supercharger vs. exhaust flow question
#1
Burning Brakes
Thread Starter
Join Date: Sep 2006
Location: Myrtle Beach, South Carolina
Posts: 1,203
Likes: 0
Received 0 Likes
on
0 Posts
![Default](https://rennlist.com/forums/images/icons/icon1.gif)
I'm trying to find the perfect (for me) level of volume from my exhaust. Right now I have the stock manifold into Motorsports X pipe into Flowmaster Super 40's (installed in place of resonators) into RMB.
I'd like to add a bit more 'muffling' without hurting performance, either by replacing the Flowmasters with something a touch more quiet or adding to the system, maybe high flow cats.
Is there a rule of thumb I should be aware of regarding exhaust flow since I'm pushing approx. 10 psi into the intake? I don't want to restrict the flow in search of the right level of BWaaaaaa.
I guess I should also ask, if I remove the RMB and re-install the stock muffler, will I be losing power? I want to keep some of that beautiful sound I get now but on a scale of 1 to 10 I'd like to turn it down about 3 or 4 notches.
I'd like to add a bit more 'muffling' without hurting performance, either by replacing the Flowmasters with something a touch more quiet or adding to the system, maybe high flow cats.
Is there a rule of thumb I should be aware of regarding exhaust flow since I'm pushing approx. 10 psi into the intake? I don't want to restrict the flow in search of the right level of BWaaaaaa.
I guess I should also ask, if I remove the RMB and re-install the stock muffler, will I be losing power? I want to keep some of that beautiful sound I get now but on a scale of 1 to 10 I'd like to turn it down about 3 or 4 notches.
#2
Drifting
![Default](https://rennlist.com/forums/images/icons/icon1.gif)
The stock muffler will be restrictive. Maybe try switching over to Borla mufflers. I like Magnaflows better, but they aren't the quietest. Borlas seem to be built similarly but are quieter.
#3
Administrator - "Tyson"
Lifetime Rennlist
Member
Lifetime Rennlist
Member
![Default](https://rennlist.com/forums/images/icons/icon1.gif)
Unless you measure the backpressure you are just guessing.
It's very easy to add a vacuum port to the exhaust. Z drilled a hole into an O2 bung plug, works like a charm.
It's very easy to add a vacuum port to the exhaust. Z drilled a hole into an O2 bung plug, works like a charm.
#4
Burning Brakes
Thread Starter
Join Date: Sep 2006
Location: Myrtle Beach, South Carolina
Posts: 1,203
Likes: 0
Received 0 Likes
on
0 Posts
![Default](https://rennlist.com/forums/images/icons/icon1.gif)
what does Tim run on his or his wifes car, he said he doesn't like them loud but I forget what he said he was using and his cell # is not working....
#5
Race Car
![Default](https://rennlist.com/forums/images/icons/icon1.gif)
This may help I did this to mine also and it sounds great: S/C, X-pipe, no cats and mid muffler.
https://rennlist.com/forums/928-foru...214-277rw.html
https://rennlist.com/forums/928-foru...214-277rw.html
#6
Administrator - "Tyson"
Lifetime Rennlist
Member
Lifetime Rennlist
Member
![Default](https://rennlist.com/forums/images/icons/icon1.gif)
We've never measured the exhaust pressure in his car. It's on the spring "to-do" list.
#7
Instructor
Join Date: Mar 2008
Location: Pacific Northwest
Posts: 158
Likes: 0
Received 0 Likes
on
0 Posts
![Default](https://rennlist.com/forums/images/icons/icon1.gif)
As a rule of thumb, the exhaust system is going be handling significantly more flow from forced induction. More intake charge = more gaseous volume. The stock manifolds and pipes are almost certainly no longer ideal.
Some generally accepted rules of thumb are:
Exhaust gas velocity should not exceed 250 ft/sec
Back pressure should not exceed 1-2psi
A 10% increase in exhaust tube diameter will yeild approx. a 20% increase in flow
There are many who will say that lowering exhaust gas back pressure will either lower torque or move it up in the RPM range. Clearly as a motor's tuning is moved toward the upper limit, zero back pressure is the goal.
The muffler will most likely be the biggest obstruction in the flow path. I would think that a person would probably want to work from that point back to the exhaust valve.
Given our engine's power producing characteristics I would also think it unlikely that a moderate oversizing of the exhaust components would rob usable power............under boost that is.
Hacker nailed it on the head. One needs to KNOW at what pressure the exhaust system is operating.
That being said, we have a Borla system on the 79, M28/11, Euro and it's a much more subdued tone than FM 40s. I have no idea how they flow.
David
Some generally accepted rules of thumb are:
Exhaust gas velocity should not exceed 250 ft/sec
Back pressure should not exceed 1-2psi
A 10% increase in exhaust tube diameter will yeild approx. a 20% increase in flow
There are many who will say that lowering exhaust gas back pressure will either lower torque or move it up in the RPM range. Clearly as a motor's tuning is moved toward the upper limit, zero back pressure is the goal.
The muffler will most likely be the biggest obstruction in the flow path. I would think that a person would probably want to work from that point back to the exhaust valve.
Given our engine's power producing characteristics I would also think it unlikely that a moderate oversizing of the exhaust components would rob usable power............under boost that is.
