Jump to content
SAU Community

Recommended Posts

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.

This is pretty much what I wanted to say but I lack enough knowledge to write it coherently, but basically the turbine wheel and the space in which it sits is not a 3 inch diameter, so why would a split dump pipe with an actual dump pipe of say 2 and a bit inch that expands to 3 be a restriction? As long as its larger than the turbine wheel itself it should be able to handle the air flow sufficiently.

Also my waste gate pipe is the entire length of the dump and joins just before the front pipe, is that long enough to be effective? Just out of interest

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.

Totally fair. You and others have made the same claims and I reckon that with what's available off the shelf, and the variability of those products your results are valid. I'd like to think that someone could/would spend enough time to design and build a split dump for RBs in Skylines that incorporates the most ideality it can and see what happens.

I reckon that if you were gung-ho about it you'd piss off the air-con, move the turbo forward so you could have a longer straighter conical exit from the turbine, etc, etc. The sad thing about that is that there's not way you'd do it, because going to that effort for a standard dump pattern turbo seems well silly. You'd put on a much bigger turbo with external gate as your first choice.

Also my waste gate pipe is the entire length of the dump and joins just before the front pipe, is that long enough to be effective? Just out of interest

Yeah, they're the only ones worth considering. Mine rejoins at the bottom bend. I'm in two minds about where the best place down there to re-introduce the gases actually is. Practicality, buildability and installability often get in the way of what is probably the best aero solution.

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.

100% correct. You can see this in carbie design too. There's a short radius into the venturi, then a long, small angle cone out of the venturi. That's because the transition from large to small produces less turbulence than the transition from small to large, so you need to take more care with small to large. IIRC its 7 degrees but as you say, its impractical to go too small with the angle because it makes the cone too long.

With the mess of turbulence coming out the back of the turbo I don't think perfect transitions matter too much. Get the exhaust out as quickly using the largest tube possible, with the least amount of bends works for me, with gentle transitions preferably. Sometimes a decent split dump just can't work, due to space constraints, or the costs involved in running two pipes where one should be.

I have never seen a good internal gate split dump design, they all step out from the gate, then back into the hole in the flange, causing flow restrictions at the step down. I have never seen one where the merge was any good either, I would be die grinding them out to the size of the gate pipe after welding if I made one, but you don't want it merging into the small dump diameter either. When I merge an external gate I make sure the pipe after it is similar (or greater) in area than the two merging pipes, then step it down gently later if required. Hardly any split dumps on the market do that, most can't even get a decent merge angle.

  • Like 2

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

    • That would be interested if it's the case as seems like it would make the job a lot messier but subaru sells OEM gaskets so seems it might of had it.
    • Fuel tank is out. Looks pretty bloody good under (ie above) the fuel tank.  Amazing how good the factory e-coat holds up in the hidden areas.  No where near as much body deadener to get rid of as the front but still not looking forward to it!  Shit of a job.  I'm thinking I might grab a needle scaler and give that a go before I attack it with the angle grinder and wire wheel which is how I did the front 2/3's of the car.  Feels good to be working on the car again but who knows how long it will last!  Hopefully I can get it further forward than where it was prior to this last block of work when I decided to go back 10 steps further down the rabbit hole from where i already was! The problem is, I'm realising how hard it will be to repair and paint the bumpers, which cannot be put back on the car, and have a good paint match. There are also areas where it would be VERY hard to hide the new paint where it transitions from old paint. Like this- There is only one way to do that perfectly and I really dont want to have to do what that means I would have to do... 🤦‍♂️
    • Pretty sure the factory assembly doesn't have any gasket at all, just RTV.
    • I will get a couple cans for sure thanks, I do have 2lt fish oil to spray around the car already.
    • Thought I’d post pics of the packages for those interested. The premium package is #16. Remote start, security with tilt sensor and remote and backup battery, auto fold mirrors and door illumination.    Can post up pics of all the factory options and images from the factory options brochure if people are interested. I have all the dealer books etc for all rv37 models. 
×
×
  • Create New...