A new thought for twins - Compound Turbos? Maybe?

[vex]

Originally Posted by dudemaaan

Oh I didn't know you were suggesting the turbine outlet of the SmallT would be dumping into the the LargeT. That would work fine then, the velocity would still be high enough to spool the LargeT. You do realize the compound turbos have wastgates too right? They are plumbed in the same manor you're talking about. Sounds like there is really no difference in operation for the exhaust side of what you're suggesting. And in both cases you still run into the problem of two exhaust housings/wheels in the stream of the exhaust which will increase back pressure/EGT, especially if the first is a small housing to help spool.

I'm sure you know this already, but exhaust gas velocity plays almost no part in turbine spooling. It's temperature and pressure differentials that play the biggest role in it. What I was suggesting was that the small turbo not take up 100% of the exhaust gas flow from the manifold, but that the manifold feeds both the small and the large--which is different than all the compound turbo setups I've been shown thus far. The manifold would need a "wastegate" or a gas diverter to ensure that the large turbo would not "steal" all the temperature/pressure of the exhaust, but have just enough to spool when the small is reaching its efficiency limit.

The only difference I see now, is the cold side. In a compound turbo the LargeT cold side feeds into the inlet (where the air filter goes) of the SmallT. This is where the compounding takes place. The boost is "compounded" or multiplied. While in a sequential system, there is usually a butterfly valve to keep the turbo that's "working", flowing only into the manifold, then the butterfly opens and allows the second turbo to contribute to the total volume of air, but it in itself would not increase boost pressure. The difference is much like the following diagram of 2 pumps in series vs parallel. Series would be compounding, and parallel would be sequential.

I'm aware, but the exhaust flow in the compound setup is different from what I'm suggesting. Couple it with the fact that a small turbo has a very limited efficiency range you really become limited in the turbos one will be able to run. Couple this with a rather large increase in AIT and I see a bad time coming. Even if you held the AIT's low by some super intercooler process the pressure levels one would tune for would not be worth headache the system offers.

EDIT: Just saw your direct comparisons, care to elaborate on those plots?

[TitaniumTT]

I have two routes that I'm going to be taking with the FD. If one doesn't work because of mechanical/temperature limitations, I'm still going to try this out. The way that I see it working best is basically 3 wastegates. The Smaller Turbo being internally gated as well as a rather large WG right at the merge coupler. The Larger Turbo being internally gated, but with a LARGE internal gate. The exhaust shall merge at a collector prior to the ST. The ST being internally gated to control boost to a preset level. Once the ST's internal gate is 100% duty cycle, the gate on the collector will begin to open to control boost yet again. This will happen rather low in the RPM range, I'd shoot for ~3k. After the other gate begins to open and the LT is spooling, it becomes a juggling act as to when the LT becomes "active" I think y-valve or a exhuast cut-out would need to be used. This is more of a sequential system though.

So, Exhaust flows from the block to the ST. From there, the DP if you will of the ST feeds the turbine of the LT. The external gate will alse feed the LT. The LT is internally gated and has ultimate control over the boost so it needs to be large.

I believe, after doing a bunch of research that the compound will generate too much boost. As a multiplier, I think it's might be a little much. Although, there's still a ton of research to do on the subject and it is something that I still believe has merrit. I think there are simpler ways to get the torque curve that I want. If anyone says V8 I will kill them

A super intercooler system isn't something that is too hard to accomplish. My setup works retardedly well and the core is somewhat small.

Aux injection is something else that can be employed although I'd rather not. We'll see in a few months when I really start humping on the FD.

[dudemaaan]

I spoke to a guy running a compound turbo setup on a eagle talon running 8.97 at 156 mph. He said he did it because the turbo he was running, a 1.32 T6 s475 wouldn't spool until 7000 rpms on his car, so he was using nitrous to help spool it. Then he added a t3 50 trim in a compound setup and it sees full boost around 4000 rpms.

This is what I found out from him, peak power is pretty much the same at the same boost levels. In his case he runs about 40 psi, which is medium boost for his engine. You do not have to run extremely high boost levels in a compound setup.

You choose the large turbo for whatever max power you want, and you choose the smaller turbo for whatever spool you want. Obviously they will have to be somewhat matched or you won't have the large turbo spooled before the little one runs out of breath.

His intake temps are the same, though he never measured pre-intercooler temps which would be more accurate. He said his drive pressure to manifold pressure is really good being 1:1, "Which is not going to happen on a single turbo that will actually spool on the car" (his words)

The advantage of the setup is much faster spool with a high top end, disadvantage is complexity and cost. He's running 2 wastegates on the first turbo to keep it from boost creeping, the second turbo also has a wastgate that he uses to adjust the boost level.

It sounds like a really good way to go, and the concern of needing to run high boost is not the case. The pressure ratios are divided between the 2 turbos. Almost like it's one turbo with a very broad map. Exhaust restriction and high AFR's are also not the case.

[neit_jnf]

His intake temps are the same, though he never measured pre-intercooler temps which would be more accurate. He said his drive pressure to manifold pressure is really good being 1:1, "Which is not going to happen on a single turbo that will actually spool on the car" (his words)

As I mentioned, they have higher boost than backpressure on compound setups for diesels! If I had the $$$ I would do it with the 13B!

[TitaniumTT]

That could mean 40lbs of boost and 39 psi though. EMAP on a rotary needs to be kept as low as possible. Anything over a bar and the EGT's get a little high, or so I've found.

I would like to know what Dudemann's EMAP is.

I would also like to know what the EMAP is of a bunch of other singles

[dudemaaan]

The stock turbos would definitely have a high emap considering how restrictive the manifold is. It would be difficult to have a fast spooling turbo with a low emap, since restriction is what really helps get the turbo moving. One day I'll check mine. I have a feeling the manifold back pressure won't be too bad in the compound setup since so much gets diverted around the first turbo once it is spooled and the wastegates open. The only issue is it will require multiple gates or a very large gate to divert say 600 hp worth of exhaust.

[TitaniumTT]

The stock twins EMAP isn't nearly as high as everyone thinks. It's internet lore. I stay within a 2:1 ratio. I've posted the datalogs before, but they're not as bad as everyone thinks.

The thing with the rotaries is that .... and this is MY theory.... that the EMAP needs to be kept as low as possible for a variety of reasons. I think with large enough gates and proper turbo's it can be done.

I would love to know what some of these mani's are making in terms of EMAP. The problem with the stock mani is the internal maze. Make something tubular and I bet it would go way down. Then you're outflowing the turbo's though (Roen) so the stock hitachi's won't work.

Meh, it all depends what the manufacturer says when I call them in a few months. I'm getting the itch to work on the FD. I'm getting the itch to work on the FD when I have two FC's to completely assemble and one to paint in the next 5 weeks from yesterday

[Roen]

lol, I got called out without even being present in this thread.

It's a tough mission finding something to provide greater power than 450 whp while maintaining Mazda's sequential system.

Fix your damn sig, Brian!