compression ratio?

[Big Block Bob]

Hi, I was wondering what is the most cylinder compression apropriate for a turbo? like would 11:1 compression be too much? or 10:1?
also would installing a thicker head gasket lower compression ratio?

[454Casull]

It depends a lot on the % octane of the gas you fill up with.

[MustangRoadRacer]

it also depends on the level of boost you are running, and your timing advance/retard.
contact a turbo manufacturer for more info.

[Polygon]

With good tuning and higher octane fuel you can run 10:1 but the engine won't be reliable and I would never run an engine on 11:1 with a turbo unless it was a pure drag car. I would suggest between an 8:1 and 9:1 compression ratio for ANY turbo application.

[SaabJohan]

Most OEM turbocharged cars are running with a compression ratio of a high 8:1 or a low to mid 9:1.

[johnnyBgood]

I know a fair amount about eclipses, so here is what I know about the CR. The first gen eclipses came with a CR of 7.8:1. The 2nd Gens came with a CR of 8.5:1. The most powerful 3rd gen 4cyl I have seen is turbo'ed with a raised CR of 10:1. he is currently looking for is 400 whp dyno around 20 psi. His last dyno was 316 whp on 14 psi.

Raising and lowering will affect how much boost you can run, but it will also have a detrimental affect on how your car is off boost. If you have a large turbo running lots of boost and your CR is 8:1, don't expect to get great response at lower RPM's. But when that turbo does spool up, watch out.

A thicker head gasket will lower compression just as a thinner head gasket will lower compression.

[MustangRoadRacer]

that's a pretty ghetto way of adjusting compression if you ask me.
changing the pistons/ heads is the way I would prefer to do it.

[454Casull]

<<A thicker head gasket will lower compression just as a thinner head gasket will lower compression.>>

I think you meant:

A thicker head gasket will lower compression just as a thinner head gasket will raise compression.

Anyway, you can use a higher compression ratio if you have an optimal combustion chamber shape (which depends on the shape of the piston crown and the head).

[johnnyBgood]

Yeah, that's what i meant. Typo in the brain.

[2turboimports]

i wouldn't want to be running more boost on a thicker head gasket unless other precautions were taken like o-ringing the heads.

[Evil Result]

Lets say in theory if i had a 8:1 ratio and i was running 14 psi what would the NA compression ratio be? like 10-11:1? or how much is internal cylinder pressure increased during compression or BMEP, ect?

[454Casull]

Originally Posted by Evil Result

Lets say in theory if i had a 8:1 ratio and i was running 14 psi what would the NA compression ratio be? like 10-11:1? or how much is internal cylinder pressure increased during compression or BMEP, ect?

The static compression ratio would stay at 8:1, since the dimensions don't change at BDC and TDC. I don't know much about dynamic CR.

Cylinder pressure is inversely proportional to volume.

P1V1 = P2V2 - you can't use this formula once the spark ignites.

[Evil Result]

<DIV>http://www.rbracing-rsr.com/comprAdvHD.htm

Camshaft, Rod Length, Boost and Altitude Correction to Compression Your engine summary is as follows: Bore 3.600 inches, stroke 3.000 inches, rod c-c length 5.000 inches, with a static compression ratio of 8.0 :1. Your camshaft specifications call for an inlet valve closing of 0 degrees ABDC (after bottom dead center).Your chamber volume is 71.49 cc's. With this camshaft your dynamic, or effective stroke is 3.00 inches. Your dynamic compression ratio is 7.30 :1 corrected for cam timing, altitude, and rod length. Your dynamic cranking pressure, corrected for cam timing, rod length and altitude is 141.08 PSI. Your dynamic boost compression ratio, reflecting static c.r., cam timing, altitude, and 18 PSI is 16.24 :1.
Knowledge is power.

These are the setting i used, something of interest to me is no matter how far degrees ABDC the dynamice compression kept going down as if the VE was always 100% or more even without boost, well its rough math i suppose.

[SaabJohan]

P2 = P1*CR^1.32

where P2 is pressure after compression and P1 before.
CR is geometric compression ratio.

Most exhaust valves closes after BDC, this doesn't mean that air will always leak out under the beginning of the compression. Air can leak out on low revs but at higher revs the increased static pressure at the intake valve will make it possible to fill the cylinder even under the first part of the compression stroke.

High compression turbocharged engine often cause high exhaust temperatures since they require a very late ignition. Can cause problems for the turbine/exhaust valves and so on.

[454Casull]

Would you guys believe boost at 27psi in 11.2:1 compression on pump gas?