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These don’t look like k20a2 OEM retainers to me.
Seems like you k24 uses something different.
Here is a recent picture from my head with Toda springs and all OE retainers.

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Height is dictated by the valve and not the spring.
Seat pressure will be higher with the Toda springs compared to stock, but likely lighter than Supertech double springs.

The K24 will extend its powerband with the JRSC even at very low boost. It‘ll climb to wherever you set the limiter. do you have any class restrictions?
Which Toda did you get?

My CT-E SC (also Eaton MP62) does not work well with the Toda A3 on my K20 build. It severely lacks torque independed of VTC angle and boost is low. It likely suffers blow-through. For next season, I drop in the k20a2 stock cams or go back to NA and the RBC intake manifold.
 

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Toda C on a K24 should make a very nice NA engine. Did you fit forged pistons and steel rods or are you running a stock bottom end? If the latter, don't overdo it with the rev limit.

a 3 PSI it won't generate a lot of heat, but the VE won't drop no mater how high you rev the engine. It is a very nice combo despite the low boost. It will easily get you over 250wHp. I am not so sure on Toda C plus supercharging. So overall sounds like a good idea to start off as NA first.
 

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Jeff Cooper in the UK got about 280 crank HP and 214 lbs ft out of his K24 NA with cams and RBC intake and 330bhp and 222lbs ft torque with the JRSC.
It just did not last long reving it to 8k with a stock bottom end.

Gains with the SC were mainly up to 5000 and post 7000 rpm.






Check out his videos. He build a dry sumped K20 caterham on ITBs and the like.
 

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average piston speed at 7800 is 25.74 m/s. That is F1-level on a cast piston.
For a rally application on cast pistons and rods, I'd stick to something like 7200 to 7300 . That corresponds to the piston speed of a K20 spinning about 8300 or 23.8 m/s.
That is near stock, but it will last. Make it 7500, but not higher on a stock bottom end.

Joe McCarthy once posted here 2013 about the K20a that is essentially using the same rod and piston technology.
"I'm basing that bit of advice on many years of experience of endurance racing stock bottom end K20's in the 25 hour at Thunderhill. If the rev limiter is set at 8,200-8,300 the engine will finish the race without exception. If its set at 8,600 one of the pistons will disintegrate by the 11-12 hour mark. At 9,200 I've seen stock pistons last under 10 laps. I can't even begin to count how many times I've seen people destroy perfectly good engines by doing this. I've also seen engines limited to 8,300 last through 2 complete seasons of endurance racing, which is about 100 hours total.
Its metal fatigue, which has 2 components, load and time. That's what determines the lifespan of any metallic part.
On the street the time span is SO short that a lot of people get away with over revving the engine for a long time, but its just a clock ticking off the seconds at that load level and eventually the piston WILL be destroyed."
 

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I just had a look at the Pracworks website after suspecting this being a resonance issue.
RRC vs Pracwork (blue)
Rectangle Slope Plot Font Parallel


They show on this EP3 (K20a2) the same 6500 rpm resonance as you. the next positive mode only starts getting active at 7500 rpm to about 8500 rpm.
You're stuck in the torque dip between 6500 and 7500.
On the other side, reving it only to 7000 will make your k20 last.
 

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From an acoustics standpoint very little as this is mainly geometry driven.
It makes a difference in cam duration requirements. A k24 needs a longer duration cam for same peak power revs than a k20. I would not be surprised if the Toda C in a k24 peak at about the same rpm as the Toda A3 in a k20. For the latter this is usually somewhere slightly north of 8000 rpm.
 
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