Performance difference between 2 1/2" and 3" tailpipes

So, I must be getting old, because driving around in my car that has turndowns right after the mufflers is starting to be less enjoyable. I am ready to put tailpipes on the car, and I wanted to know if anyone had any real world test results comparing 2 1/2" and 3" tailpipes? The head pipes and mufflers on the car now are 3", and they will be staying the same. I remember reading somewhere that once the exhaust gasses have cooled down, the exhaust pipe size can decrease without any horsepower loss. I would love to use 2 1/2" tailpipes, because they will be much easier to fit on the car, but I don't want to give up anything performance wise. The car is a 67 B-body with around 600 flywheel horsepower. I have a electric fuel pump, fuel filter, and braided lines coming from the gas tank sump all taking up room under the back of the car, so space is at a premium back there. Any input would be greatly appreciated!

Depending on the muffs there isnt that much of
a HP drop.. just a few HP... I did testing at work
checking the difference.. but on those engines it was 6 hp
difference... in the end your muffs can and will make a
bigger difference.... there was a article on a Pontiac that
showed what little difference there was in ET just in changing
the tail pipes... then they went back and did the same tests
with different muffs... and with certain muffs and the 2 1/2
pipes... the same muffs and the 3" gained less than .02....
I want to say they were Dynomax muffs... but part of the
test was noise also.... I cant remember all the data on it
but it was a good article... it was all at the track
EDIT
and your right the temp is less so the volume/area
is less... the longer the car the cooler it is

All I can say is 2 1/2 tailpipes are very tight but doable on a 67 B Body in a stock location. I had a set on my 67 Coronet from Don's Custom Exhaust.

Were they press bends like any muff shop or mandrel...
I'm talking all mandrel bent exhaust... press bend you
lose 15% per bend... thats why MOST all production exhaust
is mandrel bend... aftermarket... different world

This episode of Engine Masters may not look at tailpipes only, but it gives a good idea of 3" vs 2-1/2". I have 2-1/4" tailpipes on my 67. I'm not so worried about street performance though, since I don't street race. I uncork with 3" cutouts at the track.

https://www.youtube.com/watch?v=_PVXvHkr-Vs

Thats a great vid.. BUT.. he has the 3" all the way
out of the muffs... not changing from a 2.5 out the
back... he wants to change the tail pipe... and the muff
difference is a major part... so its not apples to apples

Mike was your testing on 600+ hp engines?

I was wondering the same thing when I was trying to fit my 3" tailpipes around my shocks, fuel system, and leaf springs
It seems that it would be much easier if they were 2 1/2"

Gus

i have 3" tti tail pipes on my 65 coronet. fit like a glove. very happy.

I have the Flowmaster pipes and they don't seem like they were even made for my Savoy. Ron 383man has the TTi's on his 63 and they look real nice maybe I will just sell the Flowmaster pipes and pick up some tti's.
Thanks

Gus

I very badly want tailpipes for my Duster, I have a 3" H-pipe system so I'm debating on getting TTI's 3" tailpipes or just getting the 2.5" tailpipes for ease of installation...?

No.. about 400-450hp so the percentage will change
but they were changing the complete exhaust from 2 1/2
to 3 and their muffs were straight through... if you have
a 3" muff(if its truely 3".. a lot are 2 1/2 to 3") the
data would have changed... guarantee it
EDIT
Everyone that buys what they did love what they
bought.. its always the best.... try using stuff
thats given to you to test and do it on the dyno
or the track... then say whats best... all the
stuff was given to use to test for production...
it meant MILLIONS to the companies

Yea as Gus said I use a full TTI 3" system. The tailpipes fit great as I installed the full system in my driveway with the car just jacked up about a foot at the most. It fits great and was easy to install on my 63. I do use 3" tailpipes but I have heard that the exh gasses have contracted enough by the time they hit the tailpipe that 2-1/2" tailpipes work fine on a 3" system. Ron

Google "Jim Hand exhaust system testing" and see if his results are still available online.

That's the guy that did a bunch of muffler & pipe diameter comparisons on his bb Pontiac street/strip wagon.

http://www.pontiacstreetperformance.com/psp/exhaust.html

I had a complete three inch sytem on my old Duster, that car had coil overs on it so the exhaust shop ran the 3 inch tailpipes under and around the rear end housing and mounted the three inch by 14 inch long oval Magnaflows between the rear end and the rear bumper outside 1/4 panel and the rear subframes behind the rear end. It had a stock Gas tank also. I'm puttng a 1966 Coronet Deluxe two door sedan together right now,440 4 speed car now I bought the Summit three inch B body exhaust system for it but I haven't taken that system out of the shipping box yet . I did buy a set of 3 inch Ultra flows for the car and plan on mounting them back behind the rear end like my Duster had, I had the rear leaf springs move into the rear subframes and moved the rear end forward two inches I'm using a new stock gas tank also so it might be lots of fun routing the exhaust back there

They were not mandrel bent.

