Exhaust Back Pressure

[KiwiRS4T]

I have read quite a bit on "back pressure is bad" but found very little in the way of actual numbers for RB engines. My RB30 is making only 280 kw and it seems needs cam adjustment but also I belatedly noticed that the "3in" cannon that I bought actually narrows down to 2 3/4 in which doesn't sound a lot but will be significant.

I set up a rough test rig - put a fitting in the O2 sensor bung and ran a line to my boost gauge - it probably leaks a little so pressure readings will be a little under the real number.

With my stock exhaust, 3in everywhere except for the 2.75 bottleneck in the muffler, I got 0-1psi at idle, pretty much unchanged at 2000rpm and 5 psi at full boost WOT around 5500rpm.

Then I stuck in my "silencer" which reduced it to about 2 1/2 in (not much you might think but 16% less flow) ) but at idle to 2000 rpm it read 1-2 psi and at WOT 8 psi so a noticeable increase in back pressure.

This leads me to believe that if I change my back muffler to a full 3in straight through muffler i should see 18% more flow and a decent reduction in back pressure.

Does anyone else have any back pressure numbers they can quote (together with engine type and exhaust size) preferably from a dyno but even from a jury rig like mine would be of interest!

[Lithium]

I set up a rough test rig - put a fitting in the O2 sensor bung and ran a line to my boost gauge - it probably leaks a little so pressure readings will be a little under the real number.

The more significant number is the exhaust manifold pressure imho - that is the one that can wreck havoc, and if you are getting 8psi post turbine then it is potentially getting quite ugly pre-turbo.

[KiwiRS4T]

The more significant number is the exhaust manifold pressure imho - that is the one that can wreck havoc, and if you are getting 8psi post turbine then it is potentially getting quite ugly pre-turbo.

Well I'm getting 5psi without the silencer and hope to improve that with a full 3in muffler but what would you consider an acceptable number?

[GTSBoy]

5 psi is excessive.

Ideally you would have <2 psi in the exhaust at full noise. In practice you might have to put up with more. But 5 psi is a hell of a lot of pressure. Remember, that is pushing back on the gases leaving the turbine. So the exhaust manifold pressure has to be that much higher to get the stuff to flow out through the turbine in the first place. If you can get less back pressure, you make EVERYTHING better. Except perhaps that amount of noise.

[KiwiRS4T]

OK so 2psi is the max. In theory 3in should be ok for 350kw or no? 3.5 in systems are hard to come by I would probably have to spend $1000 to get one made.

I will get a proper 3in cannon and if that doesn't work I have a resonator which I have always assumed is 3in but I 'll check that next and if that is ok then sadly i might have to swap out my 5in Venom cat for a pretend cat.

[KiwiRS4T]

double post

[wolverine]

What happens when you drop the exhaust Bob?

[KiwiRS4T]

What happens when you drop the exhaust Bob?

With no cert therefore no wof therefore no rego not game to try it on the street - will do when next on the dyno! Actually my focus over the Xmas hols will be getting the driveshaft loops on (required for cert) and getting the car all legal!

[GTSBoy]

wof = warrunt of futniss ?

[t_revz]

wof = warrunt of futniss....bro ?

Fyp

[scotty nm35]

lol.

So why the tailshaft loops Bob? Is that a compliancing thing? I didn't think you had to do anything to cars over there, making importing Jap cars chep as cheps?

[KiwiRS4T]

The good thing is almost any mod is legal here so long as you can get it certified as being done properly by an approved engineer. If you have a bigger than stock engine (RB30 in my case) and or 50% or so more than stock power you also need driveshaft loops in case the extra power breaks the universals and the driveshaft digs into the road and flips the car (it happens!). There are plenty of uncertified cars running around but if the testing station notices it they will not issue a Warrant of Fitness without which you can't get rego. My car should comply in all respects but the driveshaft loops are a pain (seats and carpets out ) as I haven't even got a concrete drive to work on but am moving soon and will have a drive and a garage again!

[Dan_J]

had no clue how to measure back pressure, so useful info to be had in here.

you sure the cat isn't on its way out ?

or the muffler actually muffles sound well hence back pressure build up ?

i'm current running 3" to two 2" tipped straight pipes and its about the same sound level as my 3" muffler ....under 5k rpms, at wot on high rpms its a tad louder

Edited November 20, 2012 by Dan_J

[GH05T]

you also need driveshaft loops in case the extra power breaks the universals and the driveshaft digs into the road and flips the car (it happens!).

Busted by mythbusters

[mattie4270]

The facts are backpressure robs horsepower big time.Ive built systems for racecars for years,normally aspirated and boosted,it doesnt matter,backpressure NO GOOD,especially turbos mate.Anything over 2pound at full noise is bad.Good to see you checked it as most people dont.Best spot to check is just in front of the cat,if you have one as its normally the most restrictive part of the system.Check what size systems all the big guns run on this forum,its very important

[ARTZ]

.at least 3.5...especially with a 3lt..your exhaust needs to go bro..at the very least the muffler..

[WMDC35]

take photos of inside driveshaft loop mounts otherwise they might want carpet back out to see, 2 piece needs 2 loops? saves alot of tickets when you pick up the fush & chups

[KiwiRS4T]

Yep taking the pics.

I was starting to think 3in was not enough. My muffler is a cannon type straight through brand new but narrows to 2 3/4 in. My cat is also brand new a 5in body with 3in inlet Venom so I hope that's not the problem. There is also a resonator in the middle which i have always assumed was 3in inside but now I hope its not so I can fix my problem by changing that too. I can't afford a whole new exhaust ATM so I will change the rear cannon for a full 3in for now and the resonator if necessary and start saving for a 3.5 in system.

[OoskylineoO]

The more significant number is the exhaust manifold pressure imho - that is the one that can wreck havoc, and if you are getting 8psi post turbine then it is potentially getting quite ugly pre-turbo.

+1

No point going larger exhaust if you've got a poorly designed manifold creating high back pressure. In this instance, running the gate straight off the manifold pressure should bring response up and keep pressure lower, as majority of the time this point is where exhaust back pressure would build up first.