Exhausts with good flow velocity

[Ashikabi]

Originally Posted by NoHaveMSG

Have you seen Porsche exhaust? I get that it is handmade blah blah but jeez.



https://www.dundonmotorsports.com/pr...-crack-pretzel

Oh I'm aware the more expensive the car, the more expensive the parts. Axle back on a GTR isn't any different than on an FRS but costs at least twice as much

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[Ultramaroon]

Originally Posted by NoHaveMSG

You could almost buy a new GR86 for what a full exhaust cost from Dundon.

The Dundon guys are really cool though. Sometimes they bring their support trailer out to the Ridge and will help anyone out.

Haha... I can sell my exhaust and buy 4 cars!

https://www.counterspacegarage.com/a...-axleback-gt86

[NoHaveMSG]

Originally Posted by Ultramaroon

Haha... I can sell my exhaust and buy 4 cars!

https://www.counterspacegarage.com/a...-axleback-gt86

lol

Better investment then Bitcoin.

[nikitopo]

There is a direct relation between the scavenging effect and the gas velocity. How big the vacuum behind each pulse is depends on the gas velocity. The higher the velocity, the bigger the vacuum the pulse has behind it. So, you get the maximum scavenging effect if the gas velocity is high. By maximizing the scavenging effect, you help to pull pulses out of the combustion chamber, which means the engine doesn’t have to work as hard to do that. The one exception of exhaust sizing is on turbo cars. Since the turbo is in the exhaust stream, the gas flow spinning the impeller tends to come out of the turbo with the pulses greatly diminished. In this case, you can get away with running a larger pipe than on an equivalent NA engine because you can’t take as much advantage of the scavenging effect.

[ZDan]

Originally Posted by nikitopo

There is a direct relation between the scavenging effect and the gas velocity. How big the vacuum behind each pulse is depends on the gas velocity. The higher the velocity, the bigger the vacuum the pulse has behind it.

By this logic you should make the exhaust as small as possible, which will give max flow velocity.

The fact is, you pay a price to increase the gas velocity with a smaller-diameter pipe. That price is back pressure. A smaller catback exhaust pipe diameter doesn't magic vacuum pulses to help evacuate the cylinders during exhaust stroke.

Yes, there is a point of diminishing returns on going bigger with exhaust pipes.

[Ashikabi]

Originally Posted by ZDan

By this logic you should make the exhaust as small as possible, which will give max flow velocity.



The fact is, you pay a price to increase the gas velocity with a smaller-diameter pipe. That price is back pressure. A smaller catback exhaust pipe diameter doesn't magic vacuum pulses to help evacuate the cylinders during exhaust stroke.



Yes, there is a point of diminishing returns on going bigger with exhaust pipes.

By this logic you should make the exhaust as large as possible which would give the least resistance to flow. But that doesn't work either. You put 5"exhaust on a 2 liter motor and I guarantee you'll lose power over a 2.5. Bigger is better until it's not. But it can't be too small either. Turns out 2.5-3 inches is just about perfect for small motors. For the VAST majority of people, the only difference between the 2 sizes will be sound. I would never go over 3 inch on a 4 cylinder unless it had a turbo or made like 500+hp

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[NoHaveMSG]

Originally Posted by nikitopo

There is a direct relation between the crossflow scavenging effect and pressure differential.

FIFY

[ZDan]

Originally Posted by Ashikabi

By this logic you should make the exhaust as large as possible which would give the least resistance to flow. But that doesn't work either. You put 5"exhaust on a 2 liter motor and I guarantee you'll lose power over a 2.5.

So run the test.

Only test I'm familiar with on smallish ~150rwhp engine showed gains throughout the rev range from aft-of headers exhaust going from 2" to 2.25" , more modest gains from 2.25" to 2.5", and still more modest from 2.5" to 3". They stopped there though...

But anyway, the idea that "more flow velocity = more vacusucking of the exhaust out the cylinders, more power!" is a bit misguided, particularly if we're talking about catback rather than header primaries, but again for sure there is a point of diminishing returns and I'm not advocating for 3"+ exhausts at all.

[churchx]

ZDan: but as tube diameter affects exhaust gas velocity .. i wonder if using larger crossection tubes won't ease designing/packaging of headers, allowing using shorter runner lengths to still get same optimisation/scavenging as smaller & longer tube headers ..

[Dirty Harry]

Originally Posted by Ashikabi

STI shit is so fucking over priced

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It can be, if you’re patient and wait for deals or regularly check eBay etc., you can get it cheap. Maybe get it second hand?

I got the STI lowering springs for $700 AU fitted and the STI axle back for $1150 AU fitted. The lowering springs were part of an email campaign from Subaru (I admit this one was a fluke). I kept searching for the STI axle back on eBay and one day saw one advertised for $1300 and realised the seller was a local Subaru dealer. I phoned the dealer directly and got it fitted for $1150, it was clearance stock.

The springs were normally $1100 fitted and the axle back around $2500 fitted, so pretty happy and a bit lucky I guess and yes those prices are too much.

[nikitopo]

Originally Posted by ZDan

Only test I'm familiar with on smallish ~150rwhp engine showed gains throughout the rev range from aft-of headers exhaust going from 2" to 2.25" , more modest gains from 2.25" to 2.5", and still more modest from 2.5" to 3". They stopped there though...

Not sure if you know the GRMN 86 car with the 219 PS engine (19 PS more @ 7,300 rpm). I have an access to all the part numbers and midpipe is exactly the same with the one of the normal car. Axleback on the other hand is changed in a similar manner to the STI axleback. Don't you think if there was any real potential of getting a better result with a bigger midpipe they wouldn't do it?

Just my 2¢ of thought ...

[ZDan]

Originally Posted by nikitopo

Not sure if you know the GRMN 86 car with the 219 PS engine (19 PS more @ 7,300 rpm). I have an access to all the part numbers and midpipe is exactly the same with the one of the normal car. Axleback on the other hand is changed in a similar manner to the STI axleback. Don't you think if there was any real potential of getting a better result with a bigger midpipe they wouldn't do it?

Just my 2¢ of thought ...

If it's a series production car to be sold for money, and has to be suitable and not annoying to customers and also meet noise regs, there are other factors at play regarding axleback exhaust selection beyond absolute peak power.

But personally I doubt it's giving up much to an "optimized" axleback...

FWIW I'm still running stock exhaust aft of the header/overpipe although I know I could could gain a couple/few hp. I just don't think it would gain me much at the track to justify. Also street car/daily, and also lazy and cheap (or rather spent too much on other car stuff this season)

[PulsarBeeerz]

This



And this changed with the GRMN 86




I'd wager that these modifications made more difference in response, performance and tuning than any axleback could ever hope. The OEM front pipe controls sound suppression well. This a requirement for most OEMs.. A 2.5" front pipe would have been louder yet, even if it did make more power. Is 3-4PS worth 6dB? Probably not to most OEMs. Not including the extra cost involved sourcing and re-certifying a different catalytic converter for production. They had to compromise on some things.

[jflogerzi]

Originally Posted by ZDan

There's no magic point of "balance" as far as catback exhaust diameter. From what i've seen, bigger is better but beyond probably ~3" you're just taking up more space and adding weight without much if any more to be gained. Wouldn't/shouldn't be hurting low/midrange torque though.

Mine peaked at 7000 stock, and also with header/tune.

Anyway, nothing to particularly worry about here I don't think, overkill may be mostly pointless but it's not likely to be hurting power/torque.

2.5 is fine for NA and even small turbo setup as long as everything flows well. Bigger HP boosted builds can benefit from a 3inch exhaust. You can make the same power with less boost.