Exhaust Size - The 1947 - Present Chevrolet & GMC Truck Message Board Network

GBFD 453 · 03-31-2018, 12:23 PM

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 · 03-31-2018, 12:37 PM

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** · 03-31-2018, 12:55 PM

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 · 03-31-2018, 02:08 PM

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 · 03-31-2018, 02:53 PM

Quote:

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 · 03-31-2018, 03:06 PM

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 · 03-31-2018, 04:17 PM

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 · 03-31-2018, 07:44 PM

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

jessemthompson · 03-31-2018, 09:33 PM

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 · 04-02-2018, 03:46 AM

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

Ekliptix · 04-02-2018, 03:47 PM

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

sprint_9 · 04-02-2018, 06:44 PM

Quote:

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 · 04-02-2018, 08:37 PM

Quote:

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** · 04-02-2018, 08:53 PM

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 · 04-02-2018, 11:10 PM

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 · 04-03-2018, 10:03 AM

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 · 04-03-2018, 08:38 PM

2.5 too big??

��

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

03-31-2018, 12:23 PM	#1
GBFD 453 Registered User Join Date: Jan 2015 Location: Great Bend Ks Posts: 9	Exhaust Size 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 __________________ 1971 LWB LS/350 Factory A/C 2002 GMC 2500HD 8.1 2005 Silverado BDS 6.5 with 35's 2012 Chevy Tahoe

03-31-2018, 12:37 PM	#2
davepl Registered User Join Date: Aug 2006 Location: Redmond, WA Posts: 6,332	Re: Exhaust Size 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. __________________ 1970 GMC Sierra Grande Custom Camper - Built, not Bought 1969 Pontiac 2+2 427/390 4-speed Coupe 1969 Pontiac 2+2 427/390 4-speed Convertible

03-31-2018, 12:55 PM	#3
kwmech Senior Member Join Date: Mar 2003 Location: Colfax-California Posts: 8,650	Re: Exhaust Size 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

03-31-2018, 02:08 PM	#4
Killer Bee Registered User Join Date: Jan 2016 Location: Barber City, CA Posts: 4,816	Re: Exhaust Size 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! __________________ *I started out with nothing - and I still have most of it*

03-31-2018, 03:06 PM	#6
7 Devils Registered User Join Date: Mar 2018 Location: Kuna, Idaho Posts: 117	Re: Exhaust Size 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 __________________ Forgive my typos, big fingers and little Android digital keyboard....what a mess 06 GMC Sierra crew cab Z71 DD leveling kit, 34's, bumper, LED lighting. Still doing stuff to it. 67 El Camino Custom 300 Deluxe. In progress. 55 GMC 371 2 1/2 ton. Upcoming project.

03-31-2018, 04:17 PM	#7
GBFD 453 Registered User Join Date: Jan 2015 Location: Great Bend Ks Posts: 9	Re: Exhaust Size 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. __________________ 1971 LWB LS/350 Factory A/C 2002 GMC 2500HD 8.1 2005 Silverado BDS 6.5 with 35's 2012 Chevy Tahoe

03-31-2018, 07:44 PM	#8
harpo231 Registered User Join Date: Nov 2014 Location: Lincoln City DE Posts: 1,292	Re: Exhaust Size Loss at low end ... pick it up on top __________________ Stan 67 swb BLU MULE posted via flip-fone

03-31-2018, 09:33 PM	#9
jessemthompson Registered User Join Date: Nov 2011 Location: Williamsburg, VA Posts: 960	Re: Exhaust Size 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.

04-02-2018, 03:46 AM	#10
adricar1 Registered User Join Date: Sep 2013 Location: United kingdom Posts: 127	Re: Exhaust Size i second that i went 3 inch on a dropped truck took days to get a bend route over the axle __________________ GMC K1500 1971 350 cu in th350 4wd Bondo Bob c 10 1971 stepside th 350 c10 1970 350 cu in manual air con stepside.

04-02-2018, 03:47 PM	#11
Ekliptix Registered User Join Date: Aug 2013 Location: Calgary, Alberta Posts: 683	Re: Exhaust Size This answer all your questions. Actual tested data. https://www.youtube.com/watch?v=_PVXvHkr-Vs __________________ 1972 Cheyenne Super SWB PS, PB, Tach, A/C, Tilt. Drivetrain: Built 402BB, TCI Streetfighter 700R4, 2,500 stall, Eaton posi, 3.73. Suspension/Chassis: ECE 4.5/6.5 drop, CPP nitrogen shocks, CPP front and rear sway bar, CPP cross members. Exhaust: 3" exhaust with x-pipe, Spintech 3000 mufflers, Hooker ceramic coated headers. Best 1/4 mile time: TBD.

04-02-2018, 08:53 PM	#14
'70_402 Senior Member Join Date: Dec 2003 Location: Pampa, Texas Posts: 1,553	Re: Exhaust Size 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?? __________________ https://www.ebay.com/itm/29620574751...mis&media=COPY

04-02-2018, 11:10 PM	#15
Robznob11 Registered User Join Date: Feb 2004 Location: brownfield texas (west texas) Posts: 1,936	Re: Exhaust Size 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 __________________ The Ghost 67 short wide full custom. 550 hp LSX, 4l80E, narrowed 9". Body dropped w/air ride! ........................ ___ .........________//__{\_____ ,,,,,,,/__(⊙)___//___/__(O)_/ @disciple2.rc 💪instagram💪 LED TAILS

04-03-2018, 10:03 AM	#16
Ekliptix Registered User Join Date: Aug 2013 Location: Calgary, Alberta Posts: 683	Re: Exhaust Size 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. __________________ 1972 Cheyenne Super SWB PS, PB, Tach, A/C, Tilt. Drivetrain: Built 402BB, TCI Streetfighter 700R4, 2,500 stall, Eaton posi, 3.73. Suspension/Chassis: ECE 4.5/6.5 drop, CPP nitrogen shocks, CPP front and rear sway bar, CPP cross members. Exhaust: 3" exhaust with x-pipe, Spintech 3000 mufflers, Hooker ceramic coated headers. Best 1/4 mile time: TBD.

04-03-2018, 08:38 PM	#17
indymachinist Registered User Join Date: Jun 2015 Location: Indanapolis, IN Posts: 603	Re: Exhaust Size 2.5 too big?? �� I'm going single 4" into a 4" in dual 3" out straight thru muffler. ☺ __________________ -Curt My 1968 Chevy shortbed stepside build



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