Lightweight underdrive pulley

[California98Civic]

I just bought a lightweight VMS Racing crank pulley for my Honda Civic engine. OEM diameter. Moderately lighter (weighing 2.6 lbs instead of something like 4 lbs or maybe a bit more). I already deleted both the AC and the PS and put the alt on an on/off switch. So, not much to under drive!

Even though 2 lbs reduction on a sock 6 or 8 inch diameter pulley size is maybe less than 1 HP up to 2500 RPM, I drive a 106 HP car. The relative effect is greater. Also, I drive mostly city, and even on the freeway I do a lot of pulse and glide driving, so I run the engine up to 2000 or 2500 far more often than most people. In the city, at a light, I store a bunch of energy in that rotating mass quite quickly in first gear and then maintain it through the subsequent shifts. Ultimately, in the city or the freeway, I put the clutch in to coast and waste all that energy. It is a small amount, but it is wasted over and over and over.

Next will be the flywheel, whenever I get into a transmission swap or service again. Somewhat bigger gains to be had there.

Here is a great discussion and calculator:
https://www.w8ji.com/rotating_mass_acceleration.htm

[serialk11r]

The diameter of the pulley is small enough that the reduced mass shouldn't be a big factor in terms of how much energy is wasted accelerating it. IIRC there is actually an effect on main bearing #1 friction from a heavier pulley, but again 2lbs is not much force. Lighter flywheels have a much bigger effect since they have twice the diameter.

Still, I am a fan of lighter pulleys, because in terms of lbs saved/dollar they are usually a good deal. Might as well do a little underdrive while you're at it.

One thing I've thought a little about is a water pump relief valve that feeds water straight from the outlet back to the inlet when the pressure exceeds some level to delay cavitation, but it's probably not worth the added complexity.

[California98Civic]

Originally Posted by serialk11r

... but it's probably not worth the added complexity.

There is a point of diminishing returns for sure, where multiple changes add complexity and reduce reliability and ease of maintenance. I could have gone with some racing pulleys that are even lighter. There used to be a 1.5 lbs crank pulley (~3 lbs lighter). And there are underdrive pulleys that weigh less than 0.5 lb that would give even greater effect because the little weight they have is also closer to the pivot point, but I would lose my alternator belt and would be pushing the tolerances of my engine. I have a DD, not a race car (lol).

Definitely going to start looking for a lightweight flywheel, though.

[Ecky]

How heavy is the one you have?

I put a ~10lbs flywheel on, OEM was ~20lbs. I was surprised how easy it was to stall my car with a 2.4L engine, compared with the stock 1.0 with more rotating mass. It revs like a sport bike though.

[California98Civic]

Originally Posted by Ecky

How heavy is the one you have?

I put a ~10lbs flywheel on, OEM was ~20lbs. I was surprised how easy it was to stall my car with a 2.4L engine, compared with the stock 1.0 with more rotating mass. It revs like a sport bike though.

My kinda old notes on my garage page: the stock DX flywheel is about 20lbs and the stock VX is about just under 16lbs. ACT Streetlite flywheel is 11.6 lbs. A Ralco RZ Aluminum Flywheel is only 7lbs.

The Ralco would be too light. But 11.6 sounds pretty good. Shaving 8.4 lbs would be a significant shedding of rotating mass and not affect drivability all that much.

[Ecky]

I found it to be "all that much" personally but I'd probably do it again. I like how easy it is to bump start the engine, and how quickly it revs.

[California98Civic]

Originally Posted by Ecky

I found it to be "all that much" personally but I'd probably do it again. I like how easy it is to bump start the engine, and how quickly it revs.

With almost no parasitic draw left on my crank pulley, I can feel that 20 lb flywheel roll on when I put in the clutch and kill the injectors. It spins so much that I learned to wait for it to slow down so the engine does not spontaneously restart when I turn the injectors back on. In fact, I often turn the alternator on for a second to act as a brake on the flywheel (get a tiny bit of "free power" back from the flywheel).

[serialk11r]

I had a 10lb flywheel on my 2ZZ-GE and never found stalling to be a problem and was very disappointed that it didn't rev faster. I'm definitely putting the lightest "racing" flywheel on all my future cars.

I feel like people who complain about stalling tend to be people who try to take off with little to no throttle input. Give it a constant, healthy jab of throttle, and as long as the clutch isn't jumpy, it should be a piece of cake.

I think this is actually another reason to want an underdrive pulley; the crank pulley is one part, but it's spinning the alternator at >2x crank speed, whose rotor has some decent size and mass to it, and the pumps are consuming several newton-meters of torque.