What size belt for a 3.50 inch upper and 4# lower crank pulley on a KB or Whipple?

[Dominick32]

What size belt should I use for a 3.50 (13lb.) inch upper and 4# lower crank pulley on a KB or Whipple?
There will be no aux. idlers in the setup..

Thanks,
Dom

 

[Silver 03 Cobra]

I have a 78.62 Gates on my KB with a 3.5 upper and a four pound lower and a 90 mm idler. It is tight and I have to slip the belt on the blower pulley and then screw the pulley to the blower to be able to get it on the engine. Then tensioner is fully compressed. It will work. You may be able to get a 78 on your blower without an idler though.

 

[Dominick32]

Originally posted by Silver03Snake
I have a 78.62 Gates on my KB with a 3.5 upper and a four pound lower and a 90 mm idler. It is tight and I have to slip the belt on the blower pulley and then screw the pulley to the blower to be able to get it on the engine. Then tensioner is fully compressed. It will work. You may be able to get a 78 on your blower without an idler though.

Thanks alot man.
How much boost are you making with that setup?
Dom

 

[Silver 03 Cobra]

I have a problem that I am trying to work out. I had it dyno tuned in Charlotte last week and it made around 17 psi. The tuner has a portable dyno so I was having to look up from the ground while he was making pulls but it was definately over 15 psi. On the way home I took it easy until a few miles from home. That's when I found a problem. It would nail 16 psi at WOT but then would drop back to 13 psi so fast the gauge needle almost looks like it snaped. I have an aftermarket boost gauge. The boost drop was so fast I really doubted it was belt slip. I have my KB boost lines plumbed differently than KB shows. I have checked this out. I have checked out my dump valve. All are sound. I tried another boost gauge yesterday. It was cool here in the NC foothills and the engine made good boost and held. I thought it was the gauge. Just before I got home I did a couple more runs and sure enough the problem was still there. I looked at my KB and I saw that belt dust was beginning to build up. I am now convinced it is slip.

There are a few things I wonder about the KB. I think it is probably as prone to belt slip as the Eaton. The KB makes more boost at less revs. That is where it's gaining it's effeciency gains over the Eaton. It's still taking a lot of HP to turn the KB but it's pumping more air at less revs. When you go to a lower you are increasing the belt speed and I think that may cause too much slip on the KB pulley. The other thing I found was that Gates belts just do not grip nearly as well as a GY Gatorback. A belt that does not have notches has trouble making turrns over small diameters because of the resistance to bend. Goodyear makes a 78 inch belt. I think the dealer told me the next size was 81 inches. Way too big. Here is what I am doing.

I just ordered a 3.25 non slip KB pulley from South Florida Pulley. The 3.25 is 15 pounds versus the 3.5/13 pound I'm running now. I am going to order a two pound lower form Metco Monday morning to replace my four pound. Going to these two sizes will allow my 78 inch Gatorback belt to fit. The belt speed will be slower going to a smaller lower but I have a smaller upper so I still have the same psi on the blower. I'm hoping the trade off in belt speed will overcome the smaller grip area on the smaller upper plus it's a non-slip. I would like to get 17 psi. Yesterday at around 16 psi when it held this thing was fast even compared to the 500 rwhp I was making with the Eaton. I wondered how 100 more wheel HP would feel when you are already at high levels. You definately notice it especially mid range up. The car has a lot more punch at speed. At 80-100 mph in third it would lay down rubber with just throttle but I also have 3.90 gears that are about to be swapped back to 3.55's.