I'm looking to doing a forged rebuild and want to run 12-15 lbs of boost.

What CR would be the best for reliability and performance, 8.5:1? And in what ways could I lower compression besides buying aftermarket heads???

I don't want to spend a whole lot

How important would be my cam selection be? I'm running a 218/228 hotcam now, I'm just looking to have a very good setup

Thanks in advance.

 

three things you can do, a thicker head gasket, or dished pistons, or heads, but you said u didn't want to do the last one. I have seen engines run with as high as 9.5:1 compression with a blower, for mild street use, it depends on how you want to drive it and how much boost you will plan on using 8.5:1 is a good place to start, but again, it depends on the amount of boost you plan to use. Good luck

 

 

sry, i missed that part, was in a car all day driving to ohio to pick my car up, a little tired. Anyway, a thcker head gasket wouldn't do too much for you, you will have to go with dished pistons to lower compression, but, have you thought about leaving the compression as is? I couldn't tell you how much boost you can run with stock comp ratio, but that would be something to research. But ya, unless you buy heads with a bigger combustion chamber, you will have to go with dished pistons, which aren't expensive, but pulling the engine and having them put in could get expensive if you don't do it yourself, and thats quite a hassle. You have to outweigh the costs of each and see which one you want to do, but I would first see how much boost you can run with stock compression, and go from there.

 

The Hotcam will work ok with the boost. Ideally you want very little overlap to prevent blowing boost out the exhaust and generally a 114 LSA is prefered over the Hotcam's 112 LSA because of this. The good thing the Hotcam has is a big split to the exhaust side, which will work good with the boost.

Botch

 

Running dished pistons is preferable to running a thicker head gasket because with a thicker gasket you'll lose quench, which is probably the last thing you want to do if you are adding boost.

 

Heres what I've collected so far"

Head Volume: 58cc (Stock)
Piston Top Volume: 24cc (JE Pistons Forged)
Deck Height: 0.025 (After Decking -0.005)
Head Gasket: Fel-Pro (0.0395)

After the above numbers and using the SCR calculator I reached a Ratio of 8.6! Sounds good!

Or if I went with the stock gasket of 0.049 I could get 8.5:1, but I think the Fel-Pros are well worth the 1 tenth of Compression to get.

 

With a given setup (stroke and chamber volume already determined) your main variables in CR will be rod length, piston compression height, and piston volume.

But there are alot of ways to change compression. If kinda bugs me when people just think you should throw on dished pistons or use a thicker gasket. The compression has to be precisely calculated using the stroke, deck height, rod length, piston compression height, piston volume (dome, flat, dish), gasket, and combustion chamber design (i think i got 'em all).

And thats just for static compression. The cam design and boost will affect dynamic compression. When you get your setup made, you can easily use stock combustion chambers to achieve your goals. Talk to a qualified engine builder and he will give you the specs you need for your specific engine setup. If you want to double check everything, we will gladly help you determine the CR with a given setup.

 

thanks, I'll be using 5.7 Rods, 0.025 deck height, 24cc Inverted Dome pistons, .0395 Felpro Gasket, and at this point a 218/228 112 Cam with .525/.525 Lifts...

 

If you're looking to run 12-15#, you'll need a static CR of about 6:1 on pump gas. Here's some good info to look over.

http://www.holley.com/HiOctn/TechSer...fo/SCTech.html

 

There's guy running pump gas doing 14# @ 9:1 on this board somewhere....

According the that chart from Holley, their target effective compression ratio(ECR) is 12:1. But when you look @ a stock LT1 SCR, and 6# of boost it equals 14.8:1.

I see people running 6# all day on a stock LT1. There ECR is also using only 92 octane.

I only run 94 octane and according to the 14.8:1 ECR I could run 10-12# with 92 Octane....

Quote from their page "Note that for most street applications with 92 octane pump gas, you should keep your effective compression ratio below about 12.0:1. "

I don't think they're referencing to LT1s, which of course are reverse cooled to have high compression without detonation. I'm pretty confident with a HUGE *** intercooler I could run my 14# of boost with an 8.5:1 SCR.