Exhaust to large?

PCT · Sep 4, 2017

Back in March I change my exhaust around from the y-pipe 3 inch, to 2.5 inch parallel pipe, I have a true dual system with a flow master 50 series so I have 2.5 in 2.25 out. (Both side need to merge at one point after cats to pass CA Smog) In March I have noticed a drop in hwy fuel econ, I'm wondering if the lack of back pressure moved the torque curve out of the highway range there for causing my usable power to drop? maybe I don't know what I'm talking about, but I'm thinking of going from 2.5 to maybe 2, or 2.25 from the cats to the muffler.

monteholic · Sep 4, 2017

going to an exhaust that is too large....

it's not back pressure, it is the exhaust gases slowing and losing velocity because for the larger volume pipe, this velocity aids in pulling the exhaust gases from the engine (scavenging)

exhaust like carburetors and camshafts...more is not always better

I am fighting the same thing with my SS454, I have a 3 1/2" dual exhaust from when I had my 12:1 468 race motor, now I have a 10:1 .030 over 454 built for bottom end torque for street use and the exhaust hurts the drivability/responsiveness

PCT · Sep 4, 2017

monteholic said:
going to an exhaust that is too large....

it's not back pressure, it is the exhaust gases slowing and losing velocity because for the larger volume pipe, this velocity aids in pulling the exhaust gases from the engine (scavenging)

exhaust like carburetors and camshafts...more is not always better

I am fighting the same thing with my SS454, I have a 3 1/2" dual exhaust from when I had my 12:1 468 race motor, now I have a 10:1 .030 over 454 built for bottom end torque for street use and the exhaust hurts the drivability/responsiveness

Do you think 2.5inch per side is too much? What would you reduce it too? Used to be 3 after the y pipe.

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PCT · Sep 5, 2017

I'm going to make an appointment to change the 2.5 to a 2.25, hopping that helps it out...

SYKRAMMAN · Sep 5, 2017

I had come across a mathematical equation on this exact topic but can't seem to find it anywhere, but there's a way to find out exactly how large of an exhaust should be in comparison to the ci's. Sorry dudes!

PCT · Sep 5, 2017

I'll post the chart, I didn't do the math but based off the HP, the 4.7 which makes 310hp, the 3 inch allows up to about 330hp(calculated out) worth of flow. 2.5 allows 232 per pipe, and 2.25 allows 185 per pipe, so if you x both sides the 2.5 is waaaaayyy to big for the 4.7 but just fine for a hemi. I need to go to 2.25.

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PCT · Sep 5, 2017

SYKRAMMAN said:
I had come across a mathematical equation on this exact topic but can't seem to find it anywhere, but there's a way to find out exactly how large of an exhaust should be in comparison to the ci's. Sorry dudes!

I found a chart with hp and diamiter

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Casper · Sep 5, 2017

PCT said:
I found a chart with hp and diamiter

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So, if I read this correctly (and that's not guaranteed) in the stock set-up the single 3" from Y pipe through the Cat is actually choking the Hemi and even the dual 2.25" after the muffler aren't allowing 390 (+) HP.

Or did I miss something??? Not to say I actually expect 390 at the wheels on a near stock truck :disgust:

monteholic · Sep 5, 2017

I think that chart is way off...watch this video

https://www.youtube.com/watch?v=_PVXvHkr-Vs&t=672s

PCT · Sep 5, 2017

Casper said:
So, if I read this correctly (and that's not guaranteed) in the stock set-up the single 3" from Y pipe through the Cat is actually choking the Hemi and even the dual 2.25" after the muffler aren't allowing 390 (+) HP.

Or did I miss something??? Not to say I actually expect 390 at the wheels on a near stock truck

Don't forget I have 4.7

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Casper · Sep 6, 2017

PCT said:
Don't forget I have 4.7

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Saw that, just being selfish and thinking Hemi.

It appears you're probably on the right track based on the chart.

PCT · Sep 6, 2017

monteholic said:
I think that chart is way off...watch this video

https://www.youtube.com/watch?v=_PVXvHkr-Vs&t=672s

But that is for 600+ hp, my guy produced 310 in it's hay day

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monteholic · Sep 6, 2017

PCT said:
But that is for 600+ hp, my guy produced 310 in it's hay day

yes but they were producing 600hp on a 2 1/2" exhaust and hardly any difference when they went to 3" and the 2 1/2" actually produced better torque down low

so in my eyes I think a 2" is more than enough for a 4.7, and probably don't need that big

blackbetty14 · Sep 6, 2017

Take that chart with a grain of salt. It's common for a single 3" exhaust to flow enough to handle 450hp. I know this bc a 82-92 firebird camaro it's all we can run and that's where they max out and start to hurt performance. Even at that point a larger 3.5 or 4" would only gain so much and it would be at the higher Rpms. I've built so many exhaust systems it's hard to count but here's the rule of thumb.

Smaller is always better(to an extent), even if you restrict power on the top end the restriction compare to a well designed smaller system is so minimal you wouldn't even be able to tell. What you will feel is the laziness of the loss of velocity. I currently run a single 3" system and it supports a 650hp supercharged LS motor. I originally ran a single 4" and it lost too much low end.

