bajajoaquin
Senior Member
Probably had more to do with scavenging than velocity or back pressure.
Full dual exhaust system
- Thread starter Mr Grumpy
- Start date
Kernel Sanders
Member
I did up a 2.5" on my 318 with headers. Although a little overkill for the size, I figured if I go 360 or even big block stuffed into my grampa 4dr Abody, I wont have to redo the whole exhaust 
tbm3fan
Old Man with a Hat
I haven't been able to on as much lately due to several cars needing general maintenance. I first questioned the 2.5" because for many people it is the default size picked most of the time. I knew that when I started taking in all of my cars to get new dual exhausts and one a single exhaust. The engines varied from a stock 360, to a stock 410, a stock 390 and a modified 302. My research lead me to calculating air flow through different sizes as I already had experience with the bigger is better when it comes to camshafts, valve sizing and intake runner size. Consequently my calculations showed that 2.25" was more than enough for my duals for all those engines. Had my 302 been stock, like my 289, then it would have gotten a 2" exhaust but it was no longer near stock. When I took my cars in and the shop guy asked what size, and I told him 2.25", he went smart choice as everyone else would have said 2.5" These guys have done my cars since 1990.
I actually did it the long way using math some years ago to make a quick table for myself at the 2.25 and 2.5 sizes.
Today there are similar tables now on the web that have done all the math for you without going into volume, mass, moles, % of O2 and so forth. All that stuff is for physic majors.
How To Calculate Muffler Size and Exhaust Pipe Diameter | Exhaust Videos
One for diesel trucks
http://www.asia.donaldson.com/en/exhaust/support/datalibrary/1053747.pdf
Mr Grumpy
Member
There are "pipe bends" and then there are "mandrel bends". The main issue is with the over-the-axle bend, which can also have some lateral bends in the mix, too.
Getting 2.5" pipe to bend in the same piece of underbody real estate as the factory 2.25" pipe can take some doing, without ripples or outright collapses of the tubing body. Just depends on the equipment the shop might have on their bender.
OR you can get a "build-it-yourself" exhaust header kit with a selection of u-bends and cut/shape/weld the over-the-axle pipe yourself. "Cut and fit". All mandrel bends.
I'm not so sure about all of the backpressure things. I believe some are "wives tales" from older times and others are conjecture from putting something where it wasn't designed to go. OR what was done was not compensated for in other tuning calibration changes. There were al lot of things not generally known about back when these "stories" were more popular. Be that as it may. I recall one recommendation that with duals, you needed to richen the carb jetting so it wouldn't be too lean.
CBODY67
Getting 2.5" pipe to bend in the same piece of underbody real estate as the factory 2.25" pipe can take some doing, without ripples or outright collapses of the tubing body. Just depends on the equipment the shop might have on their bender.
OR you can get a "build-it-yourself" exhaust header kit with a selection of u-bends and cut/shape/weld the over-the-axle pipe yourself. "Cut and fit". All mandrel bends.
I'm not so sure about all of the backpressure things. I believe some are "wives tales" from older times and others are conjecture from putting something where it wasn't designed to go. OR what was done was not compensated for in other tuning calibration changes. There were al lot of things not generally known about back when these "stories" were more popular. Be that as it may. I recall one recommendation that with duals, you needed to richen the carb jetting so it wouldn't be too lean.
CBODY67
TallTex
New Member
Was looking for source for new exhaust system when I saw this thread. Thanks for the mention of the few aftermarket sources. So this is a reply to old these old posts as I am new on this forum but not new to performance cars, trucks, boats.
This is a pure urban street nonsense myths/opinion. No engine "likes" back pressure and all engines even my old AMC straight 6 benefits from as big as exhaust system as the engine bay layout and car will allow. You are speaking of a large air pump. The most air in and most air out makes most torque and power. If you do not believe my novice opinion check out all the you-tube videos on the subject by the guys at Hot Rod Mag, Road Kill or Engine Masters videos by past "Editor In Chief" David Freiburger and his team members. They have shown on engines and dynometer bigger is better.
LocuMob
Fluid Technician with a hat
I was just speaking from experience with the 360 I mentioned, and it didn't like the open exhaust, maybe the difference in load on the engine made a difference? Myth legend or whatever, it ran and made more power with a full length exhaust system. I'm not arguing anything, I don't have any other data than what I previously posted. Yep, it's an air pump. It's back in a van, and doing what it did before, running very well.
