Questions on 2.5" vs 3" Exhaust - Chevelle Tech
Headers and Exhaust Discussion related to Exhaust systems and headers.
Sponsored by: Pypes Performance Exhaust

 70Likes
Reply
 
LinkBack Thread Tools Search this Thread Display Modes
post #1 of 91 (permalink) Old Dec 3rd, 18, 3:47 AM Thread Starter
Tech Team
Rob
 
Join Date: Nov 1999
Location: Warner Robins GA.
Posts: 973
Questions on 2.5" vs 3" Exhaust

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?
66rat is offline  
Sponsored Links
Advertisement
 
post #2 of 91 (permalink) Old Dec 3rd, 18, 5:36 AM
Lifetime Premium Member
 
Join Date: May 1999
Location: Under the hood of some musclecar
Posts: 27,963
Re: Questions on 2.5" vs 3" Exhaust

I have an 11 second street car that makes 500+ HP real HP at the crank and I run a 2.5" exhaust with manifolds.
So,IMHO and experience you will be fine. If you were over 550 HP I would say go 3" and have at least a 1 7/8 primary tube headers with a BBC
Alan F likes this.


To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.


To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.


To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.


To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.




To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.



To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.
mr 4 speed is offline  
post #3 of 91 (permalink) Old Dec 3rd, 18, 6:52 AM
Senior Tech Team
 
Join Date: Dec 2005
Location: Lancaster, PA
Posts: 10,869
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?
Nope. 2.5 is fine.

Ray


To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.

Best pass 8.87 at 150.98 1.212 60'. Sold 3/2018

1970 SS 396 project in progress. Bought 5/2018
bracketchev1221 is offline  
 
post #4 of 91 (permalink) Old Dec 3rd, 18, 8:02 AM
Senior Tech Team
jeff
 
Join Date: Jan 2008
Location: yukon ok.
Posts: 4,798
Re: Questions on 2.5" vs 3" Exhaust

Did you see this ..sped up video. 2.5" vs 3" much more HP than you make.


Now if you get some crappy bent tubes I feel the 3" will win over the 2.5" even on a smaller HP engine.
Mandrel bends are a must have if you put a lot of bends in the system.
Local muffler shops around here kink the bends.
jeff swisher is offline  
post #5 of 91 (permalink) Old Dec 3rd, 18, 8:06 AM
Tech Team
Jeff
 
Join Date: Jan 2017
Location: Greensboring, NC
Posts: 991
Re: Questions on 2.5" vs 3" Exhaust

Quote:
Originally Posted by mr 4 speed View Post
I have an 11 second street car that makes 500+ HP real HP at the crank and I run a 2.5" exhaust with manifolds.
So,IMHO and experience you will be fine. If you were over 550 HP I would say go 3" and have at least a 1 7/8 primary tube headers with a BBC
But you could be in the 10's.
bigmachd likes this.

*Jeff*

Project Salty - 1964 4 door Malibu, beaten, neglected, red headed foster child
LQ4 / T56 Swap
To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.

To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.
Hotwire is offline  
post #6 of 91 (permalink) Old Dec 3rd, 18, 8:15 AM
Lifetime Premium Member
 
Join Date: May 1999
Location: Under the hood of some musclecar
Posts: 27,963
Re: Questions on 2.5" vs 3" Exhaust

Quote:
Originally Posted by Hotwire View Post
But you could be in the 10's.
It does exactly what I built it for and has worked very well for 18 years.
I have no desire to run in the 10's with it.
It is certified 100% stock appearing and gets driven 180 miles round trip to the track. Run high 11's and drive home.No headers,no 3" exhaust,no steep gears,no high rise intake.No special carb.

My point is, the OP will be fine with a 2.5" exhaust.


To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.


To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.


To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.


To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.




To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.



