callys

iTrader: (2)

The LQ4 in my Buick spun a cam bearing, so as I put things back together I am making some changes.

The combo:

01 LQ4 with ebay gen iv rods and .020" over pistons
Stock 317 heads
LS9 gaskets
Lil john stage 1 cam, btr duals, etc
KB log manifold
On3 7875
80lb siemens in truck intake

4l80e with shift kit
Circle D multi plate

Before I was running the on3 7665, but I sold that and I am switching to the 7875.

I have a 3" downpipe that is ceramic coated, so I really don't want to redo all of that - $$$. I am running 3" exhaust clear out the back behind the rear tire in the factory spot. I am using some off-brand, big straight through muffler the shop recommended. Before the wiped cam bearing I was seeing boost pressures about 3 psi lower than the spring - 7 psi spring produced 4si, 10psi spring made 7-8psi. Is this due to exhaust back pressure? I realize this is probably a guess without measuring, but that makes sense.

I need to buy a better straight through muffler anyway - a 3.5" is not anymore money. Does it make sense to pay to have the exhaust from the downpipe back changed to 3.5"? I read conflicting results on this in search. I don't want to run a cutout.

This article shows a big gain leaving the downpipe at 3" but changing to 3.5" exhaust on a Supra - but they do not retune, so the bigger exhaust could have leaned it out

http://www.superstreetonline.com/how...ust-test-tech/

Thanks for any info. I'd really appreciate any real dyno numbers from people running 3" without a cutout over the axle like a stock exhaust (not just a straight dump before the axle).

coltboostin

Yes, BP is pushing the gate open early.

And yes, if you can go 3.5 at the firewall you will see a difference. Every little bit helps. Very common in the import community with small turbo to step the down pipe.

And lets silence the naysayers before they start yappin.

http://www.superstreetonline.com/how...ust-test-tech/

callys

iTrader: (2)

Does anyone have more dyno results?

I went ahead and got a 3.5" muffler. I would go 4" but doing some measurements under the car I don't think it will clear if I have any kind of weight in the trunk or while squatting on launch.