Compound turbos

Heaten m90

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I want to run a compound turbo setup. Im planning on having 2 wastgates that bypass the low pressure turbo around 10 PSI, then having an extremely large 120+MM take control from there. big turbos have billet wheels.
Question- will the small turbo create a bottle neck when trying to push 30-40 pounds through it?
 

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01yellerCobra

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I don't really know anything about these set ups, but bleeding boost off before the big turbo seems counterproductive. You have one turbo slowing down and one speeding up. Are you just doing it for the cool factor?

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Heaten m90

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The wastgates open up into the big turbo, no boost is bleed off, it simply allows some exaust gas to bypass the small turbo after it has reached its max efficiency. The goal is to have minimal turbo lag, low IAT and be able to support high hp at high RPM.
 

groundpounder

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If at 10 psi the wastegates open and direct the exhaust gases around the small turbo it won't know how much boost is being made by the big turbo. The big turbo can be making 100 lbs and the small turbo will just make boost on the gases that bleed passed the wastegates. When you drop under 10 psi you might have a decent lag as the smaller turbo will have to spin up from a really low RPM. Fluid Dynamics was never my strong suit. Why not find a VGT turbo from a Super Duty?
 

Heaten m90

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Variable geometry turbos, ill have to look into that, it never crossed my mind until you brought it up. John Deere has done good things with them.
 

Heaten m90

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Well the big turbo has its own wastgate as well, it controls the max boost. The 2 wastgates for the small turbo just prevent the exaust gas from needing to flow through the small turbo.
 

stkjock

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KISS

Why reinvent the wheel. Ask, how many set ups like U r suggesting as u seen work successfully.

Twins would likely be a better set up
 

Heaten m90

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Not many, but eliminating turbo lag makes it seem worth the effort. Just think of a turbo car that could load the turbo with 4.10's in 1st .....
 

NUTCASE

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Our larger displacement gas v8s don't need compound turbos.

compound turbos are needed on smaller displacement diesels. The thing diesels usually don't go over 3500rpm. Rarely 4k and even more rare 5k. So this leaves the manufacturer with some issues concerning gearing and customer expectations. when you need a small engine to spool hard at 1500rpm but still need the top end compound boost is the ticket.

compound boost is also the ticket when your trying to build a no-smoke diesel truck out of a 6.7 cummins or a powerstroke or something because you don't need to dump fuel to get the boost to come on.

I am guessing that 3500rpm or slightly later is ok rpms for you to get your boost. We already have singles that will do this and make lots of power. making huge power under 3500rpm on a gas motor can really stress things out anyway.

fun fact: audis newest compound turbo diesel v6 uses an electric motor to turn the small turbo.
 

NUTCASE

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RS5c140019-1.jpg


the electric turbo is on the bottom right in the pictures. I am pretty sure it also uses a check valve to stop pressure from sneaking around the back of the little turbo.
 

Heaten m90

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Im aware of the low RPM power concerns, although I've never understood the concept. Like sure the valve is open longer I get that, But isn't the force applied to the bottem end like 1000% less then it would be at the same power level at say 9K RPM.
 

702GT

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I tend to agree with stkjock. God that's painful to say. Anyways, I actually love the concept of compound turbos, ask just about any Supra owner and they'll jump right on the band wagon. In the world of drag racing, I think a compound setup is an absolute waste of money and effort on a gas powered modern Ford motor. If you were doing a diesel swap, I'd say get 'er done. So far, you've been talking about modular 3v max effort setups. But hey, it's your money and effort and we'll certainly tag along for the ride.

To your question, will the small turbo create a bottleneck. The answer is no. All of your piping post-low pressure turbo (the smaller turbo) has to be conducive to the high-pressure turbo (larger turbo). However, it's up to you to do the math on turbo sizing, CFM, and how much PSI is behind that CFM being pushed into the intake manifold. If you size your turbos poorly, you'll have a low-pressure turbo spinning its guts out and a high pressure turbo playing catch up, defeating the purpose of a compound turbo setup. You only need 1 WG for the low pressure turbo, which should be located between the combined exhaust source and the LP-Turbo. This WG will act as a bypass to the HP-Turbo. Once the smaller turbo is spooled to the desired PSI, that WG will go wide open and bypass the exhaust to the HP-turbo, and continue to freight train the larger turbo. That's all that 1 WG needs to do. Not sure where you think you need a 2nd WG on the LP-Turbo. But again, there shouldn't be a bottleneck unless you're beyond its efficiency range, and if your LP-WG has the correct spring pressure, the LP-Turbo should never spin beyond its max efficiency. If you size both turbos correctly, account for all the different pipe sizes, length of pipe routing, it should spool as designed. There are plenty of smaller turbos that can push 30-40 psi efficiently.
 

