My new 3" d/p back exhaust

[S2_Dave]

Surely the area of two 2.25" pipes is around 3.9".
Isn't that more than 3"?
Ok, I understand that there will be turbulent action at the "Y" but under pressure a flow in a 3.9" then a 3" pipe will still be better that a flow from a 2.25" to a 3" especially if there is a cat in the way (even if it is a sports cat)!

I think it's something to do with the drag on the exhaust wall. With 2 x 2.25in pipes you have a total area of 7.95in² compared with 1 x 3in pipe giving 7.07in², so this has the two smaller pipes ahead by 12.5%. However, there's 14.14in of wall with 2 x 2.25in pipes, compared with 9.42in of wall with 1 x 3in. I'm sure I've read somewhere that the closer to the wall, the slower the air moves, so more wall isn't a good thing and there's 50% more with two pipes (although I suppose it may conduct a bit more heat away).

[Knight Rider S4]

You gonna get your slide rule out again Jamo?

Surely the area of two 2.25" pipes is around 3.9".
Isn't that more than 3"?
Ok, I understand that there will be turbulent action at the "Y" but under pressure a flow in a 3.9" then a 3" pipe will still be better that a flow from a 2.25" to a 3" especially if there is a cat in the way (even if it is a sports cat)!

Rubbish I'm afraid.......

P.S you asked for that

[S2_Dave]

I think two 2.25in pipes is the equivalent area of one 3.182in pipe.

[Animal Mother]

I think two 2.25in pipes is the equivalent area of one 3.182in pipe.

Bingo was his name'o!

[Animal Mother]

Rubbish I'm afraid.......

P.S you asked for that

Deserved!

[DJones]

I was always told that the best systems are those that stay the same diameter front to back, (with the exception of leaving the turbo) OK you have to have bends but do not change diameter.

I wanted to have twin out of the rear box but they did not recommend that either..

Made my own in the end but still followed there advice...

[DJones]

Another..

[DJones]

Another...

[DJones]

Another....

[S-Express]

Amazing quality and attention to detail
You did that yourself?

[DJones]

Amazing quality and attention to detail
You did that yourself?

I did... got the burns to prove it too.... This is the other end..

[S-Express]

Permission to use the phrase "Top Bollock" !!!
I'm more than a little interested as I am thinking of fabbing up a new manifold and side exit systems for my kit car - did you hire a mandrel bender? did you have to TIG it?
(I have welded 0.9mm stainless sheet with stainless rods on an arc welder with good results so may stick with the simple way!)

[DJones]

Permission to use the phrase "Top Bollock" !!!
I'm more than a little interested as I am thinking of fabbing up a new manifold and side exit systems for my kit car - did you hire a mandrel bender? did you have to TIG it?
(I have welded 0.9mm stainless sheet with stainless rods on an arc welder with good results so may stick with the simple way!)

Cheers, I got all the parts from Jetex, bends tube joints etc (web site is pretty clear) All the welds were done with a MIG welder using stainless wire with argon gas with the exception of the tail pipe i used a TIG on that but by my own admission I'm not good with a TIG welder....

[Jamo]

You gonna get your slide rule out again Jamo?

Surely the area of two 2.25" pipes is around 3.9".
Isn't that more than 3"?
Ok, I understand that there will be turbulent action at the "Y" but under pressure a flow in a 3.9" then a 3" pipe will still be better that a flow from a 2.25" to a 3" especially if there is a cat in the way (even if it is a sports cat)!

No because 2x2.5 will have more back pressure than a single 3" y

Your thinking of total cross sectional area which isn't the primary concern as your dealing with a gas. Flow is the concern.


The biggest cause of restriction is at the front of the exhaust section i.e downpipe and then centre section, by the time it's hit the rear the gas has lost heat.

It is why on the Coupe you can have 3" downpipe and centre and then have 2.75 over the rear without affecting performance.
Think of it like a car journey.

It's quicker going on a motorway for 70mph (3") from the moment you leave your house
than travelling along an A road at 40mpph (2.5") before you can get to the motorway.

[Animal Mother]

I'll see how good mine is on the 24th on the Dyno as exhaust design theory is sketchy at best.
At the end of the day its all a bit academic with an OE manifold.