Questions on 2.5" vs 3" Exhaust - Page 2 - Chevelle Tech
Headers and Exhaust Discussion related to Exhaust systems and headers.
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post #16 of 91 (permalink) Old Dec 4th, 18, 3:44 AM Thread Starter
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Rob
 
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Re: Questions on 2.5" vs 3" Exhaust

Thanks gang, I really hate to leave power on the table. I think I'll figure out how to run a 3" system, even thou it might not help in my situation power wise, but I'll have the exhaust in place should I decide to upgrade the heads and cam for more power later down the line.
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post #17 of 91 (permalink) Old Dec 4th, 18, 3:53 AM
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Re: Questions on 2.5" vs 3" Exhaust

Jim Hand did indeed do some extensive testing on his street driven Pontiac 455.

He tested H versus X type crossover, mph were 110.29, 110.63

He also tested 3" v 2.25" tail pipes, mph were 106.96, 106.63.

The OPs engine has more cubes & makes more HP, so larger pipes are more important.
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post #18 of 91 (permalink) Old Dec 4th, 18, 4:38 AM
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Read Jim Hand’s Test:

http://www.pontiacstreetperformance....p/exhaust.html

A copy of the wagon exhaust testing was included with the paperwork for my Torque Tech exhaust back in the late ‘90’s - had photos of the wagon too.

It’s funny how they tested two different Flowmaster Mufflers and found two completely different results. I’m a fan of the sound but quiet would be cool too.

To summarize:
3” head pipes better suited for this power range. If you are running manifolds, don’t bother with larger than 2.5”.

A well-executed X-type crossover helps crutch an undersized exhaust. Transitioning from 3” down to 2.5” either after the crossover or after the mufflers (if no crossover) minimizes power loss. Use reducers with the longest taper, whenever possible.

To me, crossovers are hard to execute properly and the most functional are the least serviceable (access for maintenance under the car).

Use the largest case, lowest restriction muffler possible. The large case will help dissipate sound and heat (which decreases demand on the tailpipes).

Proving every day hindsight is 20/20.
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post #19 of 91 (permalink) Old Dec 4th, 18, 8:30 AM
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Re: Questions on 2.5" vs 3" Exhaust

Quote:
Originally Posted by 66rat View Post
Thanks gang, I really hate to leave power on the table. I think I'll figure out how to run a 3" system, even thou it might not help in my situation power wise, but I'll have the exhaust in place should I decide to upgrade the heads and cam for more power later down the line.
I'll go ahead and tell you NOT to go with the Summitracing 3" exhaust. The tailpipe bends are ridiculous and hit everything back there. I'm running their setup from headers to mufflers with turn downs right now. Just ordered flowmaster tailpipes, arrived yesterday and hopefully can mess with them tonight. Look like they'll fit a lot better.

Got this kit: https://www.summitracing.com/parts/sum-680122

Tailpipes bend IN towards rear diff, running diff girdle, it hits, close to trunk floor and fuel lines, runs along front of gas tank, then runs alongside gas tank, too much heat around the fuel for me.. Mufflers sound good but are LOUD, makes your ears ring with the turn downs..

Tailpipes that just came in: https://www.summitracing.com/parts/flo-15819

*Jeff*

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post #20 of 91 (permalink) Old Dec 4th, 18, 3:39 PM
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Re: Questions on 2.5" vs 3" Exhaust

Quote:
Originally Posted by Hotwire View Post
I'll go ahead and tell you NOT to go with the Summitracing 3" exhaust. The tailpipe bends are ridiculous and hit everything back there. I'm running their setup from headers to mufflers with turn downs right now. Just ordered flowmaster tailpipes, arrived yesterday and hopefully can mess with them tonight. Look like they'll fit a lot better.

Got this kit: https://www.summitracing.com/parts/sum-680122

Tailpipes bend IN towards rear diff, running diff girdle, it hits, close to trunk floor and fuel lines, runs along front of gas tank, then runs alongside gas tank, too much heat around the fuel for me.. Mufflers sound good but are LOUD, makes your ears ring with the turn downs..

Tailpipes that just came in: https://www.summitracing.com/parts/flo-15819
I'd like to hear how they fit.
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post #21 of 91 (permalink) Old Dec 4th, 18, 4:51 PM
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Re: Questions on 2.5" vs 3" Exhaust

The Flowmaster set looks like something that could be made to fit pretty good.

Also, not sure if you thought about adding an X/H, but we just built nearly identical 427 sbc's (engine dyno'd at 630hp), one with duals and one with an X (from Torque Tech). The X has a much different (smoother/mellower) sound but I think the performance (based on 1/4 mile times/speeds) seems almost the same. The separate duals come off easily, but the X is a lot more tricky.

I think I'd do true duals if the header collector, extension, and exhaust pipe diameters were all good for the combo. Maybe the X/H helps when something in the system is restricting things?

