exhaust pipe diameter for a mildly modified 340

[ir3333]

I have read 2 1/4" diameter will support 400 hp. Any idea on how much power might be lost going to 2" diameter duals.
2 inch will be much lighter, leave more room and allow the use of quieter mufflers!

 

[str12-340]

Original headpipe was 2 1/4 except for around the starter, tailpipes about 1 7/8. Accurate limited's standard is 2 1/4 all the way through and fit very well. Aluminized pipe. Various types of mufflers can be had for various inlet and outlet sizes.

 

[MoparMike1974]

2.25" minimum. I always run 2.5" on my street 340's.

 

[273]

I have read 2 1/4" diameter will support 400 hp. Any idea on how much power might be lost going to 2" diameter duals.
2 inch will be much lighter, leave more room and allow the use of quieter mufflers!

More than likely the mufflers will kill more hp than the pipes, ideally you want zero back pressure, if you already have an exhaust you could measure the back pressure and guesstimate from there.

 

[RustyRatRod]

Even a stock 340 will see benefits from a full 3" exhaust with 3" constant diameter mufflers. That is, AS LONG AS the collector diameter is at least 3". If you're running 340 manifolds, then 2.5 is about as big as you can go.

 

[RustyRatRod]

More than likely the mufflers will kill more hp than the pipes, ideally you want zero back pressure, if you already have an exhaust you could measure the back pressure and guesstimate from there.

Well.......actually, ideally, you want negative pressure and that's possible with some of the mufflers available today, such as the Ultra Flow welded. Some of them flow better than an open pipe, due to their design that actually causes a vortex action to help evacuate exhaust.

 

[ir3333]

I thought a street engine needed some back pressure to keep lo rpm exhaust velocity up for low speed power?
Isn't that why street driven engines benefit from a small cfm carb and why the factory reduced the tail pipes
to 1 7/8" from 2 1/4"? This wouldn't apply for an open header race engine at 6000 rpm.
....but the question is how much power might be lost going from 2 1/4" to 2" exhaust? I'm wondering if there
may even be an improvement below 3500 rpm?

 

[Icetech]

 

[Icetech]

I'm going to be running headers into a single 3" exhaust with the biggest muffler i can fit (want quiet as possible) and a pair of electric cutouts behind the headers for when i want to be annoying

 

[273]

I thought a street engine needed some back pressure to keep lo rpm exhaust velocity up for low speed power?
Isn't that why street driven engines benefit from a small cfm carb and why the factory reduced the tail pipes
to 1 7/8" from 2 1/4"? This wouldn't apply for an open header race engine at 6000 rpm.
....but the question is how much power might be lost going from 2 1/4" to 2" exhaust? I'm wondering if there
may even be an improvement below 3500 rpm?

Back pressure doesn't create exhaust velocity.

 

[toolmanmike]

I have read 2 1/4" diameter will support 400 hp. Any idea on how much power might be lost going to 2" diameter duals.
2 inch will be much lighter, leave more room and allow the use of quieter mufflers!

Personal preference, I wouldn't put 2" on anything but a slant 6. Now saying that, a 2" dual system on a stock 318 is probably borderline overkill but, unless it has mandrel bent pipes, the bends are restricted down to 1 1/2" or maybe less.

 

[ir3333]

I think it does, to a point. High velocity gasses exiting the exhaust will create some vacuum to help pull successive
exhaust pulses out and it also helps fill the next cylinder on the intake stroke.Think of an engine that has no exhaust pipe..the fumes would just come out of the valves and blat haphazard against the atmosphere. This is why tuned headers only work on long duration cams and why exhaust pipes that are too big hurt power.
For a street application and small cams this really only helps low speed power.

 

[273]

I think it does, to a point. High velocity gasses exiting the exhaust will create some vacuum to help pull successive
exhaust pulses out and it also helps fill the next cylinder on the intake stroke.Think of an engine that has no exhaust pipe..the fumes would just come out of the valves and blat haphazard against the atmosphere. This is why tuned headers only work on long duration cams and why exhaust pipes that are too big hurt power.
For a street application and small cams this really only helps low speed power.

back pressure fights the exhaust from leavening the chamber.

Say you need a 2.5" pipe for zero back pressure, obviously a 4" pipe would have zero too but with less velocity.

Velocity is important but don't let it scare you into decision that strangle your engine that kill power overall.

