• When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.

2 1/2 vs 3 inch exhaust

One thing is for certain about gas or liquid flowing through a pipe. It's called the boundary layer. This is a portion of the gas that sticks to the wall due to friction and doesn't move as fast as the gas toward the center of the pipe. The result is the effective diameter of the pipe is reduced by about 1/8" and should be factored in.
I wonder if Ma Mopar took this into consideration.
 
I'm with Kern Dog on this one, the whole idea of needing back pressure to create torque makes no sense. The size of the exhaust pipes from the primary tubes back needs to be correct for the RPM range that you are targeting.
Although this is an article out of a Chevy magazine, there is some good info here:
http://www.superchevy.com/how-to/exhaust/0505phr-exh/
 
1 3/4 on a 440 is kind of small - I would have stayed with 1 7/8 or so.

You don't want back pressure - don't open headers with properly tuned collectors produce the most power?
 
I remember when I use to 1/4 mile everyone ran with open headers. The theory was less backpressure, more power. Based on that wouldn't larger exhaust be better all around? I see hot rods, street rods, ect with big side pipes all the time.
 
What works on a race car which runs at WOT all the time is not necessarily the hot set-up for a street car. Be honest about what the car will be doing a majority of the time. This is one of those questions that will get 10 different answers if you ask 10 people. BTW, on my max wedge street car I run 2" TTIs, 3.5" collectors, an X pipe to 2.5" in/out Dynomax mufflers and 2.5" pipes to the bumper. I'm just one of the 10...
 
@ 6 speedgtx very good article that exactly what i was looking for, according to the info I lucked out and am very close to where I need to be, @ car nut I know a back in the day chyrsler engineer who developed the heads he told me the epa/ cost factor alway squashed what they wanted to do so I know they played with the exhaust tunning. I always wondered why they used resonators on the r/ts, now I know. brillant! they were forced to de-tune a lot of components

- - - Updated - - -

@ big Al open headers yep thats true but I run on the steet too so i have to find the middle ground
 
My head is spinning my head is spinning!! lol. Headers are designed to 'scavenge' the cylinders so why would back pressure be needed down the line? If back pressure works for you then do it but it's telling me something else isn't right if BP helps. There's been a LOT of debate on this subject with a lots of knowledgeable people weighing in on the subject from all corners of the hot rod world. Only your own testing will tell you if your car runs better with a free flowing exhaust or one that's restricted. Dyno tests are all over the map on but they don't test real world results, ie, tests on actual cars running down the road. I have found that my junk runs better with a good free flowing exhaust. Even gas mileage improves a bit....but, just about everything I've ever owned was run with straight pipes since the 70's. How many tickets have I got from 'all the freakin noise'??? NONE!

Uhhh... NO.
Most muffler shop guys I've met are just guys that can bend pipe and weld it together. Most do not race or care to race. They only know what they read in the trade magazines.
Cylinders dependant on backpressure? NO again. All engines make more power with less restriction. I laugh when I hear old timers say that their 340s and 440s with Magnum manifolds make as much power as the same engine with headers. Faster in, faster out, MORE power.
A stock engine will lose a little bit of power at low speeds with large headers, but take that same engine with iron exhaust manifolds and do tests on power with then without mufflers. The engine will make more power with less restriction.
Getting back on topic: 2 1/2" exhaust works fine up to around 425-450 HP. Beyond that, you will see benefits from going to a 3" system. I had a 2 1/2" system put in my Charger when I swapped from the stock 318 to my first 440. The 440 made around 380 HP and the car ran great. Currently I have a 440 based 493 and my 2 1/2" pipes were too small for my combination. I went with a 3" mandrel bent center section with an X crossover along with 3" in/out mufflers, yet retained the 2 1/2" tailpipes. Power jumped up dramatically. You'll find that tailpipes can be smaller than the center section with no loss in power. The exhaust gases have cooled off some at that point to where large tailpipes are not as important.
Amen!!

One thing is for certain about gas or liquid flowing through a pipe. It's called the boundary layer. This is a portion of the gas that sticks to the wall due to friction and doesn't move as fast as the gas toward the center of the pipe. The result is the effective diameter of the pipe is reduced by about 1/8" and should be factored in.
It's called 'slow moving boundary layer flow'.

I wonder if Ma Mopar took this into consideration.
Most likely didn't in the early days of the muscle car era.....
 
Another thing to consider is the exhaust flow is happening in pulses. There is a pressure wave that travels back up the tube as the exhaust exits and that wave must be in resonance with the valve timing for everything to scavenge. Meaning you don't want the wave to hit the valve with it open. The way to accurately measure this is quite complicated but the result was the invention of the first step exhaust in the early 1900's.
 
This is some good info guys I'm not to exhaust yet but deff something to keep in mind this is one of those subjects that know one will agree on .I have to say I've played around with one of my chargers I took it for a test drive with open headers (very fast ,and pissed off a lot of people ) I put the exhaust on , 2" and it was like dropping an anchor.but the car felt more responsive. I tossed the 2" stuff aside and went 2 1/2" and there deff was a power increase but the responsiveness went down a little bit , so there's deff something there with the size now that car was a street car with only 260-300 hp and 2.71 gears so if you guys a still talking about this when I get the 340 going I'll post up what it does it currently has 3" under it and I have a 2" system sitting here and I was going to order a 2 1/2" set up so I'll try all 4 ,open headers ,2,2.5,and 3" so maybe I can see a big difference.
 
" I went with a 3" mandrel bent center section with an X crossover along with 3" in/out mufflers, yet retained the 2 1/2" tailpipes. Power jumped up dramatically. You'll find that tailpipes can be smaller than the center section with no loss in power. The exhaust gases have cooled off some at that point to where large tailpipes are not as important."

Thanks Kern Dog, very helpful. I need to put a system on my car & have little idea what really works. My Satellite had side exit 2 1/2" w/ round glass pak like things. At track 12.60 @104. Open headers, 11.80 @111. Both disappointing for 500", 10.5:1, 2"TTI, Performer heads, Comp .502/.511 236 / 242 @.050, Torker 850 DP, 4.10 & 10" TA conv. No pull above 5000 rpm. Over the winter put in an old .565/.585 265/270 @.050 cam & Victor intake. I will need a good exhaust setup, so this is really helpful. I agree, back pressure isn't the issue, reversion timing relative to cam timing is the deal.
 
Chrysler did recommend tri-y headers back in the late 60 over the 4 tube to the collector. But have never seen any test results as to what that decision was based on.
 
I met a retired engineer who worked at the r&d lab for engine heads way back in the day, I think it was in ventura c.a., and he told me they had developed some pretty wicked heads for the regular production line, so I bet the exhaust guys had some kick *** designs too. He said the potential for what they wanted to do was left in the engines so the racers could reclaim the hp left on the table. I guess thats what direct connection was all about.
 
Hmmmm according to Stacy David I should have 550 hp! lmao I got lots of torque so didn't looooooose anything with 3" duals ha!
 
Last edited:
Loose or lose?
Loose exhaust can fall off. It isn't good to lose your exhaust pipes.
 
Auto Transport Service
Back
Top