To follow this up, I've been speaking with IABED in canada about his new throttle bodies and have learned a few things and come to some conclusions.
We know of two "failed clutches" that have led us to the conclusion that there is an issue. IABED run a 191mm pulley on their own CREC engined car with no issues.
Issam suggested that actually what we've seen is not clutch slip but belt slip. Crucially, logic would suggest that if the measured supercharger shaft speed falls outside a permissible error (calculated relative to engine speed which is also known) then the only possibility is clutch or belt slip. Since the CREC engine is the first to have a clutch to go wrong, it makes sense that there is a warning in the ecu to simply calculate if there's an issue.
I'm fairly certain that the only wiring to the clutch is to engage it. There's no second shaft speed downstream of the clutch, because there's already a timing wheel measuring engine RPM. It would make no sense to spend on hardware to check clutch engagement when there's enough info already being measured to infer it. In short, I'm convinced Issam from IABED is spot on, it is simply belt slip causing a false positive clutch slip warning.
MRC had two issues that flagged the same thing, but they reported clutch issues and replaced the clutch to find it was still happening. It turned out to be a faulty supercharger which required too much torque to turn and hence was dragging. This would manifest in either clutch slip or belt slip, mechanically I think the latter is more likely. After replacing with a supercharger that wasn't dragging no more issue. This was supported by the fact that MRC's stage 2+ set up uses a lower ratio than the APR one that caused the error.
In addition, dual pulley set ups mean a smaller super charger pulley has to transmit a greater amount of torque through a smaller circumference which increases the chances of belt slip.
On a separate note, I have heard rumours about the belt quality in the APR not being so great, which might give rise to slipping belts. No idea if these are true or not, I have no dealings with APR, but a few internet sources have suggested it to be a known issue. This might also explain why MRC just don't have concerns any more as perhaps they use a different quality of drive belt.
So I wonder if this means that actually, all APR high ratio dual pulley set ups are slipping to some degree but it is only measurable or causes a fault to register on CREC engine cars that happen to have the hardware to log the issue, even though it is incorrectly attributed to the wrong component.
Of course this is theory, but I am now much more confident that with a single larger drive pulley, a known high quality belt (gates) and a ratio closer to 3 than 3.2, everything should be fine.
I will be doing cooling via a forge set up, but I am also looking into the plug and play throttle body at the same time. Porting the supercharger looks like a decent option too. I think I'm going to get Litchfield to do all this for me in one go.
