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Discussion Starter #1
I am looking at exhaust design at the moment and I want to see if some of you guys can help me out.

the Kakimoto Dual exhaust (not full Dual) goes from the decat pipe into 2x 70mm pipe into 2 rear silencer.

While the HKS Ti goes from the decat into 85mm pipe that expend into a 102mm pipe.

I know the 102mm pipe works as Nito has proved on this To4Z R34. However, ground clearance will be an issue along with the cost (4 inch bore pipe are expansive).

My question is will a design like the Kakimoto exhaust (2x 70mm) be as efficence in reducing back pressure as the HKS Ti (1x 102mm)? Or is the only way is to buy it, put it on a dyno, and find out? They both have a bottle neck which is the downpipe and decat pipe, so they all start out at around 75mm.
 

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Without going into infinite detail, the crude calculation:

70mm=35mm dia

2 pipes therefor 2*2*Pie*35=440mm squared

102mm=51mm dia

2*pie*51=320mm squared.

Therefore for this section of the exhaust the evacuation capacity per unit length is 440mm2 for the Kaki and 320mm2 for the HKS.

This is an extreme simplification, and really you should have as big as possible all the way from the turbo exhaust flange, however I'm sure you get the idea.

Ideally, 2 pipes each of 100mm!

DaveG
 

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Atco - hate to have to correct you but your maths is a bit amiss....

2 x 70mm dia

2 x PI (3.1415926) x 35^2 = 7696mm^2

102mm dia

PI x 51^2 = 8171 mm^2

Also have to factor in boundary layer effect - the small pipe will not flow proportionally as well as a large pipe. The flow along the pipe wall is slow and effectively reduces the effective diameter.

So the HKS will flow better - give me one fat one over 2 halfs any day.
 

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oops 2PiR instead of PiRsquared! Thanks, should have fully engaged brain before fingers. Did circum'ce not x-section.......

Still say two 100mm are best!

True about boundary layer, however in this case how significant is it likely to be in the overeall scheme of things?

And yes, Kakimoto's are loud.

Regards,
DaveG
 

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Discussion Starter #6
Whats the difference though. Would anyone know how much air you need to be able to flow in order to achieve say 650 bhp?
 

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Discussion Starter #7
Okay, another question that just pop up.

Say the HKS goes from 85mm into 102mm pipe, so the pipe just expend.

However, if it is a cat-back exhaust (not the Kakimoto Full Dual design), it mean it will connect to the de-cat pipe, then split into 2x70mm pipe.

How much restraction will that "split" cause in the flow of the exhaust? Or will the exhaust be moving so fast that it wouldn't really matter?
 
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