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Gear-x make a brilliant close ratio gear set for the 6-speed that can take 600Nm. More than you want in an Elise.

Watch Marc Julians videos from canada. 395 WHp k20a2 turbo with that gear-x kit. Love it.
$2.5k though. Long 1st and then close ratio to 6th. Sounds like a liter bike motorcycle tranny.
His power was originally a bit higher, but he had a hard time maintaining charge temperatures.
He then lowered boost from some 14 to some 12 PSI and got much more consistent power.


if you overdo it powerwise, handling becomes challenging. K24 Rotrex C38. You need to know what you are doing on track as it get seriously fast. :cool:




Some prefer the torquey nature of the k24 while others like to rev their build k20 to 9k.
The k20 revs scream race car while the big lower reving k24 sounds like a meaty big inline 4.

For a track car, I’d go k20. if k24 and stock, don’t rev them higher than 7200 or 7300. Road users rev them higher, but on track, you’ll spends miles on end reving higher than 7k.

I have a currently SC build K20 in my Elise, but also helped converting a 2004 Toyota Exige to a JRSC k20.
Much better drive train than the SC 2ZZ.

The Rotrex kits will make a lot of power and provide for a thrilling experience.
if you like to add low down torque, go for the charge cooled big TVS kits (TVS 1320, 1900 and 2300 available) from Mercracing. This covers power from 350 wHp to well above 500.

Supercharging either a k20a2 or a k24a2 in complete stock form expect of a decent baffled sump or eventually removal of the balance shafts provide a path to well above 400 HP.
Just to rev them too high as sometimes suggested or done by the my stock k24 revs to 8500 crew (only on the highway for 10 seconds). The cast pistons will eventually give in.

if you build them with rods and pistons, ported oil pumps, you open the path to a durable 8k rev limit for the k24 or like 9k for the k20.
 

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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:

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"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.
 

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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.
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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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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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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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