Irresponsibly Large Exhaust Piping and 3800 Power - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


Performance, Brainstorming & Tuning Talk about modifications, or anything else associated with performance enhancements. Have a new idea for performance/reliability? Post it here. No idea is stupid! (please use Detailing and Appearance for cosmetic ideas)

Reply
 
 
 
Thread Tools Search this Thread
Old 02-22-2008, 03:57 AM   #1
Senior Member
True Car Nut
Thread Starter
 
radomirthegreat's Avatar
 
Join Date: Sep 2006
Location: Indianapolis, IN
Posts: 4,606
Thanks: 0
Thanked 0 Times in 0 Posts
radomirthegreat is on a distinguished road
Default Irresponsibly Large Exhaust Piping and 3800 Power

I was going through the two catalytic converter posts I'm participating in, and I came up with this question. It is reported that removing a cat. converter causes low end power loss. Here'* some of what I would like to ask:
Quote:
Originally Posted by JMH1950
I'm having trouble wrapping my brain around the idea that removing the cat will cause a loss of low end power, but installing a high flow cat is a good idea.
Is it because of the way that the 3800 makes power that it needs back pressure, or laminar flow out of the exhaust ports, to get better low end power? I would like to start with a Radomir classic: A hypothetical. If I were to install 5" exhaust piping after 2.5" header pipes coming from each exhaust port and port matching, with of course a 5" in/out catalytic converter and muffler, what would it take in my engine to get either the same low end power as I have now or more of that?

Now moving away from that to a bigger picture, what element of the 3800'* design causes loss of low end power when a loss of back pressure occurs?
radomirthegreat is offline   Reply With Quote
Old 02-22-2008, 12:15 PM   #2
Senior Member
Posts like a Northstar
 
Join Date: Oct 2004
Location: Purgatory
Posts: 533
Thanks: 0
Thanked 0 Times in 0 Posts
Greyhare is on a distinguished road
Default

Dig around with the search. There is a lot of info here.

The over simplified version is "gas velocity". With properly sized piping the high velocity exhaust from one cylinder "pulls" the exhaust from the next. If the piping is too large the exhaust "stalls". Think of a fast moving on ramp merging into a traffic jam.
Greyhare is offline   Reply With Quote
Old 02-22-2008, 12:23 PM   #3
Senior Member
True Car Nut
 
Join Date: Jun 2003
Location: Purgatory
Posts: 6,313
Thanks: 0
Thanked 0 Times in 0 Posts
PontiacDad is on a distinguished road
Default

In talking with an exhaust expert this is tough one to nail down and requires much trial and error to get it right.
As an example Nascar teams have an exhaust guy that continualy tests and tunes.
If it were a formula or a hard set of rules then crew chief would/could do it.

Mike has it covered though, the exhaust of one pull the other

How to offset this? Make more boost (power) than needed but it would still benifit from a good exhaust tune.
PontiacDad is offline   Reply With Quote
Old 02-22-2008, 12:33 PM   #4
Senior Member
Posts like a Corvette
 
Join Date: Jan 2007
Location: San Diego, CA
Posts: 1,451
Thanks: 0
Thanked 0 Times in 0 Posts
agrazela is on a distinguished road
Default

There is also the issue of cylinder scavenging. With proper exhaust velocity, the exiting exhaust helps "pull" intake charge into the cylinder during the cam overlap period.

In your hypothetical example (2.5" header pipes), you would have so little exhaust velocity that you would get almost no scavenging effect.
agrazela is offline   Reply With Quote
Old 02-22-2008, 02:33 PM   #5
Senior Member
Posts like a Northstar
 
Join Date: Feb 2003
Location: Sacramento, CA WCBF '06, '07 survivor
Posts: 658
Thanks: 0
Thanked 0 Times in 0 Posts
rrounds is on a distinguished road
Default

This is a quote from Scott Stutler(engineer) of DynoMax
" For any given power output and displacement, a generic sizing chart can get you in the ballpark when choosing exhaust-pipe diameter. However, engines equipped with power adders have a different set of needs. "If you're running a turbo or supercharger, there'* no such thing as an exhaust that'* to big," Scott explains. For forced induction, the bigger the better. "In a race application, on a nitrous motor, the exhaust should be bulit in respect to the max horsepower it makes on the bottle. Conversely, in a street application with nitrous, the exhaust system should be built in respect to the maximum horsepower the motor makes off the bottle. That way, you'll optimize the exhaust system for the operating conditinos that the engine sees the most"

A motor that has positive intake pressure(boost) does not need the scavenging effect from the exhaust that an n/a motor needs to make power. And don't confuse scavenging with backpressure, you want exhaust "scavenging" in an n/a motor not backpressure.

