Questions on 2.5" vs 3" Exhaust - Page 5 - 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 #61 of 91 (permalink) Old Dec 10th, 18, 4:54 AM
Senior Tech Team
Joe
 
Join Date: Nov 2003
Location: Thornton, Pa
Posts: 13,251
Re: Questions on 2.5" vs 3" Exhaust

Quote:
Originally Posted by 66rat View Post
LOL....yeah Gene, seems like we're all over the map on this one, but it makes for some interesting discussion.

I guess I should come clean as to why I'm considering a 2.5" vs the 3" exhaust on the 496. I mentioned earlier that it was due to "space limitations" and what I meant by that is, I plan on installing an air ride system on the Impala and slamming it on the ground. So the exhaust has to be tucked up tight to the floor pans and not hung down below the frame. Like everything else in life, there's always a compromise to get what you want. And in my case, maybe giving up some performance to achieve the slammed look maybe the case in my situation.


This is what I'm considering......Shorty headers (el' cheapo's from Ebay) 1.75" primaries with 3" collectors. 3" spintech oval tubing piped back to the rearend (no mufflers). reduce down to 2.5" to route over the rear axle and tuck in a 2.5" straight thru exhaust resonator (glass pack) and dump exhaust down at the rear bumper. I'll probably add an H or X pipe in there if there's clearance, and I'm also considering some electric exhaust cutouts to help the 496 breath a little easier when the youngster with the 5.0 coyote wants a little bit of the old school hot rod, lol .


No offense. When I read Impala/Air Ride/Slammed to the Ground, first thing comes to mind is one of those low riders with the tiny VW tires!!!!!!!!!!


Back on topic. I kinda doubt you will ever get close enough to the ground to make a difference between 3" and 2.5" head pipes. With that said, the limiting factors are going to be performance potential (at the track) with air ride suspension and ground clearance with an X-pipe installed. Air ride tells me this is a street car, not worried about how it 60' at the track, 2.5" is fine. From my experience, the X-pipe will be lowest point in the exhaust system, I can almost guarantee it will rub at some point if the car sits low enough, especially on a longer wheelbase Impala.

Status: Car-Less


To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.
GRN69CHV is offline  
Sponsored Links
Advertisement
 
post #62 of 91 (permalink) Old Dec 10th, 18, 12:54 PM
Lifetime Premium Member
Steve
 
Join Date: Jan 2001
Location: Near Reading PA
Posts: 3,755
Re: Questions on 2.5" vs 3" Exhaust

Quote:
Originally Posted by fishhead View Post
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.

I'm no expert, but I don't think you can look at it in terms of size between intake and exhaust openings. When the intake valve opens, atmosphere is forcefully pushing into the engine to fill the vacuum created in the cylinder. Exhaust gasses are also rapidly expanding in volume from heat through compression and combustion while being pushed out by the piston through the (larger) exhaust valve.



That leads me to another question. Can the ratio between intake and exhaust valves be used for a rough estimation between intake and exhaust size, not accounting for length?
ZZ69chevelle is offline  
post #63 of 91 (permalink) Old Dec 10th, 18, 3:10 PM
Senior Tech Team
Ed
 
Join Date: Aug 2013
Location: Pittsburgh, PA
Posts: 4,008
Garage
Re: Questions on 2.5" vs 3" Exhaust

Quote:
Originally Posted by ZZ69chevelle View Post
I'm no expert, but I don't think you can look at it in terms of size between intake and exhaust openings. When the intake valve opens, atmosphere is forcefully pushing into the engine to fill the vacuum created in the cylinder. Exhaust gasses are also rapidly expanding in volume from heat through compression and combustion ...
<- This

Quote:
Originally Posted by ZZ69chevelle View Post
That leads me to another question. Can the ratio between intake and exhaust valves be used for a rough estimation between intake and exhaust size, not accounting for length?
Nope, but what might get you close is go by the weight of the incoming air AND fuel, then figure out how many moles of exhaust gas, then work on the volume of that gas at something like 1200 degrees (W.A.G.) at some small pressure in the system. I'm sure the number are out there somewhere, and another wild guess, exhaust gas volume out compared to air volume in is 20:1.

