eyecandy

iTrader: (19)

I am trying to decide between replacing the main pulley (rx7.com, pettit, or rotary extreme) or the water pump and alternator pullies (rotary extreme or greddy). I do not want to say I will be tracking the car all the time, but it will see quite a bit of high revs.

I know I do not want to get a full kit do to slowing the accessories about 40-45%, which I think will be to high for street use. I am not running an air pump so I want to be sure there will be enough bite on the water pump pulley, may main reason for choosing one of the two.

Now would I be better with the main pulley or the water pump/alternator pulley? Or will they both do about the same, in terms of belt/water pump contact? I know there would be HP gain with a main pulley, which why I am leaning towards that. Is underdriving the water pump and alternator (plus PS depending on kit) a good thing?

HAI-TEK7

iTrader: (31)

You are correct, with the main pulley i think you will gain some HP. With the Alt/WP you wont gain any, but slowing down those two is good. Alternators only need about 2000 rpms to do its job, so anything thing above that is not needed, so bearings and brushes come in the picture to see if they can take the rpms. The waterpump dont like high rpms either, this im not really sure why, i think it has to do with the it spinning so fast it might collapse the hoses???? (don't quote me on that)

HAI-TEK7

iTrader: (31)

oh i forgot, i have the greddy kit, it works great.

1234rotor

iTrader: (1)

What you are talking about would be referred to as "Cavitation" from the pump spinning so fast that it does not pump the water effectively and creates air pockets, as far as I know.

I am not sure if it creates enough vacumm to collapse the hoses.... maybe someone here does though.

KaiFD3S

iTrader: (37)

I have all, Main underdrive, waterpump and alternator and I love it, no problems at all

eyecandy

iTrader: (19)

I know about Cavitation with the pump, I was just not sure about the alternator. Is there a "recommended" rpm for the water pump as well? Or let me restate that, what is the minimum speed you want for daily driving, since at high engine speeds would not be a worry. I guess my main concern is the contact with the water pump. The few HP gain if any is not a big deal, I could take it or leave it and I am not counting on it.

KaiFD3S, is your car a street car, or street/track? What kit do you have?

maxcooper

For maximum belt contact on the water pump with the air pump removed, get larger-than-stock underdrive pulleys for the alternator and water pump and keep the stock main pulley.

Drive pulleys (like the main pulley) get smaller to underdrive. Driven pulleys (like alt and WP) get larger to underdrive. An underdrive main pulley with the stock alt and WP pulleys offers the least belt contact on the water pump. Underdrive main, WP, and alt pulleys offer more contact. All stock pulleys is also in the middle here, but I am not sure if there is more or less contact than the all-underdrive setup (but I think it offers more contact since the size of the main pulley seems to make a significant difference). And underdrive WP and alt pulleys with the large stock main pulley offers the most belt contact (which still isn't all that much). All of these setups seem to work based on the many cars that are running these combinations, but it is clear that the belt slips on the water pump.

Note also that some underdrive pulley kits don't change the size of the driven pulleys. I don't think the Unorthodox WP pulley is any bigger than stock -- they use a smaller main pulley to underdrive all the accessories (which probably gives the best power gains, but reduces WP belt contact if you don't have an air pump). I think the GReddy WP pulley is larger than stock, but I am not certain (anyone care to share their first hand experience?). And I know that the SR kit did come with a large WP pulley at some point (because I have one). SR changed their kit since I bought mine, I think, but I suspect it still has a larger-than-stock WP pulley.

-Max

KaiFD3S

iTrader: (37)

street driven, and I have the whole RE kit

eyecandy

iTrader: (19)

See I am glad that I asked, because the term "underdrive" implies that the main pulley would be larger, yet from what Maxcooper says it actually smaller.

Let me ask this, how would it they underdrive the accessories, wouldn't it speed them up with it being smaller? So say you take the RE kit, wouldn't really balance out, the main being smaller but the WP/Alt being laerger??

maxcooper

Say you have a drive pulley that has a circumference of 10" and run it at 5000 RPM. The belt will move at a linear speed of 50,000 inches per minute (10" x 5000 RPM). If you have a driven pulley with a 10" circumference, the driven pulley will also spin at 5000 RPM (50,000 in/m / 10").

If you reduce the size of the drive pulley to one that has a 5" circumference, the belt speed will drop to 25,000 inches per minute (5" x 5000 RPM). With the same driven pulley (10" circumference), the driven pulley will now spin at 2500 RPM (25,000 in/m / 10").

Smaller drive pulleys slow down the belt speed and thus underdrive the accessories.

The opposite is true for driven pulleys. With a drive pulley with a 10" circumference running at 5000 RPM, the linear belt speed will be 50,000 inches per minute (10" x 5000 RPM). A driven pulley with a 10" circumference will turn at the same 5000 RPM speed (50,000 in/m / 10"). But if you increase the size of the driven pulley to a 20" circumference, it will only turn at 2500 RPM (50,000 in/m / 20").

These sizes are not representative of the pulley options available for the RX-7, but I just chose them because them because the math was easy and my intent was merely to demonstrate the effect of making pulleys bigger or smaller and how the effects are reversed for driven (WP, alt, etc.) versus drive (main) pulleys.

-Max

rx7n3wb

eyecandy, you have two things mixed up. your understanding of how belt drive/driven systems work is switched up with another, gear drive/driven (e.g transmission.)

eyecandy

iTrader: (19)

I am understanding now.

So since I have removed my air pump its best to say to get the Water/Alternator pulley kit then. How necessary are upgraded pullies when the air pump is removed?