Down pipe size

[couchboy]

Hi Guys

I'm at the point of my build were I'm ready to get the exhaust done and wanting input on down pipe size.

Spec are as follows:

Rb25det series 1

1000cc injectors 

Custom T04 turbo

Front mount intercooler 

Front facing plenum 

Power goal is 400hp to 500hp 

The exhaust will be side exit as it's a r32 race car.

 

[GTSBoy]

As large as you can fit. The larger the better.

[hardsteppa]

Damn straight. 3.5in minimum.

[couchboy]

Cool cheers 🤙

[Duncan]

Sidepipes are difficult in large diameters because you lose ground clearance where you cross the chassis rails and sills. You may want to consider smaller twins rather than large single for side pipe. Or major surgery around chassis rails/sills to get the clearance.

Or....sad as it sounds, a rear exhaust is just way more practical...plenty of space and premade options if you want.

[couchboy]

Cheers for the advice 🤙. The rear exit isnt really a option because I've got a fuel cell low in the boot.

[Duncan]

Right, well that makes it surprisingly hard because larger diameters have much more area/glow.  Say you want 3.5" flow (90mm) in twins, you need 2x 65mm. You do get 35mm more clearance that way but it's definitely not half.

Ride height and side pipes are a problem; you can't have the chassis rails 100mm off the ground and then lose 65 or even 90mm clearance below it; you end up having to run the chassis rails 190mm off the ground (single) or 165mm (twin)....

[GTSBoy]

If I wanted to pass a 4" exhaust pipe under the chassis rails, I'd just make a flat oval section up, no more than 50mm high, but as wide as it needs to be to add up to more than the equivalent cross sectional area of a 4" pipe. That looks like a 2" pipe split down the centre, with a 5" wide flat spliced in top and bottom, or a ~6"x2" rectangle. Add offset transitions at both ends to 4" pipe, or just at one end and take the rectangular outlet all the way out under the sill.

[bcozican]

This is mine its 3" there is a gap at back of chassis rails that gives you more clearance 

[couchboy]

Bcozican- so you have a 3 inch down pipe?

[Unzipped Composites]

Why does side exit influence what size down pipe you have? Go as big as you can on the downpipe and taper down to the side pipe if you need to.

[bcozican]

9 hours ago, couchboy said:

Bcozican- so you have a 3 inch down pipe?

yeah straight through 3" all the way through

when I first built the car I built it all for response at your desired power level 

i got the smallest PWR flipped for shorter piping intercooler 2.5" inlet outlet they said would take up to 600hp and up till now 2.5" cooler piping as well

still got me 650hps  

but obv could be better flow and now would go a bit bigger etc

 

alot more people know way more than me as above. maybe im not the best tech example, that seems to happen with me, but it somehow just works and still gets there. @Unzipped Compositescan vouch for that ha

Edited May 13, 2021 by bcozican

[couchboy]

Bcozican - thanks for that info 🤙, I'm thinking about going for a 4 inch