I have a stock bottom end LQ4 with 7875 turbo with 3" downpipe and 3" exhaust all the way out the back. I used a Magnaflow 99009HM cat, a 12259 muffler, then a 12289 muffler. It's super quiet and easily fit under my 2001 1500HD.

 

What kinda boost are you running? Ive been kinda worried about 3" exhaust with trying to make some power.. All the other turbo builds ive done were all 4" minimum.

 

I'm currently running 11psi of boost on the stock bottom end LQ4. 4" may make more power, but this is a street truck. So the cost and pain of fitting the 4" downpipe and exhaust was not worth searching for every last HP in a street truck that rarely sees more than 5k RPM. I also run E85 most of the time, so the stock bottom end is my true limiter.

 

My last turbo truck I did 4" all the way and it wasn't bad. It was a ton of welding since it was all pie cuts. But no tight spots on a 1992 c1500 and a 2000 silverado 1500. Both had KB racing kits. I daily drove them and just wanted to 4" just incase I wanted to turn them up. But never did on either one.

Back in the day (2008? I think) I had a KB racing S475 kit run to the stock exhaust and a muffler on the wastgate. It was silly quiet. But anything over about 6psi and it was done.

 

The turbine side of the turbo mainly cares about the pressure difference across it. If you have a more restrictive exhaust (back pressure, i.e. less pressure difference across the turbo), you basically have less pressure to drive the turbo. So at some point you should see slower spooling and/or power drop off at high rpm when the backpressure should be highest. My 6L with 7875 turbo spools so fast. It hits the commanded PSI very fast and I can tell a difference between my 5, 8, and 11 psi settings on the boost controller, so it's still making significantly more power with more boost. Can't comment on power drop off at high rpm because I've never had it on a dyno, but wouldn't be surprised to be giving up some high rpm horsepower. I was initially concerned about using a 3" exhaust, that's why I went with flow through absorptive type mufflers to maximize flow. My trucks stainless exhaust was the first full exhaust I welded together myself. I didn't know how much of it I would screw up and those mandrel bends are not cheap in 3" and crazy expensive in 4". My truck runs great with no noticeable ill effects of the flow through mandrel bent 3" exhaust.

 

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Checking back on this thread: How'd the Quiet Turbo project work out ?

What muffler combination did you end with ?
More important: How'd the Huron Speed kit come together ? Pipes fit up nice ? Any glitches or difficulties fitting up cold parts through front plastic?

Thanks for details/feedback.

 

Engine block is still at machine shop on this one truck. So once i get that and rotating assembly balanced ill have the truck together.
Every Huron kit ive put on and tuned did very well. No issues on several of them yet.
Most of them i do W/A intercoolers so i only buy their hot side kit.
on the A/A ones everything fit fine.