Downside to Underdrive Pulleys?
09-14-2006, 11:29 PM
#11
2nd Gear Member
Join Date: May 2006
Location: Tucson, AZ
Posts: 196
RE: Downside to Underdrive Pulleys?
Yeah, thanks a million CrazyAl, this has been a subject that I've been in the dark about for awhile and now I've been enlightened, so to speak! May I ask another question though? Can you put in a better battery and/or alternator to handle the extra load on your electrical system with UDP's and an electric water pump or does that basically do nothing to help the problem? Total n00b to cars here so sorry if thats a no brainer. 
09-15-2006, 12:01 AM
#12
5th Gear Member
Join Date: Mar 2006
Location:
Posts: 2,544
RE: Downside to Underdrive Pulleys?
A battery is simply a tool to store electrical power. You can get "better" batteries, but that isn't going to change the fact that UDPs alter the ratio between alternator output and engine power demands. The battery is only useful for starting the car, or for keeping little things (lights or stereo) operational while you wait in the car for short periods of time. When the car is running, the alternator should be supplying all the electrical needs of the car.
Here's another way to think of it:
Battery = Savings account
Alternator = Your salary
Electrical demands of car = Bills (rent, insurance, car payment, etc.)
Hopefully, your salary is more than your bills. When that is the case, everything is all well and good. If your salary isn't enough to cover the bills, then you can take some money out of your savings to cover the bills. But, this situation can only continue for so long--eventually the savings account will run out of money.
When you add UDPs it's like keeping your expenses the same, but getting a pay cut. You have to hope that you're still making enough $$$ to cover the bills. The system will still work OK if you're making enough money to cover the bills. But if you're making less money than the bills, then you're in trouble.
You could (In theory anyway) get a bigger alternator to alleviate the electrical load issue. Another option would be to put a smaller pulley on the alternator shaft (an "overdrive" pulley for the alternator). In fact, these things ARE done by people that have very high electrical demands in their cars: for example, huge stereo systems or 4x4 guys with winches. Some big trucks even have two alternators from the factory for this very reason. BUT, this doesn't really get you anywhere from a hot-rodding perspective because a bigger alternator or an "overdrive pulley" is going to require more HP to run--effectively undoing the gains from the UDPs in the first place.
Here's another way to think of it:
Battery = Savings account
Alternator = Your salary
Electrical demands of car = Bills (rent, insurance, car payment, etc.)
Hopefully, your salary is more than your bills. When that is the case, everything is all well and good. If your salary isn't enough to cover the bills, then you can take some money out of your savings to cover the bills. But, this situation can only continue for so long--eventually the savings account will run out of money.
When you add UDPs it's like keeping your expenses the same, but getting a pay cut. You have to hope that you're still making enough $$$ to cover the bills. The system will still work OK if you're making enough money to cover the bills. But if you're making less money than the bills, then you're in trouble.
You could (In theory anyway) get a bigger alternator to alleviate the electrical load issue. Another option would be to put a smaller pulley on the alternator shaft (an "overdrive" pulley for the alternator). In fact, these things ARE done by people that have very high electrical demands in their cars: for example, huge stereo systems or 4x4 guys with winches. Some big trucks even have two alternators from the factory for this very reason. BUT, this doesn't really get you anywhere from a hot-rodding perspective because a bigger alternator or an "overdrive pulley" is going to require more HP to run--effectively undoing the gains from the UDPs in the first place.
09-15-2006, 01:07 AM
#15
5th Gear Member
Join Date: Mar 2006
Location:
Posts: 2,544
RE: Downside to Underdrive Pulleys?
A capacitor and a battery are basically the same thing.
The difference is that a capacitor can charge and discharge extremely quickly, whereas a battery cannot respond that fast becasue it depends on chemical reactions to store and release the power. (While capacitors are "faster" than batteries, they usually can't store as much energy in the same amount of space).
Bigger/better batteries or capacitors are merely devices that let you save up energy when you have extra, and then let you use it later, when you need it. That's great if you have a high-draw electrical device that's only on sometimes. For example, a bigger battery is useful when you're using a winch. Capacitors are good if you have a huge stereo and the bass hits hard. But neither one can help you in the case of CONSTANT power demands.
The potential issue with UDPs isn't related to the "speed" at which power is available. The issue is that UDPs diminish alternator output ALL THE TIME. Again, most people won't have trouble. But, be aware that they cut into the safety margin that your car has designed into it.
