Choosing Exhaust sizes

[Dan_Gyoba]

I run 3" on my NA,
and I have yet to find a faster non-turbo that will out run it.
In fact, I have out run Every stock Turbo Mk3
that I have ever come up against.

I certainly am not goig to tell you that your car isn't fast, since it obviously is, but I find your explanation of "just add more fuel" to be somewhat simplistic.

It's obvious that clearing the exhaust gasses away from the cylinders is the function of the exhaust, and it certainly seems like bigger should just be better if that's the case, however, I've seen dyno graphs, with A/F ratios plotted as well (Showing A/F being extremely similar, if not hte same) that show losses putting big exhausts on NA engines. The theory that the exhaust velocity helps with scavenging seems to be the simplest and most logical explanation for this.

Now it's also true that most of these losses are shown at lower RPMs, and certainly at high RPM, the larger exhausts are more likely to help, rather than hinder.

For your car, if you're out-running stock turbos, then I'd bet that your list of mods aren't just a 3" exhaust and tuning to add fuel. You're certainly not running the stock exhaust manifold!

I still stand by that a 3" exhaust on an NA is a waste, though I will add the caveat that it depends on what other mods have been done.

 

[Poodles]

The place where scavenging does the most work is in the collector on the header...

3" on an N/A isn't going to hurt too bad, maybe a bit bad ont he bottom end, but I wonder what the turbo cam would do to the powerband with that kind of airflow...

Plus the far larger amounts of 3" exhaust systems on the market for our cars...

 

[xXSupramanXx]

I agree with Dan. 3" is too large for the NA

 

[Boost Lee]

I like what I see.

Good, Fun, Healthy discussions, no fighting.

Obviously everything that's been said in this thread is based mostly off of personal experience. I can't imagine anyone going out of their way to lie over something so silly, so I'll stick to believing that everyones setup is different, and the outcome can be different every time.

My experiences with 3" Exhaust?

Full 3" Exhaust out back:
With the exception of typical lag, has a peppy powerband through ALL rpms.

Yank the 3" cat-back off, (Open DP):
Low rpms 2,000-3500, Will feel a bit like a dog, and sound like a dump truck.
After hitting full boost (4,000rpm), There is a noticable difference in power gain.

For giggles, Ran open 3" Turbo elbow:
Even MORE hesitation in the low rpm range, but once it hit boost, again, a bit noticeable up top.

____

4" Exhaust experience?

Of course, all depending on what you've had previously, (Stock, 3", etc)
Coming from stock, it's incredible how much it opens up the cars overall pull. Completely different car. But as it's become pretty obvious, unless you're shooting for or have large power goals, Going 4" really isn't necessary.

The concept of hot gases cooling in the middle of a large pipe, is pretty simple science. It makes since.
But I beg to differ it decreasing (or hurting) the power output as opposed to running a stock exhaust.


Cliff notes:
3" is ideal for 90% of people on these boards.
3.5" is a bit throatier, still showing power gains.
4" will still show power gains, but not recommended for low hp goals.

-Jeff

 

[rocktownmkIII]

im having alittle trouble deciding on what size of piping to use on exhaust (1990 Turbo Supra). i realize you dont want back pressure with turbos but how big is to big? some advice please...

I would go with the blitz exhaust its a little bigger than 3 in at 3.3

 

[MRSUPRA]

A friend of mine picked up close to 30rwhp by opening up his cutout on the dyno. That's solid evidence that turbos hate backpressure. His catback was the HKS 70mm exhaust. I believe he went from 420rwhp to 450rwhp with his SP61GT.

 

[Mr. Sinister]

His catback was the HKS 70mm exhaust. I believe he went from 420rwhp to 450rwhp with his SP61GT.

Don't you mean 75 mm?

 

[MRSUPRA]

/\Yup, brain fart.