Ok so I have a hybrid pickup it's a chevy with a 1st gen motor. I went to the muffler shop today to see what my options were. I learned some things one they can't bend 4" pipe so it's going to be completely custom. That normally means $$$. What the shop thinks is that they would have to order the downpipe from cummins and the pipe from Chevy. As we talked he thought that 4" would be better than 5" so as not to loose to much backpressure. The other thought was just to delete the muffler. My questions are would I gain any power or mileage just deleted the muffler and going with 3 1/2" straight pipe? What size would I get the best performance and milage? Since the turbo is still 3" wouldn't that be the point of restriction? Do they make a 3-4" or 3-5" adaptive downpipe that would open up the flow? Hopefully that make since thanks

with a turbo diesel you dont need back pressure as it goes against the turbo with = more turbo lag. i straight piped my 1993 and it spools much faster

FYI: THIS IS DIESEL!!!! Can I get the guy from the movie 300 to say this???? Less restriction to flow means more power.

Ok I guess I am a bit corned-fused. If the turbo outlet is 3" and you go larger isn't your restriction still at the turbo? Any thoughts on only doing a muffler delete? I get the less restriction the faster the spool up on the turbo however if I'm not going to be pushing more than the 3 1/2" pipe can handle trying to justify the cost. Thanks

if ur gunna do 3.5 just delete your muffler. if not 4in will work great. better spool up and probly a little lower egts too. but ya ur turbo is wat creates the back pressure.

It may not make complete since but ya you're right it is 3 inch than has to go bigger but still helps tons to go bigger right after. You could easily feel it when I did a 4 inch downpipe to 5 inch straight on my old 24 valve. I will never run below 4 inch on a diesel again although I think 5 inch is just overkill but sounds great. On my conversion just get the downpipe and then figure out the rest ended up just sticking out the side in the middle behind the cab welded my own stuff up with nineties etc.

Luke did you do this on your 78? What kind of mileage are you getting with the '78? Looks like your setup is like my 89. Thanks

The closer to zero backpressure you can a make diesel exhaust system the better for performance and fuel milage. The bigger within reason the better IMO.Rocky

78 is drivable right now but under knife don't know exact mileage just started project a little while back has a hundred gallon fuel cell and no speedometer so never knew wasn't too bad tho had stock exhaust ya not hard to do but exhaust is off now too I'm building a hood stack instead it's my toy Just welded my own exhaust on my 07 too my chev truck started leaking from pump so it's waiting on parts new governor spring and I got a denny t for it

Ok so Friday I went to the muffler shop and deleted the muffler on my beast. Instantly I saw some changes in which I'm still baffled (no pun intended) about.

1) Instant difference in power, and pickup remember this was just deleting and still using the 3 1/2" straight pipe. Quick responce.
2) The obvious sound difference
3) pulling the same load I pull and on the highway my RPM's at 65mph went from 1700 to 2100. This to me doesn't make sense and I was able to achieve 70mph much easier as before it was acting like a had an anchor just getting to 65.
4) My egt ran at or a little below what it did before. I expected this.

My question would be is the difference in RPM's make sense? I suspected that the muffler was either clogged or was just that restrictive. I did have to pull the trailer 160miles which sucked 3/4 of a tank but I will fill up tonight to see if there really was a difference in mileage.

I still want to take it in and have the mods done but just can't justify the $1400.00 unless I knew for sure that I would be able to save on fuel. What gearing would be typical and what rpm at 70 should I be in for a 12v? Thanks

yes i makes some sence. the motor isnt being restricted so much. when my truck was still running at 70nph i was at a little under 2k. but my tach is not set to the exact motor rpms. i tuned it in by the sound but i think its pretty close.

I love my 5" exhaust!!! It's deep and throaty!!!

tell your muffler shop to get you a 3-4" bell coupler and some 4" elbows. Cut the down pipe right below the first elbow. weld in the bell coupler. add 4" straight pipe to the elbow. then run straight 4" pipe back. Miter in a 45 degree and exit in front of the back wheel. minimal fuss, work and cost.

The local guru said that going to a big pipe was not worth the cost and was for folks that didn't know how to set things up right. He's very well known in the area for his knowledge and upgrades and I was a bit surprised when he said this. I wonder if I missed something during the conversation as this is completely opposite of what I have read. He does own a Diesel Performance Shop so I would figure he would know.

maybe in his world. But i bet his performance shop would sell you an exhaust system if you had cash in hand. Anytime you reduce restriction you improve performance. Ask the gas burner drag racers and you'll find different.
Admittedly on the H1c if you leave that first 90 you get some restriction, butanything help, since that second 90 chokes it down to about 2 1/2"
Remember it doesn't matter how big a turbo and sticks you use, if you can't get it out the exhaust pipe it useless.