Divided Down Pipes And Boost Creep

[RBrennan25]

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

[GTSBoy]

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.

[GTSBoy]

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.

[MrStabby]

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.

[scotty nm35]

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.