I went 13.30 at 101.75 mph with a full 2 1/2 exhaust with 1 3/4 inch headers with a lot of tuning left. Lousy 60 foot because converter was bad so only made one decent run with the combo. Lost a second the next run because of the bad converter. It would of been a solid 12 second car with a working converter and some tuning.

4000 lb car, 3.55 gears, small tire, light converter, home ported 906 heads, 9.1:1 compression and a .550 lift hyd roller cam that floated the valves on the one decent run.

I was pretty happy with the combo for a cruising street car.

Cab.

you have a picture of the 66' Deluxe Sedan? Those are some neat cars to set up ala Stahl Tuned Exhaust look.

Thanks for all the replies, and for the link to that great Pontiac exhaust testing article! My car has the larger body size 3" Dynomax Super Turbo mufflers on it, and those will be staying on the car. I will probably get the TTI tailpipes, everyone seems to say those fit the best. After reading that Pontiac article I am leaning towards the 2 1/2" tailpipes. They compared 3" to 2 1/4" and there wasn't much difference at all, so 2 1/2" should be even less of a difference. The headpipes that are on the car were done at a local exhaust shop, so they are not mandrel bends. The tailpipes will be mandrel bent.

You should have all of it mandrel bent... specially
the front where the temp is.. thats where you need
the area and the bend reduction hurts it

I ran 11.0s with 3" mufflers and 2.5" compression-bent tailpipes, FWIW.

Friend of mine went from a dual 2" (86 camaro) to a single 3.5" with a 3" Y-pipe and picked up 6/10th's. No joke.

I think you won't see an ET difference with 3" to the mufflers & 3" tailpipes to 2.5" tailpipes, due to the mufflers.

Muffs are where the biggest loss is at.... and the only
reason I am saying anything is due to your statement
"DUE TO MUFFLERS"... I got way too many years testing them
and I designed the F-body police car exhaust with dual
fronts to dual muffs and a rear Y pipe... where do you
think your buddy got the idea from... it was done so the
cop cars could run a bigger fuel tank but we didnt have
the room for dual tail pipes... by the way that was
in 76

I imagine he got the 3" y-pipe into a 3.5" single idea from some chevy magazine.

I remember reading about quad 68 Cadillac mufflers on the silver bullet Plymouth. No ET difference from open headers.

Edit;
http://www.silverbulletgtx.com/index.html

Do you have a pic of this system?

No... we werent allowed to take that type of stuff
out of work.. I got a patient on it(back in those
days... well expired now)
EDIT
The left pipe went across the rear end area and
the Y was at the right side of the rear end area..
that system was only on cop cars

The video in question, they lost 20hp, gained nothing below 3200rpm. That's with a 600hp engine. Then they slanted their comments toward the larger system. I think it was a biased opinion because the sound of the larger system played into what they liked.

R.

Sure it was biased.. and they did a complete 2 1/2
vs 3" system... the OP already stated everything from
the muffs forward is staying... he wants tail pipes ONLY
and if you looked at the Pontiac data it showed just tail
pipes then muffs vs noise and hp... if I was the OP I would
put dumps on it also... on my Rampage I built the headers
to match the engine but went with a less noisey muffs and
it was still to loud.. I added resonators and elec dumps
and I see a big difference when the dumps are open(the
resonators are straight thro... they only dropped the DB
6 DB.. I was hoping for more... right now its 92DB

Guess I thought it was so obvious that I didn't have to say it. Using 2.5" to get over the rear axle won't harm performance one bit.

It'd be interesting to map temperatures during a high horsepower run on the road and find out exactly what temperatures were. Then we could calculate quite easily the decrease in volume due to cooling.

R.

I did that stuff back at work... we would run about
200 thermocouples for the length of the exhaust and
data logged all of them... do I remember the numbers..
hell no.. did it every year the exhaust changed...
that was most of the reason we moved the muffs to the
rear.. more power and less noise
EDIT
and we would read about 4 pressure readings at
different points

Don't need fancy measuring equipment. Carefully touch the exhaust pipe just before the muffler, then after. After is a lot cooler. So I revise my earlier statement.

I think a 3" exhaust to the mufflers w/ 2.5" tailpipes will be a tad faster than 3" tailpipes. Due to the cooler exhaust gasses getting a velocity increase from
the smaller pipes.

