2.5" exhaust vs 2.25 or even 2"

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Hamerhead

Hamerhead

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Car is coming along. 68 GTS 383 auto. Engine is running stock manifolds and has an unknown rebuilt from a previous time. Now it's time for new exhaust. Been planning on a 2.5" as I plan to add headers min in the future. Does anyone have a similar combo and how satisfied are you.
 
Stock would have been 2 1/4" and should support over 400 hp.
I don't know if there would be a performance gain with 2 1/2".
 
I’d bet good money on, yes, there will be a gain.
If it were I that is doing what your doing, even if the engine was otherwise dead stock, I’d go to a 2-1/2 system with an X or H pipe.

Dyna Max Super Turbo mufflers would be used. The short version for some muscle sound, the long ones @ 20 inches in length (if you can fit them) for quieter sound.
 
That’s true. It was like, IIRC, 1-7/8 on the dual exhaust. Small! If I was doing this exhaust as a base up grade to an otherwise restored car (with those ever so minor upgrades) it would be dual 2-1/4.

I should have write this in my first post above.
 
When you see 2.5" up close, it looks like a monster Boa Constrictor compared to stock diameter pipes, always a good option. Probably too large for my current 318, but what the hell.

Boa.jpeg
 
My '69 340 Dart came stock with 2 1/4" duals to the mufflers and 1 7/8" from the mufflers back.I thought the 383 GTS would be the same but i've been wrong before?
And dibbons is right...2 1/2" are quite a bit larger and heavy too.
 
...the first couple of feet of headpipe on the drivers' side is actually 2" to get by the starter and torsion bar, then 2 1/4" to the muffler.
 
I like to run "whatever size" the header collector is all the way out.
 
It's not so much the size of the pipe as much as the type of equipment used to make the bends. Crimp bent exhaust is the worst. Shop bent is a little better. Mandrel bent makes a tremendous difference in flow as the flowpath literally stays 2.5" the entire time. On several cars in the 400hp range, our tests revealed a dyno difference of close to 50hp over a decent shop built 2 1/4" and a 2.5" mandrel bent system. Real world results; A Yenko Nova running 14.6 in the quarter mile with shop bent exhaust switching to Pypes 2.5" mandrel bent exhaust and no other changes. 14.1 That .5 of a second. For those that race, you know that is huge.

Since there it is very rare to see mandrel bent 2 1/4", 2 1/2" is a great solution and only hurts a little on the bottom end. Since most rebuilds have loads of bottom end torque, 2.5" is the most efficient and cost effective solution for someone that wants to maximize performance at a very reasonable cost.
 
@Ccas Good post. Very true. A .5 ET drop is generally recognized as an easy 50hp improvement. And yes! That is a huge gain racers know about.

@Hamerhead IF! Headers are a certainly in the future, I would do both at the same time. On a DIY level, it is easier. You won’t have to cut and splice piping.
 
rumblefish360 said:
@Ccas Good post. Very true. A .5 ET drop is generally recognized as an easy 50hp improvement. And yes! That is a huge gain racers know about.

@Hamerhead IF! Headers are a certainly in the future, I would do both at the same time. On a DIY level, it is easier. You won’t have to cut and splice piping.
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Yep, I didn't mention headers but the same logic applies. Anyone ever see a shop bent set of headers? ;) They are all mandrel bent for a reason. Anyway, If you have a nice pair of headers, you are going to free up 25 hp +- when in the 400hp range but it will be largely nullified with a compression bent exhaust system attached to it.
 
Ccas said:
Anyone ever see a shop bent set of headers? ;) They are all mandrel bent for a reason.
Click to expand...
Few local shops have this ability because of the sheer volume of customers that just need exhaust work done to shut there car up.

I did have the good fortune of having “Kooks headers” only about 15-20 minutes away. When they moved off of Long Island, not all the talent left with them and the ones that stayed behind started “American” exhaust and headers. Same or better super quality headers and exhaust parts. Though I haven’t been inside the new shop, only drove past, there are enough stacks and rows of headers to outfit FABO in a week.

Under rated and not enough press for those guys and there work. Pricey buggers but so worth it! So so so worth it!
 
rumblefish360 said:
@Ccas Good post. Very true. A .5 ET drop is generally recognized as an easy 50hp improvement. And yes! That is a huge gain racers know about.

@Hamerhead IF! Headers are a certainly in the future, I would do both at the same time. On a DIY level, it is easier. You won’t have to cut and splice piping.
Click to expand...
I did the stock resto 25 years ago with my 65 Cuda. I plan on just having fun with this one. Likely put numbers matching motor under bench and build a stroker. Gojng for a period day 2 vibe with this one. Just like before getting married.
 
Hamerhead said:
I did the stock resto 25 years ago with my 65 Cuda. I plan on just having fun with this one. Likely put numbers matching motor under bench and build a stroker. Gojng for a period day 2 vibe with this one. Just like before getting married.
Click to expand...
I hope you don't mean you want to put old school exhaust on there.:realcrazy: lol Stroker = first step to old school fun. :) Now make sure it breaths and you can have even more fun. ;) You can get that old school sound by choosing the right mufflers.
 
