http://www.taperformance.com/hyperteu.htm
http://www.kb-silvolite.com/page11.htm

Atleast check this out. Even the buick suppliers don't recommend them. True....lower compression OBVIOUSLY helps...no arguement there. I just wouldn't want to chance it that my car will NEVER detonate. There are too many factors BESIDES compression ratio. Just because you run low compression doesn't mean that you WON'T detonate. If you think that then ...well...anway.... Opinions are like *******s...everybody has one...and my opinion is making me build a shortblock before it blows.

Joe

 

All this talk about low CR + boost + stock pistons has me a little confused. You can get a decent set of forged pistons for $300-400 TRW, Wiseco, Speed Pro). It's not all that hard to install a set of pistons. I'm not saying that it's not true, low CR and stock pistons may well be viable. But the whole point of going low CR is to run high boost safely, as far as I am concerned. If you want lowCR for boost, for most people, the logical way to do it is with a set of forged "blower" pistons.

Not that big a deal to install new pistons.
Drop motor 4-6h.
Disassemble bottom end 1h.
Take rotating assembly to machine shop for balancing.
Reassemble bottom end 2h.
Now's a good time to do plugs and wires.
Reinstall motor 4-6h.

Total time: 12-15h. Take it apart Sunday. Put it together next Saturday if you're on a M-F schedule. Probably $6-700 for parts and labor if you get a $3-400 set of pistons. If I had done that when I first did my first SC install in '96 my whole life might have been different .

Listen to the voice of experience.

Rich Krause

 

I ran 3 years on a stock motor and 8# Procharger. I finally popped it last October and am replacing it w/ a forged 383.

From my experience, don't do forced induction on a stock motor unless you're ready to rebuild it. I know the longevity of my motor is in the minority, I had a good tune and used my head.

 

Generally I’ve been arguing the same side of this argument as you have, but I’ve got to say that your argument would be much more persuasive with more accurate facts.

In a boosted application, the static compression ratio has almost no effect on the stress on the pistons. Cylinder pressure is much more heavily effected by the boost pressure and the resultant combustion pressure spike then your compression. Typically, the cranking compression of a street engine will be in the 140-200psi range, where the peak combustion pressure could be 10x that or more. Changing your compression ratio typically changes the cranking compression a few percent which can have a similar effect on the peak combustion pressure if the cam matches the CR change and it effected the density of the A/F mixture in the chamber. Boost will typically cram 1.5-2.5x the a/f ratio into the combustion chamber, only raising cranking compression by the psi boost (not that you could really measure that anyway, but it would be about the same as changing compression), but it could easily double your the peak combustion pressure.

What lowering compression does is makes the engine easier to tune and more fool proof so you’re not as likely to see detonation which will destroy pistons and head gaskets. When it comes down to it, there is not a significant difference in the stress the pistons will see between 8.5:1 and 10:1 compression ratio in a boosted application, ASSUMING that you can control detonation as well in both applications.

No, top fuel guys run low compressions because they run nitro, not to run massive boost. They also run boost because they run nitro. Nitro has very little if any resistance to detonation (would have a very low octane number, if at all) and typically if it’s mixed with gas it also has methanol or even water mixed with it to counter nitro’s tendency to detonate. If you ran ‘normal’ compression with any kind of serious load of nitro you’d accomplish nothing but blowup the engine (instant detonation).

 

Goodness aren't we pickey today. As I understand it, the higher the CR, the more heat is generated in the chamber(I know that the cam has a big influence on the DCR, which is what the engine actually uses). And when you pressurize the chamber further with more volatile oxygen and fuel and then combust it, the heat only gets higher. Which leads to higher cylinder pressures and temperatures (the latter being the primary source of detonation). And that is what kills pistons. Lowering the static CR helps to alleviate the cylinder of some if this extra pressure. This is the methodology my fellow boosted collegues(TB.com) and myself have followed for years and it seems to be working. Not that I want to get into any of the pages-long theory of pressure differentials, flame fronts, etc. (of which i'm still deciphering at school). Yea it's good to know and learn, but good grief we're just talking about a plain street car right now. The TF example was just to show the importance of CR in a boosted engine. And the fact that they use a much more powerful fuel just emphasizes it more.

Am I the only one that sees what 1BadBrd is doing? He's not trying to all-out race it. It's not like he's going to up the wick past 7-8psi then go drag race it every weekend or everyday (at least form what he tells us). I agree with Rich about replacing the stockers with forged units, but why bother with all the time and money if he is just going to go back into the engine with a stroker combo in the next year or two? That's the side I've been on all along, not that the stockers should be used or that it should last for 100K miles or whatever else came up in this thread along its merry way.

I'm guessing that what this thread is coming to is just to wait and do EVERYTHING at once. It doesn't make sense to build a low compression stroker then use the stock heads and a small cam on it (a DOG set-up) for however long until you can afford to buy and put on the heads and charger. Whatever, I don't know, who cares? lol

PEACE

 

Ya, you guys were correct on the buicks, they use hypereutectic, not forged pistons. The factory boost however, is very limited. If you turn it up to 12-14 psi, a broken piston is much more of a possiblilty.

On a blown LT1, why go with a cast piston? It makes no sense. Go forged all the way!

 

OMG - this is the longest thread I've ever seen.

If you read it from the beginning, you'll that what started it was someone asking if dropping the compression to 9:1 by using heads with bigger combustion chambers, while not going into the shortblock, would handle 9 or 10 lbs of boost. His intent was to put the blower on now, and build a forged engine later.

 

Factory boost is limited? You've got to be kidding! I don't know any of my buick buddies that ran less than 14# - I ran 18.5# on pump gas on my 145k mile daily driver, and when the C-16 went it, it was 26-28# without a worry. Tim Stockwell ran 31# on a completely stock internal motor (10.70's at well over 123 mph with a fully optioned, full weight T-type through full exhaust). Broken pistons were just not something anyone worried about, unless something went very wrong.