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post #22 of 91 (permalink) Old Dec 4th, 18, 5:40 PM
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Re: Questions on 2.5" vs 3" Exhaust

I went from 1-5/8" headers with 3" collectors/ 2 -1/2" pipe / 2-1/2 mufflers turndowns at the axle to the same setup but 3" pipe and mufflers on my 388....It picked up 1.5 mph in the 1/4 mile.... .05 in the 60' no x or h pipe. I was surprised it picked up anything.

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post #23 of 91 (permalink) Old Dec 4th, 18, 7:33 PM
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Re: Questions on 2.5" vs 3" Exhaust

My recent project has a 550 horse 427, I might cam it down a little so that figure may reduce. I decided on fabbing 3" headpipes out of 304 SS with mandrel elbows and pipe. Settled on 3" Borla ProXS muffers and reduced to 2 1/2" to the flowmaster 409 SS tailpipes. Those tails fit pretty good but required 1" cut out of the hoop heading over the axle. Took a major investment of time but I have less than $700 including water jet square flanges to the collector. The 304 SS won't turn brown like 409 will and the tailpipes don't get hot enough to turn brown. This system will look new for a lifetime.
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post #24 of 91 (permalink) Old Dec 4th, 18, 8:35 PM
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Re: Questions on 2.5" vs 3" Exhaust

Quote:
Originally Posted by 66rat View Post
Did a search and didn't come up with much info.

Is there a basic rule of thumb for how much power a 2.5", 3", 3.5", etc, exhaust system will support?

Due to space limitations, I'd like to run a 2.5" exhaust system on my 67 Impala. The motor is a 496 making a peak HP of 528 and 566 torque and its all done by 5300 RPM. At my performance level, would it be worth it to run the 3" exhaust?
Question is : how much time do you plan on spending at wot and is it worth dealing with the additional weight, clearance issues and increased noise and expense of a 3” system.

I do however like the idea of 3” headpipes and 2.5” tail pipes. Seems like a nice compromise.
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post #25 of 91 (permalink) Old Dec 4th, 18, 8:42 PM
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Re: Questions on 2.5" vs 3" Exhaust

Quote:
Originally Posted by 69-CHVL View Post
Question is : how much time do you plan on spending at wot and is it worth dealing with the additional weight, clearance issues and increased noise and expense of a 3” system.

I do however like the idea of 3” headpipes and 2.5” tail pipes. Seems like a nice compromise.
The pass in my sig is with a 3" X that reduces to 2.5" to 2.5" ultra flo's and torque tech 2.5" tailpipes.
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post #26 of 91 (permalink) Old Dec 4th, 18, 10:26 PM
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Quote:
Originally Posted by WHT/73 View Post
The pass in my sig is with a 3" X that reduces to 2.5" to 2.5" ultra flo's and torque tech 2.5" tailpipes. <img src="https://www.chevelles.com/forums/images/smilies/thumbsup.gif" border="0" alt="" title="Thumbsup" class="inlineimg" />
Sounds like a well-thought out system.

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post #27 of 91 (permalink) Old Dec 5th, 18, 1:58 PM
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Re: Questions on 2.5" vs 3" Exhaust

Not trying to hijack, but I'm going to the same question on the moment.
Want to upgrade the exhaust before I upgrade the 396 to a 496.
Have a full 2.5" system (mandrel bends) with magnaflow 14" long mufflers. It's just a bit to much on the highway 2500-3000rpm to have a normal conversation.
Want to go with lemons headers, from the 3.5"collector with 3.5" or 3" to X with 3" to mufflers and keep the 2.5" tailpipes.

But what size mufflers should you take? 3" to match the front pipe size and get more flow. Or 2.5 to match the tailpipes?

I don't want more exhaust sound level and are looking at magnaflow 14264 18" long case 3" mufflers or should I go with 2.5" mufflers?

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post #28 of 91 (permalink) Old Dec 5th, 18, 2:28 PM
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Re: Questions on 2.5" vs 3" Exhaust

https://www.summitracing.com/parts/j...-64g/overview/
Something like that? 3" in and 2½" out. There has to be others.
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post #29 of 91 (permalink) Old Dec 5th, 18, 2:47 PM
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Re: Questions on 2.5" vs 3" Exhaust

Use a 3" inlet/outlet muffler with a tapered reducer for the 2.5" tailpipes.
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post #30 of 91 (permalink) Old Dec 5th, 18, 3:36 PM
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Re: Questions on 2.5" vs 3" Exhaust

In my opinion either one will work with what you have. For whatever reason I have friends who think that all blocks require a 3.5" exhaust. I have a 3" on my car and will usually out run them. Their cars are also much lighter than mine(eg 600lbs lighter) and with one we ar eneck and neck and the other I am usually about a second faster.
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