The stock hemi exhaust is more than adequate from the ypipe back. It's the manifolds and ypipe that's the main restriction. Leave the muffler back or do a muffler upgrade for sound and keep the sizing, upgrade the ypipe and manifolds to get the gains.

On the 4.7 I'm sure it's the same deal, you want fast hot gasses to keep velocity and as the exhaust gets farther away from the chamber the exhaust is slowing down so u want to keep velocity up as well. A single 2.25-2.5" would be more than enough for 300hp (prob a 2.5 to be safe). What you don't want are large primary headers... keep them smaller to promote better velocity and tq.

PCT · Sep 7, 2017

blackbetty14 said:
Take that chart with a grain of salt. It's common for a single 3" exhaust to flow enough to handle 450hp. I know this bc a 82-92 firebird camaro it's all we can run and that's where they max out and start to hurt performance. Even at that point a larger 3.5 or 4" would only gain so much and it would be at the higher Rpms. I've built so many exhaust systems it's hard to count but here's the rule of thumb.

Smaller is always better(to an extent), even if you restrict power on the top end the restriction compare to a well designed smaller system is so minimal you wouldn't even be able to tell. What you will feel is the laziness of the loss of velocity. I currently run a single 3" system and it supports a 650hp supercharged LS motor. I originally ran a single 4" and it lost too much low end.

The stock hemi exhaust is more than adequate from the ypipe back. It's the manifolds and ypipe that's the main restriction. Leave the muffler back or do a muffler upgrade for sound and keep the sizing, upgrade the ypipe and manifolds to get the gains.

On the 4.7 I'm sure it's the same deal, you want fast hot gasses to keep velocity and as the exhaust gets farther away from the chamber the exhaust is slowing down so u want to keep velocity up as well. A single 2.25-2.5" would be more than enough for 300hp (prob a 2.5 to be safe). What you don't want are large primary headers... keep them smaller to promote better velocity and tq.

Would dual 2.25 still be too much? My muffler is a 2.25 in 2.25 out.
Does the back end of the muffler matter, if I make it 2 in 2.25 out will that make a difference? Should I make them turn down?

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crackerjack1957 · Sep 7, 2017

PCT said:
Would dual 2.25 still be too much? My muffler is a 2.25 in 2.25 out.
Does the back end of the muffler matter, if I make it 2 in 2.25 out will that make a difference? Should I make them turn down?

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Which Flowmaster 50 you have?
2 or 1 muffler?
Flowmaster 50 SERIES DELTA FLOW MUFFLERS

PCT · Sep 7, 2017

crackerjack1957 said:
Which Flowmaster 50 you have?
2 or 1 muffler?
Flowmaster 50 SERIES DELTA FLOW MUFFLERS

This one

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HammerHead · Sep 7, 2017

Read these articles before you make a decision. Lots of reading but you will make a better exhaust decision.

http://www.ramforum.com/f44/exhaust_systems_101_a-63137/

blackbetty14 · Sep 7, 2017

PCT said:
Would dual 2.25 still be too much? My muffler is a 2.25 in 2.25 out.
Does the back end of the muffler matter, if I make it 2 in 2.25 out will that make a difference? Should I make them turn down?

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2.25" is fine. You could carry that all the way through. Stepping up to a 2.5" might gain you 5peak hp if your lucky but the loss in low end is noticeable. This is only true if your building a true dual system and your running good longtube headers and a high flow ypipe. I personally think Long tubes, high flow ypipe into a single 3" and split out the muffler into 2.25" or 2.5" is fine. The muffler would be the largest restriction in that system for a stock ish motor (intake, exhaust and a tune). True duals would need need a crossover pipe (H or Xpipe) to help with scavaging and equalizing exhaust pulses. Im a big fan of xpipes for dual systems. I feel they work better and change the tone In a more desirable way.

Coating the headers or wrapping will help with velocity as well not to mention keep things under the hood alittle cooler.

Flowmasters are junk, I would throw them in the trash personally. It's old tech and not even good tech at that. Go for a straight through perforated tube style muffler with a roving fiberglass sound deadoner. They are the highest flowing and best for noise reduction/drone.

PCT · Sep 7, 2017

blackbetty14 said:
2.25" is fine. You could carry that all the way through. Stepping up to a 2.5" might gain you 5peak hp if your lucky but the loss in low end is noticeable. This is only true if your building a true dual system and your running good longtube headers and a high flow ypipe. I personally think Long tubes, high flow ypipe into a single 3" and split out the muffler into 2.25" or 2.5" is fine. The muffler would be the largest restriction in that system for a stock ish motor (intake, exhaust and a tune). True duals would need need a crossover pipe (H or Xpipe) to help with scavaging and equalizing exhaust pulses. Im a big fan of xpipes for dual systems. I feel they work better and change the tone In a more desirable way.

Coating the headers or wrapping will help with velocity as well not to mention keep things under the hood alittle cooler.

Flowmasters are junk, I would throw them in the trash personally. It's old tech and not even good tech at that. Go for a straight through perforated tube style muffler with a roving fiberglass sound deadoner. They are the highest flowing and best for noise reduction/drone.

I went from super 40 to the super 50, I'm guessing from my prior set up this probably isn't the cause? I guess I will switch from 2.5 per side to 2.25. didn't realize .25 inch would make a huge difference but it must be

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