Lightweight underdrive pulley

[ssullivan]

I think you are on the right track. Weight farthest from the center stores the most energy so grooving any smooth pulley is probably a good idea. My new Al WP pulley is pretty thick but I am concerned if I make it thinner it could crack. If this is anything but the harmonic damper (sounds like you are talking about a regular pulley) I would not worry about any additional harmonics. Definitely would not add weight. Any balancing would happen with a grinder or hopefully more sophisticated machine,

Originally Posted by serialk11r

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.

[serialk11r]

Nope, I was referring to machining the OEM crank pulley for a smaller diameter. The groove angles need to be machined correctly for belt life. It shouldn't be overdone, I would go only 10% on the crank pulley (maybe less, if there's not enough material), because the alternator in particular is not supposed to be run that much slower.

Mass needs to be added back to the harmonic damper to make it work correctly. Usually, the pulley is the harmonic damper mass, but some crank pulleys have a separate harmonic damper, in which case life is easy and you can just cut away.

I would personally not mess with the harmonic damper's rotational inertia unless the crank has been modified. The OEM did their homework on crank harmonics.

[ssullivan]

Oh, got it. Yeah I agree, don't mess with the damper's weight (much). Thinking of the calculations: IF the pulley is part of the mass spring mass system that is a harmonic balancer then this precludes lightening the pulley for other purposes. That said cutting some small grooves would probably be close enough to still get most of the damping effects. The newer TDIs have dual mass flywheels. Mine was converted to SMF (I think it is ~4lb lighter) at some point. That combined with motor mount bushings leads to some vibes but I'm not worried/bothered.

My wife's 2015 Impreza inadvertantly got an overdrive AC compressor pulley: The AC clutch failed at 65k and they wanted $1400 to change the whole compressor and recharge the system. The '15 pulley/clutch was not available yet so I bought the one for the previous year. It fit the shaft but was smaller. AC is cold

[serialk11r]

Actually now that I think about it, it's possible changing the damper's rotational inertia by a little is not a big deal. The maximum crankshaft displacement would increase by however much you reduce the damper mass, but there has to be some safety factor built in, and no one is holding the engine right at the worst rpm for a long time.

The belt could possibly get chewed up faster, but serpentine belts are flexible and I want to say a little bit of vibration wouldn't kill them.

[California98Civic]

Originally Posted by serialk11r

Actually now that I think about it, it's possible changing the damper's rotational inertia by a little is not a big deal. The maximum crankshaft displacement would increase by however much you reduce the damper mass, but there has to be some safety factor built in, and no one is holding the engine right at the worst rpm for a long time.

The belt could possibly get chewed up faster, but serpentine belts are flexible and I want to say a little bit of vibration wouldn't kill them.

I agree. When I research aftermarket crank pulleys, or milling mine, to reduce rotational mass, I ran into these debates. But these little four cylinders and the RPMs we're runni g them at... not much risk. The OEMs, bless 'em, overbuild... just the way I like my Hondas... overbuilt by the OEMs. I did not add the lightweight pulley in the end (price and availability).

[serialk11r]

Well even with a cross plane V8, it's not like the crankshaft is going to fatigue and snap with a little less damping mass. Some light misfiring probably puts more stress on the crank than that. If you remove some of the mass from the middle of the pulley, that also reduces the inertia at the front of the crank and will increase how much energy is transferred to the damper.

Do people running aluminum aftermarket crank pulleys see increased belt wear? If they don't, then I feel pretty good about machining down a pulley to underdrive accessories.

[redpoint5]

I replaced 3 pulleys (including crank) with aluminum 20% underdriven ones somewhere around 2011 and went 100k miles with the same accessory belt and no issues.

[Nautilus]

Originally Posted by 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."

Indeed: if all rotating accessories (alternator pulley, crank pulley, power steering pulley, cam gear, flywheel) are replaced by lightweight versions, if the stock alternator is replaced by a small light 60A racing alternator, and if the car runs only in racing or in long, constant-rpm highway commutes, then MPG gains and engine responsiveness are visible and significant. In city driving, when the accessories fail, their repair or replacement is far more expensive than any monetary gain from fuel economy.

PS Manufacturers have been already moving towards intermittent-running accessories for the last 10-15 years. Electric-hydraulic or electric-motor power steering, variable-displacement A/C compressor, decoupler alternator pulleys. They sought the same kind of gains, not from making a part "lighter and easier to run", but not running it at all when it's not needed.

[serialk11r]

It's definitely more about weight than saving power. Usually there's several pounds of excess steel in all the pulleys that doesn't really serve much of a purpose; replacing with aluminum or FRP is an easy way to take a bunch of mass off the nose of the engine.

AFAIK aside from low efficiency alternators, the belt-driven water pump is the biggest parasitic load since they have no pressure relief valve and cavitate at high rpm. Unfortunately, belt routing can make it a challenge to electrify the water pump. If all your accessories are belt driven instead of electric, a slightly smaller diameter crank pulley is a clean way to reduce overall wasted power.

[Nautilus]

VAG experimented with electrically adjusted variable-displacement oil and water pumps, which they patented.

From the Audi patent of 2011:

"Smaller in terms of delivery rate, these volumetric-flow-controlled oil pumps operate only as required, and no longer need to circulate oil continuously.

These regulated pumps switch from the low to the high pressure stage either at a defined engine speed or in a continuously variable fashion, depending on the design. If the design includes spray jets for cooling the piston crown, they are activated at this point. Variable operation is possible with both of the common designs: gear pumps and vane pumps. A number of methods can be used to switch the pump between stages – an electrohydraulic mechanism, for example."



The logic of an electric water pump is that driving the pump at high rpm when not needed just wastes horsepower. If the pump could be made to run intermittently and controlled by ECU, it should run faster when the coolant is hot and not at all when it's cold. On a start-stop engine this is vital: electric water pumps keep the coolant circulating during stops, preventing the damage that comes with excess heat building up in engine block and head.

An electric oil pump on the other side should need to be far more powerful than a water pump, drawing a lot of amps to move viscous oil. So they leave the pump to be driven by the crankshaft and increase or decrease the displacement as needed via an electrohydraulic actuator.