LandSpeed-DSM
10+ Year Contributor
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- Dec 5, 2011
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Chicago,
Illinois
Your example is extreme.
Johns car makes 650whp through an auto (That's 750whp on a manual car) with the H1C at 18psi and 27psi on the HX52. What you're saying is that if the boost is turned up on the 52, it won't make more power because of mass flow?
If what you're saying is true, there there is no advantage to a compound setup in the power making aspect besides area under the curve.
This also means that someone needs to come up with "progressive wastegates" Where we can constantly vary the amount of boost through each turbo. Punch the small charger up, then fade it out when the big turbo comes on to make top end power. Essentially making a sequential turbo system.
OR that any compound setup needs a MUCH larger charger for the larger turbo.
I'm also willing to bet that we've all been doing it wrong and you don't need to recirculate the wastegates from the smaller charger, as now they're adding mass flow and limiting power by increasing exhaust backpressure.
Sorry for all the rambling.
I can't tell if there is some hyperbole in your position here because you are upset that I am disagreeing with you or not.. but lets go through this real quick:
This statement:
If what you're saying is true, there there is no advantage to a compound setup in the power making aspect besides area under the curve.
Is more or less true. From a perspective of complexity and efficiency. However it is sometimes the case where when match real well, some have demonstrated a small increase in total flow than otherwise be possible out of the LP stage on its own. The 16G and the 60-1 from Paul's old setup.. I think Ricky has it now. Not sure if that configuration was retained though.
There are more than a couple of reasons to run a compound. Area under the curve being a big one.. bringing up effective displacement to run a turbo that motor would never have been able to light in the first place (like a 96mm turbine on a 2.0, as cited earlier), using a smaller turbo to get on a converter that is tighter than would otherwise be possible in a class where nitrous is not allowed, or from an OE consideration.. help a diesel pass stringent emission standards by keeping airflow up at lower revs/between shifts when fuel is introduced.
On to your point on John's car - It's important to remember (in case you haven't) that the HX52 is a big mofo, I run one as well.. so with sufficient charge cooling, stable fuel and aggressive tune put 650awhp @ 27psi by its' self well within the range of what I would expect. John's seemingly efficient intake and exhaust routing combined with a nice big cam help a good bit in keeping boost that low, but its the massflow as a function of demand that is what makes the power, not necessarily a given boost pressure. There is one HX52 powered 5MT DSM that made ~770awhp @ 30-31psi FWIW
If you are logging boost and drive between stages, you will see the pressures and proportions are not static. This concept, as you put it:
Where we can constantly vary the amount of boost through each turbo. Punch the small charger up, then fade it out when the big turbo comes on to make top end power. Essentially making a sequential turbo system.
Is indeed a good idea, and what some folks already try to do. This is because the bypass from the HP turbine is hotter and faster moving. You are not introducing more massflow, it was already there. You would just need a lot of valve area for when the big charger comes on and needs to breathe through the gate, how much it needs to go through the gate depends on the size of the HP turbine, and how much air you are asking of the LP compressor.
Wastegate manipulation with electronic control certainly would be advantageous. Recirculating the HP gate for this purpose is a good idea, dumping the LP gate to atmosphere is too and for the same reason you would on a single turbo.
Dumping the HP gate would be a mistake.. you need this to provide the energy to sustain the LP turbine. You just need to size the LP turbine like you would for a comparably sized big single on a bigger motor.. the LP turbo after all doesn't know or care what it's attached to.
Find your corrected flow on the system concerning just the HP turbo and the motor, and plot that on to a a bigger turbo's compressor map. It's the same way you would choose a single for a bigger motor.
OR that any compound setup needs a MUCH larger charger for the larger turbo.
From my perspective, yes this is true if you are just going for a power goal and don't have a need for any of the uses mentioned earlier (emissions/converter/nitrous/etc) It's precisely what I am employing it for on my SOHC 1.5L Compound project. A significantly larger LP turbo than would otherwise be usable.
If you want to use something like a 50 trim @ 2.0PR as your small turbo to determine spool, and you plan to make in the neighborhood of ~1100whp you are going to have a real tough time because that is going to have you looking an ~88mm compressor and the equally gargantuan turbines those come attached too.
This goes back to the same example from earlier.. ~4:1PR on a 67mm as a single will do mid-upper 800s maybe 900 depending on fuel and other details.. but if you are running 2:1 on a 16G, and then 2:1 on a 75mm to make your total of 4:1 in the manifold, where does that put you on the LP turbo's map?
Just turning up the boost won't help at that point. Especially on a high rpm, high VE motor. Several real world examples back up the math. I'm not just being argumentative for my own entertainment.
This is where that idea of manipulating the wastegate scheme and PRs on each stage comes into play.
I don't mind the rambling at all, these staged turbo discussions are some of my favorites and inevitably they will occasionally bring up things all parties involved may not have have considered.