Ccas said:
I hope you don't mean you want to put old school exhaust on there.:realcrazy:
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What’s an old school exhaust?
True duals? LMAO!
Before you bring mufflers into it.....
You can get that old school sound by choosing the right mufflers.
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Very true. The rhythm to the sound is the cam.
I always suggest a good high flowing muffler but many want a certain sound and I get that but it is also subjective and it is a personal choice on what you think sounds right.

To that end, I then suggest to attend a car meet and when you hear what you want, track down that car owner and ask questions. Not just what mufflers he is running.

Engine size, compression, cam specs and “How the exhaust is constructed.”
 
IMO , you are giving up both horsepower and torque by going to a 2 1/4" system . I have been told by several manufacturers of exhaust headers and systems that 425 engine horsepower is the cutoff point between 21 /2" and 3" systems . If you have a low production/numbers matching car then 2 1/4" is your deal . If you are planning headers I would go 2 1/2" . If you plan on h.p. upgrades I suggest 3". whats't he harm .
 
On several cars in the 400hp range, our tests revealed a dyno difference of close to 50hp over a decent shop built 2 1/4" and a 2.5" mandrel bent system.

Wow!
i've seen tests where open headers only added an average of about 12 - 15 hp.
... but that was on a 300 hp 360.
 
Hamerhead said:
Car is coming along. 68 GTS 383 auto. Engine is running stock manifolds and has an unknown rebuilt from a previous time. Now it's time for new exhaust. Been planning on a 2.5" as I plan to add headers min in the future. Does anyone have a similar combo and how satisfied are you.
Click to expand...
Here's my .02 about weighing whether going 2.5" the whole length now is "worth it" in this situation.
  • Peak RPM. Higher the rpm, the more exhaust. More exhaust, the greater the need for larger diameters for best top end flow.
  • VE. The higher the VE, the smaller diameters needed through the same rpm range. Street hot rod engines its prob best to assume lower VEs.
  • Mufflers are the main restriction. 5 or 10 feet of 2.25" pipe is generally nothing compared to the losses in the muffler.
  • Use a pair of good flowing mufflers! In general a 2.5 in-out will outflow a 2.25 in - out of similar design.
  • Assume when you install headers new pipes all the way to the mufflers will be needed. The angles and positions are generally too different.
IIRC '68 383 came with an H-pipe. If you do an H, put it in the right spot. With manifolds, copy the factory position.

Exceptions to my comparison of plain pipe to muffler restrictions:
* Sharp bends, cheated bends, and closely placed bends, especially when the tubing diamerter is reduced by the bending.*

In concept I'm fine with pipe diameter reductions after the muffler. The exhaust gas has cooled considerably by that point, and it would be good to keep velocity up all the way to the exit. However, in reality it depends how its done. That's why these tailpipes are coming off in the next week or so. I'm going to test a mandrel bent 2.5" tail pipe and see how it fits. I may end up splicing in some 2.25" straight pipe. Since your car is a '68, you should have less trouble with the tailpipe fitting than a '67. If you have a local shop that can make or weld up the bends, that's the cat's meow.

This gives some idea of reduction in size and turn combination that is just not going to allow for good flow. Even at mid and low rpms its probably hurts velocity.
upload_2019-5-12_19-39-6.png


Driver Side is worse.
upload_2019-5-12_19-40-10.png
 
Mattax said:
Here's my .02 about weighing whether going 2.5" the whole length now is "worth it" in this situation.
  • Peak RPM. Higher the rpm, the more exhaust. More exhaust, the greater the need for larger diameters for best top end flow.
  • VE. The higher the VE, the smaller diameters needed through the same rpm range. Street hot rod engines its prob best to assume lower VEs.
  • Mufflers are the main restriction. 5 or 10 feet of 2.25" pipe is generally nothing compared to the losses in the muffler.
  • Use a pair of good flowing mufflers! In general a 2.5 in-out will outflow a 2.25 in - out of similar design.
  • Assume when you install headers new pipes all the way to the mufflers will be needed. The angles and positions are generally too different.
IIRC '68 383 came with an H-pipe. If you do an H, put it in the right spot. With manifolds, copy the factory position.

Exceptions to my comparison of plain pipe to muffler restrictions:
* Sharp bends, cheated bends, and closely placed bends, especially when the tubing diamerter is reduced by the bending.*

In concept I'm fine with pipe diameter reductions after the muffler. The exhaust gas has cooled considerably by that point, and it would be good to keep velocity up all the way to the exit. However, in reality it depends how its done. That's why these tailpipes are coming off in the next week or so. I'm going to test a mandrel bent 2.5" tail pipe and see how it fits. I may end up splicing in some 2.25" straight pipe. Since your car is a '68, you should have less trouble with the tailpipe fitting than a '67. If you have a local shop that can make or weld up the bends, that's the cat's meow.

This gives some idea of reduction in size and turn combination that is just not going to allow for good flow. Even at mid and low rpms its probably hurts velocity.
View attachment 1715334376

Driver Side is worse.
View attachment 1715334377
Click to expand...
Thanks for your insight. I was unaware an H pipe was standard. Another reason to ask opinions. Car at present does not have H pipe. Anyone know correct position?
 
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