Exhaust... could it be too big?

[Smokey Eddy]

I'm building a rather ludacris exhaust for my 1.6....
so im thinking a 3 inch down pipe that goes to a "Y" where it splits to two 2.5 inch pipes that go to the front of the back tires. So essentially its going from just a foot or two of 3 inch to ... 5 inch exhaust.
How much back pressure is required for proper engine operation... I know there is no definate answer but does this seem stupid? I just wanted to have the exhaust in front of the back tires but are my pipes too large? My exhaust fell off once and I ran it with no exhaust for just a day or two and it almost ran better but that was before I changed the pump/boost and that was with stock restrictive down pipe and exhaust. Any thoughts?
Thanks for your time,
Ed.

[flapjack]

Two 2.5" dia tubes have the are of one 3.5" dia tube.
Being that big won't hurt anything.  I don't think you need back pressure but if you're worried about it, the turbo is a big restriction and provides plenty.
The kids getting close to 200 hp were running 3" dia from the turbo back. I can't find the thread at the moment.

[arb]

Should be 2x 2.5" = 3.75"


area = Pie R squared, so:

3.145 * (1.25 x 1.25) x 2

[subsonic]

You want as little back preassure as possible in the turbo setup.  About the only debate on large exhausts is flow vs velocity.

[Duzty]

I have made 204hp on 2½" pipe.

[arb]

Should be 2x 2.5" = 3.75"


area = Pie R squared, so:

3.145 * (1.25 x 1.25) x 2

Pi is 3.14159265....

That rounds off to 3.142 rather than 3.145

Andrew

Yeah, fat fingers ;-)   But its still 3.75"

[saurkraut]

I'm building a rather ludacris exhaust for my 1.6....
so im thinking a 3 inch down pipe that goes to a "Y" where it splits to two 2.5 inch pipes that go to the front of the back tires. So essentially its going from just a foot or two of 3 inch to ... 5 inch exhaust.
How much back pressure is required for proper engine operation... I know there is no definate answer but does this seem stupid? I just wanted to have the exhaust in front of the back tires but are my pipes too large? My exhaust fell off once and I ran it with no exhaust for just a day or two and it almost ran better but that was before I changed the pump/boost and that was with stock restrictive down pipe and exhaust. Any thoughts?
Thanks for your time,
Ed.

Words you'll never see on this forum:

"Jeez, did I screw up by putting too big of a pipe under my VW"

Any back pressure after the turbo is a negative thing.

I have been contemplating a 3" system myself for my '79.

One of the pioneers of this GTD monky business (Jake Russel) found that if he ran his 2.5" down pipe only, he had more measurable power (Gtech) than with the full 2.5" exhaust.  He thought that a 3" system would show more performance than the 2.5" system.

Go for it.  3" is not too big.

[flapjack]

ARB: Here is what I did, please show me where I made a mistake, or show me what you did.
Ignoring wall thickness, the area of two 2.5" dia circles are equal to one 3.54" dia circle

A=pi r2

 For 2.5" dia
    A= pi ( 1.25 )2 = 3.14 * 1.56 = 4.9 square inches
    Two tubes of 2.5" dia = 4.9 sq in * 2 = 9.82 sq inches

For 3.5" dia
    A= pi ( 1.75 )2 = 3.14 * 3.06 = 9.62 square inches

For 3.54" dia
    A= pi ( 1.77 )2 = 3.14 * 3.13 = 9.83 square inches

For 3.75" dia
    A= pi ( 1.88 )2 = 3.14 * 3.53 = 11.1 square inches

[MJF]

When I had dynoed 174hp@crank, I had 3" downpipe with 2,5" exhaust. I measured 0,3bar from downpipe. Then I dropped exhaust, drove just 3" downpipe and got 0,1 seconds off from 1/4 time.

[Smokey Eddy]

Hmmm I see
Sorry about my math screw up. I should have caught that but it was late when i posted that apperently im s'posed to be good at math don't let my prof's read that post about the 5 inch exhaust hahaha

I'm also glad this has a geometry lesson intertwined with the wisdom on downpipes and exhaust. So, to all of you, thanks!

Ed.