Divided Down Pipes And Boost Creep

[3L r32]

I am looking at a fairly standard rb25det skyline for a friend that has a dump pipe which looks the same as this on it with a divided plumb back.

http://www.ebay.com/itm/JDM-PRO-NISSAN-SKYLINE-RB20-RB25-R32-R33-EXHAUST-TURBO-DUMP-FRONT-PIPE-RB25-/181239136372?pt=LH_DefaultDomain_15&hash=item2a32b1c074&vxp=mtr

The car is having some serious boost creep issues. It is running a standard 5psi wastegate actuator with no boost controler and is creeping to over 12psi above 5000rpm

Boost holds steady 5psi at lower RPM

I have checked the flapper movement angle and it is a little less than 90 degrees

Has anyone had any similar issues with these?

[jay-rod]

Waste gate flap is probably hitting the split pipe

Happens all the time on those.

[Kinks]

bellmouth FTW

[MrStabby]

Waste gate flap is probably hitting the split pipe

Happens all the time on those.

But he said the flapper is moving almost 90 degrees.

Maybe try disconnecting the actuator so the gate will be blown full open and go for a run (with epic lag) to see if it still creeps up the top end. If so, the its a hot side/exhaust problem.

[jay-rod]

How can u possibly know if its moving 90 degrees though with the dump on?

Edited June 24, 2015 by jay-rod

[Ben C34]

Cos yu move the lever the flap is directly attached to.....

[Trex]

lol

[jay-rod]

Assuming you take off the actuator arm to move it freely and be aware than most of the time the lever is not parallel to the gate flap.

I would be measuring it with the dump off/on and compare the 2.

Also making sure there is no crazy preload on the actuator arm if it has been taken off before..

[Ben C34]

Parallel doesn't matter though, it's the fact it moves 90 from the closed position.

The flap could still be on strutted y a shit flange, or the rerun merge of the waste gate pipe could be horrible.

[jay-rod]

Either way, get rid of the split pipe ;P

[Ben C34]

Either way, get rid of the split pipe ;P

Word

[STATUS]

divided sucks, bellmouth is superior everytime.

[Trex]

Any back to back testing?

And what are we talking about here?

Split dump alone? Or a split dump + split front? Divorced dump + front? Standard turbo? Highflow? Other? Etc..

[RBrennan25]

I haven't had any issues with my dump pipe. It's got 2 different pipes altogether and merges just before the front pipe. It was custom made to fit my standard turbo and assumed it would never work with the SS2 turbo I'm running now but had no issues with install or tuning, and no issues with boost creep or anything else.

Made 320rwkw on an unopened 25det neo with E85 on the standard manifold and full boost is right on 4000rpm

Maybe I got lucky? Or because its custom made it fit better?

[scotty nm35]

Think about it. If you are running decent boost, then 99% of the time the wastegate is closed, which means the turbo has a 3 or 3.5 inch dump to get rid of the exhaust flow. Only when the gate opens does it add to that flow.

Most split pipes do restrict, as the turbo pipe is only 2 1/4 or 2 1/2 instead of 3 or 3 1/2. I can nearly guarantee upgrading to a decent bellmouth will improve response, just due to the increase in pipe size available to the turbo.

Realistically at 300kw you want a 3.5 inch dump at least, this can be tapered back to 3 inch after the cat as the exhaust gas has contracted as it cools, and it cools fast. The dump isn't the place to restrict the exhaust, here bigger is best.

[KiwiRS4T]

Think about it. If you are running decent boost, then 99% of the time the wastegate is closed, which means the turbo has a 3 or 3.5 inch dump to get rid of the exhaust flow. Only when the gate opens does it add to that flow.

Most split pipes do restrict, as the turbo pipe is only 2 1/4 or 2 1/2 instead of 3 or 3 1/2. I can nearly guarantee upgrading to a decent bellmouth will improve response, just due to the increase in pipe size available to the turbo.

Realistically at 300kw you want a 3.5 inch dump at least, this can be tapered back to 3 inch after the cat as the exhaust gas has contracted as it cools, and it cools fast. The dump isn't the place to restrict the exhaust, here bigger is best.

Good point. I had a split dump on my 25DET and people talked about the dangers of the W/G flap catching (it didn't on mine) but no-one mentioned the much smaller dump pipe - obvious in retrospect ...but not at the time!

[3L r32]

Been a bit busy at work but I got a chance to attack it the other night.

He picked it up yesterday and I haven't heard back from him but i"m assuming it will fix his issues.

Plumb back was partially blocked, I think this was the main part of the issue.

[Ben C34]

Wow. That was so shit!

[GTSBoy]

Truly an awful effort on that one!

I must say though, that I disagree strongly on the whole "biggest volume after the turbine outlet possible" school of thought. Respected turbo people (read, dev engineers at Garrett) have stated publicly that the very best turbine dump is a conical taper expansion from the diameter of the turbine outlet up to the diameter of the required dump pipe size. Said conical expansion needing to be about 10° included angle, or somesuch small angle. Of course that is a little difficult to package in an engine bay, so we do have to accept compromises. But as far as I am concerned, the "smaller" turbine pipe that results from a split dump is not an issue, provided it is as nicely shaped in terms of the expansion and the bend downward as possible. In fact, a smaller turbine pipe actually makes it easier to get a more respectable conical expansion and a slightly more decent looking bend down than going straight to the largest pipe. The fringe benefits of keeping the nasty turbulence of the gate exhaust flowing sideways into the turbine exhaust, right at the point where the turbine flow exits the casting is almost a bonus, rather than the original aim of the design.

Of course, I have nothing good to say about the divided dumps (the HKS ones and the copies thereof) where the return was really close to the top. They do need to take the WG gases as far downstream as possible before putting them back in.

I agree that obtaining the best pressure drop across the turbine is the goal. But I disagree that a drainpipe dump is the way to achieve that.

Edited July 1, 2015 by GTSBoy

[hardsteppa]

I've read that too, the conical tapering out from the turbine housing is the 'ideal' design (due to gasses exiting in a spiral fashion?) and with this in mind thought the split pipe would work best, as it did have a nice conical taper at the start of the pipe from the flange. But, I bought both a split pipe (wen the extra mile of ported flanges to give a good smooth entry taper to both pipes) and a bellmouth pipe, and tested back to back....bellmouth was much better, car pulled noticeably harder. I put it down to one of those 'ideal on paper but not in use' things....as in, if you were designing a setup completely from scratch you could come up with a better setup, but fitting a premade product into a premade space and throwing in only 1 aspect of the 'ideal' design like the conical taper, just doesn't cut it so the 'as much space as possible, as soon as possible' design of the bellmouth worked best.