BigRich954RR

iTrader: (1)

ok i had a 7 inch crank pulley with a 4.75 procharger pulley belt length= X have 3 belts for X . I put a 8 inch crank pulley on with 4.75 procharger pulley belt length =Y

Now with 8 inch crank and 3.75 procharger pulleys would belts X fit ?


Where the best deals on F1C pulleys?

Aster

iTrader: (9)

Short answer no that will not give you the same belt length. Instead with that combo your new belt length would be increased. Read on for the long answer if you like math.

When dealing with belt size it is more important to look at the circumference of the pulleys and how many degrees the belt travels across the pulley.

Say you have a 7" crank pulley and a 4.75" head unit pulley and you are getting 180 degrees of belt wrap on each.

Say your belt length is 48" with the 7" crank and 4.75" hu pulley.
7" Crank Circumference = 38.485"
Each degree = .107"
Belt covers 180 degrees = 19.242"

4.75" pulley Circumference = 17.721"
Each degree = .049"
Belt covers 180 degrees = 8.86"

So we subtract the 19.242 and 8.86 from our original 48" belt to tell us how much distance the belt is traveling between the pulleys, this won't change much regardless of pulley size.
This gives us 19.898"
We will use this number when figuring our new belt size or to decide how large a head unit pulley you need for an 8" crank to keep the same belt size.

8" Crank Pulley circumference = 50.265"
each degree = .14"
180 degrees = 25.133"

3.75" HU pulley Circumference = 11.045"
each degree = .031"
180 degrees = 5.522"

So to find the belt length now for these pulleys would add the 19.898 above to the 25.133" and 5.522" of the new pulleys which gives us a belt length of 50.553" round down slightly to a 50.5" belt.

If you wanted to keep using the 48" belt you would take 48" and subtract the 19.898 static length and subtract the new crank 25.133" = 2.969"

As above we determined there is 180 degrees of belt wrap so divide 2.969" by the belt wrap degrees = .0165"
Now multiply that by 360 degrees to get circumference of the needed pulley = 5.938"
Now we divide 5.938 by PI (3.14159...) = 1.890" then square root that gives us the radius = 1.375". multiply by 2 for diameter = 2.75" head unit pulley.

As diameter increases the distance a belt must travel is exponentially larger than before. Generally speaking when making large changes to your drive pulley you will always need to increase your belt size and in some cases depending on your goals and head unit limitations may also need to increase the size of your head unit pulley to keep you from over spinning the blower or to keep you in the desired boost range.

Hope this helps a little.

Aster

iTrader: (9)

As a side note if you're using an F1C an 8" crank pulley and 3.75" head unit pulley will max your rated RPMs for the blower by 6500 RPM.

BigRich954RR

iTrader: (1)

Aster thank you that was exactly what i was looking for. But most likely going with a 4.25 or 4.50 pulley and i have the 8 inch crank on all ready i shift at 6700 which should put me around 64k and 68k Im trying to add just a little more boost on to what i have now but not sure what size pulley to try.

EPP

iTrader: (22)

I sell them for $85.00 plus shipping. Bob

Aster

iTrader: (9)

Depends on what you mean by just a little more boost...
How much are you making now with the 4.75" pulley?
How much would more would you like to make?

BigRich954RR

iTrader: (1)

Im at 15psi want to get about 17 maybe 18psi

Aster

iTrader: (9)

I'm no expert on boost per blower rpm, but it sounds like if you want to gain 2+ psi you may want to try the 4.25 pulley.

Have you run it by ATI or any of the vendors like Bob at EPP? He has a lot of experience and may be better at gauging how much boost you'll pick up with the pulley swap.

BigRich954RR

iTrader: (1)

Yea im going to order pulley from Bob at EPP so i will run it buy him then. I just feel bad he's always in the shop when i call

Side note what type of engineer are ?

Aster

iTrader: (9)

Software.