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Dump Pipe Advice.

Plattsy

By Plattsy,
in Engines & Forced Induction

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MrStabby Grand Master

MrStabby

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If you were to use it, the separator is an essential part. There is no point having a split dump unless the flows are actually split, and the only way to do that is to have the separator in there. This design is "fabricated" in that the separator is attached with fasteners. Others are welded.

The benefit of having the separator attached with fasteners, is that you can take it off so the dump is also compatible with housings where the whole turbine outlet runs all the way to the mating surface. More recent turbos are more likely to have the turbine outlet separated all the way to the mating surface, as its a more efficient shape for the gas flow.

But even if you were to want to use a split dump, you wouldn't buy one of these because the short ones are shite. The only ones worth considering are the ones that are a full dump and front pipe unit in one, and the split is dropped back in a long way down.

I think "shite" is really overstating things, i'd say "doesnt work as effectively". But IMO, should still work a little better than bellmouth.

GTSBoy Grand Master

GTSBoy

Posted
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The benefit of having the separator attached with fasteners, is that you can take it off so the dump is also compatible with housings where the whole turbine outlet runs all the way to the mating surface. More recent turbos are more likely to have the turbine outlet separated all the way to the mating surface, as its a more efficient shape for the gas flow.

Yeah, but you could grind off the welded in type flush with the flange anyway, so it's 50:50.

I think "shite" is really overstating things, i'd say "doesnt work as effectively". But IMO, should still work a little better than bellmouth.

I (being an aerodynamicist type engineer) prefer the concept of the split dump. But I think it is a waste of time to squirt it back in up high in the dump pipe just to make a dump that is easily compatible with the original engine pipe. A long split makes sense where a short one doesn't.

is it going to be better than the standard dump? and we are 100% saying that on the rb25det, that separator needs to stay in. (standard turbo.)

Standard RB turbo, yes, you need the separator.

MrStabby Grand Master

MrStabby

Posted
Posted

I (being an aerodynamicist type engineer) prefer the concept of the split dump. But I think it is a waste of time to squirt it back in up high in the dump pipe just to make a dump that is easily compatible with the original engine pipe. A long split makes sense where a short one doesn't.

So, from a theoretical point of view, would you say that splits should spool better (ie consider what happens when the gate is shut, so the interference of the merge is not a problem yet)? I used to know something about fluid dynamics, but that was a long time ago and i've forgotten it all....

GTSBoy Grand Master

GTSBoy

Posted
Posted

So, from a theoretical point of view, would you say that splits should spool better (ie consider what happens when the gate is shut, so the interference of the merge is not a problem yet)? I used to know something about fluid dynamics, but that was a long time ago and i've forgotten it all....

Absolutely. There was once a very well informed Garrett engineer who used to post on performanceforums and contributed an enormous amount of good data to our knowledge base. He said that Garrett's testing showed that a separate wastegate/turbine outlet arrangement always tested better than if the wastegate flow was allowed to spill in an uncontrolled fashion into the turbine exit flow.

But there were limits and caveats on that statement. The turbine exit dump should be a smoothly expanding cone from the diameter of the turbine exit opening up to whatever pipe size you are going to run as a dump. The ideal expansion angle was somewhere in the 7 - 11° range, I can't remember exactly. So if you have a 2" turbine hole and want to run a 3" pipe, you need a nice conical transition betweeen them. The trouble is, you can do that on an engine dyno easily enough, but in a real engine bay the pipe usually also has to bend through 90° very soon after it comes off the back of the turbo. Makes it hard to go for a smoothly increasing cone angle. Nevertheless, there have been others (such as CES) who showed that you can get good results by making sure you use all the length/room you can to put as good a cone direct onto the turbine outlet as possible before beding it downward.

So the above statement holds true for any turbine exit, regardless of whether the turbo is internal or external gated. It is certainly easier to organise on an external gated turbo, because they usually have a circular outlet. In fact, some of them even have the beginnings of the appropriate cone angle cast into the housing (those where the turbine in snugged a bit deeper back into the housing).

Where is gets difficult to make anything that lines up with the Garrett and CES findings is where your internal wastegate shares a big open space with the turbine outlet like the Nissan turbos usually do. In this case, the Garrett findings were that the nasty cross flow of wastegate gas flowing over the swirling flow comeing out of the turbine tended to increase the pressure in the dump. Obviously, for the best response and the best outright power potential, you want the pressure in the dump to be as low as possible. You want the turbine outlet flow to be able to get out and expand as smoothly and quickly as possible. Making it have to fight its way through wastegate flow is not good.

Granted, when you are coming onto boost the wastegate will be shut, but the wastegate will eventually start to open even before you have full boost, and of course once at full boost the wastegate has to be open. It does mess up the flow.

So the ideal split dump pipe would have a nice conical expansion from the turbine outlet to the turbine dump's diameter and would not re-enter the wastegate gas any closer to the turbine outlet than absolutely necessary. And it would re-enter it in the best manner to avoid interfering with the flow - so down at the bottom bend makes good sense. The shorty dumps that have to mate up with the original engine pipe simply don't have the room for any of that. The CES split dumps have been shown to work well. The cheap copies of same have different mileage, depending on how much effort they put into coning out the turbine exit and how nice the fabrication is, particularly at the point where the flows merge. My cheapy is OK, but probably nowhere near as good as the CES dump.

But, I can definitely say that the cheapy split I have made a huge change to the spool behaviour compared to the OEM dump. The OEM dump has a bigger volume than the turbine only part of my split dump, yet the split dump spooled much earlier, and made the boost setting increase by quite a bit (had to adjust it back down after fitting the dump, but can't remember how much by because it was so many years ago). I attribute this to the single circular cross section pipe used for the turbine dump on the split being a much better way for the gases to get out of the turbine than the larger more rectangular shape of the original dump.

I haven't back to back tested a split like mine against a big bellmouth, so I really cannot say that one is better than the other, but I do know that the long split dump is a hell of a lot better than the original dump.

MrStabby Grand Master

MrStabby

Posted
Posted

Absolutely.

<snip>

I haven't back to back tested a split like mine against a big bellmouth, so I really cannot say that one is better than the other, but I do know that the long split dump is a hell of a lot better than the original dump.

Thanks for taking the time to write that up :thumbsup:

The Dan Grand Master

The Dan

Posted
Posted

I'm pretty sure 90% of the people posting here don't know what the f**k they are talking about or why. They are just regurgitating what they have read somewhere. Split or bellmouth makes stuff all difference when you make tiny amounts of HP. The fact that it's 3inch makes it shit and means that you are definitely making less than 400hp at the wheels anyway.

Say you put on the bellmouth and make 50rpm better response? The next guy doesn't or he fits the split dump and say he buy s a cheap ass one from China and it doesn't fit correctly or he fits it at home and does an awesome job. There are just as many people saying the split dump is better as the people saying the bellmouth is better. They are the same - Comparable results aren't comparable.

wolverine Grand Master

wolverine

Posted
Posted

^^ yeah what Dan said.

I had a CES split on the Stagea and back to back with a bellmouth there was a bees dick in it but bellmouth made slightly more power everywhere with a GT-RS. A whisker more power everywhere hardly counts because taking a dump might have netted more performance.

The reason I would choose a bellmouth is the price difference and head off the risk of the wastegate fowling.

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