With the added bonus of better fitment & overall quieter.

Your results may vary with like 610 cid engine vs a 360 cid.

Just my

When building a car you have to be a bit closer
than... yep thats hotter than that... and temps
come into play with making power

Yeah. XX inches from the collector. A crayon will tell yah the same thing. Just without all them fancy numbers.

That is so much BS I cant believe people still believe it..
what happens if the engine is fat or lean... OH that line
changes by INCHES... and I have already proven it
EDIT
And all crayons dont melt at the same temp

How so? Crayon wax melts at 150f. You can clearly see where the melt stops on an exhaust.

But we ain't talking fat or lean.. just where the air starts to cool/slow down. Few inches fore/aft is inconsequential.

Anywho gonna go to Burger King.

"that was most of the reason we moved the muffs to the
rear.. more power and less noise"

Never heard that before.

To be honest this isnt worth my time... 34 years
in the exhaust lab at Chrysler and I've tested more
ways and tried theories that you will EVER think of..
built a ton of headers for Chrysler for competition
and all crayons dont melt any where near 150*
EDIT
Its much easier to use a program for the headers
and collectors so your right. not a guess.. its math
and I have bult headers that were different that the
math said and they didnt make as much power at any point

You want the muffs after the exhaust gases stop expanding. Gases expanding in the muffler will cause a restriction/pressure build up.

It makes sense, but again, the solution never has been discussed much here, from what I have read, nor was it mentioned that there is enough difference to make it worth the effort. I hope Mr P follows up on this . I did it with one of my cars years back, but thought it was out of the norm.

I would also be curious when he was doing non crayon tailpipe temp testing (sorry, I couldn't resist), since it sounds important enough to measure small temp differences, did outside moving air speeds and location, factor in much on effecting inside tailpipe temps, and if so, how did they account for those conditions?

yep.. and where is it coolest... at the rear
EDIT
the cooler its also easier to control noise..
all in all its way better
this is where I put my muffs

Lol ok Mr P. I yield to your 35 years experience in the exhaust lab.. I'm not as smart as you, I got no problems admitting it. That said, for the rest of us who want huffs in the stockish location,
the bigger the muffler (internally), the better. Helps dissipate heat & gas expansion.

^^ Crayola Crayon approved.

On the muffs.. your correct... bigger is better
EDIT
Give the crayons back to the kids where they belong

I would never run a 2 1/2" tail pipe on anything with 600 hp, that is just silly.

Oh, curious, how do you know it is 600 hp?

How does things change when boost comes into the mix??

The pressure goes up... your moving more air..
you know that

Yea,I know,,,even with a 4 inch going into 3.5 there is ALOT of backpressure.I bought the material to install cut outs after the "X" and before the mufflers.

I read a test where s dude with a big inch Hemi powered street based car had dual 3.5" out of his collectors to a pair of under-the-seat mufflers then into 3.5" tailpipes and he hated the volume of the exhaust.

So he dynod it on the chassis dyno to get a baseline.

He then proceeded to go from 3" to 2.5" to 2" tailpipes out to the rear bum;per and recorded the differences in HP/TQ.

The end result was a loss of a total of 6 HP from the 3.5" tailpipes to the 2" tailpipes yet the sound was WAY quieter and comfortable for him.

Make of it what ya'all will

Don't the FAST guys only run a 2 1/2 system...
Of course, I'm sure their build is centered around that restriction..

I'm gonna ask a dumb question just so I can get it out of my head, have any of you tried the Flowmaster B-body tailpipes on a Duster? Can they be made to fit or is it a completely different deal?
I want the TTI tailpipes but I'm gonna have to really be in the mood to do it as they are around $300 with shipping.

I'm sure with some mods they can fit.. lineup the hump
area(over the axle) and a B-body being longer you do a
couple of cuts to make them fit.... I dont remember the
hump being any taller or shorter

This is when it was running the 3" mufflers way at the rear. Right before the track closed he had his 2.5" Ultraflows back on and it was running 11.40s all night. The weather was better in late 2011 and and he had been playing with lifters and cams i think. He now has the 2.5" Superturbos on it since they are so quiet back under the rear with both outlets on the pass. side.

I did all the exhaust work in his book and set up all the tests we ran at the track and on the engine test stand and coordinated all the supplies we used at Dad's house when we made the sound CD. Dad shelled out all the money! Like Dads do for their kids. If I knew how to post a picture, I have some taken of the car yesterday. It is still beautiful and faster than heck and so quiet.
Tom Hand