Anybody used an Underdrive Crank Pulley?

One of these:

 http://cgi.ebay.co.uk/MAZDA-MX5-94-97-UNDERDRIVE-ENGINE-CRANK-PULLEY_W0QQitemZ270302919457QQcmdZViewItemQQptZUK_CarsParts_Vehicles_CarParts_SM?hash=item270302919457&_trksid=p3286.c0.m14&_trkparms=72%3A1121|66%3A2|65%3A12|39%3A1|240%3A1318

 I’m assuming they are smaller diameter for your auxilliary items such as your altenator etc… therefore meaning the engine works less hard. 

I doubt it would add 8hp, but it may make things free’er. I’m doing the cam belt later this month, worth it? Or steer clear?

 Thanks

Phil.

I’ve not used one but I seem to remember discusion that due to them not having the torsional damper found on an OE pulley they are not recomended or give problems. Not sure which.

You’re right that the idea is to save power by turning the engine accessories slower, but the gains are probably tiny and it’s not completely risk-free.

The alternator is not going to provide any power gains at all - it supplies whatever amount of power the electrics require so if you spin it slower, it just increases its torque to compensate - no improvement there.

Water pump - yes you will save some power by pumping the coolant through the engine more slowly.  Don’t know how much, though.  The coolant leaving the head will inevitably be somewhat hotter too (the head, of course, will be hotter too).

Aircon - no power gain, because the MX-5 AC system is smart enough to cut out when the throttle’s fully open.

Power steering - I don’t know enough about how these pumps operate to say if any significant power is wasted when they’re not doing any work.

Risk factor:  Some years ago there was a group of owners in the US who suffered oil pump failure after fitting underdrive pulleys.  All of the affected cars were non-standard (all turbocharged IIRC) but the shattered pumps were blamed on crankshaft vibrations which ought to have been absorbed by the harmonic damper in the crank pulley.  The stock pulley is a “sandwich” of two steel parts with a rubber layer between the inner and outer, but the underdrive pulley was a solid part. 

In short, you’ll probably be OK with a normally aspirated car, but for minimal gains, personally I wouldn’t fit it.

Is it not only the 1.8 that has the damped pulley?

Not as far as I know.

 
 Confirms my thoughts exactly! The most concise answer I’ve ever had! Thanks guys!
 Will leave it, soon realised it was for the 1.8 anyways! But I’m sure this will be helpful to other new modding mx5 owners!
Cheers
Phil.