Jump to content
SAU Community

Recommended Posts

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?

Link to comment
https://www.sau.com.au/forums/topic/457742-divided-down-pipes-and-boost-creep/
Share on other sites

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.

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..

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.

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?

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.

  • Like 1

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!

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.

74b58bd2-63e0-4ac7-ad2d-443dee33ee2c_zps

53ae6aa4-ab32-4bb6-a03e-1ddb0b318082_zps

IMG_2863_zpsn1u9libs.jpg

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 by GTSBoy

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.

  • Like 1

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now


  • Similar Content

  • Latest Posts

    • @Haggerty this is your red flag. In MAP based ECU's the Manifold pressure X RPM calculation is how the engine knows it is actually...running/going through ANY load. You are confusing the term 'base map' with your base VE/Fuel table. When most people say 'base map' they mean the stock entire tune shipped with the ECU, hopefully aimed at a specific car/setup to use as a base for beginning to tune your specific car. Haltech has a lot of documentation (or at least they used to, I expect it to be better now). Read it voraciously.
    • I saw you mention this earlier and it raised a red flag, but I couldn't believe it was real. Yes, the vacuum signal should vary. It is the one and only load signal from the engine to the ECU, and it MUST vary. It is either not connected or is badly f**ked up in some way.
    • @Haggerty you still haven't answered my question.  Many things you are saying do not make sense for someone who can tune, yet I would not expect someone who cannot tune to be playing with the things in the ECU that you are.  This process would be a lot quicker to figure out if we can remove user error from the equation. 
    • If as it's stalling, the fuel pressure rises, it's saying there's less vacuum in the intake manifold. This is pretty typical of an engine that is slowing down.   While typically is agree it sounds fuel related, it really sounds fuel/air mixture related. Since the whole system has been refurbished, including injectors, pump, etc, it's likely we've altered how well the system is delivering fuel. If someone before you has messed with the IACV because it needed fiddling with as the fuel system was dieing out, we need to readjust it back. Getting things back to factory spec everywhere, is what's going to help the entire system. So if it idles at 400rpm with no IACV, that needs raising. Getting factory air flow back to normal will help us get everything back in spec, and likely help chase down any other issues. Back on IACV, if the base idle (no IACV plugged in) is too far out, it's a lot harder for the ECU to control idle. The IACV duty cycle causes non linear variations in reality. When I've tuned the idle valves in the past, you need to keep it in a relatively narrow window on aftermarket ecus to stop them doing wild dances. It also means if your base idle is too low, the valve needs to open too much, and then the smallest % change ends up being a huge variation.
    • I guess one thing that might be wrong is the manifold pressure.  It is a constant -5.9 and never moves even under 100% throttle and load.  I would expect it to atleast go to 0 correct?  It's doing this with the OEM MAP as well as the ECU vacuum sensor. When trying to tune the base map under load the crosshairs only climb vertically with RPM, but always in the -5.9 column.
×
×
  • Create New...