mpg gains with underdrive crank pulley?

[Fr3AkAzOiD]

I know an aluminum underdrive crank pulley will be lighter weight then stock, 1.3 lb vs 3.6 lb and the smaller size will help reduce accessory drag.

I know this will not only free up some hp but also give you mpg gains as well.

Given that at 50 mph I get about 50 mpg would you guess are we talking a miniscule mpg increase of 0.5 - 1.0 mpg or something more around 1.5 - 2.5 mpg gained?

Here is the part...
5 ribs - Ecotec Underdrive Crank Pulley - 2.0LNF/2.2/2.4: by Make and Model - Chevrolet - Cobalt - NA Ecotec UDP - Car Performance Parts And Auto Accessories | MRZ Performance

I know I'm talking several years before breaking even but was just curious.

[tim3058]

If there was any improvement it'd be infinitely minuscule. I've got one on my car, I think the main goal of U/D is to improve mechanical advantage under acceleration of the motor... as I posted elsewhere, the alternator still has to charge the battery, power lights, heater fan, ignition, etc, so the same amount of work has to be done regardless of pulley size, the alternator pulley in an U/D setup turns slower, so it has to turn longer to generate the same output. In performance applications thats fine, because this delays alternator output into the future once the car is cruising, freeing up power during hard accel. As far as mpg's I don't see it changing anything, its still a burden (smaller but for longer duration) on the motor. A lot of money for a net-sum zero add-on in my opinion.

[easy]

I put an underdrive pully on my ford ranger a few years back. I picked up 1 mpg and my "seat of the pants dyno" reflected a few extra hp.

easy

[Piwoslaw]

Why are underdrive pulleys so much more expensive than OEM pulleys? Smaller production volume? Lightweight, hi-tech materials? An underdrive pulley is smaller, so it requires less material, and so would be cheaper and lighter.

Just my common sense.

[MadisonMPG]

All of the components that the engine drives via belt are slowed down. They spin slower, so they don't produce as much friction/parasitic loss. How many RPMs they slow down vary on the size of the pulley. 99.99% an underdrive won't "pay for itself".

[greasemonkee]

Might buy a set of rod bearings to go with your underdrive crank pulley, especially if the factory had an isolator in the oem unit.

[roflwaffle]

I'm thinking you're looking at several decades before payback, but I've been wrong before so the best bet is a test. Yank the belts (cept the WP) and measure how much fuel consumption increases. Take that figure and multiply it by the ratio of the two pulleys in order to guesstimate how much you'll increase mileage.

[Red Lion]

I would guess your MPG will increase in the sub 1mpg range. I doubt you'll see the cost come back in gains for a VERY long time, especially considering how light your stock pulley is (my 240sx's OEM pulley weighs about 9lbs compared to an Unorthodox 1.5lb pulley; now that's some weight loss).

The performance you'll mostly see in a lightweight pulley, which most people buy them for, is engine response, which I guess in a round-about way increases MPG because the engine doesn't work as hard to turn it's rotational parts. However, I never liked underdrive pulleys and prefer stock-size pulleys that are lightweight AND harmonically dampened (which 99.9% of aftermarket pulleys aren't).

Though I have heard some people say that using a lightweight (or underdriven, I can't remember) pulley on the A/C decreases power lost during A/C usage, so I guess that kind of increases MPG while running the A/C, but I don't have any first-hand experience.

Originally Posted by Piwoslaw

Why are underdrive pulleys so much more expensive than OEM pulleys? Smaller production volume? Lightweight, hi-tech materials? An underdrive pulley is smaller, so it requires less material, and so would be cheaper and lighter.

Just my common sense.

Simply put, because it's a "performance" mod and can be sold as such. I'm sure the automakers have good reasons for not using aluminum pulleys too (maybe fatigue?).

Originally Posted by greasemonkee

Might buy a set of rod bearings to go with your underdrive crank pulley, especially if the factory had an isolator in the oem unit.

Since he's probably not going to be spinning the engine to the upper RPM's often (I'm guessing), he probably won't have to worry too much about that.

[redpoint5]

I'm dragging this thread back to see if there are any more responses by way of scientific explanation or real world figures.

I have installed an underdrive pulley on my car, which reduces the rotational speed of the alternator, power steering pump, and A/C by 20%. As Tim has pointed out, I wouldn't expect much mileage gain by underdriving the alternator, as the energy must still be expended to create electrical power. However, I would suspect that underdriving the power steering pump would save some fuel, although I don't know how exactly it operates. Would under driving it 20% result in less power steering pressure? Does a pump continuously move fluid, or only while turning?

The idea of delaying energy consumption during acceleration and moving it into the future is along the same lines as turning the A/C off while accelerating and turning it back on when reaching cruising speed. This should save a small amount of fuel.

Unfortunately I switched to regular gasoline from premium at the same time I did the pulley swap, so I have no valid data to provide. Perhaps I have noticed a 0.5 mpg increase (from 28.5mpg to 29mpg). It's very difficult to show conclusively that a mod returning only 1% better fuel efficiency is doing so because of the mod.

At 1% fuel savings and given a yearly expenditure of $2000 on fuel, I would save a measly $20/year. At $300 for the pulley set, it will take me 15 years to break even on the investment, and that does not account for the half day lost putting it in.

It was still a fun project though, and worth the expense to learn something.

[C3H8]

i doubt an under drive pulley will help mpg very much. the weight savings would only effect acceleration, and since pulleys are small, and close to the centre of the crankshaft rotation, the gain would be miniscule.

on the other hand, it underdrives all of the belt driven compoents. so you should see the waterpump, power steering, a/c going slower. that might help a bit. the alternator probably wouldnt change anything because alternators are voltage regulated. if the under drive spins the alternator slower, then the alternator just applies more oomph to the brushes inside it to maintain its required voltage.