For example. Let's say that at 3500 RPM your car needs 50 amps of electricy to run. That runs the lights, gages, fuel pump, ignition, injectors, and so on. On a stock-configured car, the alternator output is going to be more than this. Perhaps 70 amps. That leaves 20 amps of "extra power", which typically is used to charge the battery and also might be used to operate any accessories you might have. (for example, stuff plugged into the cig lighter). If it's not needed, the internal electronics in the alternator will decrease output, running at a lower duty cycle.
Now, let's say you put those UDPs on there. Now you're only getting 55 amps (for the sake of argument) out of the alternator. That's barely enough to operate the car. There's not going to be much battery charging going on, so you might get into trouble if you do a lot of short trips.
...now lets say that a couple of years down the road, things start to wear out a bit. That's normal. On a stock car if the output drops 10% you're still OK. But if you've got the UDPs and you suffer the same drop, then you're in trouble. Your are forcing the alternator to run at 100% all the time...and it will wear out that much faster.
The difference is that a capacitor can charge and discharge extremely quickly, whereas a battery cannot respond that fast becasue it depends on chemical reactions to store and release the power. (While capacitors are "faster" than batteries, they usually can't store as much energy in the same amount of space).
Bigger/better batteries or capacitors are merely devices that let you save up energy when you have extra, and then let you use it later, when you need it. That's great if you have a high-draw electrical device that's only on sometimes. For example, a bigger battery is useful when you're using a winch. Capacitors are good if you have a huge stereo and the bass hits hard. But neither one can help you in the case of CONSTANT power demands.
The potential issue with UDPs isn't related to the "speed" at which power is available. The issue is that UDPs diminish alternator output ALL THE TIME. Again, most people won't have trouble. But, be aware that they cut into the safety margin that your car has designed into it.
For example. Let's say that at 3500 RPM your car needs 50 amps of electricy to run. That runs the lights, gages, fuel pump, ignition, injectors, and so on. On a stock-configured car, the alternator output is going to be more than this. Perhaps 70 amps. That leaves 20 amps of "extra power", which typically is used to charge the battery and also might be used to operate any accessories you might have. (for example, stuff plugged into the cig lighter). If it's not needed, the internal electronics in the alternator will decrease output, running at a lower duty cycle.
Now, let's say you put those UDPs on there. Now you're only getting 55 amps (for the sake of argument) out of the alternator. That's barely enough to operate the car. There's not going to be much battery charging going on, so you might get into trouble if you do a lot of short trips.
...now lets say that a couple of years down the road, things start to wear out a bit. That's normal. On a stock car if the output drops 10% you're still OK. But if you've got the UDPs and you suffer the same drop, then you're in trouble. Your are forcing the alternator to run at 100% all the time...and it will wear out that much faster.
09-15-2006, 09:00 AM
#16
3rd Gear Member
Join Date: Aug 2006
Location: North East Texas on the Red River
Posts: 886
RE: Downside to Underdrive Pulleys?
Although Al is always quick and accurate with his theories, he tends to get buried in them and sometimes overlooks the practical application of it all.
With the UD pulleys and most any gears, you will have more than enough delivery from your alternator for all but the heaviest of loads such as a zillion watt aftermarket sound system.
The practical side of the matter is that UD pulleys are generally a good thing and will work well in the vast majority of cases. With 4.10 gears, your AVERAGE driving and speeds will work out such that slowing down the accessories will jive well with the added AVERAGE RPM increase of the gears.
I live in Texas and have to deal with pretty sorching heat in the summer. My car will ultimately have 4.10 gears and underdrive pulleys. As I said before, I have run them on my Vette for many years in ALL types of conditions with absolutely no downsides. I fully expect to have the same experience with my GT.
With the UD pulleys and most any gears, you will have more than enough delivery from your alternator for all but the heaviest of loads such as a zillion watt aftermarket sound system.
The practical side of the matter is that UD pulleys are generally a good thing and will work well in the vast majority of cases. With 4.10 gears, your AVERAGE driving and speeds will work out such that slowing down the accessories will jive well with the added AVERAGE RPM increase of the gears.
I live in Texas and have to deal with pretty sorching heat in the summer. My car will ultimately have 4.10 gears and underdrive pulleys. As I said before, I have run them on my Vette for many years in ALL types of conditions with absolutely no downsides. I fully expect to have the same experience with my GT.
09-15-2006, 01:43 PM
#17
3rd Gear Member
Join Date: Dec 2005
Location: Southwest Florida
Posts: 538
RE: Downside to Underdrive Pulleys?
Damn, you people think way to much, don't be a sissy just put them on, haul some *** and don't worry about it. IF something goes wrong sway them back for stock and the dealer has to warranty it, nothing will go wrong so don't worry about it. All this comparing this to that is pointless, just do it and don't worry, sheesh.
[8D]
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