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 offline  
 
post #64 of 91 (permalink) Old Dec 10th, 18, 3:23 PM
Senior Tech Team
Ed
 
Join Date: Aug 2013
Location: Pittsburgh, PA
Posts: 4,008
Garage
Re: Questions on 2.5" vs 3" Exhaust

Quote:
Originally Posted by Pioneer4x4 View Post
<- This


Nope, but what might get you close is go by the weight of the incoming air AND fuel, then figure out how many moles of exhaust gas, then work on the volume of that gas at something like 1200 degrees (W.A.G.) at some small pressure in the system. I'm sure the number are out there somewhere, and another wild guess, exhaust gas volume out compared to air volume in is 20:1.
Cool, found this...
https://www.eng-tips.com/viewthread.cfm?qid=104735
to sum up
"The combustion formula is C8H18 + 12.5 O2 ==> 8 CO2 + 9 H20. For each 114 grams of C8H18, there will be 12.5 moles of oxygen consumed, producing 8 moles of CO2 and 9 moles of H2O. For gas volume purposes, since equal moles of gas produce equal volume, the volume of exhaust gas replacing oxygen will be equal to 17/12.5 = 1.36.

The volume percentage of oxygen in air is about 21% (not exact, but work with me here). This volume will be removed, and replaced by exhaust gas with a "volume" of (21*1.36) = 28.56%. The resulting post combustion volume is (79% + 28.56% = 107.56%) of the pre- combustion volume -- assuming no temperature increase.

So what do we have? Combining the increase in volume from combustion reactions and thermal expansion, an engine with a 3 liter displacement running at 3000 rpm with the throttle wide open will have an exhaust volume (at 1800 deg F) of 665.4*1.0756 ~~ 715 cubic feet per minute." Or, say a 454 at 6000RPM is close to 4000CFM of exhaust gas.

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 offline  
post #65 of 91 (permalink) Old Dec 10th, 18, 9:02 PM
Senior Tech Team
Bill R
 
Join Date: Aug 2002
Location: Gold Canyon, AZ
Posts: 2,807
Re: Questions on 2.5" vs 3" Exhaust

Quote:
Originally Posted by ZZ69chevelle View Post
I'm no expert, but I don't think you can look at it in terms of size between intake and exhaust openings. When the intake valve opens, atmosphere is forcefully pushing into the engine to fill the vacuum created in the cylinder. Exhaust gasses are also rapidly expanding in volume from heat through compression and combustion while being pushed out by the piston through the (larger) exhaust valve.
When the air fuel mixture is ignited in the cylinder (s) energy is produced which results in heat and energy. This pressure of combustion pushes the piston downward and the crankshaft turns the linear force into rotational force, thus applying power to the drive wheels. As the piston is pushed downward the energy is consumed and much of the heat is transferred to the engine cooling system. Because of this action, there is very little pressure in the cylinder (just residual exhaust gasses), which is pushed out by the piston into the exhaust ports, manifold/header system/exhaust system, etc. I have never seen a modern internal combustion engine that has an exhaust valve larger than the intake valve . . . . . . . .

Bill
64 Chevelle Malibu Sport Coupe (original owner) car now has 427" BB, Jerico 5-spd. trans., Dana 60 rear end.

SAE Member (25+ years)
ASE Master Certified Engine Machinist
AERA Certified Professional Engine Machinist
engineguy is offline  
post #66 of 91 (permalink) Old Dec 10th, 18, 10:20 PM
Lifetime Premium Member
Steve
 
Join Date: Jan 2001
Location: Near Reading PA
Posts: 3,755
Re: Questions on 2.5" vs 3" Exhaust

Quote:
Originally Posted by engineguy View Post
When the air fuel mixture is ignited in the cylinder (s) energy is produced which results in heat and energy. This pressure of combustion pushes the piston downward and the crankshaft turns the linear force into rotational force, thus applying power to the drive wheels. As the piston is pushed downward the energy is consumed and much of the heat is transferred to the engine cooling system. Because of this action, there is very little pressure in the cylinder (just residual exhaust gasses), which is pushed out by the piston into the exhaust ports, manifold/header system/exhaust system, etc. I have never seen a modern internal combustion engine that has an exhaust valve larger than the intake valve . . . . . . . .

Duh, ignore that, I misworded my quote when thinking about the valve size proportions, good catch I meant to say larger exhaust path, not valve but must have been tripping before my morning coffee. Yes, you are absolutely correct. Too late to edit.
ZZ69chevelle is offline  
post #67 of 91 (permalink) Old Dec 11th, 18, 1:29 PM
Senior Tech Team
Bill R
 
Join Date: Aug 2002
Location: Gold Canyon, AZ
Posts: 2,807
Re: Questions on 2.5" vs 3" Exhaust

Easy fix: just start drinking earlier (coffee that is) . . . . . . LOL Have a great day.
ZZ69chevelle likes this.

Bill
64 Chevelle Malibu Sport Coupe (original owner) car now has 427" BB, Jerico 5-spd. trans., Dana 60 rear end.

