Exhaust question

[Kirk]

Can someone explain to me why a typical front pipe for the GTR has two 70mm pipes connecting into one 80mm or even a 85mm pipe? They have created a bottle neck right there.

I was having this conversation with someone, and they weren't quite grasping what I was saying. They kept replying that they flow better than the stock one. Where that of course is true, it doesn't resolve the "pinch" in the straw.

[Ezzie]

Space constraints. 2x 70mm. has a total area of 440 sq. mm. To keep the exhaust gases flowing at the same velocity (zero back pressure), you'd need about a 140mm. exhaust pipe (5.5" exhaust). The velocity of the exhaust gases inside the 80mm. tubing are higher than the downpipes which causes back pressure. The best way for a turbo is to reduce the back pressure as much as possible - try running a big dump pipe exiting below the passenger door with no mufflers!!

[WRick]

I have a hks down pipe made like this.

The car backfires ALOT.

Can this be the cause ? backfiring stresses me.

[Kirk]

Space constraints. 2x 70mm. has a total area of 440 sq. mm. To keep the exhaust gases flowing at the same velocity (zero back pressure), you'd need about a 140mm. exhaust pipe (5.5" exhaust). The velocity of the exhaust gases inside the 80mm. tubing are higher than the downpipes which causes back pressure. The best way for a turbo is to reduce the back pressure as much as possible - try running a big dump pipe exiting below the passenger door with no mufflers!!

I don't doubt space constraints, but your area calculations are a bit off.

70mm = 3848.45mm sq.
2 x 70mm = 7696.90mm sq.

so you would need 1 pipe of 99mm to not create any back pressure. This is based on a 2x70mm pipes which is only equal to 3.897" exhaust.

On most of the turbos that I have been looking at, they have an exhaust of about 3.5" or 88.9mm or 6207.17mm sq. and again, to not create any back pressure, you would want 2 x 88.9mm or 12414.33mm sq. or 488.753" sq. or 4.949" single pipe. After the catalytic converter, I am sure you could shrink the size of the pipe down a bit as the exhaust gases have cooled and therefore the volume of gas has shrunk. But I have not done any thermal dynamic testing, so that part is only theory.

I highly doubt that I am the first to calculate this, but it does make me wonder why companies that make exhaust, don't offer something of a bigger nature for those of us (maybe just me) who want "port" matched piping.

So with that in mind, who knows of a good exhaust shop so I can get a custom exhaust made (doesn't have to be local so please don't move to eastern ).

I tried discussing this with someone else and their mechanic and they just couldn't grasp what I was trying to explain so I gave up.

Could someone else let me know if I am out to lunch on this and provide reasons why.

I still think it could fit though.

Thanks all.

[rockcrete]

That's the point of the full dual exhausts like the Kakimoto Racing one, you can't fit a pipe that big under your car and drive over anything.

[Danno]

I say we find someone that's up for making a custom downpipe with two 70mm pipes to a 100mm (at least) with a dump pipe/exhaust cutout to replace the stock downpipe and cat!!!

Who's up for the challenge??? I'd certainly buy one.

[Ezzie]

..... but your area calculations are a bit off.

70mm = 3848.45mm sq.
2 x 70mm = 7696.90mm sq.

so you would need 1 pipe of 99mm to not create any back pressure. This is based on a 2x70mm pipes which is only equal to 3.897" exhaust.

Thanks for correcting me. It's been a long time since high school math. I just used the simple formula pi "D" (where "D" = inside diameter) and calculated the circumference. The proper formula for the area of a circle is pi x the square of the radius.

[rockcrete]

Here's Kakimoto Racing's full 70mm dual exhaust

http://www.kakimotoracing.co.jp/image_NF1C08.html

[Kirk]

Here's Kakimoto Racing's full 70mm dual exhaust

http://www.kakimotoracing.co.jp/image_NF1C08.html

Thanks, I was looking for that after your last post and couldn't find it.

[WRick]

I have the kaki regu 06 & R....

I LOVE it.

it's awesome. A little pricy, but looks and fitting is perfect

[Ezzie]

Here's Kakimoto Racing's full 70mm dual exhaust

http://www.kakimotoracing.co.jp/image_NF1C08.html

Interesting that they don't do a balance tube or crossover between the two exhaust streams. Each turbo has an individual pipe all the way out.

Any thoughts on why this works or would it be better with a balance tube/crossover like they do in V8 engines?

[Kirk]

Interesting that they don't do a balance tube or crossover between the two exhaust streams. Each turbo has an individual pipe all the way out.

Any thoughts on why this works or would it be better with a balance tube/crossover like they do in V8 engines?

I could be wrong here, but I think the only reason they do that is for sound. The Thunderbird SC guys (my old group), we always x'd or h piped the exhaust otherwise they sounded like complete shite. I mean really, really bad.

I can't see any other reason, the less bending and cramming into small pipes the better.

[rockcrete]

Supposedly the balance pipe adds a bit of power too, but I can't say I've ever seen them anywhere, except on American exhaust systems. I guess the Japanese don't make as many dual systems as the Americans do, maybe.

[Danno]

Supposedly the balance pipe adds a bit of power too, but I can't say I've ever seen them anywhere, except on American exhaust systems. I guess the Japanese don't make as many dual systems as the Americans do, maybe.

X/H-pipes are typically on V-style engines to equalize the exhaust pressure. Because exhaust gasses actually "pulse" through the exhaust system, a pipe joining the two individual sides will help in eliminating the exhaust gas from being "sucked" back when there is a vacuum on each pipe.

I guess that would also make it sound better, as the pulses of the exhaust after the cross pipe would sound quite a bit smoother.

I hope this all makes sence. It's kind of hard to explain without pictures.

Cheers,
Dan

PS: Exhaust systems don't "add" power, only free up power that's currently restricted with the original exhaust system.