View Full Version : FLOW: stock vs aftermarket exhaust systems!
jbradsh1
12-05-2008, 01:16 PM
For all you mathematicians out there, I posed the following question to myself. How much more flow do you get in going from stock (2") to cat back systems (2.5") to LTs (3")? Well, to save you the trouble, here is the answer.
Stock Flow: is what it is
Cat-back: 56% more flow than stock
LTs: 44% more flow than cat-back & a whopping 125% more flow than stock. In other words, the LTs flow over twice as much exhaust as the stock system, if you agree that the stock system is limited as described below.
I'm using 2" for the stock system because that's the diameter of the pipes between the single and dual resonators so it effectively limits the flow at that point to a 2" flow. Most cat-back systems seem to have 2.5" pipes and most LTs have 3" pipes. There you have it. It's no wonder that LTs make so much more rwhp!, as I've said all along (joke).
The only thing I haven't mentioned in this learned dissertation is the fact that the stock mufflers have at their entrance a 2" pipe which one would think would severly restrict the flow no matter which of the three above mentioned systems you had on the car. But evidently not, so I guess that proves symbolic logic is about as full of beans as I always suspected it was.
99-LS1-SS
12-05-2008, 01:21 PM
LOL. So...what did we learn today?
jbradsh1
12-05-2008, 01:23 PM
LOL. So...what did we learn today?
Beats me but your guess is as good as mine!
Chewy
12-05-2008, 01:24 PM
I thought the ARH and Kooks were 2.5"...
jbradsh1
12-05-2008, 01:32 PM
I thought the ARH and Kooks were 2.5"...
See, that's what I get for not having a beer after work. You're right, the LT's are 2.5", so hey just drop out the last part and everything will work out fine.
99-LS1-SS
12-05-2008, 01:33 PM
see, that's what i get for not having a beer after work. You're right, the lt's are 2.5", so hey just drop out the last part and everything will work out fine.
lmao! :poof:
Crazy Mathmatical post about exhaust flow.... FAIL!!
Tate
caretaker
12-06-2008, 06:08 AM
Correct me if I'm wrong but,
it has always been my understanding that the only way to
change the amount of air flow through an engine is to increase
the displacement or add a forced induction system.
The amount of air going through a naturally aspirated engine is
determined by the bore, stroke, and R.P.M.
Changing the exhaust lets the engine breath more efficiently.
Like blowing your nose when you have nasal congestion.
Headers and higher efficiency CATS and mufflers just give the air
a less restrictive path to exit the engine. Thus less back pressure
and more horsepower.
Caretaker, to add on to what you say, without an increase 18+% then the full exhaust system will not help much at all. You wil not have enough back pressure to have it do much good. With the power gains, the larger system will show its true potential as it will again have some sort of back pressure to help it out.
GRRRR8
12-06-2008, 06:33 AM
The ARH system is the best bang for the buck. You use your stock mufflers so there is ZERO drone.
I was about to rip the thesis to threads, but it has already been tattered so I will let it alone.
Bottle necks are bottle necks they do not define the total flow charecteristics of a pipe.
A couple of us did some experiments with our GTPs, Comp Gs regarding exhaust systems. By using sensors to measure backpressure we found that size alone is not a good indicator of flow performance.
One of the keys to good flow was constant size or long tapered transitions from large to small OR even small to large. We looked at 3" catted downpipes that transitioned nicely (cones) to the 2 1/2" catback flange and the OEM downpipe with its 2 1/4" flange and compared the backpressure numbers with the stock catback and aftermarket catbacks that had 2 1/4" flanges and pipes or 2 1/2" flanges and pipes going into the front resonator and 2 1/2" pipes on aft.
In every case the backpressure numbers were better when there were no step transitions, either way-up or down. The 2 1/4 system (OEM downpipe and GMPP catback) actually had lower backpressure than the stock 2 1/4 downpipe connected to a 2 1/2" catback (Corsa). With the Corsa connected to the 2 1/2" outlet down pipe, backpressure was lower. When a 3" catback was connected to either of the 2 1/2" or 2 1/4" outlet downpipe backpressure was higher than the match up 2 1/4 or 2 1/2" combinations. The 2 1/2" downpipe was 3" into and out of the cat then did a nice tapered transition to the 2 1/2" flange.
Any Step Transisition in the exhause, either up or down, will cause relections or standing waves in the flow and these can reduce the flow.