SAE Member (25+ years)
ASE Master Certified Engine Machinist
AERA Certified Professional Engine Machinist
engineguy is offline  
post #68 of 91 (permalink) Old Dec 11th, 18, 2:01 PM
Lifetime Premium Member
 
Join Date: May 1999
Location: Under the hood of some musclecar
Posts: 28,027
Re: Questions on 2.5" vs 3" Exhaust

132.26 MPH thru exhaust manifolds and 2.5" exhaust
Just saying...
Attached Thumbnails
Click image for larger version

Name:	daros2.jpg
Views:	29
Size:	57.8 KB
ID:	580488   Click image for larger version

Name:	daros1.jpg
Views:	11
Size:	19.7 KB
ID:	580490  
66rat, 71350SS, novadude and 5 others like 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 #69 of 91 (permalink) Old Dec 11th, 18, 10:59 PM
Tech Team
 
Join Date: Feb 2013
Location: CO
Posts: 542
Re: Questions on 2.5" vs 3" Exhaust

Quote:
Originally Posted by mr 4 speed View Post
132.26 MPH thru exhaust manifolds and 2.5" exhaust
Just saying...
Awesome!!!
mr 4 speed likes this.

69 461" BBC 11.25:1 Compression
840 Rec Heads (2.25/1.88) AED 850 HO carb
Orig Exhaust Manifolds
Straub (Clay Smith) Hyd Roller cam, 288/300 .600/.569 109.
TH400, 3500 stall, 4.11 12 bolt posi
rp930 is offline  
post #70 of 91 (permalink) Old Dec 12th, 18, 3:37 AM Thread Starter
Tech Team
Rob
 
Join Date: Nov 1999
Location: Warner Robins GA.
Posts: 974
Re: Questions on 2.5" vs 3" Exhaust

Quote:
Originally Posted by mr 4 speed View Post
132.26 MPH thru exhaust manifolds and 2.5" exhaust
Just saying...
OK.....that's pretty incredible, I would like to know the details of this combo because that's almost too good to be true
66rat is offline  
post #71 of 91 (permalink) Old Dec 12th, 18, 7:07 AM
Gold Member
Gene
 
Join Date: Sep 2000
Location: Buffalo NY
Posts: 14,136
Re: Questions on 2.5" vs 3" Exhaust

Yeah, but one example isnt a proper data set kids, especially an "outlyer" . Unless you can match the engineering to give you that much HP thru manifolds its irrelevant.
That takes many tricks they dont share, and is a great engineering task btw!
WHT/73 and Hotwire like this.

Gene
ACES 3112/Team Chevelle Gold #62
Be big, be a 'builder'!
427L88 is offline  
post #72 of 91 (permalink) Old Dec 12th, 18, 8:27 AM
Tech Team
Lew
 
Join Date: Nov 2015
Location: Potomac, MD
Posts: 811
Garage
Re: Questions on 2.5" vs 3" Exhaust

On the green Nova maybe he would do 9.90 at 140 mph if he had headers and 3 inch exhaust. I realize the rules for his class would not allow it. Also is that a full exhaust out to bumper or a tube out the side? I see a side tube but hard to say with all the tire smoke. I think where some of the problem lies is (in this discussion) the full exhaust over the back with non-mandrel bends.
Lew540 is offline  
post #73 of 91 (permalink) Old Dec 12th, 18, 9:25 AM
Lifetime Premium Member
 
Join Date: May 1999
Location: Under the hood of some musclecar
Posts: 28,027
Re: Questions on 2.5" vs 3" Exhaust

Quote:
Originally Posted by Lew540 View Post
On the green Nova maybe he would do 9.90 at 140 mph if he had headers and 3 inch exhaust. I realize the rules for his class would not allow it. Also is that a full exhaust out to bumper or a tube out the side? I see a side tube but hard to say with all the tire smoke. I think where some of the problem lies is (in this discussion) the full exhaust over the back with non-mandrel bends.
[email protected] is irrelevant as you mentioned since the rules require the correct exhaust manifolds and full exhaust.
The car is certified 100% stock appearing


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 #74 of 91 (permalink) Old Dec 12th, 18, 10:38 AM
Senior Tech Team
 
Join Date: Dec 2005
Location: Lancaster, PA
Posts: 10,904
Re: Questions on 2.5" vs 3" Exhaust

Quote:
Originally Posted by 66rat View Post
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.
This is probably more of an issue than whether to use 2.5" or 3" exhaust.
427L88 and WHT/73 like this.

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 #75 of 91 (permalink) Old Dec 12th, 18, 12:27 PM
Gold Member
Gene
 
Join Date: Sep 2000
Location: Buffalo NY
Posts: 14,136
Re: Questions on 2.5" vs 3" Exhaust

Its all about the cross- sectional area!

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