 

[ir3333]

From icetechs' chart a stock 340 at 275 hp would seem to be be ok with 2" duals.

 

[ir3333]

and will a 320 hp 340 only lose a bit of power above 5000 rpm?

 

[273]

and will a 320 hp 340 only lose a bit of power above 5000 rpm?

I just looked at an engine masters dyno 2.5 vs 3 with mufflers on a 600 hp big block, was a 13 hp and 16 tq different at peak, but did lose a bit more mid high rpm.

To me what mufflers you run will have the biggest impact.

 

[4spdragtop]

Watching/learning... I know an upgrade will be needed when I pull 273 for 340. Mo' money....

 

[Snake]

I thought a street engine needed some back pressure to keep lo rpm exhaust velocity up for low speed power?
Isn't that why street driven engines benefit from a small cfm carb and why the factory reduced the tail pipes
to 1 7/8" from 2 1/4"? This wouldn't apply for an open header race engine at 6000 rpm.
....but the question is how much power might be lost going from 2 1/4" to 2" exhaust? I'm wondering if there
may even be an improvement below 3500 rpm?

You need to watch engine masters.

 

[MoparMike1974]

"I thought a street engine needed some back pressure to keep lo rpm exhaust velocity up for low speed power?'

Old wives tale. Engine masters proved it was BS with dyno testing.

 

[rumblefish360]

I thought a street engine needed some back pressure to keep lo rpm exhaust velocity up for low speed power?
Isn't that why street driven engines benefit from a small cfm carb and why the factory reduced the tail pipes
to 1 7/8" from 2 1/4"? This wouldn't apply for an open header race engine at 6000 rpm.
....but the question is how much power might be lost going from 2 1/4" to 2" exhaust? I'm wondering if there
may even be an improvement below 3500 rpm?

Your minimum pipe size is 2-1/4. Smaller pipes will not improve low end anything.

I think it does, to a point. High velocity gasses exiting the exhaust will create some vacuum to help pull successive
exhaust pulses out and it also helps fill the next cylinder on the intake stroke.

Correct

Think of an engine that has no exhaust pipe..the fumes would just come out of the valves and blat haphazard against the atmosphere.

Ridiculous

This is why tuned headers only work on long duration cams

100% false

and why exhaust pipes that are too big hurt power.

There is truth in that.

For a street application and small cams this really only helps low speed power.

Not correct

and will a 320 hp 340 only lose a bit of power above 5000 rpm?

Good question

 

[rumblefish360]

I have read 2 1/4" diameter will support 400 hp. Any idea on how much power might be lost going to 2" diameter duals.
2 inch will be much lighter, leave more room and allow the use of quieter mufflers!

A 2-1/4 pipe size should be able to support 400hp. The problem is the muffler is cutting power.

My question to you is, how much HP does your engine make?
How quite do you want your exhaust?

 

[1971 CY Dodge Dart]

I put a TTI 2 1/2 system on my mild 340 that had the stock exhaust from Accurate with Black Widow mufflers. I reused those mufflers on the TTI system and felt a seat of the pants improvement.

 

[RustyRatRod]

I thought a street engine needed some back pressure to keep lo rpm exhaust velocity up for low speed power?
Isn't that why street driven engines benefit from a small cfm carb and why the factory reduced the tail pipes
to 1 7/8" from 2 1/4"? This wouldn't apply for an open header race engine at 6000 rpm.
....but the question is how much power might be lost going from 2 1/4" to 2" exhaust? I'm wondering if there
may even be an improvement below 3500 rpm?

Back pressure is an urban myth that just needs to go away.

 

[toolmanmike]

I have read 2 1/4" diameter will support 400 hp. Any idea on how much power might be lost going to 2" diameter duals.
2 inch will be much lighter, leave more room and allow the use of quieter mufflers!

What's on there now? If I was to install a new system and 2 1/2" were an option I would go with that for a warmed over 340.

 

[furyus2]

I thought a street engine needed some back pressure to keep lo rpm exhaust velocity up for low speed power?
Isn't that why street driven engines benefit from a small cfm carb and why the factory reduced the tail pipes
to 1 7/8" from 2 1/4"? This wouldn't apply for an open header race engine at 6000 rpm.
....but the question is how much power might be lost going from 2 1/4" to 2" exhaust? I'm wondering if there
may even be an improvement below 3500 rpm?

An episode of Engine Masters shows that an engine does not need backpressure. Check it out on YouTube.