Explain 3" exhaust system advantage.

[Racer57]

I'm old school as in 60s and 70s muscle cars. Majority of people feel that 3"+ pipes are only good for Hemi's and big block strokers because there isn't enough restriction (velocity) for smaller engines for the exhaust system to work properly. Please explain why they work on these engines.

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[WildHorse]

Having backpressure is a myth. ANY restriction in the exhaust is detrimental to performance (which includes economy). Header primary diameter, & collector length is much more CID vs SIZE sensitive. You want all the velocity in the header.

Coyote combined exhaust valve size is 2.45".
Add 7000+ RPM.
IMHO These engines would love a 2" primary & 3" exhaust.

 

[Racer57]

Having backpressure is a myth. ANY restriction in the exhaust is detrimental to performance (which includes economy). Header primary diameter, & collector length is much more CID vs SIZE sensitive. You want all the velocity in the header.

Coyote combined exhaust valve size is 2.45".
Add 7000+ RPM.
IMHO These engines would love a 2" primary & 3" exhaust.

I wrote "restriction (velocity)" intentionally instead of saying "back pressure". You want an exhaust pipe that aids flow. Too large a pipe decreases gas velocity and it’s velocity that scavenges the exhaust from the cylinder.

 

[Snakebyte]

Here's a great analysis performed by one of our great forum contributors...
https://www.mustang6g.com/forums/threads/exhaust-pressure-testing.162791/page-28#post-3876691

 

[Racer57]

Having backpressure is a myth. ANY restriction in the exhaust is detrimental to performance (which includes economy). Header primary diameter, & collector length is much more CID vs SIZE sensitive. You want all the velocity in the header.

Coyote combined exhaust valve size is 2.45".
Add 7000+ RPM.
IMHO These engines would love a 2" primary & 3" exhaust.

You think its funny for me to say "Too large a pipe decreases gas velocity and it’s velocity that scavenges the exhaust from the cylinder. " ?

 

[WildHorse]

You think its funny for me to say "Too large a pipe decreases gas velocity and it’s velocity that scavenges the exhaust from the cylinder. " ?

Ahhh yer one of those eh ?

Fine.

Every time you get a cross sectional change, the pressure wave reverses.
So reducing the possibility of that, is the goal.

You don't have a 1 -size is perfect for all the RPM range. so find the sweet spot
for your intended purpose. Mines WOT.

These engines make next to no power below 4k.

The 302 coyote exhaust port flows more than a 1964 426 HEMI intake port.

You want velocity ? Stick with the factory tri-y header & use a 2" catback.

you'll have loads of fun at low RPM. Not so much at WOT.

 

[Roady68]

Ahhh yer one of those eh ?

Fine.

Every time you get a cross sectional change, the pressure wave reverses.
So reducing the possibility of that, is the goal.

You don't have a 1 -size is perfect for all the RPM range. so find the sweet spot
for your intended purpose. Mines WOT.

These engines make next to no power below 4k.

The 302 coyote exhaust port flows more than a 1964 426 HEMI intake port.

You want velocity ? Stick with the factory tri-y header & use a 2" catback.

you'll have loads of fun at low RPM. Not so much at WOT.

I think there could be gains on using 3". As an owner of a nice well built Gen 2 (426) hemi in my other car, I can say 3" was the way to go. If this engine has more cross section area, and it certainly revs higher, I would expect better performance for bigger pipe.

 

[Racer57]

I think there could be gains on using 3". As an owner of a nice well built Gen 2 (426) hemi in my other car, I can say 3" was the way to go. If this engine has more cross section area, and it certainly revs higher, I would expect better performance for bigger pipe.

I agree 100% about a BB 426 hemi. But a SB 305 with a 3.64 bore/3.62 stroke doesn't move as much air as a 426 with a 4.24 bore/3.75 stroke.

 

[GregO]

But a SB 305 with a 3.64 bore/3.62 stroke doesn't move as much air as a 426 with a 4.24 bore/3.75 stroke.

