ok im having a little trouble understanging how these work... i think theyre bigger(or smaller..) then stock pullies and cause the accesories to spin slower then normally? but if an accesory drive is spinning slow or fast wouldnt it still put the same amount of resistance on the belts and crank? and if my alternater, waterpump, p/s, ect are all spinning slower wouldnt that also mean theyre doing their job less efficently?
As I see it, a larger pulley would require less force to be spun, yet would spin the accessory more slowly. Just adding more fts to the lbs. Longer lever for more torque. Less power needed to turn it. Many of our accessories work once they hit a certain rpm. Then they don't work much better or much worse if going faster or slower. For example, your alternator works from 1500 rpm on up (I think, I might be thinking of older ones), but it will put out the same amperage at 1500 as it would at 7000 rpms. So if it is spinning a little more slowly at the cars running speed, but pulling less torque off the main drive pulley, then you are freeing up a little power to use at the wheels. I think for most of us this is a moot point. We don't need that extra .8 hp.
As far as underdrive pulleys go.Slowing down the water pump isnt a good idea if you have a marginally effective cooling system.Allthough slowing the coolant down a bit to give it more time to transferr heat isnt necessarily a bad thing either.Alternators will work spinning backwards upside down etc...It just has to be spun up to the pont where it starts charging.One wire alts start once the engine is running so... but yeah the HP gained is negligible unless you run bracket racing classes where you are limited on mods then every hundreth you can gain makes it worth while.Just my two pennies.
ok, i was just having trouble wraping my head around the sciance of the whole thing. im not looking to put them on any of my cars but they definatly seem to be a big fad among the local ricer and mustang boyz lol
under drive pullies are used primarily for high rpms used in racing. It slowes down the rpms of the alt and/or water pump. You can actually harm an alternater by spinnung to fast and with higher rpms at the water pump can cause cavitation/airation at the impellor and also moves water too fast to efectively pull heat from the engine and also transfer heat at the radiator. By slowing the water down it allows for more heat transfer. This of course does not really apply to street cars because they can't sustain high rpms very long dude to cops and other things that wont allow fasr driving. This is just my $.02 from personal experiance.
Mine had an underdrive crank pulley when I got it, and I actually had to take it off because of the negative effect on charging. Basically, at idle the alternator and power steering were both useless. Not a big deal in a simple stoplight or highway situation, but at night with the headlights on you could run the battery flat dead waiting for your burger at the drive-thru. Not to mention compounding the charging problem and overheating if you have an electric fan. It might be worth .01 on the strip, but it's not worth the inconvenience and embarrassment if your car's a driver. Maybe with a 3g alternator you could pull it off. You'd probably get more charge at low revs - BUT, that alternator takes more muscle to turn for that reason, so what was the point? That's why you use a dual grove pulley with those, or a serpentine type belt. Damn that conservation of energy....
This is the reason why they don't make sense for a street car, underdriveing your pullies on a normal/low rpm ( where the engine spends most of it time) will have negative affects. Street cars rarely hit let alone spend any amount of time at or above 6 grand like race engines do.
...Exactly my point. If it's a racer and occasional driver, go for it. If it's a driver and occasional racer, wouldn't recommend it. Kinda like a lot of other go-fast parts.
I 100% agree with you, just because something might make your car faster doesn't mean it you need it on a street car.
i had under drive pullys on my 87 mustang. they defintly free up some power. i could feel it in my seat of the pants and knocked off 1 or 2 tenths in the 1/4 (cant rember exactly how much because it was so long ago). my lights would dim at idle. the power stearing was unafected. my mav has a elec water pump and no power steering, only a 3g alt. i started out with a race under drive pully on it (the pully diameter is smaller than the bolt circle) and it wouldnt charge below 2000 rpm with the elec load of a fuel pump, msd ignition, elec water pump and elec fan. i had a standered fox body mustang under drive crank pully but it still didnt charge to well untill 1500 rpm. so now i have the stock fox mustang crank pully on and its charging at idle at aroun 13.2 volts and jump up to 14 volts at any rpm above 1000 rpm. so if you have the standard group of belt driven accessories and are looking for every ounce of performance then get underdrives. if not then dont.
The stock crank pulley is twice the diameter of the alternator pulley. That means that the alternator is spinning twice the engine speed. At 5000 rpm the alternator is turning 10000 rpm. If, for some reason your alternator went to full load at that rpm it would destroy itself catastrophically. That was the reason for the switch in the field wire when drag racing. It was never for the HP savings that was later claimed for it. The water pump will cavitate if it turns too fast. Cavitation is when the vacuum on the tips of the impeller makes the water boil (due to the low pressure) and it is so violent that the metal can be broken off in small amounts. (looks like rust pits) When cavitation strikes it causes large amounts of steam around the impeller and it quits pumping water all together. You can't pump too much water through an engine as long as you have a thermostat in place. If it cools the engine down the thermostat will close and the water stops flowing. When it gets hot again it opens and lets the water flow through the radiator. The water pump is a centrifugal pump - it doesn't produce much pressure but it can push a lot of volume with no pressure. Whe you turn an engine over 5000 it should be fitted with an under-drive crank pulley and a switched alternator field circuit. Remember that alternators produce their rated output at about 1500 RPM (750 engine rpm with standard pulley). If your engine can turn 6000+ rpm then your idle is probably high enough to spin the alternator to 1500 rpm at an idle even with an under-drive pulley.
Paul, thanks for clearing up that it is 1500 ALTERNATOR RPM that activates it, and not engine rpm. I always revved mine thinking i needed to to activiate the alternator. I lately have only gone out and started my car in the garage because the trans is leaking WAY too much for me to trust it on the road. So I run it 15 minutes or so a month to keep everything lubed and charged. During this time I always thought I needed to get the engine rpms up to get the charge started.
My dad's '95 Mustang has had under drive pulleys on it for over 130,000 miles. The voltage bounces around a little when it's sitting at idle, and the head lights noticeably get dimmer, but the car never has any charging problems and will sit in traffic all day long in the middle of summer with the A/C blasting and doesn't over heat. It has a stock 5.0 H.O. that has never been apart since it left the factory, 267,000 miles, the car will turn 6,000 RPM with ease. Valves start floating at around 6,800.
Oh, there are definitely some cars that can get away with it. Mostly ones that have 90 amp or higher alternators. Stock Maverick type alternator can't hang though, not for long periods of idling... Really it wasn't an issue most of the time, but it just bugged the hell out of me to have to think about that kind of thing just because I want to get into a slow moving line at a bank or a fast food place. ...And, some underdrive pulleys are no doubt better than others. My crank pulley was only 4" in diameter, which is probably kind of extreme.
