Master list of 65+ proven mods for increasing MPG

[Frank Lee]

Hee hee, I just might be!

[bensenvill]

Frank, if your putting on larger pulleys, you are also putting in a longer belt, you are NOT increasing the tension of the belt. (the purpose of the idler pulleys is to take in that extra slack, keeping a constant tension).

Overdrive pulleys are typically around 10% larger (it entirely depends on clearances), and almost always milled out of aluminum, so they are considerably lighter than the stock thats being replaced.

[Christ]

Originally Posted by bensenvill

Frank, if your putting on larger pulleys, you are also putting in a longer belt, you are NOT increasing the tension of the belt. (the purpose of the idler pulleys is to take in that extra slack, keeping a constant tension).

Overdrive pulleys are typically around 10% larger (it entirely depends on clearances), and almost always milled out of aluminum, so they are considerably lighter than the stock thats being replaced.

If you're putting on larger pulleys, you're increasing the angular momentum that the pulley carries, making it more difficult to turn because of increased weight further from the center, and because of increased drive ratio.

I'm not sure where anyone said anything about overdrive pulleys, though, other than in jocularity, a few posts back.

[bensenvill]

Originally Posted by Christ

If you're putting on larger pulleys, you're increasing the angular momentum that the pulley carries, making it more difficult to turn because of increased weight further from the center, and because of increased drive ratio.

That is a poor blanket statement to make. And considering most aftermarket overdrive pulleys are aluminum, they are already significantly lighter than the stock pulleys they are replacing... and at approximately a 10% size increase, this increased angular momentum GENERALLY does not come close to offsetting the weight savings of an aluminum pulley.

[Christ]

It's not a poor blanket statement, it's an accurate observation. If the pulley is larger, there is weight in places that it never used to be, meaning that there is more weight in those same places.

Then, you still have an increased drive ratio, which means that it's loading the engine more at a given speed than a smaller pulley would.

Milled and cast aluminum is generally not much lighter than a pressed pulley, either. It's like the difference between OE steel and OE aluminum wheels. There isn't much, generally. In specialty cases, the aluminum pieces are specifically designed to be lighter, but the average aftermarket piece isn't really there to increase performance, it's a shiny bit with the added benefit of a slight performance increase.

I still have to ask who was talking about overdrive pulleys?

I think you're referring to underdrive, in which case, the normal "fix" is to lower the size of the main pulley, rather than change each of the accessory pulleys. This means that the engine/accessory drive ratio is numerically lower, so the engine can turn easier with less accessory loading.

An overdrive setup, like you mention a few posts back, would either increase the size of the main pulley, or decrease the individual accessory pulleys in size.

[bensenvill]

Originally Posted by Christ

An overdrive setup, like you mention a few posts back, would either increase the size of the main pulley, or decrease the individual accessory pulleys in size.

other way around

[Frank Lee]

Originally Posted by bensenvill

other way around

wrong, bucko

and who said anything about belt tension?

[Christ]

Originally Posted by bensenvill

other way around

Are you talking about overdrive, or underdrive?

Overdrive is what the pulleys already are, where the driven cog is spinning faster than the drive cog.

Underdrive is used to slow the accessories down, to create less load on the engine. This is done by either making the drive pulley smaller, or making the individual accessory pulleys larger.

We've got to work out terminology here, because you've used both terms, and they're NOT interchangeable.

[Frank Lee]

Ha Ha, today I saw on another forum where someone was defending their use of "loosing" by saying it was interchangeable with "losing". Oh, and they used "chaff" where it should have been "chafe".

Best part was, it was commentary on education! L M A O

[Christ]

Gotta love it.