compression ratios

I did a search for this but I couldn't find anything that had an answer I was looking for. The best page I found was this one

http://www.automotiveforums.com/vbulletin/showthread.php?t=123722&highlight=piston+compression

I'm putting the Hahn racecraft stage I kit on my car(eclipse 420A). I am now buying internals so I can eventually upgrade the system. I'll initially only be running 5-6 psi, but I plan on increasing this up to somewhere between 15-21 (whatever the lowest number is that gets me the performance I want). I'm putting in forged rods/head studs/valve springs... but I don't know what compression pistons to put it. I've seen some for sale that are both 8.5 and 8.8. the above article also mentions compression as low as 7! I can see how lag is going to get worse with lower compression pistons. I'll try and combat this as much as I can with other mods, but I only want to go as low as I have to. I was wondering what the highest compression pistons someone would recommend for up to 21psi on a 420A

Thanks!

Well, ok. Congrats on the Hahn Kit. When ever you build an engine you have the choice. You can rely on what other people tell you. or you can take the oppertunity to lean somthing new, and do things right. No matter what anyone else says.

I would strongly suggest 3 books.
*How to Tune and Modify Engine Managements Systems by: Jeff Hartman
*Maximum Boost by: Corky Bell
*Auto Math Book (cant reamber)

That $50-60 could potentualy save you thousands of dollars. I can't tell you how valueable Jeff Hartman's book has been to me. It's the best one i've found in 2 years. Tuning in a nutshell, but rather a 300 page, boring, long drawn out, neardy nutshell! Love it!

That said; Mechanical Effeceny of an engine depends alot on design & combustion. When I use the term design with you, I really mean how you built the block, not the factory. Your changing the way the engine was designed to run, and effectivly every other aspect of its air/fuel delivery to support what you'll be using it for.

Desiging your engine based on the type of machineeing you do, for your HG, Pistons, Rods, all start in the selection. Some pistons/rings work well with diffrent hones. Some HG require a special Deck smoothness and Head Milling for best contact. Most importantly of all is correct ballancing whenever you change somthing from that of the factory. Even somthing as small as a flywheel can alter the ballance of the rotating assembly. Make sure you spend the extra $115 or so and get your crank, rods, pistons, fly ballances.

Deckheight, Head Mill, HG, Piston shape, all play into the equation of compression. thats where the Auto Math book comes in handy. It has all kinds of shit in there that alows you to memorise other less important shit =). Compression ratios are important, but not significatly so in your case. I would suggest a 8.5 pistons. You can change your gasket thickness if you want to increase or decrease the ratio, simple fix.

I wont get into the Mechanical, Thermal, Volemetric effencys of building engines, but adiquite provisions in oil supply, cooling, need to be addressed. Your timing will depend on it.

The most important thing after getting everything installed and done will be making power. If i reamber correctly that hahn kit comes with a 5-7lb spring and no tuning device. Buy yourself an air/fuel controller, and that book.

For whatever it's worth, the difference between 8.5 and 8.8 will be too insignificant to notice, so don't stress out about that. The difference from 7.8 to 8.5 as in the 1g to 2g example is also quite small. 7 flat would be a substantial jump though. Good for a race motor that will run assloads of boost and wants some extra afety margin built, not so good for street use, but I have never gone that low. I have been everywhere between 7.8 and 10 and saw no real difference in spool or off boost power. I think it's one of those situations where the theory makes sense (and is completely true I'm sure) but in reality you just can't tell the difference. I would just go with what other successful turbo NTers are using. Hard to argue with what is proven when it comes to things that are so vague and hard to quantify like this.