I have a lightened aluminium fly on my ST170 it weighs 5.5kg the standard one is 11kg. I understand the benefits etc etc and the results have been great, i can hit 60 in under 7 seconds. I have just fitted my latest addition which are underdrive pulleys. The standard Water pump and Crank pully weight a combined 1.8kg, my alloy replacments combined weight is 600grams. Will this effectively give the same results as losing another kilo of the fly? The pulleys release a garenteed 5bhp due to underdriving the accessories but its the extra acceleration in low gears im interested in
I am yet to drive the car as it is going through a full restoration so will not be tearing up the streets for a couple of months yet

 

NO in short, this is due to the effect of the different size of the pulleys, ie they are much much smaller in diametre, if they were the same diametre as the flywheel then would be the same, but take so much less power to drive as the weight is a lot closer.
Think of being on a roundabout as a kid your mates spinning it as fast as they can to make you fly off, get in the centre no probs, try hanging on from the very outside edge much more effect the spin has on you.
tabetha

 

tbh you wont notice the difference in parasitic loss in terms of acceleration but you will in terms of engine response and engine braking that is all. We run alloy pulleys on all sorts like smaller diameter pulleys for ancilleries to offset higher revs, for example pas pump maybe ok to 7krpm but were often pulling well over 9k in race cars this caused lots of pump failures, reducing diameter solved this cheaply and lighter was just a bonus really.

 

I assume from his "guaranteed 5bhp" this was an american product!!
tabetha

 

Most of it is simply of no real conseqence