HKS GT Supercharger (Rotrex) in development!

[serialk11r]

Restrictor? WTF? Why not slow the compressor down?

[uspspro]

Originally Posted by madfast

what was the redline of your build? do you know what the max impeller speed was on your set up? here's what im thinking. stock 7400 rpm is kinda low. so if you build the engine to handle 8500 rpm, you would have to size the pulley to keep it under max impeller speed right? and how much of an impact would that have at lower rpm?

i believe you posted a dyno before, but do you have a boost graph of that run? what rpm was peak boost?

I did not use a restrictor, so peak boost was at redline. I had my redline set at 7500 RPM and the pulley size was chosen so it would hit the maximum recommended impeller speed right after redline.

Sorry, I don't have a plot of boost vs. rpm. At least not anymore!

Originally Posted by serialk11r

Restrictor? WTF? Why not slow the compressor down?

I know it sounds weird, but look it up on the HKS website. Without a restrictor, the max boost comes in at redline. If you are boosting 15+ psi, then that's fine. However if you wanted lower boost (say 8psi), and you chose a pulley that gave you 8psi at redline, you would not have much boost in the midrange. With a restrictor, you can spin the crap out of the SC, such that it would be close to the max impeller speed at redline, resulting in more midrange TQ , hitting max boost (restrictor limited) at a lower RPM.

Esentially the restrictor serves to pinch off air to the compressor. In a perfect world, you would want to vary the compressor speed, but you cannot. It is a fixed gear ratio (Crank pulley: SC pulley). There was a device invented by a guy name Antov, that was a 2-speed "gearbox" for the Rotrex compressor. However, it looks expensive, and make the SC quite large.

The ideal solution is employed on the Koenigsegg. The Koenigsegg uses a second throttle butterfly on the inlet of their SC compressors, that is controlled by the ECU which serves as a "variable restrictor." Essentially it is wide open at low rpm and closes off at higher RPM. Just a fancy version of the fixed opening restrictors, which allows for even better low-end torque (since the restrictor is not there at low revs).

Unrestricted will make the most power for sure, but the restrictors let you play around with the powerband. Just another tuning option!!!

Here is a picture of a Rotrex with a restrictor (this one is pretty small). I willing to bet that one is built into that inlet pipe that bolts on the compressor inlet on the HKS kit.



And yes. Holset makes the Rotrex compressor housings.

[serialk11r]

Wow, that's pretty crazy, but I guess it works.

CVT or electric driven supercharger is the ideal solution, hopefully those OEMs are close to bringing them to reality.

[Sully]

CVT or electric would own immensely.

[madfast]

Originally Posted by uspspro

I did not use a restrictor, so peak boost was at redline. I had my redline set at 7500 RPM and the pulley size was chosen so it would hit the maximum recommended impeller speed right after redline.

thanks for the info!

for you lazy folk here's some graphs courtesy of HKS
http://www.hks-power.co.jp/en/produc...ger/index.html

Pulley test:


restrictor test:


and last but not least, a CVT Rotrex is not far off:
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[serialk11r]

That chart makes no sense, why is boost dropping off at the high end, compressor surge? Or am I missing something obvious? Is it a restrictor?

Going off the chart 60kPa max boost sounds like it will get us the ~280hp we are looking for on this car.

And by the way, I dunno how that Torotrack supercharger is going to do since electric superchargers and mild hybrid systems ("start stop" systems that actually provide a small degree of regeneration count too) go hand in hand, and electric supercharger development is underway by OEMs already. Electric is technically better since CVT will realistically have a limited range of reduction ratios available while electric can operate on full blast at idle and give a really big kick of torque.

[uspspro]

Originally Posted by serialk11r

That chart makes no sense, why is boost dropping off at the high end, compressor surge? Or am I missing something obvious? Is it a restrictor?

Yeah the "pulley test chart" from HKS is weird. The part before 5000 rpms makes sense. That is not the response of a restrictor. The restrictor would have the boost flatten out (like in the other charts).

It could be the engine running out of flow.

Not sure, since there is no info provided about the rest of the setup.

[BLikeWater]

This could be the simple solution.

[phattyduck]

Originally Posted by serialk11r

That chart makes no sense, why is boost dropping off at the high end, compressor surge? Or am I missing something obvious? Is it a restrictor?

Going off the chart 60kPa max boost sounds like it will get us the ~280hp we are looking for on this car.

I have no way of knowing 100% for sure, but it looks like the pulley test was with the same restrictor for each test (note the max power is similar, but peaks at different times). The restrictor is then the other version - same pulley, but different restrictors giving different max power.

And technically, the restrictor doesn't limit boost - it limits max horsepower. When the engine can start to 'suck' more air at higher RPM compared to what the compressor is putting out, then boost pressure goes back down.

In any forced induction application, boost is the air that didn't make it into the engine - really, boost doesn't give you horsepower, airflow does.

-Charlie

[uspspro]

Originally Posted by phattyduck

I have no way of knowing 100% for sure, but it looks like the pulley test was with the same restrictor for each test (note the max power is similar, but peaks at different times). The restrictor is then the other version - same pulley, but different restrictors giving different max power.

And technically, the restrictor doesn't limit boost - it limits max horsepower. When the engine can start to 'suck' more air at higher RPM compared to what the compressor is putting out, then boost pressure goes back down.

In any forced induction application, boost is the air that didn't make it into the engine - really, boost doesn't give you horsepower, airflow does.

-Charlie

All of this is true, but the restrictor does effectively limit boost by limiting the source of air to the compressor.

The restrictor limits the compressor's source of intake air to compress. The airflow capabilities of everything after the compressor (IC piping, TB, intake manifold, head, extractors and exhaust) remain the same.

Imagine the compressor as a separate system. With a restrictor, the compressor effectively has an "airflow cap."

But theory and calculations are only so accurate anyway. The best way to figure out the ideal system is to try different options on the dyno while data-logging boost pressure, air temps, knock sensor feedback, HP, TQ, AFR, etc.

I am sure HKS is doing this (or has already done so) with their kit.

With a Rotrex at the core, and many tuning options, I know this kit will be awesome.

[dabocx]

Was hoping for around the 300-350 whp from a supercharger.

Hopefully there is more left in this system

[serialk11r]

Originally Posted by dabocx

Was hoping for around the 300-350 whp from a supercharger.

Hopefully there is more left in this system

Well it's something like 280-290 at the crank currently so 40-50% more than stock, and they did say it's probably not safe to go higher. Lower compression pistons would reduce the peak pressure and let you have a little more probably, but those would have to be carefully designed as the fuel spray is guided by the piston bowl. It looks like that thing is pushing about 0.7 bar boost, I'm guessing you could go a tiny bit higher with methanol injection or something to cool it off a little, but the limiting factor is probably the peak pressure and consequently force on the rods.

[uspspro]

Originally Posted by dabocx

Was hoping for around the 300-350 whp from a supercharger.

Hopefully there is more left in this system

IIRC, the posting mentioned that was the safe limit for stock fuel system and internals (or at least the latter).

That sounds good to me.

If you need more: Build the motor, no restrictor, small pulley, 15+ psi, and go crazy!

[Crashoverride]

I see good AFR's in our future, so FI seems like the next logical step