Let me quote Scott Stutler(engineer) of DynoMax again,

"Some people say a motor needs a bit of backpressure to preserve low-end torque, while others argue that the less backpressure you have the better. "The fact is, an engine does not need backpressure", Scott says. What it needs is scavenging, which is directly related to exhaust velocity. A common misconception is that you'll lose some low-end torque by stepping up to a larger after-cat system. What'* really happening is that the larger-diameter piping is moving the rpm range where the scavenging takes place higher up the powerband. Since exhaust velocity increases with rpm, you're not losing power but moving it higher up in the rpm band instead"


ROD
rrounds is offline   Reply With Quote
Old 02-22-2008, 02:41 PM   #6
Junior Member
Posts like a Ricer Type-R
 
Join Date: Aug 2002
Posts: 10
Thanks: 0
Thanked 0 Times in 0 Posts
willwren is on a distinguished road
Default

On an engine designed and built for low-end torque, that'* the same as 'losing'.

From the cat back, we typically see no difference other than several of us who have noticed a remarkable loss in low-end for short periods of time without a Cat. From there back, you're not going to affect performance much.

Messing with the EM runners, collectors, and DP will dramatically affect performance.
willwren is offline   Reply With Quote
Old 02-22-2008, 07:08 PM   #7
Senior Member
True Car Nut
Thread Starter
 
radomirthegreat's Avatar
 
Join Date: Sep 2006
Location: Indianapolis, IN
Posts: 4,606
Thanks: 0
Thanked 0 Times in 0 Posts
radomirthegreat is on a distinguished road
Default

Wow, a lot of information here! Thanks for all the input so far. I'll read up and get back to you. There'* some very good info here, and now I feel a lot better about exhaust details.
radomirthegreat is offline   Reply With Quote
Old 02-23-2008, 09:09 PM   #8
Senior Member
True Car Nut
Thread Starter
 
radomirthegreat's Avatar
 
Join Date: Sep 2006
Location: Indianapolis, IN
Posts: 4,606
Thanks: 0
Thanked 0 Times in 0 Posts
radomirthegreat is on a distinguished road
Default

I was told by 1337ssei'* dad that in diagrams that show how to maximize traffic flow, cars should slow down to let others merge in front of them, and that keeps everyone moving the whole time. Is this practically how exhaust gases should flow as well?

While I'm on really weird hypotheses, what if I were to find a way to attach independent vacuum hoses to the exhaust ports? Would engine efficiency increase? Even though I may never install vacuum hoses on my exhaust ports, I'd like to know if there'* a limit to how well exhaust gases can flow, and what they'd do for the engine at peak flowing efficiency.
radomirthegreat is offline   Reply With Quote
Old 02-23-2008, 09:59 PM   #9
Junior Member
Posts like a Ricer Type-R
 
Join Date: Aug 2002
Posts: 10
Thanks: 0
Thanked 0 Times in 0 Posts
willwren is on a distinguished road
Default

Vacuum is created by the manifold. What are you going to do under boost?

Even if you didn't have boost, you're going to suck in burnt up exhaust fumes (carbon monoxide) that doesn't combust and has no oxygen in it, and try to combust it again?
willwren is offline   Reply With Quote
Old 02-23-2008, 10:17 PM   #10
Senior Member
True Car Nut
Thread Starter
 
radomirthegreat's Avatar
 
Join Date: Sep 2006
Location: Indianapolis, IN
Posts: 4,606
Thanks: 0
Thanked 0 Times in 0 Posts
radomirthegreat is on a distinguished road
Default

Oh, I meant running an electric vacuum cleaner or something. Like a Shop Vac. Probably not going to work, but it'* just a thought considering how we've found it'* important to keep moving gases evenly through the pipes.
radomirthegreat is offline   Reply With Quote
 
 
Reply

Related Topics
Thread Thread Starter Forum Replies Last Post
Large California Fires Picture (1200x1400, be warned, large) Bonneville92V688 Lounge 2 10-24-2007 12:56 AM
S1 L67 heater core piping vital49 1992-1999 2 03-14-2006 11:03 PM
Another Bonneville exhaust clip (3" piping -stock muffl OLBlueEyesBonne Your Other Rides: Pics & Videos 15 02-26-2006 04:05 PM
95 SE Exhaust Piping triple18 Performance, Brainstorming & Tuning 4 04-23-2005 03:42 PM
Large golden car, large golden problems largegoldencar 1987-1991 12 02-09-2004 10:06 PM


Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not 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 Off
Pingbacks are Off
Refbacks are Off



All times are GMT -4. The time now is 11:41 AM.


We are a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for us to earn fees by linking to Amazon.com and affiliated sites.