Lightweight underdrive pulley

[Nautilus]

Been there, done that, ended up poorly

There was some formula floating around the net, showing how to calculate joules/second needed to spin a disc of given weight from zero to 6000rpm. It fell somewhere around 616 J/lbs. Then it could be converted from J/s to hp.

Example: by changing a standard-driven pulley from 2.11 lbs steel to 0.66lbs aluminum, gain is only a miserable 0.84 hp at 6000 rpm.

Fluidamprs are about 2 lbs heavier than stock rubber-steel pulley, yet they recover a couple of hp as proven on the dyno, due to dampening effect. It takes energy to vibrate a 30 lbs forged steel crank, and it has nowhere to come from, except from engine itself.

Tried a lightweight, non-clutched pulley for the alternator. Stock pulley is one-way clutched and almost 2 lbs heavy. The purpose was to compensate for the weight of the Fluidampr, which is about 2 lbs above a stock harmonic crank pulley.

There was a slight (almost too slight to feel) increase in engine responsiveness and nothing more. But after 1 year and half, accessories on the serpentine belt began to make grinding noises at idle rpm and alternator voltage went down. There was no noise or vibration above 1500 rpm.

Removed the serpentine belt and checked: alternator slight grinding (bearings worn away), belt tensioner also slight grinding. Replaced both. Also went back to OEM clutched pulley.

In racing, where the engine runs for short time and mostly above 6000 rpm, the slightly better engine responsiveness may help.

In city driving, it will, sooner or later, wear down accessories (alternator, A/C compressor, power steering pump) because solid metal pulley does not allow any compensation for any fault - the smallest wobble in a bearing is amplified by belt tension until it gnaws away at the bearing or the tensioner. Or both.

[redpoint5]

Originally Posted by Nautilus

In city driving, it will, sooner or later, wear down accessories (alternator, A/C compressor, power steering pump) because solid metal pulley does not allow any compensation for any fault - the smallest wobble in a bearing is amplified by belt tension until it gnaws away at the bearing or the tensioner. Or both.

Curious what would cause an aftermarket part to be more out of tolerance than an OEM pulley? I realize they could be cheaply made, but I don't know why an aftermarket pulley would be inherently less precise than OEM.

[Nautilus]

They are made from aluminium, therefore softer than a solid steel pulley.

Hydraulic / spring belt tensioners from modern days keep the belt tight like a guitar wire, it vibrates when you knock it with a finger. There is zero compensation for the slightest lack of balance in a pulley, the belt pulls so tightly that even a hundredth of an inch of wobble in a pulley is transmitted away. This is why clutched pulleys were introduced in the first place: allow the alternator to free-wheel at the higher rpm and absorb crankshaft vibrations before they gnaw through alternator bearings.

Back in the days of manual belt tensioners with bolts and brackets, there was some slack in the belt, it gave back half-inch when pushed by finger. This compensated the vibration and rapidly-changing engine rpm, one could see the belt oscillating up-down when running at idle.

Racing 1.8T engines may run with fixed light-alloy set of pulleys, no crankshaft damper, even with manual tensioner in the timing belt, but they run for short time, at high constant rpm and never in city driving.

[ssullivan]

Thanks for the story Nautilus. Lots of good points. I do want to debate you on the fact that these parts are useless for mpg. Their impact may be almost negligible on the dyno but over 100k of gas/brake gas/brake, the cumulative affect of: simple weight reduction, rotating inertia reduction, reduced pumping load would add up. Drag racers have a saying "Start saving ounces and you will save pounds."

As far as underdriving the alternator, avoid it: The shaft load is proportional to the electrical load. As Nautilus found, spinning them slower (at same load) will cause the bearings or electrical parts to die. Thanks for the knowledge.

Originally Posted by Nautilus

There was a slight (almost too slight to feel) increase in engine responsiveness and nothing more. But after 1 year and half, accessories on the serpentine belt began to make grinding noises at idle rpm and alternator voltage went down. There was no noise or vibration above 1500 rpm.

Removed the serpentine belt and checked: alternator slight grinding (bearings worn away), belt tensioner also slight grinding. Replaced both. Also went back to OEM clutched pulley.

In racing, where the engine runs for short time and mostly above 6000 rpm, the slightly better engine responsiveness may help.

In city driving, it will, sooner or later, wear down accessories (alternator, A/C compressor, power steering pump) because solid metal pulley does not allow any compensation for any fault - the smallest wobble in a bearing is amplified by belt tension until it gnaws away at the bearing or the tensioner. Or both.

[redpoint5]

I'm about 100k miles in on a UR underdriven pulley set on my TSX. Can't say I noticed any differences since I made the mod fairly early on in my ownership of the car. One worrying thing to me is that the original crank pulley had a damper on it, and the UR one did not.

[ssullivan]

Does your crank pulley have a dampener built in? I am only doing WP on my truck, also got the Al water pump for net 7lb. Mainly because I am in there to do the timing chain and really wanted to throttle the pump: high flow and high pressure :/

[serialk11r]

Originally Posted by redpoint5

I'm about 100k miles in on a UR underdriven pulley set on my TSX. Can't say I noticed any differences since I made the mod fairly early on in my ownership of the car. One worrying thing to me is that the original crank pulley had a damper on it, and the UR one did not.

I am not sure how much machine shop time costs, but an idea I had was machining stock pulley grooves to reduce the diameter and then adding mass to the pulley to restore its original harmonic damping.

You can also lightweight it at the same time by machining out the center, which usually has a lot of excess steel.

[Mustang Dave]

I installed a (damped) underdrive crankshaft pully on my Mustang in May, 2015. As stated years ago, it Gave me a 1 MPG increase.https://ecomodder.com/forum/em-fuel-...vehicleid=2691
I won't be undoing this mod.
YMMV

[redpoint5]

I had to jump on the end of a 3 foot pipe attached to a wrench, and block the tires on the car to break the crank pulley bolt, and when it went, it sounded like a gunshot. I won't be doing that again either.

[ssullivan]

If I was only going to do one pulley it would be water pump. Make sure the cooling system is good obviously but it is very overdesigned by OEM. I imagine the PS load is more determined by the back pressure regulating valve in the line, meaning under driving it does not help. Pumps at constant rpm and varying pressure, or pumps at reduced rpm and reduced pressure/flow have well established relationships for calculating power requirements