Depends on the RPM and VE.

IMO the full length exhaust sweet spot diameter is 70mm (2.75”) for the S550 5.0 & 5.2. Sourcing domestic thin wall 70mm 304 stainless isn’t as easy as 3.00” so the aftermarket defaults to 3.00”. Plus customers are more likely to gravitate to a 3.00” system vs. 2.50”.
I think Milltek is the only company making a 70mm S550 replacement exhaust.
I believe the Mach1, GT350 & GT500 are factory equipped with 70mm dia. exhaust tubing.

 

[Racer57]

Depends on the RPM and VE.

IMO the full length exhaust sweet spot diameter is 70mm (2.75”) for the S550 5.0 & 5.2. Sourcing domestic thin wall 70mm 304 stainless isn’t as easy as 3.00” so the aftermarket defaults to 3.00”. Plus customers are more likely to gravitate to a 3.00” system vs. 2.50”.
I think Milltek is the only company making a 70mm S550 replacement exhaust.
I believe the Mach1, GT350 & GT500 are factory equipped with 70mm dia. exhaust tubing.

I agree with you. But I wonder how many people install 3" pipes only because of sound ? Kinda tricking people like Ford putting 4 3/4" tips on a much smaller pipe.

 

[CrackedHorn]

This guy might explain it better than I can.

 

[junits15]

Ahhh yer one of those eh ?

Fine.

Every time you get a cross sectional change, the pressure wave reverses.
So reducing the possibility of that, is the goal.

You don't have a 1 -size is perfect for all the RPM range. so find the sweet spot
for your intended purpose. Mines WOT.

These engines make next to no power below 4k.

The 302 coyote exhaust port flows more than a 1964 426 HEMI intake port.

You want velocity ? Stick with the factory tri-y header & use a 2" catback.

you'll have loads of fun at low RPM. Not so much at WOT.

this this this 1000x

An exhaust is a pulsed acoustic system, at every "interface" in the system there is a matching problem. When the pipe abruptly changes diameter, you have a mismatch which will cause the pulses to experience destructive interference and reduced velocity.

A "good" exhaust will have tapered transitions where needed and use entirely the same diameter pipe the whole way through. Consider the collector on any of the many LTH's out there, that collector is designed to bring the pulses together without creating a matching issue. This is the same reason that the headers are equal length, this ensures that the pulses from each cyl enter the collector at different points in time, further reducing interference.

The reason bigger is better is because the pulsed acoustic system that an exhaust is acts as filter, it will offer less restriction at some pulse rates than it does at some other pulse rates. Typically high pulse rates (high RPM) are more restrictive than low pulse rates (low RPM). Using a larger pipe changes the cutoff frequency(ies) of the exhaust to a different pulse rates which typically helps performance at high RPM.

This is something that isn't regularly talked about, but an exhaust is not a linear system, its a reactive system. It's restriction changes with RPM.

This is why when people go to LTH's they don't lose any low end performance, because there was no restriction there anyway and the LTH conversion doesn't effect that area of the RPM range the same way it effects the top of the RPM range. The volume of gasses flowing through the exhaust is just as important as the rate of the pulses leaving the motor.

 

[ice445]

Why do the stock post cat connections neck down to like 2 1/4, does anyone know? Seems like it will make any 3 inch system kind of pointless.

 

[GregO]

Why do the stock post cat connections neck down to like 2 1/4, does anyone know? Seems like it will make any 3 inch system kind of pointless.

Well the GT350 and I believe the GT500 are 63mm (2.375) and the 5.0 is 2.25”.
It was explained to me the sizes are more than adequate within the designed requirements by the Ford engineers.
Personally I’m still on the fence that the reduction is similar to a header collector choke but that’s going down a deep rabbit hole.

 

[Racer57]

This guy might explain it better than I can.

Nice video. The exhaust system needing to pull gases back out of the cylinders during each cycle is what a lot of people miss.

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