To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.
mr 4 speed is offline  
post #7 of 91 (permalink) Old Dec 3rd, 18, 8:42 AM
Tech Team
Lew
 
Join Date: Nov 2015
Location: Potomac, MD
Posts: 781
Garage
Re: Questions on 2.5" vs 3" Exhaust

You are on the edge how much gain would remain to be seen. You do have a big engine though. I think the flowmaster guide was around 500 hp you should begin to look at 3 inch. I agree with Mr. 4 Speed - 550 hp you should definitely look at 3 inch but as I said you also have a lot of cubic inches too. You could do a 3 inch back to the muffler and 2.5 over the axle as a compromise. Furthermore, you need a mandrel bend not the Midas muffler shop bend where a 2.5 inch is squished down to a very ugly 2 inch or less looking 90 / 180 degree bend (like Jeff said above).
427L88 and Kirk's67SS like this.
Lew540 is offline  
post #8 of 91 (permalink) Old Dec 3rd, 18, 9:14 AM
Gold Member
Gene
 
Join Date: Sep 2000
Location: Buffalo NY
Posts: 14,001
Re: Questions on 2.5" vs 3" Exhaust

I think the mathematical answer is to run 3" to the mufflers, and then 2.5" behind if that alleviates your space concerns. Or 3" torque tubes off the headers that cone down to 2.5" ID maybe as the front of the headpipes. The further back the EX gas gets, the cooler and less pressure it has. This allows smaller space for full flow.

That MotorTrend video speaks to mufflers only? Please keep in mind the above thermodynamic relationship then. By the time EX gases get to the muffs, 2.5" v 3" may be rather academic from a pressure /volume pov.
Kirk's67SS likes this.

Gene
ACES 3112/Team Chevelle Gold #62
Be big, be a 'builder'!
427L88 is offline  
post #9 of 91 (permalink) Old Dec 3rd, 18, 10:07 AM
Team Member
Mark
 
Join Date: Dec 2007
Location: Tampa, FL
Posts: 1,103
Re: Questions on 2.5" vs 3" Exhaust

I had a 383, maybe 450hp, when I changed from 2.5 to 3. I ran the car with and without the mufflers/tailpipes 1/4mi, and it lost 2mph with the 2.5 mufflers/tailpipes vs open but didn't lose anything through the mufflers/tailpipes with the 3. The mph didn't really improve much with the 3, but I got the performance of the open 2.5 with mufflers.
427L88 likes this.
MadmanMark is offline  
post #10 of 91 (permalink) Old Dec 3rd, 18, 11:04 AM
Senior Tech Team
Rick
 
Join Date: Nov 2006
Location: Langhorne, Pa
Posts: 4,161
Re: Questions on 2.5" vs 3" Exhaust

Quote:
Originally Posted by 427L88 View Post
The further back the EX gas gets, the cooler and less pressure it has. This allows smaller space for full flow..

Would love to see data that supports this claim.


An exhaust system has x volume of combustion gases being pumped into it at any given time. It moves along the tube at x velocity. A drop in tube diameter will increase velocity and increase pressure at the point of restriction.

I've never seen data that showed how much the gas actually cools from the header/manifold to the end of the tail pipe during a WOT pull or the resultant PSI drop. It may exist, i've never seen it.
427L88 and ChevelleFan70 like this.

1967 Chevelle Malibu 4 door


To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.


The 7.99 at 173mph pass:


To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.


8.02 at 171mph:


To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.
Calculated Risk is offline  
post #11 of 91 (permalink) Old Dec 3rd, 18, 11:53 AM
Lifetime Premium Member
 
Join Date: May 1999
Location: Under the hood of some musclecar
Posts: 27,963
Re: Questions on 2.5" vs 3" Exhaust

I believe Jim Hand did a series of exhaust tests years ago that could be relevant.
427L88 likes this.


To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.


To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.


To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.


To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.




To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.



To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.
mr 4 speed is offline  
post #12 of 91 (permalink) Old Dec 3rd, 18, 12:53 PM
Senior Tech Team
Ed
 
Join Date: Aug 2013
Location: Pittsburgh, PA
Posts: 3,952
Garage
Re: Questions on 2.5" vs 3" Exhaust

Quote:
Originally Posted by Calculated Risk View Post
Would love to see data that supports this claim.


An exhaust system has x volume of combustion gases being pumped into it at any given time. It moves along the tube at x velocity. A drop in tube diameter will increase velocity and increase pressure at the point of restriction.

I've never seen data that showed how much the gas actually cools from the header/manifold to the end of the tail pipe during a WOT pull or the resultant PSI drop. It may exist, i've never seen it.
In a gas, pressure=temperature x a constant.
Lower the temp, the pressure drops. If you actually refrigerated the exhaust pipe, you could condense the exhaust to a liquid, and get away with a small tube with no pressure.
Also, there is no such thing as "backpressure" in an exhaust, only pressure. Increase the restriction, and the pressure increases in the system everywhere.

