines are limited by pump gas, your limited on how much pressure you can add before you're out of octane. When that happens; Kablooey.

http://www.goodvibesracing.com/Compression%20Ratio.htm

Dropping CR does not remove as much as a turbo gives back. As long as you don't drop below 8:1 or so, you won't notice sluggishness when the turbo is not spooled up, and depending on how well you tuned the turbo it will spool up almost instantly after you mash the pedal anyway.

Just dropping from 10:1 down to 8:1 you might lose 10% hp, but the 10-ish psi you can now blow will increase your power output by about 70%

[nissanfanatic]

READ on the knock limit!