I suppose 20k May be overkill for an 80 that lives in northern areas and rarely, if ever, leaves it’s home area. As I stated before, I’ve had two black hub clutches open and the oil shearing area is at least double the area, if not more, than that of the blue hub based on photos I’ve seen of the open blue hub clutch. Anecdotally, then, 5k oil should be sufficient for a black hub clutch in hot climates as far as Toyota is concerned. In accordance with info posted by the experienced folks down in Arizona I filled my black hub clutch with 10k and she would pull some big air when hot keeping coolant temp under control. Start up plus two minutes is about how long it took to disengage after sitting over night.
Gas mileage? What’s that? I think mpg would be more adversely affected by modding the open up temp of the clutch thermostat leavening it engaged much more of the time. The way I see it, leave the timing as factory set, run an oil weight that will affect a strong clutch engagement to drop the radiator temp quickly so the clutch disengages as soon as possible.
My assumption is that the fan clutch comes timed to work in sync with the thermostat anyway.
So, as for timing, I'm a little rusty on this sort of thing, and I certainly don't remember the correct terms, but I don't think the fluid dynamic shear effect we're talking about is linear... I know it's a curve with regard to speed, and I think with heat as well, so the valve timing is acting on a different part of the curve with a different viscosity anyway. Also, viscous clutches don't ever FULLY disengage and they don't ever really FULLY engage. As far as startup goes, honestly, it was every bit of two minutes (probably more) for the blue to stop pushing
hard with 15k.
Another thing to consider, I'll bet there was a real "factory" spec for settings when these rigs were new, but I'm pretty sure they're just slapping them together roughly... *here* (waves hand toward a general area) these days. I assume the adjustment was there in the first place to allow for variance in the bimetal spring to match Toyota's opening spec, but I doubt they bother if you just buy a regular Aisin now. I've heard of a lot of variance in the "stock" setting mentioned in several of the threads. It seems to be all over the place.
As for mileage, when you're only getting 14, losing another 1 mpg hurts! I tried a few different timing settings on the 15k and my experience was that it really didn't matter where it was set, it was pushing a lot of air even when it was as fully disengaged as it gets... to the point where my OBDII gauge was showing the difference in temps, even with higher opening settings that I tried.
With the blue vs black comparison, I have no idea how that breaks down. Honestly, aside from how MINE behaves in MY RIG, it's all glorified speculation. I suppose we could build a fan clutch dyno... anyone want to slap some load cells onto one and fire up some data-logging software?
Anyway, I'll go back up if I ever feel the need, but I'm waiting on a few of those 100* plus days that we sometimes get to drive it hard with full AC. I've had it in and out like a dozen times in the last few months, just playing around with it. I'm not afraid to try again, but as noted previously, Toyota thought 10k was good enough for the middle east and Australia. I'm not too worried about it. Just sharing my experience.
