Kraftwerks Rotrex Supercharger Discussion

[digital_assassin]

Originally Posted by TrogDor the Burninator

Curious where you think a blow off valve would go on this kit if indeed you went with the larger Rotrex ...

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There's already a BOV on this kit. It recircs though

[TrogDor the Burninator]

Jesus ... Where is my mind ... Ive only read that a million times ... Sorry!

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[digital_assassin]

Originally Posted by PRORICAN

This! I've been thinking about the same thing but you go first and give us a review! Lol



On another note, did you receive a belt cover/bracket with your kit? Mine is missing.



This one:

I have mine. Just give them a shout. They should have them. Not required for the install so hurry up already and install. Make sure to read my notes in the owners topic to make things easier.

[PRORICAN]

Originally Posted by digital_assassin

I have mine. Just give them a shout. They should have them. Not required for the install so hurry up already and install. Make sure to read my notes in the owners topic to make things easier.

Yup I just went thru your notes and I have two more issues so far. The rotrex is cocked the wrong way like yours was and I dont think an installation manual/instructions came with my kit. I went thru the rotrex manual and it said something about voiding the warranty if you open it, so fixing the issue will void the warranty if I do it myself? I emailed them about but I'm still waiting for a reply. Where did you find the instructions for fixing the rotrex?

[digital_assassin]

Originally Posted by PRORICAN

Yup I just went thru your notes and I have two more issues so far. The rotrex is cocked the wrong way like yours was and I dont think an installation manual/instructions came with my kit. I went thru the rotrex manual and it said something about voiding the warranty if you open it, so fixing the issue will void the warranty if I do it myself? I emailed them about but I'm still waiting for a reply. Where did you find the instructions for fixing the rotrex?

Link to download the Rotrex manual. Go to page 22 for instructions on clocking the housing.

[PRORICAN]

Thanks! Also did your kit come with installation instructions? Mine didn't unless I missed it.

[TrogDor the Burninator]

Originally Posted by OjiGeorge

When I spoke with KW it didn't sound like there would be any additional charge, as the C30 and C38 are sold for around the same price.

They said that more supporting mods would be needed (of course) to run the larger SC. I'm just wondering if we can put a C38 on there with a larger pulley to stay at a safe HP level for stock internals, and slowly decreasing pulley size as we build the motor over time.

I'm assuming that the response on the c38 will not be quite as instant as the c30 at lower power levels, and may not have as nice of a powerband~ this has yet to be seen though. Once the boost gets craked up though... Oh yeaaaa

I'm considering this as well. I'm going to be purchasing a kit soon and am highly interested in a scenario where I buy the larger Rotrex C38 and limit the overall boost to comply with the stock internals and clutch. My FR-S is currently a daily driver, but I'm looking to make it track dedicated by the end of the year for NASA TT. I'd hate to buy the smaller Rotrex C30 with it so close to the 100k RPM limit at ~300 HP if I'm looking to specialize my FR-S for the track. Knowing I could regulate the desired PSI via a larger pulley would make buying the C38 a no brainer for me ...

[Ralph Spoilsport]

Looking over the compressor maps, I'm puzzled as to why anyone is even messing with the C30-94. The C38-61 appears to do everything the C30-94 does, with greater peak efficiency, broader optimums and a fair bit more headroom.

The surge limit is hardly any different at all.

It's hard to tell from the maps, but the smaller blower might be better at reduced rpm (say half of whatever you choose for your redline condition).

[OjiGeorge]

Looking at those maps, I agree... C38 appears to be the way to go. Having that extra headroom down the road would be very nice indeed.

[tendogy]

Originally Posted by Ralph Spoilsport

the smaller blower might be better at reduced rpm

Indeed! Using a few pieces of information (the C-30 makes 10psi at 93k rpm) we can tell that our little FA20 works through about 0.37 kg/s of air at redline.

