[VM0824]

ok this is just for my own curiosity but im confused to the benefits of higher compression versus boost. i found this nifty little calculator from another thread that said at 10psi boost 9:1 static compression im making the equivalent of 15:1 compression at sealevel. sounds cool. what would be the difference if i were to simply run pistons and heads which created a 15:1 compression? obviously now im only getting that compression when im in boost, whereas id have that compression at every rpm/load if i did the piston/head combo.

basically, why do most of us opt to use boost versus building high compression motors? efficiency? mechanical clearance issues?

[MustangMatt96GT]

Two things... Detionation and cost or race fuel...

that calculator is something that shouldnt be used for what your thinking about...

With high compression, the engine will detionate unless it has a slow enough burning fuel (Q16 comes to mind). High compression also creates alot of heat when idling and everything else, because you have to retard the timing back.

Boost and high compression are completely different, and then you are still starved of air, where with the boost you have alot more oxygen to burn with. This is what allows for the use of lower octane (91-93) on boosted cars, because there is alot more oxygen to mix with.

Its not as simple as you are thinking...

I have run 13 PSI with a Novi 2000 on a 11.5:1 motor! E85 FTW

[VM0824]

ok fair enough. thx for explaining it

[nolife]

Fuel requirements like already stated and the quality of components required to reliably run with a higher compression all the time like more stress on the valvetrain, sealing and heat generation for example. With normal or low static compression combined with boost, you get the advantages of a low compression engine while cruising and some of the high compression advantages when under boost. The safe "effective compression" number (basically static + compression added from the boost + some corrections for rotating parts and timing events) for your combination is the important number and that number can be achieved though a number of different static + boost combinations like low boost high compression, high boost low compression, middle boost middle compression etc. No single combination is right or wrong.

[quadracer786]

valves hitting the piston also becomes an issue at higher compression, and you have to make sure to have the perfect heads and cams for the job. but when you do go full n/a with that kind of setup, theres nothing else like that power

[69fastback]

Boost does not creat compression. It creates manifold pressure.



What does compression have to do with valves hitting pistons?

[VM0824]

Originally Posted by 69fastback Boost does not creat compression. It creates manifold pressure. What does compression have to do with valves hitting pistons?

yea i know it was "effective compression" or whatever. and the more compression you want i imagine the more valve clearance issues you may run into. if i understand right

[kenco]

69fastback, increase in manifold pressure will result in a increase in cylinder pressure, the whole point of boosting as I understand it.