My car is an every day driver, but I drag race it just about every weekend. I've been pushing 10 psi boost on the stock bottom end with an intercooled ATI D1SC. It finally grenaded. Now I'm doing a new short block that can handle the abuse. I'm just wondering what the max compression I can run with 10 psi boost. I know for a fact that 9.4:1 works fine. How much farther can I push it? How do you guys think an all forged 10.5:1 383 would respond to 10 psi boost? I know Desktop Dyno says it would be insane, but that doesn't mean it would last more than a weekend either.

 

If you want more power you should lower compression and raise boost.

 

Check out http://www.procharger.com/intercooled.shtml#anchorb

I will be running 9 psi of boost with my 10,3:1 383 next year hopefully........

 

If you run in detonation you WILL blow the motor, it's only a matter of time. I don't care if you build it from billet titanium. It's gonna go BOOM! And no, using a lot of boost retard will not save you from this fate if you have too much boost + too much compression.

If you are running pump gas then forget about crazy 10+ compression ratios with boost. Put it out of your mind completely. I am surprised that you even would consider 9.4:1 with that much boost, but if you know for sure the combo of the guy who is successfully running it and will duplicate it exactly, then more power to you.

9:1 is what I'd recommend with that much boost MAXIMUM. And you're still going to run a boost retard.

Compression is not the be-all, end-all of making power. And on a blown motor you DEFINITELY want to miss on the low side. It's just too damned dangerous (and expensive) to live on the ragged edge. That is, if you like to get 50-100K miles out of your motors like I do. If it's just got to hold together for a few races then forget everything I said.

My expereince is FILLED with friends who tried to run boost on a high compression motor on pump gas. It NEVER has turned out well. But, people keep right on doing it anyway.

 

If yu use a cam with a little over lap you can bleed off a little cylinder pressure and save the head gaskets

 

Just wondering what compression you were running with the motor that grenaded?

 

It was a bone stock L98 bottom end, 9.4:1 compression. I put headers on the car, a ZZ4 cam, and Trick Flow Twisted Wedge heads. It held for quite a few passes at the strip, but finally let go.

 

I was going to build a long rod motor, but after looking into it changed my mind. The long rod setup tends to build even more cylinder pressure, which I don't need. Instead I'm going with the standard 5.7" rod length. I have a forged steel crank, forged h-beam rods, and forged dished pistons. It will end up being a 355 with 8.8 to 9.4 compression, depending on how I deck the block. Anybody know what the recommended quench area is on the Trick Flow heads? I've been reading quench should be between .035" and .065", but want to set it up perfect for my combo.

 

ZZ4 or HOT?

 

Just the stock ZZ3/ZZ4 cam. .474/.510 lift

 

according to weiand/holley (roots bower people)

a 9:1 engine with 10 PSI of boost will see a 15:1 dynamic compression ratio. after 12:1, they say, your motor is in the "danger" zone. (timing retard / more fuel)
after 14:1 they say you are in trouble without O-ringing the block, Race fuel, timing retard, etc...

now if you can INTERCOOL it, thats another story. people run 10 PSI intercooled boost with stock timing and iron heads all day. just use the highest octane fuel....

 

I will run 10.3 and 9 psi of intercooler boost. And if I need I will buy a boost timing retard unit. Here in sweden we have 98 oct. gas but I'm not sure its calculatet the same way in us........

 

I don't believe it is. I think all of Europe uses the "other" measure of octane. 98 in the UK is about 92 or 93 in the USA.

 

there are 3 kinds of octane ratings two are scientific and the third is an average of the other two which is what is displayed on our pumps (it says so on the sticker on the pump if you look closely). But in other countries the may just use one or the other.

 

I’ve said it before and I’ll say it again: A boosted engine’s power output depends on the air pumped by the compressor. Nothing more, nothing less (assuming that you can tune for the air being pumped). Changing the static compression of the engine does not change the ultimate power output (at least not in the range we’re talking about), but will soften up the bottom end and may do the same to the midrange.

For the most part, if you want to make more power you’re better off using less compression and more boost, especially if you’ve got to run the thing on pump gas

 

I thought he was worried about detonation sensitivity and engine survivability

 

this is what Im supscribing to...

 

Yup. I'm getting ready to build my L98 for a Vortech S-Trim (V1) and I'm gonna go even lower than 9:1. I want 8.5:1 or even 8:1. For now I'm pushing 7psi (with boost retarded) but later I hope to push 12-15 psi on an intercooled upgraded unit.

Lower the CR. Turn up the boost.

 

I wouldn't go that low. Its not a 10-71 roots. The Vortech doesn't make boost all the time, so you're going to loose part throttle power. I ran 10 psi intercooled boost on the 9.4:1 stock L98 shortblock. I'm sticking with 9 to 9.5 compression. With all forged and balanced internals, it should be just fine.

 

I have a friend who has a 383 with 8.5 in comp and I didnt like it. It felt "slow" before the boost came in............

 

just my .02 im currently running 9psi. no intercooler but water injection. i just last spring changed over to afr heads at 10-1 before that i was running stock iron heads and 9.4 to 1. and had put approx 10000 miles on this setup with very little tuning and approx 1 degree of timing retard per lb of boost.now with the afr's i need no retard to keep out knock counts but have learned to burn chips and installed larger injecters and at least 100 hours of tuning. i personally think that 10-1,10.5-1 is possable as long as you run aluminum heads,some sort of an intercooler and have a scanner to tune with.burning chips is also great but i put over 10000 miles on with the stock chip and a little boost retard.
pete
ps i'm running a stock cam with 1.6 rr's