Exhaust Restriction

[SiR_RB]

hey guys

my exhaust consists of R34 GTR dump pipes(60mm outlets) going into 2x60mm to 76mm merge front pipes, into 76mm decat, then 70mm to 80mm "hks legal" cat back.

all this time i though my cat back was going from 76mm after the cat then opening to 80mm but i had it off the other day to fix a leak and measured inside the flange and found it to be 70mm inside diameter !

would this step down in exhaust flow cause a noticible restriction? also its a very quiet cat back, got afew little bends after the cat going into the center resonator(straight through type) then afew more little bends (with a nice almost golf ball dent in it too lol), then a big S looking bend near the fuel tank before exiting out the rear muffler.

im organising a final tune in the coming month or so, is it worth me ditching this and getting a nice 80-90mm stainless zorst? (kakimoto regu 06&r mmmmmmm )

[smithy85]

Biggest is always best when it comes to exhausts, although I believe that alot of the power that can be gained from the exhaust is through the front pipes and cat.

It wouldn't hurt to get a larger cat back section fitted to reduce the overall restriction on the system, but again the main gains come from the first half of the exhaust system, which is overly restrictive from the factory.

I personally had a 3 in (GTR) cat back exhaust fitted to the std cat and front pipes and then changed it to 3.5 in and only noticed small gains to the top end. It wasn't until I changed to the big Trust front pipes and 3.5 in hi flow cat that I noticed significant power gains through the rev range - particularly in the midrange.

[Nee-san]

Think about it in terms of:

everything you do to your engine and how it performs is governed by the way the exhaust system works.

This is because the entire engine terminates at the exhaust. This sounds very simple but when you actually think about it in terms of what you do to your engine can be completely neutralized by having an insufficient exhaust system.

Why spend money on bigger turbo's, cams headwork all stuff that can make you go faster and not allow it to run as optimally as possible by not putting an effort into minimizing back pressure as much as is relatively possible.

Design of exhaust and cooling system are two of the most overlooked parts of car development (in terms of our scene anyway).

[BoostdR]

its 70mm after the cat as heat at that point is very high from where the cat outlet should be...now that it has a decat your gas would have cooled more which would cause more of a restriction...prob still better not having the cat however.

Matt

[SiR_RB]

So ur saying because it has a 76mm decat testpipe, the 70mm flange on the cat back is actualy causing a restriction ?

Does the amount of bends in the catback cause abit of power loss/restriction?

[BoostdR]

So ur saying because it has a 76mm decat testpipe, the 70mm flange on the cat back is actualy causing a restriction ?

Does the amount of bends in the catback cause abit of power loss/restriction?

The size would cause a restriction...but the flange lip would cause more going from 76 to 70mm. Bends do cause restriction which is why you see the Jap Cannon angled mufflers...as they require less bends. If you have under 300rwkw I wouldn't worry to much.

Usually the biggest restriction is inside the Jasma approved mufflers. I have seen a 34 with a 3.5" exhaust but the rear cannon went to 60mm internally. Easiest way to tell is to unbolt the exhaust at the cat. Drive down the street and see how much more boost it runs. Will highlight your restriction. My side pipe see a 6psi change in boost.

Matt

Edited April 16, 2011 by BoostdR