NUTCASE

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I just had a thought, if you really want a huge turbo and no lag it might be simpler and easier to setup and tune if you put a positive displacement supercharger on the car, then put a turbo kit on it. The PD will give you your spool and low end and the turbo will push you thought the top. You can leave a fairly large pulley on the blower because you are not leaning on it for all your power thus the typical PD heat soak will be less of an issue.

You can use the heat exchanger that comes with the blower simplifying your piping.

There was a guy with a 1400hp GT500 that ran twins through the blower. There are also some people in the cobalt SS/SC world who found the best thing to do with the blower was to leave it stock and just ram air through it with a turbo.
 

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Its not quite that simple, oem setups have a lot of engineering to get it right and the one turbo feeds the other on a compound setup btw. Hence its not common in the aftermarket especial on v8 motors.

In short it looks way cooler on paper than how practical it is to get working correctly with proper sizes and wheel designs on both units. Feeding the whole motor into a single properly sized turbine or splitting the motor in parallel in a traditional twin setup to reach the power goal is generally simpler and just as efficient on a v8 motor. Mix that with a quality billet compressor design and good bearing and that "larger" single that's sized properly will have damn good response always getting fed by the whole motor, comparative to any split bank/manifold compound or bi-sequential staggered setup where you have multiple turbines of different sizes sharing the load with bypass valves and check or flow valves on the coldside to prevent reversion.

30-40psi? What kind of power are you looking to make? Do you have a realistic goal for the car, be it a hp number or et/trap speed?
 

01yellerCobra

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If compound set ups were the way to go wouldn't racers be running them? I know not everyone is a fan, but look at Big Chief's set up. Twin turbos in a conventional set up and 2000+hp. As Dave said, keep it simple.

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Heaten m90

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Well I've been practicing bending tubes for the hot side. I was going to fab up and test the setup before making it out of stainless/ceramic coating and using high quality v-bands. Im not sure if it'll be a big single, twins, compound or sequential setup but. I do plan on no prep, standing mile, as well street racing and possibly even road course....the fabrication will be time consuming but not expensive for me, which is one of the main reasons i thought i might give it a try. I appreciate the input though guys, thanks
 

JeremyH

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Not many, but eliminating turbo lag makes it seem worth the effort. Just think of a turbo car that could load the turbo with 4.10's in 1st .....

We would be talking about a mostly unmeasurable difference with this kind of setup compared to a traditional one on these cars, couple hundred rpm tops if you get it just right. Also don't get hung up on the lag stuff you read about on the internet totally different thing than transient response. With a v8 in a 3500 plus lb car its practically non existent with a proper design. While moving with load, a stab of the throttle or quick gear change will instantly put you in boost, from there target boost comes shortly there after. The gear thing is a moot point when you look at what's happening yes peak boost comes in at a higher rpm on a graph as your final gearing puts you into a higher rpm quicker. Gear and tire height selection goes more with the goal for the setup.
 

Heaten m90

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I'll be gradually taking the motor up in power until it hits an unsurpassable ceiling. A big single would be great but my concerns were with not having enough cubes to spool a big 88. So i started looking down other avenues to meet my needs.
 

JeremyH

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I'll be gradually taking the motor up in power until it hits an unsurpassable ceiling. A big single would be great but my concerns were with not having enough cubes to spool a big 88. So i started looking down other avenues to meet my needs.

You will want a parallel twin setup sounds like, your ceiling isn't as high as your thinking. If you list the motor setup, fuel and compression, these guys can give you a realistic boost/power level you could achieve with it.

1300-1500hp from a 3v is an easy $50k into the setup. Engine, trans, power adder setup and fuel system for starters plus all the other stuff you don't think of to make it work. Just ask the $70k club guys.:insane:
 

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