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So....Flowmaster "Suggests" a different diameter...tell me why?

8K views 20 replies 11 participants last post by  derryck 
#1 ·
Let me start off by saying -- my knowledge on FLOW is hardly above sea level...which is my reason for this thread.

Some of you may or may not know that I installed a GTX2971R over the weekend. With the help of Alex at Stratified, we are shooting for 400HP+ on 93 and meth.

With his "beginners" edition of the conservative tune we are at a safe 20PSI, my VDyno is showing around 330HP...which feels INCREDIBLE already.

...so I was goofing around at different mufflers and such and came across this nifty little chart from flowmaster...which tells me when I reach my goal of 400+, that they "suggest" I should be running 4.0" to a possibly 4.5" exhaust!!!!!!!???????????

Therefore, I must call out all of my engineers and professional tuners and ask...WHY? and Should I?

Teach me! :wink:

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#5 ·
Let me start off by saying -- my knowledge on FLOW is hardly above sea level...which is my reason for this thread.

Some of you may or may not know that I installed a GTX2971R over the weekend. With the help of Alex at Stratified, we are shooting for 400HP+ on 93 and meth.

With his "beginners" edition of the conservative tune we are at a safe 20PSI, my VDyno is showing around 330HP...which feels INCREDIBLE already.

...so I was goofing around at different mufflers and such and came across this nifty little chart from flowmaster...which tells me when I reach my goal of 400+, that they "suggest" I should be running 4.0" to a possibly 4.5" exhaust!!!!!!!???????????

Therefore, I must call out all of my engineers and professional tuners and ask...WHY? and Should I?

Teach me! :wink:

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Seems pretty accurate to me. I've scene that chart before and It's based on N/A engine setups where you want some back pressure. The 2015 GT uses 2.25 Dual pipes I believe. The reason 3" works good on the focus st is because we want to eliminate post turbo back pressure and maintain exhaust velocity. That's why if you're running a 3" exhaust you want a 3" DP and vice versa.
 
#9 ·
Not sure what your "Cracken" there "Douche," (sorry, I had too...HAHAHA), but I have yet to see an informative post concerning my question.

I've read "Overkill," "Ridiculous," "hmmmm," and "mentally retarded."

...yet there are more hits on Google concerning this than one could read through in a week...so...what gives?

I'm am just looking for real data...or a good explanation as to why we would not benefit with a larger pipe (for those higher hp FoST's).

Saying its overkill or ridiculous...albeit funny...is worthless.

Here's another example from Summit Racing...

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Why are they saying this?
 
#7 ·
So after the first few replies, I asked google...and stumbled across multiple links with just about the same info.

Here is one in particular:

How To Calculate Muffler Size and Exhaust Pipe Diameter - Exhaust Videos | Exhaust Videos

Just about most of the pages visited said that 2.2 CFM per engine horsepower.

So, if I am shooting for 425ish, that's 2.2 * 425 = 935 CFM.

...and according to another chart, on the SINGLE PIPE, I should between 3.25 and 3.5 inch. Which, if I were to change, I would probably go 4".

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Please correct me if I am wrong -- because I could be -- but isn't the goal with a turbo charged engine to allow the exhaust gasses to exit as quickly as possible (zero back pressure compared to a NA engine)?
 
#8 ·
Please correct me if I am wrong -- because I could be -- but isn't the goal with a turbo charged engine to allow the exhaust gasses to exit as quickly as possible (zero back pressure compared to a NA engine)?
That is correct, but the gains are minimal. You've no problem running over 400whp on a 3" exhaust. Most important is the downpipe and if you want to be safe, I'd suggest a 3.5" downpipe necking down to 3". That'll support anything the stock short block can take.

You'd see more power dumping straight under the car, but the few hp lost is worth it on a street car so you retain your hearing.
 
#14 ·
Also... Flowmaster is not well-named for what they do. They do not "master the flow" at all. They are very restrictive.

Yeah I said it.
 
#15 ·
no worries my man...lol! Too bad I'll never hear the "Vtec" kick in with my little 3" exhaust...bahahaha!
 
#18 ·
AND if you want the best of both, consider an open dump valve or something. Play when you want to play, and be conservative when you don't.
 
#19 ·
Guys in the LS world use single 3" mandrel systems for heads/cam LS engines and don't see any real restriction and these are guys in the 400-500rwhp region.

I did most of these calcs when I was building my LT1. My engine sees a maximum RPM of about 7000rpm, its peak power is around 6700rpm-6800rpm, but power falls of around 7100rpm. This is based on engine air flow, head flow, and the camshaft timing events.

At 7100rpm thats about 575cfm or so. And depending on bends and such, a straight piece will flow almost 750cfm. So a single 3" exhaust with mandrel bends is PLENTY sufficient for 500whp.

A single 3" pipe flows about
 
#21 ·
My 300ZX puts down about 640RWHP on 93 and has a full 3" exhaust. It's ridiculously loud and when I had the 3" exhaust fabricated because nobody sold one at the time, all of the 'z experts' said it was overkill and would yield minimal gains. Now everyone sells a full 3" exhaust for the 300ZX but I think you'll hit diminishing returns once you go beyond 3". With 3" piping you'll already be giving up noise level in lieu of performance and I don't see any reason to go beyond 3". My 335i puts down around 500RWHP on 93 and the exhaust is stock after the 3" catless DPs and most data shows that going with a 3" exhaust would yield more noise than HP. My Focus will be staying with stock exhaust so I have at least one car that doesn't wake the neighbors on startup.
 
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