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It's not really violent actually. And if you wanted it to be slow and tame, you could use a tiny drill bit.
I've done this process before with sealed Bilstein shocks, which are very highly pressurized (over 200 psi or so). When I've drilled into those I've used something like a 2mm drill bit and it was fine.
I had expected the REV9 coilovers to have a method of external nitrogen release, but there wasn't, so I had to drill into the sealing piston (see previous picture). I've rebuilt BC Racing shocks in the past, and those have a septum style seal. It's just a self sealing silicone plug that can be pierced with a needle.
Since the REV9's don't have this, the best method is to drill through, tap, and screw in a schrader valve. I bought some cheap schraders off of Amazon that had a pipe thread end. I tapped it by hand, which isn't the best or easiest thing to do in the world, but it works. I used some liquid pipe thread sealant to make sure the nitrogen stays where it belongs. Here are some pictures:
The black curve is the NISMO front
Blue is the REV9 custom rebuild
Orange is the original REV9
The blue and the orange are at similar click settings. One of the goals of the re-valving is to have adjustment range both softer and stiffer than the NISMO damper curve. The blue curve is basically at its center adjustment, so there's scope to be softer and stiffer.
Some notes on the re-valving:
I reused the original REV9 piston, but replaced the valve disks to achieve that curve. This presents some challenges because the REV9 piston is purely linear, but the NISMO curve is digressive (especially in rebound).
I simply stiffened the compression valve stack to get a linear curve that is a good approximation of the NISMO compression curve throughout most of the curve range. So in the very low shaft speeds it's slightly stiffer than target, and in the higher speeds is slightly stiffer than target. Overall, not perfect but not terrible.
Rebound was more fun because I used a ring shim to preload the valve stack. This is a simple way to make a linear piston behave like a digressive piston. I can draw a diagram of how that works because it makes more sense visually.
The blow-off point (knee) was as close as I could accomplish with the parts I had. I was limited in preload resolution by shim thickness. However, I think landing slightly on the lower preload side is better anyway.
Overall, I'm pretty happy with that curve. The goal wasn't to match exactly with the NISMO, but just to get close with minimal effort and parts. The only parts I replaced were the deflective disks (valves).
I'm blown away at the massive difference you were able to achieve and how close you got to matching the Nismo curves.
Do you think you could still achieve a near 100% match with the correct shims/disks?
Cheers!
-Icer
The short answer is yes, I could get to say 98% identical to the NISMO dampers.
To get into specifics.... the shims themselves aren't really an issue. I can purchase basically any size/thickness I'd want. To get even closer I'd likely want to change the piston. I think the easiest path would be a piston that is linear on one side, and digressive on the other side. I'd use the digressive side of the piston for the rebound. On the linear side, I'd probably very slightly preload it with a ring shim. That would achieve the very slight digressive nature of the NISMO damper curve on the compression side.
One thing to note: The NISMO shocks are twin tube architecture with low gas pressure. Whereas the REV9s are monotubes. They'll never behave exactly the same because of how the internal pressures are being controlled. Generally speaking, the monotube style offers better pressure control, so there's less hysteresis (slop) when the damper changes direction. But that's getting way out in the weeds on damper theory.
For this project, I'd say this is a good starting point, since it's about an 85% match to the NISMO and still retains the basic bleed adjustability.
Really really interesting. Did you nail that on your first try just based on experience or was that a few rounds of swapping disks and shock dyno testing?
Really really interesting. Did you nail that on your first try just based on experience or was that a few rounds of swapping disks and shock dyno testing?
It was definitely a few rounds. The one I showed above is build #7. I did one or two after that, but liked 7 the most. I was my first time working with that piston/shock, so I suspect a new build would be quicker.
