Ok, I am not totally new to boosted cars. I have a 73 TA that makes 1200 hp with a Garrett 4788 on a conventional Pontiac 462ci engine at 15 psi. I understood that for that old style chamber, even with aluminum heads it was best to keep static compression below 9:1. In fact, I built mine at 8.25:1 because I knew I could make back any loss with boost.

But I am thinking about doing the LS swap, and so I've been searching unsuccessfully in the forum for some basic info. Maybe I keep missing it because its basic and everyone knows it already except me. lol

With the LQ4 block and aluminum heads, what is a safe upper limit for static compression on a boosted engine? Talking about something street happy, occasional racing but again in the 1000-1200 hp arena (yes, crank hp should be fine. lol)

I had always heard rumor that with the more modern combustion chamber the LS engines were able to run a higher static compr. ratio, but beyond that rumor I don't have any info.

Thanks in advance!

 

A lot depends on the fueling. If I knew is be going e85 when I built the engine id have gone higher cr than stock. I'm at 9.5:1 now.

 

91 octane pump gas. Mostly street use, and want to be able to drive it long distances comfortably.

 

Hello Aron,
As a fellow 1973 TA and Turbo Trans Am owner I always admired your car over on Pontiac Zone and the way you finished and improved Marks old TA.
Your going to want to stay in the 9 to 9.5 compression area with 91 octane along with meth to run the boost it will take to get to your hp goal. I am running stock close to 11 to 1 with a twin turbo but am octane limited and have to keep the boost under 8 psi with lower timing than I like. The current build I am doing will be 9.7 with 16 to 17 lbs but it will be running e85. Good luck with your quest to update the Beast.
By the way if you ever decide to get rid of those Tribal turbo headers drop me a line.

Tony O.

 

Hey Tony, whats your screen name over there on Pontiac zone? I haven't posted there in ages.

So is the general consensus that at 9.5:1 and 15 psi boost I should use meth/water injection to use 91 octane? How about at 9:1? Trying to figure out where I should be on that static compression ratio for boost.

 

on an LS2 block im at 10:1 compression ratio with 15# using 93 pump gas with water/meth injection 80%meth/20%water

 

I just bought the turbo motor in the classifieds for sale by mailman. Calculated the compression at 9.17:1 and the only actual number I couldn't plug in was bore size on the cometic gaskets. Since I didn't know for sure on that I used actual bore size for that part of the calc. If the gasket is slightly bigger it could potentially be down around 9.1:1

With a maximum of 15 psi boost, and a fairly large air/air intercooler, does anyone think I need the meth/water injection to avoid problems?

 

what CR should I run on an eforce LS3 (i think it puts out 12psi) ? 9.3-9.7?

 

Provided tuning is good I dont see why you would have any problems even without water/meth

On the street I run my car to around 23psi without water/meth simply because any extra power it can release isnt needed. I'm at 9.0:1

With water/meth I've run up to 28psi with no problems

 

old motor 9.0:1 New motor 10.1:1
IMO There is no reason to shoot for 8.5 with an LS motor

 

That depends on fuel, CR etc etc, the list goes on.

If you wanted something to run 30psi all day long on pump fuel, then you certainly wouldnt want 10:1

There could be reasons either way.

 

Thanks for the responses guys. In a street/drag car, I could see going for more than 15 psi. And in that type of car you can set up the suspension/tires to handle it.

But my car has been set up for 90% street duty and 10% roadrace. I found that even with a 10 psi tune on my old engine it was fairly easy to overwhelm the tires coming out of corners, and since the car was set up for that type of action, even trying the straight line run once in a while I don't have the best suspension setup for it and tend to again overwhelm the tires even when already moving.

I guess if I decide to try running at higher than 15 psi its easy enough to add meth/water injection at that point. I have a kit and its plumbed into my old carb hat.