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Discussion Starter · #21 ·
the C38-series Rotrex units suffer from shaft brakages hitting the often hard rev limiters of Honda engines.
The newest C38R is also improved in this regard, but way to big for your power goal.
There are replacement shafts available with proper chamfers where the shaft diameter reduces towards the turbine wheel.
They put a step change of the shaft diameter in there to lead from the bigger center section of the shaft into the smaller threaded part for the wheel.
The torsional vibrations hitting a hard rev limiter shear them off. The bigger C38 turbines having a larger rotational intertia than the C30 wheels lead to fatique at this spot.

As of 2016 it seems Rotrex have addressed this themself:

View attachment 106107

"A major design change was introduced in 2016 as a direct response to customer feedback. The high-speed spindle shafts in our C30 and C38 range superchargers were redesigned and significantly strengthened to avoid spindle snapping under certain conditions."

"Customer feedback" as in pissed customers thrashing $2k superchargers and within the warranty period dealers having to replace lot's of them.

I do trackdays with a chap that killed 2 C38 of a F20c engined Lotus Seven before the shop downgraded to a C30-94 that then lasted.

So be aware of cheap older kraftwerks kit with C38 SCs. At least map them to not hit the limiter that hard.

The C30 units do not suffer from this. For a K20 or K24 you would want at least the biggest C30-94 unit. This will get you to over 400 crank HP.

The roots or screw blowers are very reliable albeit with a flatter torque curve than a Rotrex. Both will give you as much top end power as you dare. Power essentially climbs continuously to wherever you set the limiter.

Stock 2ZZ tend to brake valves if you rev them higher than 8000 for longer than a few moments. It is very common. With a stock 2ZZ stick to 8k max.

K20a2 are getting harder to come by. K24a3 seem to be plentiful.
That's awesome, thanks for that info. Seems like it'll be little or no concern at my power level, especially with a smaller c30. Does it make any sense to run a larger c38 and use a wastegate to blow off extra boost?

Concerning the TVS... My main reason for the rotrex was that it runs cooler. Considering the relatively lower power level that I'm aiming for... I'm guessing I should still be ok with heat on a TVS? Especially if I use their air to water intercooler inside the manifold. I'm typically only doing 20-25 minute sessions at most.

Fitment on the TVS seems simpler:

TVS:
Tire Vehicle Motor vehicle Automotive tire Automotive design



Rotrex, it looks like I'd have to relocate some bits... engine coolant, reservoir for air to water hex, oil catch can:
Motor vehicle Vehicle Automotive air manifold Car Automotive exterior


Current engine bay with previous TVS removed:
Motor vehicle Hood Automotive design Auto part Engineering
 

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Honestly, if I had an Elige, I'd run a NA setup. You could easily make more power than a S260, without the complexity of the boost components.
it'll be much more fun ripping it to 9000rpms.

And it sounds like fun is what you're after.
 

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if you look at the compressor maps of a TVS 1320 vs a Rotrex C30-94, there is not much in it.
Black Slope Line Font Parallel

Triangle Slope Rectangle Line Font


So with or without charge cooling, you will see similar temperatures at the same pressure ratios. For the C30, you'll spin it all the way up and will rund 2.2-2.4 PR at some 1 bar of boost.
looking at it, it seems the TVS 1900 might also work.

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1000 m3/h are 360 g/sec and these are good for about 430 HP..


If you build a K24 to spin 9k, it won't last more than a few runs down a drag strip, forged or not. Do a trackday doing 200 miles with 50% of the tome above 7k and at WOT, it won't last.
K20, different story.
 

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Discussion Starter · #24 ·
Honestly, if I had an Elige, I'd run a NA setup. You could easily make more power than a S260, without the complexity of the boost components.
it'll be much more fun ripping it to 9000rpms.

And it sounds like fun is what you're after.
Yea I've considered it and if I had to do it over again a na k motor would've been the right move at the time. But given the investment of some parts into what was a boosted 2zz setup... I'd like to reuse as much of it as possible. I've also gotten a taste of 325whp on that chassis and I'm not sure if I can go down on power without having regrets. Building na to near that level will get costly and blowing the motor will feel horrible.

Although I will say that the na 2zz was super reliable for me and never had issues. A na k motor is a very logical step. But I'm also a bit of an idiot.
 

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Discussion Starter · #25 ·
if you look at the compressor maps of a TVS 1320 vs a Rotrex C30-94, there is not much in it.
View attachment 106112
View attachment 106113

So with or without charge cooling, you will see similar temperatures at the same pressure ratios. For the C30, you'll spin it all the way up and will rund 2.2-2.4 PR at some 1 bar of boost.
looking at it, it seems the TVS 1900 might also work.

View attachment 106114

1000 m3/h are 360 g/sec and these are good for about 430 HP..


If you build a K24 to spin 9k, it won't last more than a few runs down a drag strip, forged or not. Do a trackday doing 200 miles with 50% of the tome above 7k and at WOT, it won't last.
K20, different story.
Interesting... So based on those graphs I'm not seeing much of a difference at all been say a c30-94 and a tvs1320. Unless I suppose you get a bigger intercooler with the rotrex.

Given what I'm hearing here from everyone. Seems like the k20a2 may be the better route for me? I spend a lot of time in the upper revs.
 

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k20 all day for track use... for reasons mentioned... but also for tire management. The added TQ from the 2.4 is pretty much across the whole powerband, boost it and its substantial over the 2.0. The 2.0 makes it easier to pedal the power under boost.