Hacker nailed it on the head. One needs to KNOW at what pressure the exhaust system is operating.
That being said, we have a Borla system on the 79, M28/11, Euro and it's a much more subdued tone than FM 40s. I have no idea how they flow.
David
Trending Topics
#8
Drifting
![Default](https://rennlist.com/forums/images/icons/icon1.gif)
I have a Borla on my Range Rover. It's a straight through muffler with a perforated core like the Magnaflows on my Audi and 84S. I don't see how it could be restrictive at all.
#9
Addict
Rennlist Member
Rennlist Member
![Default](https://rennlist.com/forums/images/icons/icon1.gif)
I am running stock manifolds, a motorsports x with cats, stock small resonators, and a RMB. It is a bit loud, and sounds pretty wicked at WOT. There has to be some room for flow and power improvement there. I do wish someone would make bolt on a 2.5" cat-back system to match the x-pipe with 2 high flow mufflers. Local muffler shops tend to run the other way on this kind of stuff when they see it is for a Porsche. ![grr](https://rennlist.com/forums/graemlins/cussing.gif)
Just one more thing on the wish list!
Jim
![grr](https://rennlist.com/forums/graemlins/cussing.gif)
Just one more thing on the wish list!
Jim
#10
Addict
Rennlist Member
Rennlist Member
![Default](https://rennlist.com/forums/images/icons/icon1.gif)
I think replacing the Flowmasters is worth a few HP. I love Borla but too $$. I'm using Magnaflows, straight thru design. Two ovals and two rounds where the rear muffler use to be.
10# boost lot of air to move. Are you considering headers?
Tony
10# boost lot of air to move. Are you considering headers?
Tony
#11
![Default](https://rennlist.com/forums/images/icons/icon1.gif)
I could add a port easily enough or copy Z's idea and temporarily use one of the two O2 sensor bungs but what kind of measurement am I looking for there? Just the absence of positive pressure?
what does Tim run on his or his wifes car, he said he doesn't like them loud but I forget what he said he was using and his cell # is not working....
what does Tim run on his or his wifes car, he said he doesn't like them loud but I forget what he said he was using and his cell # is not working....
You may get some different opinions on what backpressure you want, but I'd say the lower the better for making horsepower.
The exhaust on Tim's wife's car is the one that he had on his car. It consists of the stock exhaust manifolds, X-pipe, stock pipes with the smaller size stock center mufflers, and then two small, round, straight through type mufflers in the location that the stock rear muffler was. He's had an RMB on that exhaust setup, but thought it was too loud, and went with those two small mufflers back there instead.
Actually, the cats are the single biggest restriction of the stock parts that are located behind the exhaust manifolds. I'd say that it might be better to work from the front back. As the exhaust travels back along the system, it cools and the gasses contract when they do. Their volume goes down due to that, so the pressure and possibly speed goes down the further back in the system you go. With the pressure and maybe speed being lower back there, any restriction will be less significant than it would have been further forward, where the gasses were hotter, had greater volume, were at a higher pressure, and were possibly moving faster too. In any case, it's better to work from the most restrictive part to the least restrictive one, whether it's at the front or the back. Measuring the pressure drop across each individual part will tell you which is most restrictive.
#12
Rennlist Member
![Default](https://rennlist.com/forums/images/icons/icon1.gif)
I seem to remember back when I had my 90GT, back before the x-pipe, that gutting the cats added about 12 hp with an RMB and putting the stock muffler back on cost 3 hp. I've always understood that while stock Porsche headers and cats can be improved on, the mufflers are pretty good.
#13
Race Car
![Default](https://rennlist.com/forums/images/icons/icon1.gif)
I seem to remember back when I had my 90GT, back before the x-pipe, that gutting the cats added about 12 hp with an RMB and putting the stock muffler back on cost 3 hp. I've always understood that while stock Porsche headers and cats can be improved on, the mufflers are pretty good.
Dan
'91 928GT S/C
![EEK!](https://rennlist.com/forums/images/smilies/eek.gif)
#14
Rennlist Member
![Default](https://rennlist.com/forums/images/icons/icon1.gif)
Louie would be the one to answer your questions...he made hundreds of dyno runs with different set-ups (i.e., different pipes and mufflers)![thumbup](https://rennlist.com/forums/graemlins/thumbup.gif)
I think he mentioned the best runs were with 2.5" pipes all the way back from the x-pipe with bullet mufflers in place of the resonators. That will be my set up once I find the right mechanic.
![thumbup](https://rennlist.com/forums/graemlins/thumbup.gif)
I think he mentioned the best runs were with 2.5" pipes all the way back from the x-pipe with bullet mufflers in place of the resonators. That will be my set up once I find the right mechanic.
#15
Rennlist Member
![Default](https://rennlist.com/forums/images/icons/icon1.gif)
I'm running the stock manifolds, Motorsports X pipe, no Cats and the into Magnaflows. This is the 3rd or 4th exhaust that I've put together. This one is a little loud, but quieter than the flowmasters. IIRC the Magnaflowws had better flow than the Flowmasters. Tom Coultier had also done a lot of testing at the time and steered me away from the Flowmasters.
YMMV
Ken
YMMV
Ken