Optimal Exhaust Diameter/Set Up

[fastd]

My understanding of optimizing exhaust performance: the focus should not be so much back pressure as on velocity.

The wider the diameter the pipe, the lower the velocity; low velocity is generally not a good thing. So in order to make the "best" diameter decision depends whether you want focus on lower rpm power or upper rpm power. Does that sound right?

Can you all share your observations with respect to exhaust performance, especially if you changed from a smaller diameter to a larger diameter, or changed muffllers from restrictive to less restrictive? What did you have before, what did you change it to, what dyno hp and torque did you pick up, did you notice a "seat of the pants" power reduction at low rpms...anything else?

[mickmate]

I've changed out quite a few of the side pipe mufflers. The seat of the pants difference is what they show on the dyno. An amazing increase with little increase in noise.
http://www.classicchambered.com/clas...rformance.html

[mdross1]

My car with 550+hp 460,competition headers,undercar 2 1/2" exhaust,first used straight tru mufflers,(very loud).Then had a set of stainless chambered mufflers built,which made the noise more bearable,and definetly felt a seat of the pants difference in bottom end torque

[vector1]

here's something i posted on anothe thread, might be helpful. guess if you want velocity put it through a straw.

from open exhaust to 3" inlet dual race muffler that flows 2000cfm total

512tq/510hp to 506tq/506hp

from open exhaust to 2" inlet dual race muffler that flows 900 cfm total

512tq/510hp to 463tq/462hp

from open exhaust to 1.5" inlet dual race muffler that flows 500 cfm total

512tq/510hp to 436tq/422hp

[fastd]

mdross1,
Are you saying that the bottom end torque was better after you added the chambered mufflers, i.e. added some restriction and low end velocity?

[RedBarchetta]

Different engines respond in different ways. The cylinder heads play a HUGE role in overall air flow and velocity. Depending on the cam as well, one set of pipes on one engine may work better/worse than on another. Primary (from the cyl head) pipe diameter has a significant effect on HP, while the collector volume and length can influence where you move the torque curve. I don't think you are going to find the optimal answer, but merely guidance on what works and what doesn't to formulate a ballpark value. And then you'll have to make a decision on the data.

While I enjoy the sound of my 3" Stainless Specialties mufflers, on my particular motor it's arguably too big. If I were to do it again, I might have opted for 2.5" or 2.75" I.D. for a more optimum torque curve. I need less restrictive headers as well (my Edelbrock Victor Jr heads flow pretty good...AFRs would be better).

We all wish we had free access to a good welder and a dyno.

-Dean

[fastd]

Dean,
I am replacing my stock ERA 2" under car exhaust (all the way to my chrome tips) on my 351W; I am thinking of a 2 1/2" muffler with 2 1/2" pipes before the muffler and a 3" pipe after (for aesthetics) exiting in front of the rear tires.

I don't want to loose anything on the low end in favor the the high end; 2 1/2" muffler and pipe should be OK, right?

[Rare Iron]

My rule of thumb: use 2.25" dia. (or 2.5") up to 375 or so HP, use 2.5" exhaust up to 485 HP, over 485 use 3". There is not a HUGE difference in cfm flow between the 2.25" & 2.5", but there is a quantium leap in cfm flow when you go to 3" from 2.5".

You'll lose bottom-end & midrange if you go with too large of diameter for your engine's output. Maintaining proper flow velocity & backpressure achieves optimum exhaust performance. Bigger is not always better! Also, use mufflers that will flow adequately for the desired results. Running a 3" exhaust & then into restrictive 3" mufflers defeats the purpose of running the larger system!

-Eric S.
www.classicchambered.com

[Dwight]

I have a 302 stroked to 354 c.i. I have 1 5/8 equal length headers, 4 into one with 3" i.d. Classic Chamber side pipes. I'm making 402 rear wheel hp and 416 tq.
They told me 2 1/2" side pipe mufflers for up to 450 hp and for over 450 hp use 3" mufflers.
I think you have to consider the header size and the side pipe size. Total length is part of the equation also.

Dwight

[Tim Brewer]

But what's the best collecter setup...I know there are several types.

[scottj]

This gets a lot of positive feedback in the racing community.

http://www.maxracesoftware.com/pipemax36xp2.htm

[fastd]

scottj,

I checked out pipemax - I think it is over my head; do you a less sophisticated,rule of thumb method making simple assumptions?

Fastd

[vector1]

hows this for rule of thumb

dual 2" = 216hp

dual 2.25" = 291hp

dual 3" = 533hp

dual 3.5" = 733hp

[fastd]

vector1,
Nice!
Looks like that band between 2.25 and 3" is where most cobras fall into; I am targeting 400-425 hp and trying not to give up low end torque. 2.50" looks like my ticket...

[Rare Iron]

Originally Posted by vector1

hows this for rule of thumb

dual 2" = 216hp

dual 2.25" = 291hp

dual 3" = 533hp

dual 3.5" = 733hp

----------------------------------------------------------------------
I would NEVER use 2" dual exhaust on ANY V-8! The pipe diameter is so small, it doesn't allow for proper expansion of the exhaust pulses & results in a lot of popping & backrapping. My rule of thumb is 2.5" up to 480 HP, beyond that you can go to dual 3" & not suffer any low or mid-range performance loss. A small block putting out under 350 HP run 2.25" or even 2.5". There is NOT a very big difference in scfm flow between 2.25" & 2.5". There is a quantium leap in scfm flow when transitioning from 2.5" to 3" exhaust. Keep in mind however, if you up the pipe diameter & run crappy-flowing mufflers, you defeat the whole purpose of the larger diameter. There are a lot of 3" in/out "performance" mufflers out there that flow like pigs! Beware!

The thing that blows my mind is most stock-type Cobra side exhaust mufflers have flowpaths under 2" in diameter. Imagine a fire-breathing big block in the 600+ HP range flowing through an exhaust passage as large as a Toyota Corolla exhaust system! How about a dual-core muffler? Looks like a regular tube muffler on the outside, but it has two staggered 1" flowpath cores on the inside. When you look inside these mufflers, there's a WALL immediately with a 1" entry into the first flow tube! Can you say 'jam a potato' in your tailpipe? Talk about turning your engine into a pump to get the exhaust out! MAJOR HP LOSS!

This was my inspiration!

[mdross1]

Originally Posted by fastd

mdross1,
Are you saying that the bottom end torque was better after you added the chambered mufflers, i.e. added some restriction and low end velocity?

Yes, it was noticeably better off idle.My car has everything I ever wanted or imagined a Cobra could be.My focus, when building this car was road racing,which meant a lot of chassis issues had to addressed,to handle the torque a 460 can belt out.Also the exhaust,was a major issue to me.After owning a side exhaust 427 Vette,and riding in a friends 427 Cobra with side exhaust,undercar exhaust was the only way to go.After picking the grey matter of exhaust Pro's,I assembled what worked the best including having the Stainless mufflers built to fit where they had to perform.The motor runs clean,it's very crispy,off idle performance is all I expected and then some.The music out the back is intoxicating,especially having the duals 3 1/2' apart.The sound inside the car is incredible as well.

[Bernica]

I believe Keith Craft is/was doing some pretty trick exhaust setups..