I am getting my final exhaust work done next week and want to determine my needs for flow capacity / pipe size.

I am not sure what info goes into determing this but I'm sure engineers don't just guess.

• Exhaust head flow 254cfm @ .600 and 264cfm @ .700. (These are obviously flowbench numbers and don't know how useful they are in this situation.) The cam will likely be towards the middle of that range.
• Engine will turn ~6500 rpm.
• I'm not worried about loss on the low end.
• Motor is 434cid.
• Will use Mandrel bends (unlike now)
• 1 3/4" headers

I am asking because I would like to run 3" cats into 2 1/2" reducers and into a 2 1/2" straight through exhaust. I don't know if this will be enough.

 

believe it or not, engineers do guess. they won't know for sure what the final exhaust system will look like until it's tested. this is because the engine doesn't produce smooth, pulse-free exhaust, so it can't really be modeled purely as steady-state flow.

a few things to keep in mind:

* for smooth flow you need 2.2cfm per hp.

* the closer you are to the port, the bigger the pulses will be, and therefore the more flow you need in order to not restrict the engine.

* less back pressure is always better, even at low rpm. the "loss" people refer to has to do with how flow velocity interfaces with exhaust overlap, so a properly designed system (for the engine combination) will make more power at all rpm.

* unless you use large chambers to virtually terminate the system and change the exhaust to be almost smooth flow, then it's very difficult to get away from more power being equal to louder. (unfortunately.)

the first point means the smooth flow region of the exhaust (the mufflers in a vette system) needs to be 1100cfm for 500hp.

the second point means a catalytic converter has to flow a ton more than you think to keep from restricting power, unless you put the converter into a region with smooth flow. (after a good crossover of expansion chamber.)

addressing the last point becomes very expensive. an X does a lot of the same thing, so the ideal (cheapish) system would be 3" collectors into 3" pipe, a 3" X, then going into 3" cats, then neck down to 2.5" pipe and straight through 2.5" mufflers (magnaflow or borla xr-1).

if you run 3" cats right of the header collectors, they need to flow about 1.5x the smooth flow, or 825cfm per converter. i've yet to find cats that can flow that much, so i believe your intended system would indeed restrict power. how much is a different question entirely. it might not be enough to make much of a difference. would you accept a 20hp loss? 40hp?

-michael

 

There is a chart here that may help:

http://performanceunlimited.com/docu...austguide.html

 

excellent explanation Michael, thank you

Aardwolf...thanks for the link. I have seen that chart before and I think I will call them. I don't understand the diffference between race and street HP.

 

Curious as to why you would want to neck down to 2.5" at the mufflers?

 

From my understanding, the exhaust cools and velocity decreases. So necking the pipe down toward the end keeps velocity up.

 

Hey Dave how's it goin?

I only want to do it because I like the the exhaust on there. No resonance and they refuse to make a 3" exhaust for our cars.

 

RainDelay, I did a spreadsheet awhile ago that figures CFM required at different points in the exhaust system.
These guy's are right, that Mass flow through the system stays the same (mass air into the throttle + Fuel = mass out the tailpipe) but the CFM's (volume) changes with temperature.
I use 250 Ft/Sec velocity, it seams to be accepted as a good target, fast enough for good scavenging, slow enough that friction is still manageable (most use between 200 and 300 Ft/Sec).
I'm assuming 1400 Deg coming out the exhaust port, 1000 deg into the converter (needs that much to lite off), 500 Deg into the mufflers and 200 Deg out the tailpipe.
Your 434 @ 6500 and about 83% V.E flows about 677 CFM (into the throttle).
Headers 1.78 inches ID
Converter Inlet Single = 4.47 inches Duals = 3.16 inches
Muffler Inlet Duals = 2.66 inches
Tailpipes Duals = 2.2 inches Quads = 1.56 inches



This spreadsheet isn't to fussy, just basic Velocity = airflow/area, and density tables. I tried to add in friction calculations and loss for bends, etc. etc. but it got to "funky".
It should be a good ballpark estimate.....

 

Hey Steve. When are you going to let me see that puppy? Are you running Corsa's? I have got 3" mandrel bent all the way thru to 3" flowmasters. You know how loud it is

 

That is very cool. Thank you very much.

Based on this I will try to run 3" bends after the cats and then neck down. I will be losing a little here and there but likely liveable. It sure as hell makes it clear that what I have now is not cutting it.

I am running Corsa's...I've been thru three other exhausts and just can't handle the drone with the others.

Hopefully we can get together soon. Let me know if you will be around during the day next week.