Exhaust Size

[GBFD 453]

I installed LS in my 71 LWB and need to put exhaust on. The muffler shop warned me of installing 2.5" pipes due to loss of torque, they recommend 2.25" pipe. Engine is fresh rebuild with GM hot cam and decent tune. Also flowmaster 40's exiting out the rear. Any input on loss of torque with larger pipes? Thanks

[davepl]

Ignore them. That -might- matter for header tube primaries but no way it matters for exhaust, especially once you're 6-8 feet back down the system.

It will sound louder with 2.5s though, but I don't think you'll mind.

[kwmech]

Assuming dual exhaust, yes it can affect the torque, but I doubt you'll even feel the difference since stepping up into an LS platform

[Killer Bee]

I think some folks confuse the purpose of optimum diameter and length of exhaust tubing and colloquially refer to it as 'back pressure'

in almost all cases, the least amount of restriction is better due to increased flow..

performance adders like tubular headers, crossovers, etc. enhance the exhaust stream by drafting or scavenging from one another..

this 'tuning' is accomplished by arranging the appropriate diameter and length primary tubes into the collector for optimum efficiency..

the higher velocity exhaust stream in the primaries increases this effect in the collector..

'torque tubes' or header extensions are simply an extension of the collector to increase the efficiency of the scavenging effect..

mufflers and full length exhaust in any measure are simply to reduce/tune/adjust sound levels and add appearance features..

please let us know what you end up with, pictures are groovy too, good luck!

[hjewell2]

Originally Posted by Killer Bee

I think some folks confuse the purpose of optimum diameter and length of exhaust tubing and colloquially refer to it as 'back pressure'

in almost all cases, the least amount of restriction is better due to increased flow..

performance adders like tubular headers, crossovers, etc. enhance the exhaust stream by drafting or scavenging from one another..

this 'tuning' is accomplished by arranging the appropriate diameter and length primary tubes into the collector for optimum efficiency..

the higher velocity exhaust stream in the primaries increases this effect in the collector..

'torque tubes' or header extensions are simply an extension of the collector to increase the efficiency of the scavenging effect..

mufflers and full length exhaust in any measure are simply to reduce/tune/adjust sound levels and add appearance features..

please let us know what you end up with, pictures are groovy too, good luck!

Darn tootin the less back pressure the better, more important when running full system out the back and several bends to get there.

[7 Devils]

Killer be is exactly right. Back pressure for torque is a myth and countless Dyno results have proven that. Go with what you like. Even though I like the Flow master's straight through designs out perform them.

Dan

[GBFD 453]

Thanks everyone for the help. The truck goes in early next week and I should have it back for the weekend. I will try to get some pics up on the rack.

[harpo231]

Loss at low end ... pick it up on top

[jessemthompson]

Run 2.50, 2.25 will be too restrictive. That exhaust shop is probably making excuses cause 2.50 is harder to get over the rear axle on a lowered truck. I went to 3 shops before I found one who was capable of doing it.

[adricar1]

i second that i went 3 inch on a dropped truck took days to get a bend route over the axle

[Ekliptix]

This answer all your questions. Actual tested data. https://www.youtube.com/watch?v=_PVXvHkr-Vs

[sprint_9]

Originally Posted by Ekliptix

This answer all your questions. Actual tested data. https://www.youtube.com/watch?v=_PVXvHkr-Vs

I was going to post that too, pretty cut and dry if you ask me. Now 3 inch is when things get really tight

[7 Devils]

Originally Posted by Ekliptix

This answer all your questions. Actual tested data. https://www.youtube.com/watch?v=_PVXvHkr-Vs

Good post! It's accurate up to about 650-700 genuine HP.

Dan

['70_402]

The REAL question is.... has anyone run 3" pipe over the rear axle with a drop?? I've got a 7" rear drop and i'm not sure there is room... out the side before the tire??

[Robznob11]

Lol I run 3" with dyno max bullets (18") but they are dumped before the axle. Sure I have a larger cam than you but I have so much tq ,the 345/55 drag radials beg for mercy.....these trucks will benefit from a little less tq down low and more top end hp ca use they are light in the rear....I think 2.5 is optimum for your set up.. if it were me I'd get a larger cam and even bigger exhaust. The LS motors love more cam

[Ekliptix]

I have 3" dual on my lowered C10. The installer had to pipe down to 2.5" for over the axle, so I'm 3" from the headers, x pipe, to the front of the axle, then 2.5" over the axle and back.

[indymachinist]

2.5 too big??

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I'm going single 4" into a 4" in dual 3" out straight thru muffler. ☺