Okay, here's a thought. I've been thinking about piston options for a SC installation, and I was wondering if machining the stock pistons to lower the compression ratio, and machining for non interference, were mutually exclusive OR if both could be combined in one operation.
Imagine no more TB terrors and a boost friendly CR. Has anyone gone down this path ?

 

This seems like such a basic question. I'm surprised over thirty (at this time) people have viewed it and no one ventured a comment.

 

Using the stock pistons, it would not be recommended to ream out their valve reliefs more as it would leave the piston's crown too thin in those areas.

Do a search, and you will find a very good write-up by Tony regarding machining the stock pistons, complete with pictures.

 

OK, you've made me feel guilty since I was one of the 30. No they're not mutually exclusive (how's that for a double negative). Reducing the compression ratio and reducing the interference are both moving in the same direction (ie making the combustion chamber bigger). For example putting in multiple head gaskets would drop the CR and move the valves farther away from the piston. But going that way by the time you got rid of the interference your CR would be pretty low. More likely you'd need to notch your pistons or have custom made pistions.

 

When i had my pistons machined i never had the guy render the valve reliefs on CAD. he just did the piston and the dimensions of the slope i gave him. I bought a couple of scrap pistons from 928intl and cut them up to get an idea of the "guts". Like Lags said, im pretty sure if you start dropping the floor of the valve reliefs lower than they are you may be effecting the structure of the piston too much.


Anyone know how much interfernece there is? 1mm...2mm...5mm etc etc
How far would you have to drop the floor of the reliefs to actually get a noninterference condition. Those with motors on the stands may be able to get some clay and figure it out? My spare engine is too far apart to do that now.

I think there are 3 SC cars running the lower CR piston now?

 

Explain to me why it's better to have a low CR and a higher boost, than a high CR and a lower boost? I've never understood that. It seems like either way the limit is some pressure that causes detonation. Why is it better to get to that pressure with more boost rather than more compression? Is it because the air is colder? Or does a supercharger sap less energy than the crank when compressing? I would assume a SC is a more efficent pump than a piston - is that it? Or is it because you can reach the limiting level over a range of RPMs with a SC vs just one optimum RPM with compression?

 

951 = 8.0CR = Interfearence motor

 

Assume that pressure in the cylinder that causes detonation is the same with high boost and low compression as it is with high compression and low boost. The lower compression configuration is a result of there being more volume in the combustion chamber when the piston is at the top, from something like a bigger combustion chamber volume in the head, or more of a dish in the piston top. With the pressure in the cylinder being the same with the piston all the way up, the low compression with high boost setup makes more power because there's a greater volume of the fuel/air mixture in the cylinder at that pressure than with the high compression and low boost setup.

 

Thanks Z, I totally didn't think of that. So it's almost like getting a bigger engine? I could use one of those.

 

Go over that again. I've often wondered the same thing. I was going to plug different values into my S/C equations to see what effects they might have, allowing me to understand the difference between low boost/high compression vs. high boost/low compression. I know the second to be better, but I don't understand why (at this time). At first glance, it seems like 6 times 3 vs. 3 times 6.

 

About 8:1 is ideal for the 16 valve combustion chamber and you could run lots of boost on a 32 valve chamber at that CR for sure. My vote is if you are going that deep into the motor...just get a good set of forged pistons and be done with it. Too much time on the dark side of detonation will kill the cast pistons in a heartbeat. Granted the S4 has a nice chamber and you have the knock sensors. Do it best, install forged units and then figure out a way to pull timing out as a function of boost and RPM. Virtually any new boosted motor made today will have forged internals standard. The knock sensor should stay out of the picture if the car is set up properly. I suspect the reason we hear of so many boosted cars cooking head gaskets is because the knock sensor is working OT...it should be out of the picture unless something goes bad...i.e. a safety backup, not a device you rely on every time you run the boost up. On the 16 valve turbo cars I pull out 8-10 degrees on boost for a total advance of about 20 degrees on full boost. Remember, the turbo car also runs more boost across the RPM band as compared to CS. Positive displacement will run more boost across the RPM band yet. It is a balance....boost pressure/fuel octane/ignition advance. 8:1 is a nice place to start, but many cars run over 9:1 without issue. To me, adding 10 lbs of boost and fuel to a stock NA engine, but not doing anything with CR or ignition advance (or both) is asking for trouble. I suspect the S4 Cr is not the advertised 10:1. Tuning is more than just changing one variable, but lowering the CR is a nice place to start.

 

John,
Lex is asking for some advice re engine mods for boost on another thread. I was hoping you would chime in there w/ details on coating and pistons.

Andy K

 

Let's say that you would end up with the same pressure in the combustion chamber with the high compression ratio/low boost as you do with the low compression ratio/high boost. For the compression ratio to be lower in the low compression ratio/high boost configuration the combustion area when the piston is at the top was made larger. This could have been done by something like making the combustion chamber in the head larger, using a thicker head gasket, or having a piston with a larger dish in it.

Tony had a thread a while back that showed where he had material machined from the tops of some pistons to lower the compression ratio. He might then run higher boost to get the air/fuel mixture back up to the same pressure in the combustion chamber as a higher compression ratio/lower boost engine. If he does that, he'll have the same pressure in the combustion chamber, but he'll have a volume of air/fuel mixture in there at that pressure that's greater by the same volume that he machined off the top of the pistons.

Think of it this way. You can have a one quart container with 100psi of air/fuel mixture, and a one gallon container with 100psi of air/fuel mixture in it. Both are at the same pressure, but the one gallon one will give you a bigger bang.

Forged pistons are more resistant to damage from detonation,, but don't coun't on that to save you. The 944 turbo came with forged pistons, and there are more than a few of those around that have broken pistons. Don't think you can build to withstand detonation. Build and/or tune to avoid detonation and neither cast or forged pistons are going to have a problem.

 

Quote:
Originally Posted by sublimate
Explain to me why it's better to have a low CR and a higher boost, than a high CR and a lower boost?

Maybe another rationale is that you can make a non-interference engine that way. One le$$ di$a$ter to worry about.

As for clearances, if anyone has a head off, measure how proud of the head the open valves are, make sure the intake and exhaust don't hit each other if out of time, then measure the depth from the deck-top to the piston crown, do a little math with the head gasket size and -- Voila, you should have a head gasket thickness that will make a non-interference engine.

 

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