Higher compression pistons

[hgallegos915]

hmmm do u guys think that I can tell if the car has forged pistons by getting a compression test? I got a k8 but it supposebly has forged pistons and better valves and springs..SUPPOSEBLY.. it eb nicew to find out.

-hec

[babyblueMX3]

compression will be the same unless the guy has put forged pistons that isn't the same CR as stock

[tatsu]

Some things to think about with a high-compression NA setup:

Static compression numbers will really not tell you whether your car will run on pump gas or not. All that is telling you is what the volume of the combustion chamber is at TDC (when the piston is at its highest position) vs. BDC (when the piston is at its lowest).

In an actual, running motor, the timing of valve events, particularly intake valve closing, affects the compression dynamically. Basically, on practically every modern motor, the intake valves close a number of degrees past when the crank is at BDC. What this means is that the intake valves are still open for a short period of time while the piston is rising in the cylinder during the compression stroke, so a portion of the fuel/air mixture is forced back out the intake valve openings, and the overall stroke length used to calculate your C/R is effectively shortened. When this new stroke length is used vs. combustion chamber volume to calculate the dynamic C/R, it will ALWAYS be lower than the static C/R.

Theoretically, the later your intake valves close, the lower your dynamic C/R, and the higher static C/R you can run. Conversely, this can be used to explain why, when wilder, longer-duration cams are used you usually have to increase the compression ratio to see their full benefit.

This is not without its limitations, however, as you are losing more of your power-producing mixture with the longer intake duration. This is particularly true with motors that have a very long rod-stroke ratio like ours, as the inherent slower piston acceleration away from TDC means that we are pulling in less mixture to begin with. This can be alleviated somewhat in terms of peak numbers by running higher rpm, but the losses at low rpm can not be avoided.

All that being said, with the ZE, the stock (static) compression is (I think) 10.5:1. Considering how over-engineered these motors seem to be, I would expect that an increase in compression of 1 - 1.5 points shouldn't be that big a deal, but it may be a good idea to invest in a better knock-sensing ignition like the J&S Safeguard system (http://www.jandssafeguard.com) just in case. They also supply guages that can be connected to the system to show the amount of ignition timing being pulled out to prevent knock. One of their guages is a dual-purpose ignition monitor and A/F ratio unit - very slick! If the guage shows that a lot of timing is being pulled back, then you would likely have to drop your compression or retard your intake cam timing to compensate.

Regarding high-rpm operation of these motors, with the 2.5L, its 74.2 mm stroke theoretically limits you to about 8,000 rpm while maintaining some degree of reliability. This is a bit of a moot point however, as the ZE cams won't make power at those RPMs anyway. The K8 and KF motors could theoretically run up to 8,700 rpm by the way, due to their shorter stroke. To survive at those RPM, the only real weak point in our motors are the connecting rods.

Good-quality cast pistons are actually more dimensionally stable than forged ones, so they can be run with the tighter tolerances needed for a high C/R with less concern about expansion at high rpm/temps. The one advantage that the forged pistons might have is increased strength with reduced/same weight - this, however is only really necessary if you are running boost. Our stock cranks are very high-quality forged units, and the split-crankcase design of our bottom end is very rigid. Only minor balancing and blueprinting of the crank and pistons would be needed. The rods, however, should be replaced with stronger, forged units JUST in case...

Anyway, I hope all of this long-winded blah, blah helps!

<small>[ August 23, 2004, 02:08 PM: Message edited by: JP ]</small>

[mdavis]

Ok.. say you wanted to run more boost than stock ZE internals can handle... Say you purchased the 8:1 pistons from CZT. What kind of boost are you realistically looking at being able to run when tuned correctly? 20+psi? If one were to add the 8:1 pistons, what other measures would need to be taken to use them, other than normal turbo tuning measures like bigger injectors/fuel pump/fmu, etc.?

[babyblueMX3]

is your tranny going to hold the power
defenitly a stand alone fuel managment system..fmu's are good for low boost application...when you are playing with 20+psi..you go stand alone...Not sure about that one tough but I would bet custom fuel rails, like you said bigger injectors (not those millenia 280cc)