I have a 2# lower and a 3 1/2 inch upper that came with my KB kit and am using a gatorback #4080685 belt - all good. Went to put a 2 1/2 inch upper on and saw that my belt is just not gonna work with it (should have figured that out ahead of time -duhh). Anybody know what belt I need to use with the new 2 1/2 inch upper?

Thanks,

Mark

 

I've been seeking the answer to a similar problem. Apparently the Gatorbacks come in 3/4" increments as designated by the last three digits in the parts number. xxx0685 indicates that you have a 68.5" belt. My mechanic tells me that the tensioner puller has a range of adjustment of a maximum of 3/4", but optimally no more than 1/2". Now I have calculated that a reduction in snout pulley diameter of 1/2" would result in a reduction in belt length of approximately 8/10ths of an inch. But I was never very good at math, so some engineering type may want to correct me. At any rate, depending on how tight the 4080685 was to begin with, my guess is that you need to shorten it an inch. Unfortunately, I do not see a 4080675 in the Goodyear catalog, but a 4080670 would seem to come pretty close. Apparently some vendors are now selling adjustable pulley kits, but I don't know exactly how they work or if they will solve our problem Perhaps they will chime in and explain?

 

Hmmm, been doing some math myself, and came up with a different answer. Here's what I figured. Someone agree or shoot it to h#ll:
3 1/2 inch original upper - change to 2 1/2 inch upper

shorter by: 1/2 inch in height
plus 1/2 the difference in circumference (2 x r x pi)
which i figured to be around 1.57 inches

So I figure the belt needs to be shorter by a little over 2 inches. It seems our only difference Alphadoggy, is my addition of 1/2 inch for the difference in height of the pulleys.

Input???

Mark

 

That is what combo I have used for most of the time I have owned the KB. I used the stock belt.

I only got about 10 psi with the 2lb lower and 3.5" upper.
I get around 22psi with the 2lb lower and the 2.5" upper.

 

Holly Cr@p, that is quite a jump in boost!!! My autometer gauge with the 3 1/2 and 2# lower said 13 -14 #'s. and I was expecting around maybe 18 #'s with the 2 1/2 upper. JP said my burn was good for either pully, but that is quite a spread in boost. Guess I'll keep my ears open for detonation, but with my d@mn exhaust it is hard to hear.

Mark

 

My 2lb lower is 7.9".

7.9/3.5=2.26:1(blower:crank) 2.26x5500 rpm=12430rpm
7.9/2.5=3.16:1 3.16x5500 rpm=17380

I'm sure your results will be different than mine. My truck seems to be different than everyone elses. I just got a 3" upper which gives me 18psi when used with the 6lb lower(2.97:1 or 16,317 blower rpm.)



















pm).

 

Put on a stock size belt (66.7 inches). Seems to fit fine, am about ready to put it to the road. Extra 6#'s of boost should bring a grin


Mark

 

I think you're right, Mark, the stock 66.5" belt should work with the 3" and 6 lb. pulley combo. Let us know how it turns out.

 

With a 2# lower and a 2 1/2 inch upper, the stock belt size fits fine. Matter of fact it is a little tighter than my previous belt and the noise (not whine - mechanical) from the KB has subsided considerably. Guess there must be some play in that drive and the tighter belt snugged it up a bit.

Unfortunately my test drive did not go well. Drove fine at street speeds and lower throttle. When I jumped on it on the highway, it started really crapping out, so I got off it immediately. Gonna go out in the garage and pull some plugs and see how they look.
Boost had gotten around 18# when it started missing bad. Does anyone know if the Denso 22's can handle that boost?

Have 50# injectors, so if they are working right, fuel supply should not be a problem (crappy octane though-supreme is 90). The tune is a conservative one and is being used by others with that boost level. Kind of a hasstle being so far away from a tuner, troubleshooting by Fed-ex and telephone is not the way to go!!

Mark