compression ratio and it's effect
 

Are there any articles or resources that talk about the LS1 specifically and the effects of reducing compression ratio? I'm looking to up the boost and still be safe. I planned on buying a new forged internals that will reduce the compression. If an ls1 engine on 5 psi and 10.1:1 compression makes around 400 HP then how much boost will it take for me to make the same amount of HP on 9.5:1 compression or 8.5:1? Does anyone have any experience with dropping the CR instead of just adding meth/H2O. What effects does this have on the lower RPM range? This is my daily driver so I have to be worried about the lower RPM.

One impact is if you later want to sell the car and the blower separately, the car will drive like crap with a low CR and no boost.

The farther you drop CR, the more boost and flow the blower has to provide, which the belt has to support. Bigger head units and better belt setups cost money.

I'm running 8.65:1CR, a D1SC, 3.2" 8-rib Reichard pulley, and hit near 14psi. But the blower doesn't like spinning that fast and boost starts dropping above 5,500rpm until it hits 10psi at 6,500rpm. I just dyno'd yesterday and I'm at 570rwhp/560rwtq. I called procharger, and they're recommending I upgrade to an F-1 head unit.

Good Point, I had not thought that far out yet. Though I'm pretty sure it wouldn't be of much benefit to me to part them out.
Also good information, but I'll be running a STS rear turbo unit, so it's not as big a problem.
Did you ever run at lower boost and that compression ratio? If so, what were your dyno numbers like?

My 6psi dyno had the worst numbers, 411rwhp/416rwtq.

Kill procharger's engineering.....

I push 16 psi and 700 RWHP @ 8.2:1 compression. If you are using the stock ATI intercoolers, that is your problem, not the blower head unit.

Are you kidding? :eek2: Is that with a D-1SC-1? I have an early A&A FMIC (wide and thin) but their latest full wrap bracket and 8-rib pulleys.

Define crap. I originally built my 360" SBF as a nitrous motor. At 10:1 compression it made 449 RWHP and 437 RWTQ. I decided to go with a turbo, and swapped out the pistons for a set of dished pistons at 8.3:1 CR. 419 RWHP and 409 RWTQ. Could I feel the difference? Sure. Does a car making 409 ft-lbs of torque to the ground drive like crap? Not a chance. Torque loss being a bit less on the low end and a bit higher on the top end, it still is a strong motor.

If motor X makes 400 HP at 5 psi on 10:1 compression it would likely make 400 HP at 6 psi on 8.5:1 compression. Especially when we're dealing with 350 cubic inch motors, the dramatic low end power losses associated with dropping compression are greatly overemphasized on here, IMHO. For a street motor 8.5:1 compression is a nice healthy range. I would never build a 9.5:1 street blower motor, you just can't run as much boost as I would like to.

2.75" mid pipe?? just one?? or do you have two? -Tim

Yup. D1.

Mine's not a freak either. A maxed D1 should bring 600+ RWHP no problem at all.

Maybe it's the effect of driving around on boost, and suddenly not having boost (when a hose blows off), but the feeling of driving without boost on a low CR motor is as if it were worse than stock. Which it should be if stock is a full point CR higher.
Dual 2.75" mid pipes, the Melrose headers have 1.75" primaries x 22" long, and what looks like random cats.
Interesting. I'd love to talk further but don't want to hijack this thread worse than I have.

Yeah, right. Basically 2 full points in compression was worth 30 RWHP on a motor that started off at around the same level as an average heads/cam LS1. Half that difference is 15 RWHP. That's like a lid and a catback, that isn't a huge difference. And are you telling me you're going to have a fully built motor running stock heads, stock intake, stock cam, stock headers, and stock exhaust? Even at 8:1 compression you'll make a lot more power than stock.

Boost > Compression

When my belt fell off on a pass last year, it was pretty funny... felt like 1/2 throttle with a boat, know what I mean? Even with my 8.2:1 compression, my car should still run mid to high teens on motor with my 4500 stall, heads, and cam.

I have had a direct before/after comparison with compression drop several times (1-2 points) and the only difference that I could tell is that I couldn't use 6th gear at 65 mph anymore. I have to use 6th at 75 mph+. So the car lost some of its low-low end.

Had I not had a direct before/after though, I wouldn't have been able to tell.

One story that I always tell the folks that buy into this low-compression drivability problem. We had a 99 Mustang cobra come in one day with a fully built motor. We strapped it to the dyno and laid down a wopping 220 HP. The car ran great though. After a couple calcs we had figured out that the motor had inadvertantly been built with 7:1 compression. You would never been able to tell though. The car ran and drove fine. His power loss was an utter mystery until we started to ask the guy about the details of his build.

Maybe on a no-torque flat 4 you'll feel a difference, but all of the V8's that we have dropped the compression on the effects were little to none.

I'm glad you told me this. That's a pretty high speed to use a gas saving overdrive gear. I utilize 6th at 55mph when cruising. It might not deter me from dropping the CR, but definitely something to think about.

I make over 700rwhp by 5000 rpms, what is low end again?

I would drop the compression to 8.5:1 if you plan to make over 600rwhp.