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Discussion Starter · #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:

Text Font Line Number Parallel
 

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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:

View attachment 114874


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.
 

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My original thoughts were "mental retardation".

After reading this thread, they hold true.
 

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Discussion Starter · #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)?
 

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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)?
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.
 

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Discussion Starter · #9 ·
My original thoughts were "mental retardation".

After reading this thread, they hold true.
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?
 

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Discussion Starter · #10 ·
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.
FIRST INFORMATIVE POST OF THE DAY!!! HAHA!

Thank you Sir! :)
 
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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...

View attachment 114883

Why are they saying this?
Here the honest answer you wanted, lol


Lol, to be honest, theres only so much exhaust gas you are going to push based on your displacement. So at 2.0 liters i doubt anything past 3.5 will be necessary, at like 1000 hp. Regardless of your turbo size your exhaust gas is still limited by the volume of the combined cylinders. Making it flow free is important for turbos, but obviously at 2 liters the exhaust doesnt need to be diesel truck sized. Just saying.
 
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Discussion Starter · #12 ·
Here the honest answer you wanted, lol


Lol, to be honest, theres only so much exhaust gas you are going to push based on your displacement. So at 2.0 liters i doubt anything past 3.5 will be necessary, at like 1000 hp. Regardless of your turbo size your exhaust gas is still limited by the volume of the combined cylinders. Making it flow free is important for turbos, but obviously at 2 liters the exhaust doesnt need to be diesel truck sized. Just saying.
Awesome! Thanks my man! :wink: That's what I was looking for!
 

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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.
 

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Discussion Starter · #15 ·
no worries my man...lol! Too bad I'll never hear the "Vtec" kick in with my little 3" exhaust...bahahaha!
 

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FIRST INFORMATIVE POST OF THE DAY!!! HAHA!

Thank you Sir! :)
No problem! Also remember those are all based on N/A setups. On an N/A or supercharged car, there can be no restriction after the head. You want to tune your manifold and exhaust system to scavenge exhaust gasses efficiency while also providing as little back pressure as possible. There will be some if you tune for proper scavenging, but that effect is negated by the benefits of proper scavenging. This is usually done in the header design and after the collector, you want to run something off that table to support the exhaust flow.

On a turbo car, throw that out the window. You have the biggest exhaust restriction possible sitting right after the head. Everything that comes out all 8 exhaust valves is stuffed into this little opening:

Auto part Automotive engine part Carburetor Automotive super charger part Engine

It then all goes through the 3" exit into the downpipe. Get the exhaust away from the turbo as fast as possible. That is why you can put a 4" downpipe and exhaust on a stock turbo car, or open downpipe and see some serious power gains. On an N/A car running an open header, if the header design isn't made for that you'll see a decrease in power since you lose scavenging.

On any turbo car, you have to weigh the benefits of exhaust size (price, weight, ease of routing) to power. On a stock KO3, a 3" downpipe is all you need and then even a 2.5" exhaust won't affect power. The downpipe is the most important. On most reasonable turbo upgrades, 3" or even a 3.5" downpipe, and a 3" exhaust is all you'll need. Any larger than that and there's really no gains to make it worth it. Once you start getting into built motors, BIG turbos, spray, etc....then you may see the benefit from a 4" downpipe or same size exhaust. For 90% of us, a 3" catless downpipe will get 99% of the available power. For the rest, a 3.5" downpipe to a 3" exhaust will get you all that's on the table. Most of these upgraded turbos aren't running out of their max efficiency ranges either and you're limited by fuel and the short block. If a 2.5" exhaust is killing power, add another 1 or 2psi and done. The turbo has the headroom.

A bigger downpipe will also help spool (pressure delta pre and post turbine are what determines spool time). The freer flowing the exhaust is, the lower pressure will be post-turbine. When you look how fast a tuned ST spools on a stock downpipe and exhaust...you see why this doesn't matter much. On an bigger turbo, it's a bigger difference. Once again, you look at benefit vs price/weight/room.
 

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Discussion Starter · #17 ·
No problem! Also remember those are all based on N/A setups. On an N/A or supercharged car, there can be no restriction after the head. You want to tune your manifold and exhaust system to scavenge exhaust gasses efficiency while also providing as little back pressure as possible. There will be some if you tune for proper scavenging, but that effect is negated by the benefits of proper scavenging. This is usually done in the header design and after the collector, you want to run something off that table to support the exhaust flow.

On a turbo car, throw that out the window. You have the biggest exhaust restriction possible sitting right after the head. Everything that comes out all 8 exhaust valves is stuffed into this little opening:

View attachment 114888

It then all goes through the 3" exit into the downpipe. Get the exhaust away from the turbo as fast as possible. That is why you can put a 4" downpipe and exhaust on a stock turbo car, or open downpipe and see some serious power gains. On an N/A car running an open header, if the header design isn't made for that you'll see a decrease in power since you lose scavenging.

On any turbo car, you have to weigh the benefits of exhaust size (price, weight, ease of routing) to power. On a stock KO3, a 3" downpipe is all you need and then even a 2.5" exhaust won't affect power. The downpipe is the most important. On most reasonable turbo upgrades, 3" or even a 3.5" downpipe, and a 3" exhaust is all you'll need. Any larger than that and there's really no gains to make it worth it. Once you start getting into built motors, BIG turbos, spray, etc....then you may see the benefit from a 4" downpipe or same size exhaust. For 90% of us, a 3" catless downpipe will get 99% of the available power. For the rest, a 3.5" downpipe to a 3" exhaust will get you all that's on the table. Most of these upgraded turbos aren't running out of their max efficiency ranges either and you're limited by fuel and the short block. If a 2.5" exhaust is killing power, add another 1 or 2psi and done. The turbo has the headroom.

A bigger downpipe will also help spool (pressure delta pre and post turbine are what determines spool time). The freer flowing the exhaust is, the lower pressure will be post-turbine. When you look how fast a tuned ST spools on a stock downpipe and exhaust...you see why this doesn't matter much. On an bigger turbo, it's a bigger difference. Once again, you look at benefit vs price/weight/room.

Now that is awesome information! Thank you Sir!!!
 
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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.
 

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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
 
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