yeah, you def loose leverage as you go down in size. The key is to find that sweet spot, as Jake pointed out. And who knows, that sweet spot may end up being 4" and all this was nothing more than a validation of what's already being done.

My new pulley showed up in the mail yesterday so I went ahead an put it on the car. Pretty nice looking piece... paid $200 and that included the necessarily shorter 8 ribbed belt.

http://i1282.photobucket.com/albums/...ps78d05e87.jpg

Removing the pulleys made things a lot more accessible IMO. Also, I didn't monkey around with trying to remove the old belt.. I was just throwing it away anyway so I just cut it off and pulled it out (after removing it from the tensioner so it didn't snap back and put out my eye!)

http://i1282.photobucket.com/albums/...psa925d92b.jpg

I think I'm going to hold off on pursuing an even smaller pulley until I get the electric PS pump and electric water pump installed. That's quite a bit of added electrical loading and I don't want to over tax the charging system. If everything is good to go, I might throw a custom cut 3 or 3.5 inch pulley on there just to see what happens. I plan on sticking with the 4" pulley for now though to determine the impact of the other modified systems.

Looks like your engine struck GOLD! Looks cool! :cool:

You could also use a larger pulley to selectively underdrive one particular accessory, without changing the others.

That is true. If u want my honest opinion, the value of these pulleys are that they under drive the water pump, the alt, and the PS pump. The AC is being under driven as well, but ur controlling that load with the panel on the dash in accordance with ur own preference. So if u drive it slower with an UD pulley but ur still too hot (in a convertible.. :rolleyes:), u'll just turn it up anyways so the AC is a none issue in my opinion. That just leaves the other 3 loads.

Its been proven that the other 3 loads are being over driven from the factory which is why ud pulleys work.

In my case, because I'm removing two of those loads and effectively giving their work to the alt, I may just find that it needs to be driven closer to the factory speed. I seriously doubt it but maybe.

The other potential benefit of the ud pulley is that they could be made lighter than the factory pulley and therefore give less rotational mass/inertia for the engine to have to overcome to rev. However, from what I've seen, all these UD pulleys are actually slightly heavier then stock. If I were to cut a custom pulley, that's one improvement I would def pursue - weight - even if it meant going to a different material such as titanium.

Btw, replaced my idler pulleys yesterday... used just off the shelf 70mm pulleys that are used on dozens of Audi and VW cars. Price=$20/pulley. Found out too that the bearing on these pulleys are just an off the shelf bearing and super common. If you still have the stock metal pulleys, you can press out the old bearings and press in a new one for less then $4.

http://i1282.photobucket.com/albums/...ps861c8c25.jpg

Replacing just the bearings wasn't an option for me since the lower pulley was bent/warped as it it had been dropped and the upper was effectively MIA. Haha

http://i1282.photobucket.com/albums/...psd339b5eb.jpg

I think when we talk about under drive pulley diameter limits we are considering what belt speed reduction the driven accessories can safely tolerate throughout the rpm range. You probably dont want to start running your ignition off your battery. It is also important to consider belt drive dynamics with a reduced diameter. Will the arrangement cause undue belt stress or slippage? Crank pulley size controls the belt speed for the whole system and the torque load of every accessory is reduced with a smaller crank pulley. The torque load on the engine is directly related to crank pulley radius.

A lower belt speed turns each accessory slower. One can control the speed (reduce) of each accessory individually by increasing its pulley dia. The torque load for that accessory will be reduced as well, but you will not realize the power savings that a smaller crank pulley gives unless one makes all of them bigger.

Recall that, originally the aim was to offset accessory speeds downward for constant elevated engine rpm at the track, particularly the power steering pump.

I have noted my ps pump groaning under full lock at parking speeds with the UDP....so here is another design limit cue

My guess is that that crank pulley mass is not significant when compared the massive flywheel but a quick calculation will tell you whether fancy spokes are worth the machining cost.

Look into the failure rates of BBI pulleys before doing machining on the crank pulley.

Yeah, the fancy spokes aren't worth the potential for sheering off said fancy spokes. It ur PS pump is already struggling with a 4" pulley then I'm would tend to agree that might be the limit on size. With PS being a huge load on engines (more then AC or alt), its probably the limiting factor here.

The funny thing about mass is the stock pulley is remarkably light. The under drive pulleys I looked at all were about the same weight despite being substantially smaller in diameter. That's what I was referring to when I said the diameter might already be optimized. Where the next little bit of gain might be realized is with a lighter pulley (titanium for example).

Well, until the electric PS pump gets installed anyway...

My MR2 electric PS with integrated reservoir.
http://i1282.photobucket.com/albums/...ps36dc02c5.jpg