It may not have been scientific but this is what we found by measuring backpressure at the rear O2 sensor bung.
If we came to the wrong conclusion, please set the records straight for me as I do not want to pass on bad information.
jbradsh1
12-06-2008, 05:00 PM
That's what I love about this forum, everyone has an opinion and it's fun to have everyone get in on the talk. Lot's of fun. By the way, who was the idiot who dreamed up this thread, whoops, it was me? Hey, I've had better days and better threads!
Panther 2
12-06-2008, 07:17 PM
That's what I love about this forum, everyone has an opinion and it's fun to have everyone get in on the talk. Lot's of fun. By the way, who was the idiot who dreamed up this thread, whoops, it was me? Hey, I've had better days and better threads!
Your flow equations match you exhaust hp equations perfectly, I have determined you are a genius IMHO. Lol
That's what I love about this forum, everyone has an opinion and it's fun to have everyone get in on the talk. Lot's of fun. By the way, who was the idiot who dreamed up this thread, whoops, it was me? Hey, I've had better days and better threads!
LOL. Gotta luv it!
todds87ss
12-07-2008, 05:00 AM
A couple of us did some experiments with our GTPs, Comp Gs regarding exhaust systems. By using sensors to measure backpressure we found that size alone is not a good indicator of flow performance.
One of the keys to good flow was constant size or long tapered transitions from large to small OR even small to large. We looked at 3" catted downpipes that transitioned nicely (cones) to the 2 1/2" catback flange and the OEM downpipe with its 2 1/4" flange and compared the backpressure numbers with the stock catback and aftermarket catbacks that had 2 1/4" flanges and pipes or 2 1/2" flanges and pipes going into the front resonator and 2 1/2" pipes on aft.
In every case the backpressure numbers were better when there were no step transitions, either way-up or down. The 2 1/4 system (OEM downpipe and GMPP catback) actually had lower backpressure than the stock 2 1/4 downpipe connected to a 2 1/2" catback (Corsa). With the Corsa connected to the 2 1/2" outlet down pipe, backpressure was lower. When a 3" catback was connected to either of the 2 1/2" or 2 1/4" outlet downpipe backpressure was higher than the match up 2 1/4 or 2 1/2" combinations. The 2 1/2" downpipe was 3" into and out of the cat then did a nice tapered transition to the 2 1/2" flange.
Any Step Transisition in the exhause, either up or down, will cause relections or standing waves in the flow and these can reduce the flow.
It may not have been scientific but this is what we found by measuring backpressure at the rear O2 sensor bung.
If we came to the wrong conclusion, please set the records straight for me as I do not want to pass on bad information.
That's why gutted cats can actually COST HP.
todds87ss
12-07-2008, 05:10 AM
Correct me if I'm wrong but,
it has always been my understanding that the only way to
change the amount of air flow through an engine is to increase
the displacement or add a forced induction system.
The amount of air going through a naturally aspirated engine is
determined by the bore, stroke, and R.P.M.
Changing the exhaust lets the engine breath more efficiently.
Like blowing your nose when you have nasal congestion.
Headers and higher efficiency CATS and mufflers just give the air
a less restrictive path to exit the engine. Thus less back pressure
and more horsepower.
A little oversimplified...More scavenging effect from headers due to better transition points yields a "cleaner" cylinder charge = more HP. You also did not address the issue of cam changes affecting air flow. For the same cylinder bore stroke and RPM, longer duration allows more filling of cylinders, a higher effective cylinder pressure, and yes, more flow. Yet another effect, because this is a gas system and the engine is a positive displacement device, is the pulse effect and how it is affected by pipe size changes.
soooo...I guess I'm correcting you:)
caretaker
12-07-2008, 06:13 AM
A little oversimplified...More scavenging effect from headers due to better transition points yields a "cleaner" cylinder charge = more HP. You also did not address the issue of cam changes affecting air flow. For the same cylinder bore stroke and RPM, longer duration allows more filling of cylinders, a higher effective cylinder pressure, and yes, more flow. Yet another effect, because this is a gas system and the engine is a positive displacement device, is the pulse effect and how it is affected by pipe size changes.
soooo...I guess I'm correcting you:)
Agreed, but the way this thread started I didn't want to over complicate
things. I was trying to stick to the K.I.S.S. standard. :whistle:
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