Lightweight underdrive pulley
 

I am thinking of getting a lightweight underdrive pulley for my Jeep. Not sure if it’s something that should help mpg or not? Also read it could harm the engine? Any opinions?

If you mean an underdrive crank pulley, then yes it could harm the engine if the aftermarket part is missing a needed, integrated harmonic balancer. If it is just an accessory pulley, that should not harm the engine though it would change the performance of whichever component is running on it, such as the AC, alt, or PS pump.

Search this site (upper left corner) because I am sure that this has been discussed here before. So really smart folks shared a variety of opinions.

It won't help MPG in any measurable way, but might slightly improve acceleration if you get a lightweight pulley set and underdrive the accessories.

I probably wouldn't do it again given the option to save the money and effort. The manufacturers would be using them if it made economic sense, or even boosted 0-60 performance in a meaningful way.

If you do a lot of commuting miles, it's probably cheaper to let the Jeep sit most of the time and buy a very cheap/efficient commuter vehicle.

I sold my Jeep because the MPG was terrible along with reliability, and I rarely need the clearance.

Underdrive pulley helps a little bit, but it's more effective as a cheap way to add a few horsepower at high revs. If you want to mess with your power steering, electric power steering or reducing the bypass valve pressure will get better results. Water pump and alternator don't really show significant gains until higher speed, maybe a lightbulb or two worth of power.

As for MPG increase, it depends on the particular vehicle and operator.
I Installed an underdrive crankshaft pulley/harmonic damper from ASP on my 4.0 liter Mustang in May, 2015. It's a 25% underdrive pulley. ASP made a remarkable effort to keep the damping inertia close to that of the OEM pulley/damper. It's a redesigned product; not just a modified OEM pulley/damper.
The first week after I installed this pulley, I got several alarms from my UltraGauge indicating "pending trouble code". The codes were for misfires - probably due to the lower rotating mass of the new pulley/damper. (It's detected by the crankshaft position sensor.) It hasn't happened since then. I've had no charging or cooling issues, and my butt-ometer feels a bit more power.
Prior to installing this pulley, I had 6 fill-ups over 32 MPG in an 8-year period, but they were all "one in a row". After I installed it, I had 8 consecutive fill-ups over 32 MPG (2 were over 33).
In my case, it gave me an average of about 3% (1 MPG) and I have the fuel log data to prove it. :cool: It'll pay for itself in fuel savings in about 200K miles. In the mean time, I've got bragging rights. (32+ MPG commuting with a Ford Mustang):cool:
It worked for me, but, as with any modification, your mileage may vary.

In my particular case, an underdrive pulley would not work well. I already run at low enough RPM that the alternator voltage occasionally drops. Also, when the RPM gets that low, the water pump is not moving enough coolant to keep the engine at even temperature. About 1/3 of my miles are at 1000 RPM or less.

I don't need more power. I get up to speed on a short entrance ramp while shifting at less than 4000 RPM.

It really depends on if you're buying it for its underdriving abilities or its weight loss. I know for the 3v mustang the Steeda crank pulley is a pound lighter, and the water pump pulley is negligibly heavier, so weight wise pretty meh... But at 25% under drive, they really help NA cars. It will tighten up the over assisted steering, and newer cars produce more than enough amps so it won't effect electronics. We gained 9rwhp on a 265rwhp car and gained maybe .5-1mpg from the 25% UD crank pulley, and the 25% OD water pump pulley.
Also all Ford pulleys are balanced like oem. So if you find you a good udp set for your car that is balanced and underdrives a good bit, ignore the weight loss because it won't do ****.
The same guy who designs the Ford oem pulley also designed the Steeda UDP so apparently oems believe in UDP. They just want you to spend the extra doe $$$.

I bought it for its underdriving abilities to reduce the parasitic load of the water pump and power steering pump. Weight loss was less than a pound - insignificant. It was just the most expensive part of an ongoing fuel economy experiment that began on July 23, 2007. My focus on this experiment intensified in May, 2009 when I found ecomodder.com. I knew that in a worst-case scenario, the underdrive pulley would not decrease my fuel economy. It gave me a significant increase - more than anticipated. :cool:
I didn't buy this pulley for more power - this 4-liter V-6 at 210 HP has plenty of power. I can accelerate to the speed limit on all but the shortest freeway on-ramps without exceeding 2500 RPM. Cruising, I'm always under 2000 RPM. If I'm going less than 50 MPH, my cruise RPM is between 1000 and 1500 RPM. (depending on speed and gear)

Not to bring things off subject but I've never heard of a udp set that underdrive the water pump. Typically they overdrive the water pump to offset the under driving crank pulley.

This isn't a "set". It's just a 25% smaller crank pulley to underdrive everything by 25%.
The udp "sets" that I've seen advertised that include a water pump pulley generally contain a larger water pump pulley to underdrive the water pump even more than the other accessories. The water pump and fan (if engine-driven) are the major parasitic consumers of power.
Some udp sets include an overdrive (smaller) alternator pulley to eliminate charging issues. With most modern alternators, that's not an issue.