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Discussion Starter · #1 ·
There has been lots of discussion on exhaust sizes and how much each extra inch in diameter helps at various horsepower levels. Many people just keep their 3" exhaust and strait pipe it. Some go to 4" flow-through mufflers. Some go to 5" flow-throughs, and a few even go to 6" and 7" setups. IMO, a 4" flow-through muffler or strait pipe system is plenty of exhaust for most people on here. Some need to go to 5", but I believe that anything over 5" is just more expensive. Going from stock exhaust to a 4" Flow-Pro, I dropped about 300* EGT and gained about 2 MPG. I'm not sure on the horsepower gain since I haven't been on a dyno, but I think it's one of those things that helps more if you have more horses. Feel free to comment and/or add your own experiences.
 

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I picked up 2mpg when I went to the 4" DP and 4" pipe to 5" stacks.....my EGT's dropped about 300* as well....I think most of my gain was from getting rid of the clogged kitty.
 

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I've got the stock 3" straight piped getting 19-20 MPG in the summer and when I last ran the Dyno I got 381/826... Not bad for my BOMB's (Sig)
 

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From a mathematical standpoint there are too many variables (rpm, boost, charge air temp, EGT, displacement, blowby, cc's fuel injected, cylinder temperatures, etc) to take into consideration for my liking which would be needed to calculate CFM of the exhaust at each variable. To size the right exhaust for the application you would need to know the CFM and how the engine reacts to different flow rates in the exhaust. I don't know anyone with the time interest, money or resources to do tests isolating all of these variables, plot them on at graph and come up with a trend line and ultimately and equation used to compute the ideal exhaust size if you knew all the variables. It would be an interesting read though......

My thoughts are that if you should keep the exhaust tubing the size, or up to 1/2" larger than the outlet of the turbo for a good compromise in power, spool, and noise. Considering the cross sectional area of pipe changes at an exponential rate as the diameter increases, a one inch increase from 6" to 7" is a 78% increase in size compared to the 56% increase in going from 3" to 4", I t would seem that extremely large pipes would not be needed.


I would stay 3" or under with under 400hp, 4" or under under 550hp, and 5" or under under 850hp and 6" or under for any horsepower in a 5.9L displacement, and take any mild hp loss for a gain in noise control and cost savings.

Thats my :2cents
 

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However, opening up the down pipe can create a pressure gradient across the turbine wheel if you have high drive pressure, this causes the velocity of the exhaust in the turbine housing to increase and spool the turbo quicker. But this only works to a certain extent and won't drastically reduce drive pressure, just speed up the flow of exhaust over turbine wheel.
Sean
 

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Discussion Starter · #6 ·
Finally starting to get some good feedback on this. I thought this would be a bigger topic, maybe a couple arguements, but it's all good.
 

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The turbo is a huge factor as well. My stock turbo was great with spool up at lower RPMs, but was a severe choke point at higher speeds. I have 4.10 gears, so the RPMs are that much higher than with 3.55s at the same mph.

Going from the stock HX35 w/ single gated 12 housing and 3" outlet to a PDR HX40 w/ single 'gated 16 and 4" outlet gained me more mileage and lower EGTs than going to the 5" straight exhaust and stock turbo did.

Only saw a 50* EGT drop and only gained roughly .25- .5mpg going to the 5", along with improved spool up. The new turbo gained me a bit over 1mpg.

My boost went from 15# (heck that was almost at the max of 19# I made stock and no trailer) at 70mph with the stock turbo, to 10# with the HX40. Cruising @ 55mph on the flat, boost went from 6 to 0-2#. Also, the ATS exhaust manifold dropped my peak boost by 5# and lowered the EGTs maybe 50*.

Everything you can do to reduce pumping losses and improve efficiency will help. 5" exhaust really isn't needed til you run twins, or if you run a big single for sled pulling.
 
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