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Discussion Starter · #1 ·
Im getting a cam installed and im having the shop change out the pulleys while they are in there. I went with the 25% ATI underdrive pulley and the 10% A/c pulley. What belt sizes would I need to get for both? Any help and insight will be will be greatly appreciated.
 

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I have not checked what the stock serpentine belt length is for the SS.

On my Fbody the stock length was 79". Running my ATI 10% underdrive dampener, officially ATI calls for a 77.5" belt, however I'm running a 78" with no issues after first trying a 77" that was too tight (1/2 increments weren't in stock anywhere and I was way itching to fire up my motor build). When I ran a 25% underdrive it took a 76" belt.

So my advice would be try a 3" shorter belt and check that the tensioner is operating within the marks of its range.

Sorry I can't be of help for the A/C pulley. At 10% the stock may still be OK with the tensioner, since it's a short belt.

EDITED: I originally thought I read a 10% underdrive on the main serp, so I reccomended 1" shorter, my bad.
 

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Discussion Starter · #4 ·
I have not checked what the stock serpentine belt length is for the SS.

On my Fbody the stock length was 79". Running my ATI 10% underdrive dampener, officially ATI calls for a 77.5" belt, however I'm running a 78" with no issues after first trying a 77" that was too tight (1/2 increments weren't in stock anywhere and I was way itching to fire up my motor build). When I ran a 25% underdrive it took a 76" belt.

So my advice would be try a 3" shorter belt and check that the tensioner is operating within the marks of its range.

Sorry I can't be of help for the A/C pulley. At 10% the stock may still be OK with the tensioner, since it's a short belt.

EDITED: I originally thought I read a 10% underdrive on the main serp, so I reccomended 1" shorter, my bad.

Gotcha so try a 1" shorter belt for the 25% underdrive pulley. I'm sure belts are pretty universal right?. Any brands I should look for? I'm hearing gatorback is a good brand.

Once I figure out the 10% A/C pulley ill be set

Thanks for the reply
 

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Gotcha so try a 1" shorter belt for the 25% underdrive pulley. I'm sure belts are pretty universal right?. Any brands I should look for? I'm hearing gatorback is a good brand.



Once I figure out the 10% A/C pulley ill be set



Thanks for the reply


Take the stock belts to an auto supply store and measure the length then buy the next 5 sizes down. See what fits then return the rest.

Or contact the pulley manufacture and ask the what size they recommend.


Sent from my iPhone using Tapatalk Pro
 

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Discussion Starter · #6 ·
Take the stock belts to an auto supply store and measure the length then buy the next 5 sizes down. See what fits then return the rest.

Or contact the pulley manufacture and ask the what size they recommend.


Sent from my iPhone using Tapatalk Pro
Smart idea about buying the next five sizes down..I did recontact ATI they said they didn't know lol
 

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Gotcha so try a 1" shorter belt for the 25% underdrive pulley. I'm sure belts are pretty universal right?. Any brands I should look for? I'm hearing gatorback is a good brand.

Once I figure out the 10% A/C pulley ill be set

Thanks for the reply
Shoot sorry all I did was confuse things with the edit. I was suggesting 3" smaller not 1". But I was also lazy and didn't look up a stock SS belt length. :(

If I look up a replacement belt for an SS online, it appears to be a 72.5" belt. Makes sense it'd be shorter than an Fbody with a p/s pump on the serp drive.

For a 25% underdrive, I would try a 70" for the main belt but as mentioned maybe buy a couple sizes in the range, such as a 70.5 (if available) and 71. Return the ones that don't fit. Not sure what might work for the A/C belt.

Other than that it's a 6 rib. I have a Gates, but Gatorback is fine too.
 

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Discussion Starter · #8 ·
Shoot sorry all I did was confuse things with the edit. I was suggesting 3" smaller not 1". But I was also lazy and didn't look up a stock SS belt length. :(

If I look up a replacement belt for an SS online, it appears to be a 72.5" belt. Makes sense it'd be shorter than an Fbody with a p/s pump on the serp drive.

For a 25% underdrive, I would try a 70" for the main belt but as mentioned maybe buy a couple sizes in the range, such as a 70.5 (if available) and 71. Return the ones that don't fit. Not sure what might work for the A/C belt.

Other than that it's a 6 rib. I have a Gates, but Gatorback is fine too.
Yeah I have two gates belts coming to tomorrow so we'll see... If these don't work I'm trying Brando tho... We'll see...I hate buying stuff twice
 

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Why change from stock pulley sizes? What’s the benefit?


2016 SSB, stick, hole, spare
I agree, HP gain will be minimal and engine cooling and A/C performance will be reduced for a gain that will not be noticeable. Any gain you'd get would be far less than the run to run variation in dynos. Good luck too if you're running a high power aftermarket stereo. Bad call in my view but it's your money, your car. Don
 

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The OP is having a cam put in. If the new cam requires a higher idle, the charging/alternator argument is moot. I'm not sure I would put the AC pulley on, though, especially with a 25% crank pulley.
 

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Discussion Starter · #14 ·
Why change from stock pulley sizes? What’s the benefit?


2016 SSB, stick, hole, spare
I agree, HP gain will be minimal and engine cooling and A/C performance will be reduced for a gain that will not be noticeable. Any gain you'd get would be far less than the run to run variation in dynos. Good luck too if you're running a high power aftermarket stereo. Bad call in my view but it's your money, your car. Don
I'm doing a cam also and I have no plans on running a super big sound system. If I do a system it will be small... And since I'm doing the cam anyways it was recommended that I do the pulley anyways helps with harmonics
 

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Did you ever figure out the belt sizes needed for a 25% underdrive pulley?
 

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May not get an answer. Almost five years since last login.
 

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Lol I have to thank you for bumping it so I remember where to look when the Fbody needs a new belt with the ATI, because I didn't write the size down anywhere at home!
 

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May not get an answer. Almost five years since last login.
Of course, the OP might still be swapping in different belt sizes, so there's that...
 

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Anything is possible, it seems, if one just keeps believing....
 
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