CanCritter
Senior Member
put duals on my built gmc….lost low end torque...replaced duals with single 3 in..low ends torques back...duals aren't always the answer..real world ****....
Dsertdog
Old man with an old guitar, and a blue note.
I had a local pipe bender bend my exhaust for the Polara out of 2.25 pipe. My car has the 383 4bbl. These cars originally came with single exhaust and 2 inch pipe.
I wasn't looking for more power, just a certain type of tone. Something similar to what my '69 Roadrunner had. Back in the day I put stock style "crew chief" brand mufflers on it. These were 3 pass mufflers (2.25 in/out) with slightly less baffling... but they are no longer made. I used walker Sound masters for the same application. Cost of the pipe, mufflers and tips was just over $300.00 and also included new flanges.
The Sound masters have a chinky yet mellow idle I could listen to it for hours. It's not exactly my Roadrunner but it's not the Roadrunners' engine either. But you can hear the spirit of it. When I close my eyes it's 1985 again.< wipes tear from eye>
Also, I don't have have any drone and they clear out nice when punched.
Arguing with people over exhaust tuning is like trying to convince a pyscho with a knife not to stab you. You think you've calmed them down with a quiet, logical tone, and then for no reason they start raving and you get stabbed.
It's called exhaust tuning NOT because of the sound, it's setting the length and diameter of the system to perform optimally within the design of the powertrain.
I wasn't looking for more power, just a certain type of tone. Something similar to what my '69 Roadrunner had. Back in the day I put stock style "crew chief" brand mufflers on it. These were 3 pass mufflers (2.25 in/out) with slightly less baffling... but they are no longer made. I used walker Sound masters for the same application. Cost of the pipe, mufflers and tips was just over $300.00 and also included new flanges.
The Sound masters have a chinky yet mellow idle I could listen to it for hours. It's not exactly my Roadrunner but it's not the Roadrunners' engine either. But you can hear the spirit of it. When I close my eyes it's 1985 again.< wipes tear from eye>
Also, I don't have have any drone and they clear out nice when punched.
Arguing with people over exhaust tuning is like trying to convince a pyscho with a knife not to stab you. You think you've calmed them down with a quiet, logical tone, and then for no reason they start raving and you get stabbed.
It's called exhaust tuning NOT because of the sound, it's setting the length and diameter of the system to perform optimally within the design of the powertrain.
TallTex
New Member
Not buying it. Nor are all the motor guys at Hot Rod. I go to race tracks NHRA, F1, NASCAR, Moto GP, never heard anyone in the Pits saying put back pressue into my exhaust system I want to go faster...
CanCritter
Senior Member
LocuMob
Fluid Technician with a hat
I'm not going to compare an LA 360 with an F1 or NASCAR engine, but even the NASCAR guy has more exhaust than I had.
bajajoaquin
Senior Member
No engine likes backpressure but all engines love scavenging. A completely open, huge dual exhaust might eliminate scavenging impulses or make some other harmonic change.
What if that exhaust has resonance to provide a boost in torque at 5000-6500 rpm? In a low-revving street engine it might perform worse.
I love the stuff that freiburger, dulcich, and guys like that do. I reference their roller cam and manifold articles all the time. However, it’s good to remember that most of that translates well to engines that people want to rev the piss out of, not engines that are optimized for off-idle torque and smooth roll-on performance.
Edited to fix autocorrect error!
What if that exhaust has resonance to provide a boost in torque at 5000-6500 rpm? In a low-revving street engine it might perform worse.
I love the stuff that freiburger, dulcich, and guys like that do. I reference their roller cam and manifold articles all the time. However, it’s good to remember that most of that translates well to engines that people want to rev the piss out of, not engines that are optimized for off-idle torque and smooth roll-on performance.
Edited to fix autocorrect error!
Last edited:
Fratzog
Old Man with a Hat
My take away from listening to the engineer in the posted video is there is a science to this and no general rule of thumb applies to all. The objective as I understand it is to maintain exhaust speed which can be hindered by both too large and too small exhaust components. The key is to find the "Goldilocks" size for your particular motor.
Similar threads
