[steelheadrob]

I recently dropped to the 3.25" upper pulley on the Whipple to add more boost for the track. I am not seeing any noticeable boost increase compared to the 3.375" pulley. I am still running the belt whipple sent with the kit that was for the 3.625" pulley. I am wondering if I am getting some belt slip and need to change belts. Do I need to change belts and if yes which one?

Thanks

[Joe's_cool_1le]

I went with the 8" lower on the 3.500 and used the below belt for it. Made the move from 3.25 on the 7" and had a bit of belt slip hence the move. That was before they move all the way up to to an 8.55. Not exactly sure the belt size so attaching the pic.

[KingLT1]

Try a green belt.

[steelheadrob]

Originally Posted by Joe's_cool_1le

I went with the 8" lower on the 3.500 and used the below belt for it. Made the move from 3.25 on the 7" and had a bit of belt slip hence the move. That was before they move all the way up to to an 8.55. Not exactly sure the belt size so attaching the pic.

Nothing comes up with that number mine is a k100816 belt (10 rib 81.6") the next size down is a K100810 (10 rib 81.0"). Does you belt have a K# on it? Thanks

[Joe's_cool_1le]

Originally Posted by steelheadrob

Nothing comes up with that number mine is a k100816 belt (10 rib 81.6") the next size down is a K100810 (10 rib 81.0"). Does you belt have a K# on it? Thanks

https://www.jegs.com/i/Gates/465/K100840/10002/-1
Outside Circumference (in): 84.625"

[Joe's_cool_1le]

This belt left the tensioner adjusted only to about a quarter of the slot leaving 3 quarters left for adjustment. It has worked well but would have preferred a tad more length. I did have to replace my tensioner bearings last week. The only reason why I mention that is because after changing to the 8.0 lower with a new belt I noticed hearing a bit of whine a few weeks later. Really enjoyed the whine but knew it wasn't right. Sure enough it was the tensioner bearings.... when replacing it's a lot cheaper to buy the bearings vs Whipple selling you a new pulley @ 170 plus shipping.

[Joshinator99]

Originally Posted by steelheadrob

Nothing comes up with that number mine is a k100816 belt (10 rib 81.6") the next size down is a K100810 (10 rib 81.0"). Does you belt have a K# on it? Thanks

Joes belt is the opposite of what you need. He has a larger lower pulley. You actually need to go smaller than what you have now.

3.5 pulley x 3.14 = 10.99” outside diameter, 2/3 belt wrap is 7.25” of wrap
3.25 pulley x 3.14 = 10.20” outside diameter, 2/3 belt wrap is 6.73” of wrap
I’m assuming same lower pulley in both scenarios. So you need to cut out about 0.5”, so the 81.0” sounds right on IMO.

Sounds good, right? I’ve never actually done it, but since I’ve copied most of your build, feel free to be the guinea pig so I can do this change too...

[Joe's_cool_1le]

I should have mentioned the reason why I went from the 3.25 upper and 7.0 lower to the 3.500 upper and 8.0 lower. I was getting belt slip banging gears at max RPM's. It helped with my belt slip and didn't push the blower much more.

crank pulley/super charger pulley = overdrive ratio. engine RPM = SC speed
7.0/3.25 @ 6500 = 1400
8.0/3.50 @ 6500 = 1485

But if you used your 6.25 (Would have used this but Whipple sent a 3.500 with my kit)
8.0/3.625 @ 6500 = 1434

[steelheadrob]

Thanks for the responses. I found a green belt that was 81.4", so I ordered that to try first. If that doesnt do it, I will go down to the 81" or maybe go the route of Joe and change out the lower pulley. I will report back what I see after I change to the 81.4

[Joshinator99]

Originally Posted by steelheadrob

Thanks for the responses. I found a green belt that was 81.4", so I ordered that to try first. If that doesnt do it, I will go down to the 81" or maybe go the route of Joe and change out the lower pulley. I will report back what I see after I change to the 81.4

Not sure that will be enough of a difference, but hope I’m wrong! Looking forward to the results!

[Joshinator99]

Hey Rob, any update on your belt/pulley situation?

[steelheadrob]

Originally Posted by Joshinator99

Hey Rob, any update on your belt/pulley situation?

No, went to the lake labor weekend so havent had time to try it out. Going to install the new belt tomorrow morning and test it out. I will let you know how it works.

[steelheadrob]

Originally Posted by steelheadrob

Thanks for the responses. I found a green belt that was 81.4", so I ordered that to try first. If that doesnt do it, I will go down to the 81" or maybe go the route of Joe and change out the lower pulley. I will report back what I see after I change to the 81.4

Here is a update on my belt testing. I first tried the k100814hd green belt, 81.4", I noticed a very minor increase in boost, not enough to really make a difference. I ended up ordering the next size down k100810 81.0" belt. This belt ended up being too short, does not fit. Looks like if I want to add boost I going to have to go the larger lower pulley. Which is probably not something I will consider unless I go forged down the road. Heading to the track this weekend, I will see if the 3.25" shows any improvement in 1/4 times despite lack of boost increase I am seeing in street testing.

[Joshinator99]

Originally Posted by steelheadrob

Here is a update on my belt testing. I first tried the k100814hd green belt, 81.4", I noticed a very minor increase in boost, not enough to really make a difference. I ended up ordering the next size down k100810 81.0" belt. This belt ended up being too short, does not fit. Looks like if I want to add boost I going to have to go the larger lower pulley. Which is probably not something I will consider unless I go forged down the road. Heading to the track this weekend, I will see if the 3.25" shows any improvement in 1/4 times despite lack of boost increase I am seeing in street testing.

I just went from 3.625” down to 3.500” yesterday. After taking a close look at the belt situation, I’m not surprised at all that you were slipping with the 3.250” one. There wasn’t a ton of adjustment available left IMO...kinda sucks.

Did you double check the number on the 81.0” belt itself? Between your smaller pulley and the tensioner adjustment, I can’t believe that didn’t work?!?