You could rig up a C-38 to move 0.37 kg/s of air for your 1.7 pressure ratio (i.e. make 10psi at 7400rpm) but it would only be spinning about 72k rpm. That seems great until you realize that you have no usable boost until 3000 rpm when the blower finally speeds up to its minimum 30k impeller speed. In fact, the car is slower than stock below 3000rpm due to parasitic loss and does not match the compression of the C-30 until 5000rpm.

You could rig up the C-38 to start making usable boost at 2000 rpm like the C-30 does, but then you would be over-spinning the impeller by the time you were at 5800 rpm.

You could rig up the C-38 to redline at the same time as your engine. It would still only make usable boost at 2400rpm but that's not too shabby. Then, when you were at 7400 rpm you would have 26psi blowing you up.

This is probably why Kraftwerks and Jackson have both stated multiple times that the C-38 is only going to be a viable choice for well-built engines.

[Ralph Spoilsport]

Originally Posted by tendogy

Indeed! Using a few pieces of information (the C-30 makes 10psi at 93k rpm) we can tell that our little FA20 works through about 0.37 kg/s of air at redline.

You could rig up a C-38 to move 0.37 kg/s of air for your 1.7 pressure ratio (i.e. make 10psi at 7400rpm) but it would only be spinning about 72k rpm. That seems great until you realize that you have no usable boost until 3000 rpm when the blower finally speeds up to its minimum 30k impeller speed. In fact, the car is slower than stock below 3000rpm due to parasitic loss and does not match the compression of the C-30 until 5000rpm.

You could rig up the C-38 to start making usable boost at 2000 rpm like the C-30 does, but then you would be over-spinning the impeller by the time you were at 5800 rpm.

You could rig up the C-38 to redline at the same time as your engine. It would still only make usable boost at 2400rpm but that's not too shabby. Then, when you were at 7400 rpm you would have 26psi blowing you up.

This is probably why Kraftwerks and Jackson have both stated multiple times that the C-38 is only going to be a viable choice for well-built engines.

Whoa there! We need some log data. I'm pretty sure no one is trying to push that much air through a rotrex into an FA20 (yet ). My rough rule of thumb from tuning awd turbo Scoobies is 1 WHP per g/s of airflow. The twins have rather lower drive train losses, so let's guesstimate (in the absence of dyno logs) 1.1. For 300 RWHP, that suggests airflow around 0.272 kg/sec, so I think the range of interest for street guys at peak power is going to be in the 0.25 - 0.3 kg/sec range.

From this and the known maximum impeller speeds on the current C30 kits, we can infer that the pressure ratio at the blower is closer to 2.3, due to some combination of intake, intercooler and plumbing losses (or, curse my curmudgeonly thoughts, optimism in the map).

Picking the max pressure / mass flow point on the C38 map at 2.4 / 300 g/s tells us we need 80k rpm at the impeller at redline. So at half of redline, you'll get (ballpark) 1.2 PR at the blower and something a little less than this at the manifold.

Doing the same "analysis" on the C30 shows me that we might expect as much as 1.3 PR at half of redline, but there's enough Kentucky windage on all this handwaving that I don't think we'll know for sure how a C38 system works in the real world until someone builds one. Probably, there's enough difference to show up on a dyno and probably there's enough difference to say "C30 is better for street", but it looks to me as though the difference might be small. I'm certainly interested to find out.

[tendogy]

Originally Posted by Ralph Spoilsport

we can infer that the pressure ratio at the blower is closer to 2.3, due to some combination of intake, intercooler and plumbing losses

You think the blower makes 19 psi of boost and we have a 48% pressure loss?

[Ralph Spoilsport]

Originally Posted by tendogy

You think the blower makes 19 psi of boost and we have a 48% pressure loss?

I hope not... because wasted PR is wasted input power.

Maybe someone will weigh in with some dyno logs.

[mrha]

I tried searching. Is this thing CARB legal? Thanks