Questions on 2.5" vs 3" Exhaust - Page 4 - Chevelle Tech
Headers and Exhaust Discussion related to Exhaust systems and headers.
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post #46 of 91 (permalink) Old Dec 8th, 18, 8:23 AM
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Re: Questions on 2.5" vs 3" Exhaust

Rob, I dont know how the AR suspension fits, but 3" tails would make it near impossible to access the upper shock bolts on our 67. MANDREL BENT tails are a must here. Sounds like you got a good plan! it shouldnt rob any HP at all.
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post #47 of 91 (permalink) Old Dec 8th, 18, 8:52 AM
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Re: Questions on 2.5" vs 3" Exhaust

I went with this system and hoping for a good fit. I don't like mix and matching and taking it to a custom shop seems to take forever and usually as much or more $$.

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post #48 of 91 (permalink) Old Dec 8th, 18, 10:23 AM
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Re: Questions on 2.5" vs 3" Exhaust

In the 90's I had a 10.3:1 AFR headed 383 SBC in my Chevelle. 450 HP tops. It started out with the carry over exhaust from the previous 8:1 350 SBC truck engine I had in the car in high school and college. The original system was 2.5" crush bent tubing from a local muffler shop with 2.5" - 3 chamber Flowmasters. I built a mandrel bent 3" system with 2 chamber flowmasters. In back to back weekends at the track, I picked up 4 MPH in the 1/8th with the 3" system.


The inverse of that, is I ran 117 MPH in the 1/8th with 3" straight through mufflers (3.5" head pipes with reduction just before the mufflers.) ~900 HP.


My conclusion is 3" exhaust is always a safe bet for a performance focused street/strip setup.

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post #49 of 91 (permalink) Old Dec 8th, 18, 2:39 PM
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Re: Questions on 2.5" vs 3" Exhaust

2.5" exhaust (head) pipes work very well with most street/strip applications with turbo-style mufflers and 2.5" or even 2.0" tailpipes. As the exhaust pulses enter the collector, from the primary tubes, a significant amount of velocity is lost. As the exhaust gases enter and exit from the turbo-style muffler, the velocity/pressure is further reduced. With straight through mufflers, a 3.0" exhaust system may (or may not) result very slight increase in horsepower - but the sound will be atrocious.
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post #50 of 91 (permalink) Old Dec 8th, 18, 3:41 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?
I've seen those "rule of thumb" claims in some magazines, but I don't put a lot of faith in them as the "rule" rarely ever takes into account the length of the system or the number and tightness of any bends. (An easy example of why I don't pay attention to the "rule" is every school bus you see has a 4 or 5" diameter exhaust pipe on it, not a "low horsepower 2 inch" system.)

I prefer not to guess what size system is needed/preferable under my car. Whenever I make an engine or exhaust modification I hook up a pressure gauge (at the WBO2 bung near the header collectors) so I can directly measure the WOT backpressure in the system. I then know if the exhaust restriction is tolerable/insignificant or if I need to spend the time and money to improve the flow capability.
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post #51 of 91 (permalink) Old Dec 8th, 18, 6:37 PM
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Re: Questions on 2.5" vs 3" Exhaust

Way late to the conversation, but have you looked at oval pipes...https://spintechmufflers.com/exhaust-tubing/
A 3" round sorta equals a 3.5 x 2.25 oval

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post #52 of 91 (permalink) Old Dec 8th, 18, 7:01 PM
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Re: Questions on 2.5" vs 3" Exhaust

No need to go oval since the 3 inch head pipes fit just fine under a chevelle. The chevelle also has a couple compound offsets that an oval pipe just can't accomplish. Tails need to be round also.
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post #53 of 91 (permalink) Old Dec 8th, 18, 7:16 PM
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Re: Questions on 2.5" vs 3" Exhaust

I don't think there's a problem squeezing them to the muffler (although you might need to play with them by the trans brace), but 3" definitely hangs down going over the axle. I think the rovals over the axle to the tailpipe might tuck in a lot nicer.
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post #54 of 91 (permalink) Old Dec 9th, 18, 5:24 AM
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Re: Questions on 2.5" vs 3" Exhaust

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Originally Posted by MadmanMark View Post
I don't think there's a problem squeezing them to the muffler (although you might need to play with them by the trans brace), but 3" definitely hangs down going over the axle. I think the rovals over the axle to the tailpipe might tuck in a lot nicer.
Not to get of topic, but what size tires/wheels you running on rear, they look huge. Are those 295/65's? For that matter, what do you have on all four corners and with what spring setup. Stance on that car is killer.
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post #55 of 91 (permalink) Old Dec 9th, 18, 8:45 AM
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Re: Questions on 2.5" vs 3" Exhaust

OP didn't mention a Chevelle but a 67 Impala & I don't think they make a 3" system for them, none I've ever seen.

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post #56 of 91 (permalink) Old Dec 9th, 18, 3:52 PM Thread Starter
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Re: Questions on 2.5" vs 3" Exhaust

Quote:
Originally Posted by 77 cruiser View Post
OP didn't mention a Chevelle but a 67 Impala & I don't think they make a 3" system for them, none I've ever seen.
Because I'm going with an air ride system, this will be a custom install.
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post #57 of 91 (permalink) Old Dec 9th, 18, 6:35 PM
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Re: Questions on 2.5" vs 3" Exhaust

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Originally Posted by 66rat View Post
Because I'm going with an air ride system, this will be a custom install.
Are you going to have to make your own? I've made a couple sets 2.5 for my Impala & 3 for my Monte.

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post #58 of 91 (permalink) Old Dec 9th, 18, 9:24 PM
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Re: Questions on 2.5" vs 3" Exhaust

I think the bends are more important than size.

Your exhaust flow cant exceed what you have coming into the engine...that air, naturally aspirated...goes either into 2 or 4 barrels...how thick are those barrels? Then the head passages? Did anyone ever do the math on the flow of air INTO the motor? Add up the volume of the runners? The space of the exhaust runner before the header?

Curious if anyone simply added a small air pump, like a small 12v shop vac motor to their pipes and see what the increase in performance is? Run it without and with.


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post #59 of 91 (permalink) Old Dec 10th, 18, 12:23 AM
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Re: Questions on 2.5" vs 3" Exhaust

I think that the CFM of air in, mixing of gas, and adding ignition will cause the volume to expand to higher volumes than just the CFM of the induction air alone.

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post #60 of 91 (permalink) Old Dec 10th, 18, 3:47 AM Thread Starter
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Re: Questions on 2.5" vs 3" Exhaust

Quote:
Originally Posted by 77 cruiser View Post
Are you going to have to make your own? I've made a couple sets 2.5 for my Impala & 3 for my Monte.
I won't make the headers, I'm going to buy some cheapie shorty headers thru Ebay. I'll buy mandrel exhaust tubing (2.5"-3") and cut/piece it together myself.
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