I'm putting together the plan for an upcoming twin-turbo LS6 motor, and I find myself in somewhat unfamiliar territory. Prior turbo motors I've messed with ran on race gas, but the one I'm building for myself will run on 93-octane pump gas.

Given the stock compression ratio of the LS6, just how much intercooled turbo boost can the motor handle on pump gas (this is assuming I don't replace the stock pistons)? If I elect to go with new pistons, what kind of CR should I be aiming for that won't make the engine a low-boost dog?

And while we're on the subject, do piston/valve/comb. chamber coatings significantly alter this equation? I'm angling for about 500rwhp in a stock displacement, emissions-legal LS6, spinning the motor below 6400rpm. Is this a realistic figure?

 

I think this power level has been done before on a stock ls1. It depends on the turbo sizing if you want to reach that 500 mark. NOt sure the stock compression of the ls6, but assuming 10ish, Id say the same rules apply for pump gas. Around 8psi is usually safe on a good tune.

 

10.7:1

 

Would an intercooler affect this number much? Stock CR on an LS6 is 10.5:1 if I'm not mistaken. That doesn't leave much room, does it? What would I lose if I got lower CR pistons, say, something around 9.0:1 or so. How would that affect the low RPM drivability?

 

I'm not sure what this is in answer to. Care to expand on this?

 

Sorry.. ls6 compression.

 

8psi intercooled will get you there. I wouldnt NOT intercool it. I'd say 8psi should get you to 500-550rwhp.

But considering an LS6 is good for 350rwhp, headers/catback/tune/intake/minor head port and a cam is good for 400-450rwhp... Are you sure that's the path you want to take? Boosting the engine's stock internals will shorten it's lifespan. If you stay N/A and get a mild cam and a mild port, you can get over 400rwhp without a problem. Mix that with a mild nitrous wet kit (or a bigger cam) and get to your goals without as much cost, effort, and strain... but a turbo LS6 has a huge intimidation factor...

 

If your gonna rebuild with low compression (recommended) I would go with 8.5-9.0:1...stock Buick GN's come with ~8.5 and they push 14psi on top of that, most GN guys will attest that giving .5 more compression isn't worth the loss in boost from it. At the same time, lots of people will say 9.0:1 is a good all around street compression with boost, as it allows the motor to still run efficiently but you won't get quite as much boost out of it.

 

i run a liquid intercooler and roughly 9:1 compression

car has done 636 on 93 octane and probably had room for another psi but no reason to push it and screw somethin up, but i'm sure it would do 650. was 24* timing as well.

cometic head gaskets and standard arp studs. ported 98 ls1 heads and a very mild cam.

i would not push anything on those stock pistons and pump gas... especially if 10.7 and not 10.

nothing inside my engine is coated.

 

Just out of curiousity, why wouldn't you want to intercool it? Too much pressure drop for too little temp reduction? Also, is that 8psi on pump gas or race gas?

Also, I don't want to go NOS because (a) I consider it more dangerous than turbos or blowers and (b) I don't want to refill bottles. I hit WOT quite a few times a day in my daily commute and I'd run a bottle dry in no time.

As for staying N/A and going with a top end/cam package, I've done that already with my LT1 Camaro. I'm getting 373rwhp out of it with stock displacement. Quite respectable, but I can't get it to pass emissions and it idles too roughly. It's hell in stop and go traffic because of the idle quality. I like the turbos best because of the near-stock part throttle performance manners but with the ability to go absolutely insane when the boost is on. I can't think of a better solution.

 

Is that flywheel HP or RWHP?

If it's RWHP, that's one helluva motor. Stock displacement? That just doesn't seem possible...but then again I've never tried.

 

Some head units (lets take a powerdyne blower for example) cant push that much flow. A powerdyne is decently efficient, but the problem is if your IC suffers a 1psi drop and you're maybe pushing 6psi, what HP gains will you see for the cooler charge vs the loss in air? It's a tough call. But 99.9% of the time, intercool it or use some other method (run alcy injection, run race gas, run methanol instead of gasoline or race gas, use a water injection system) but something. Cooler charge is usually better.

 

The Powerdyne is a supercharger. Am I to assume the intercooler principles of a supercharger apply to turbos exactly the same way? I've never run an IC'd supercharger, so I don't know.

Also, if you're getting a 1psi pressure drop due to the IC, why not just crank up the boost an additional 1psi to compensate? Granted, the temp decrease/pressure drop is not linear (it's more likely a percentage), but if your target is 8psi at the motor, you should be able to produce that by taking IC losses into account. Or am I missing something?

 

thats rwhp, stock displacment ls1

heads dont even flow 300cfm cam is 224/226 .523/.510 lift 115*

you just have to have a nice free flowing set up, and turbotech did that for me

 

Intercooling a centrifugal compressor is the same. What drives it isnt so important. An Eaton/Roots blower bahaves a little differently then a centrifugal.

Technically for 8psi on the motor you could turn up the boost. Downside is that a loss in pressure across an intercooler isnt JUST a loss in pressure. The reason you're loosing pressure is the charge is cooling and expanding too rapidly or improperly, and thusly also slowing down. This will increase backpressure against the compressor wheel, etc etc... these values are small, but sizing an intercooler does take thought. Also if your unit is nearing the edge of it's efficiency spot in the map, making it push more may make it generate much higher temps. Again, this is where sizing the turbo head unit properly for the lb/min flow that you need, and sizing everything to work together. Look for "Maximum Boost" by Corky Bell at the bookstore or amazon.com... It's not cheap at $35 cover list price, but worth every penny if you pay that much. I'd say borrow it from the library, but the tables and references in there are nice to always have around.