I personally wouldnt build an NA engine at that limit for road course. I would set engine compression at 10.5/11.0cr and do the rest with boost. It will be substantially cheaper per HP.

You could even build reliable power on OEM components at that point. K24a2 rods and pistons in a k24a1 bottom end. K24a1 head with 3 lobe cams of your choice and flipped pin VTEC on the OEM 3 lobe rocker assembly. I myself have a k20 variant of that build, and a friend of mine built a k24 version on the same basics. I used k20z3 cams and hit the typical 220whp, he used stg 4 Drag Cartel cams and a better header/exhaust and RRC manifold and peaked right at 290whp and almost 190wtq (both NA builds, on the OEM 11.0cr and rpm 8.5k for me and 9k for him)

He has since sold his build so i cant comment on his long term reliability (but the OEM piston and rod combo are proven reliable to 9k and 15-20psi per application) but i have over 10 years and 145k miles on my build. Hes a drag racer and mine is a workhorse, daily driver, track, Mexico street racing (years ago), and the only failure ive had was my 2nd gear just recently (02 CTR trans w/LSD)... and it was really due a rebuild considering all the abuse it received.

I prefer a roots blower for SC and track use. guaranteed power at any given RPM. Its the heatsoak that hurts...
 

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Discussion Starter · #27 ·
k20 all day for track use... for reasons mentioned... but also for tire management. The added TQ from the 2.4 is pretty much across the whole powerband, boost it and its substantial over the 2.0. The 2.0 makes it easier to pedal the power under boost.

I personally wouldnt build an NA engine at that limit for road course. I would set engine compression at 10.5/11.0cr and do the rest with boost. It will be substantially cheaper per HP.

You could even build reliable power on OEM components at that point. K24a2 rods and pistons in a k24a1 bottom end. K24a1 head with 3 lobe cams of your choice and flipped pin VTEC on the OEM 3 lobe rocker assembly. I myself have a k20 variant of that build, and a friend of mine built a k24 version on the same basics. I used k20z3 cams and hit the typical 220whp, he used stg 4 Drag Cartel cams and a better header/exhaust and RRC manifold and peaked right at 290whp and almost 190wtq (both NA builds, on the OEM 11.0cr and rpm 8.5k for me and 9k for him)

He has since sold his build so i cant comment on his long term reliability (but the OEM piston and rod combo are proven reliable to 9k and 15-20psi per application) but i have over 10 years and 145k miles on my build. Hes a drag racer and mine is a workhorse, daily driver, track, Mexico street racing (years ago), and the only failure ive had was my 2nd gear just recently (02 CTR trans w/LSD)... and it was really due a rebuild considering all the abuse it received.

I prefer a roots blower for SC and track use. guaranteed power at any given RPM. Its the heatsoak that hurts...
Thanks for the post.

In my mind, the k20 can work nicely with a roots and a rotrex with the k24. Because the blower in each case compensates for where the base motor might be weaker at. I don't know if that's actually the case in real life. I'm still debating which engine-blower combo works best.

And yes, I want to keep the block as stock as possible. The tvs kit has extremely tight to the firewall in the stock body lotus elise. The rotrex will fight easily, but I need to move around a bunch of stuff to make it work.
 

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The biggest disadvantage with roots and k24 is losing about 2psi versus the same setup on a k20 (same engine speed). Gotta fill up those cylinders, and cant spin much faster to make a difference.

My fave roots setup was 12psi on a stock JDM k20a. For a street car it was excessive with the JDM CTR trans. 6th to 3rd downshifts at 45mph and WOT was so much fun.
 

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Discussion Starter · #29 ·
The biggest disadvantage with roots and k24 is losing about 2psi versus the same setup on a k20 (same engine speed). Gotta fill up those cylinders, and cant spin much faster to make a difference.

My fave roots setup was 12psi on a stock JDM k20a. For a street car it was excessive with the JDM CTR trans. 6th to 3rd downshifts at 45mph and WOT was so much fun.
Yeah some of my concern is certainly the heatsoak on to the tvs. Everything is just harder to cool when it's mounted at the rear of the car. My heat exchanger is up front. And I'm in a generally cooler climate....so I might be ok for 20-25 minute sessions. Also the fitment of the tvs is super tight.

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boost levels themselves are largely irrelevant when it comes to positiv displacement blowers. given everything is the same, a K24 will see about 20% less boost than a k20, but the airflow is largely the same and therefore power. If you take a run of the mill JRSC and strap ii on a k20a2 and on a k24, you'll get about the same 300HP from them, albeit with lower boost on the k24. In practise you see some differences in power delivery and how they feel while driving with the k24 being more torquey at low rpm. The k24 power band is essentially compressed.
 

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Discussion Starter · #31 ·
boost levels themselves are largely irrelevant when it comes to positiv displacement blowers. given everything is the same, a K24 will see about 20% less boost than a k20, but the airflow is largely the same and therefore power. If you take a run of the mill JRSC and strap ii on a k20a2 and on a k24, you'll get about the same 300HP from them, albeit with lower boost on the k24. In practise you see some differences in power delivery and how they feel while driving with the k24 being more torquey at low rpm. The k24 power band is essentially compressed.
Question... So if that's the case (needing less boost on a k24 to make the same power). Is it safe to say that the air temperature will be lower on the k24? Or is it the same, because it's not about the psi specifically?
 

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on first order yes, lower boost yields lower temperatures.
As second order effect you need to also look at the compressor efficiencies. If the pressure ratio is very low, compressor efficiency drops. What then happens is that the temperature "advantage" is not as high as the ideal gas low suggests.
 
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