Compression Ratios with Boost?
 

Gentlemen,
I'm going to be adding a M3 Procharger with the large 504 intercooler on my MPI 540. The CR is currently at 9.25:1 with aluminum heads and I also run at 3600-5000 feet the majority of the time. I'm looking to run 5 lbs with this setup which I believe brings my boosted CR to 12.4:1. This would naturally be at sea level. If say 4000 feet is applied to the mix it drops my boosted CR to 11.6.
This, in my limited knowledge of boost, sounds like 5 lbs. would be safe at sea level or 4000 feet. It seems I could possibly run 6 lbs. at 4000 feet and still only be at 12.2.
It seems I've heard that 12.4 is the limit you would ever consider if non intercooled on premium fuel. I would think it would be the limit for an intercooled application for good longevity.
Am I thinking along the right/safe path??
Any thoughts would be appreciated.
Thanks
Dave

:cool:

Hey Dave,

Good luck with your Procharger. That 540 should sing!

Have you talked with Bob lately? We got some good dyno numbers on my modified 454 MAG w/ the Whipple. He's finishing up putting her back together and I'll lake test her this weekend. The 2300 mile round trip will be worth it!

Scott :D

Your reasoning sound good to me. Your alum. heads will help. I feel when you are at sea level you will be on verge of detonation. The problem I think with your number is the CR. I have been told that motor is more likely to detonate with higher cr & less boost then with less CR & more boost.

I have friend that detonated 454 to death with 5 lbs boost & 8.75 CR on procharger stage1 kit. He had timing set to high.

I've been running a stage 4 kit on a 502 with 7# of boost, iron heads, a static cr of ~8.5 which brings my effective cr to 12.7 and total timing of 35* with no problems.
I'm using 94 octane and a cam with a 112 lsa.

I think there are a couple of keys to my succes, however.

Sunoco 94 octane
no t-stat
colder than stock plugs
full MSD.
112 lsa takes away a little cylinder pressure which equals heat. The advantages/disadvantages of that point are of course arguable. But, it's working for me.

To answer your question, I'll think you'll be fine.

DAVE

You will make more power with a low CR and high boost than with a high CR and low boost.

That said, I was told you can run 3 psi boost with 9.5 CR on 92 octane with 32 degrees, but I havn't tried it (still afraid I'd toast a motor trying to put blowers on with the existing high CR).

-Greg