A good practical example of reducing diameter is your headers. I bet if you worked it out, the primaries flow about the same as a 5"=6" pipe all added up.

1967 El Camino Malibu - Deepwater Blue
ZZ4 350, FiTech 600 complete!,Roadmaster tank, 700R4, Underdash A/C in progress, JGC Steering, 1990s truck serpentine.
Pioneer4x4 is online now  
post #13 of 91 (permalink) Old Dec 3rd, 18, 1:53 PM
Gold Member
Gene
 
Join Date: Sep 2000
Location: Buffalo NY
Posts: 14,001
Re: Questions on 2.5" vs 3" Exhaust

Rick, I dont have the data, but got that first hand from Dr Gas, who claimed they did. I did NOT make it up just because I "beleive it" but was given that by a engineer at Dr Gas who claimed empirical data. It squares with what little I know about fluid dynamics - not much! They also referred be to the Hand testing, but unsure if that A/B testing showed anything about down stream pressure reduction as the gas cooled.

Anyway, after my post I tried to find those nice long "torque tubes" that an outfit called "Torque Tech" out of Ga used to sell. A long off collector extension that went from 3 to 2.5". None found. So the math may be academic, as I could not assemble a system "off the shelf" that went from 3" across 24-30" down to 2.5" into the mufflers anymore.

It does mean that 3" tails are totally academic at these levels tho, as the mufflers are also a heat sink as well as a pressure reducer. As so the engineers told me.

Gene
ACES 3112/Team Chevelle Gold #62
Be big, be a 'builder'!
427L88 is offline  
post #14 of 91 (permalink) Old Dec 3rd, 18, 6:24 PM
Tech Team
Lew
 
Join Date: Nov 2015
Location: Potomac, MD
Posts: 781
Garage
Re: Questions on 2.5" vs 3" Exhaust

Many years ago I had a engine dynode 600 hp 468 in my Chevelle and had T400 trans 4.11 gear. I went to the drag strip the best I could muster was 112-113 mph at like 12.30. I had 2 inch headers and 3.5 inch collectors. At that time I had 2.5 inch mufflers and 2.5 inch pipes all the way back to bumper. The pipes over the axle were not the best and were no mandrel. So kind of let down about the performance - I decided to make one more run but open the headers up. So I moved the exhaust over and bolted each one side of the collector and tied it up . Well went down the track and car felt different got the time slip and WOW 11.7 at 118 (I think 5+ mph is like 70 hp). Well I went home and started working on an exhaust did a full 3 inch system and got the time with exhaust to within 1 mph and 1 tenth of open headers. The car could have gone faster with some better jetting and more consistent driving but the main thing here is all I did on last run was open the headers up. I blame myself and the 2.5 inch exhaust for holding the car back (in some areas who knows my 2.5 inch exhaust might have been 2 inch with the old muffler shop non-mandrel bend). The peak power was at 6,200 rpm and with the old exhaust it was almost like the car stop making power at 5,000 rpm. The following week, I will never forget leaving Speedy muffler with the 3 inch mandrel set up and trying car out it was a night and day difference you could tell by the way the car lifted up on a roll that something had changed dramatically. Anyway that's my exhaust story.
Lew540 is offline  
post #15 of 91 (permalink) Old Dec 3rd, 18, 7:37 PM
Gold Member
Gene
 
Join Date: Sep 2000
Location: Buffalo NY
Posts: 14,001
Re: Questions on 2.5" vs 3" Exhaust

The bends are CRITICAL!
Dan Orgill and jeff swisher like this.

Gene
ACES 3112/Team Chevelle Gold #62
Be big, be a 'builder'!
427L88 is offline  
Sponsored Links
Advertisement
 
Reply

Quick Reply
Message:
Options

Register Now



In order to be able to post messages on the Chevelle Tech forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
Please enter a valid email address. Note, you will be sent a confirmation request to this address.

Email Address:
OR

Log-in










Thread Tools Search this Thread
Show Printable Version Show Printable Version
Email this Page Email this Page
Search this Thread:

Advanced Search
Display Modes
Linear Mode Linear Mode



Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

 
For the best viewing experience please update your browser to Google Chrome