JackShaft Pulley, How big can you go?

I'm about to make my own jackshaft pulley in a few days and I'm wondering how big is too big? Ideally I wanted to make it a 15% pulley and with the addition to my 5% on the blower I'd have a total of 20%. I'm not so worried about the actually percentage, but rather how big can I go diameter wise on the jackshaft end before I start to run into clearance problems?

I believe the stock diameters (pitch diameters) are:

3.923" (Crank end)
4.805" (SC End)

I wanted to just increase the SC end of the pulley by 15% to get a new pitch diameter of 5.526", and leave the crank end alone, just to avoid buying a new length belt on that end. I could however make the diameter change on bolt ends of the pulley, something like:

3.80" Crank end (3%)
5.38" SC End (12%)

I haven't actually had a chance to determine what would fit in there but has anyone experimented with how large you can go before you start to run into clearance or tensioner issues?

Too big is related more to physical dimensions and fitting. Hitting the tensioner and not having enough room to move the tension to release belt tension. The MP overdrive jackshaft pulley is close to that.

Is it really worth your efforts to do this. Magnum Powers alredy has a 20% od kit if you use the 10% jackshaft and pulley. And you also get extra 2 extra ribs for a better belt grip. Add the stronger tensioner springs and you got yourself a slip proof 20% OD

10 RIB 20% Supercharger and Jackshaft Pulley Kit. This 20% overdrive pulley set has 8 rib pulley to the crank and a 10 rib between the Jackshaft and Supercharger. With the 10% crank pulley below total overdrive is 25% includes Upper Belt. Recommended for All MPx and MPIII superchargers. NOTE: You MUST turn your nose down to 2.20" to run this pulley. Contact MP for details.
$376.00

I guess the reason behind it all was that I wanted to avoid getting a new crank pulley. I have a set of stock size polished aluminum pulleys that I really like, and a 5% ESM pulley that I also would like to keep. Not to mention that I got the materials and the machine time for free.....

I guess I'll have to decide what clears, and what doesn't. I should be able to do the 15% with no issues if I split the ratios between both sides of that pulley. I know they make a 10% jackshaft, so I'm sure a 12% would fit just fine, and I'm assuming that the 3% on the crank side of it can still allow for enough tensioner travel.

I'm not so worried about slip since I think the added contact surface should help. I'll have to make up some templates to see what fits and what doesn't.

One note on the Magnum power overdrive pieces. You can mix and match them with stock pullies just fine... it's just that you end up having to be careful of belt position as you are stuck running a narrower belt on the wider pullies.
I.e. a stock jackshaft pulley with a MP crank pulley means the belt goes on one rib forward of the rear of the crank pulley. The MP blower pulley using the stock jackshaft pulley also has the belt one rib forward of the back. Basically right in the middle.

I see, But if you sell your 5% for like $75-100 bux. Then the MP kit is costing you about $300. And you get a slip prof 20% aluminum pulley set that you can polish to match the crank you already have on. And there is no guessing if there is a belt available that will work with what you want to do.
And it also helps the community.

If this is worth it to you then go for it.

well i got nothing to lose except my time. Material and machine time was free. I'll see what I can come up with in the next few days

It will be cool to see your progress on this, take some pics if you can.
You going to turn the whole thing on a lathe? Seems like making the ribs would be difficult.

bigpoppa822 said:

It will be cool to see your progress on this, take some pics if you can.
You going to turn the whole thing on a lathe? Seems like making the ribs would be difficult.

Click to expand...

Not if you know what you're doing. The ribs should be the easiest part. I'm actually using a threading bit to do the grooves. The angle in addition to the compound rest, I can get the groove at 40*. The rest is just a boring bar and a turning bit. I'll have to face it off, flip the part and then do the details on the front. Then finish it off in the mill to do the hole pattern. If you have the prints, with the correct datum points, it should go fairly smooth. A DRO helps too...

fastsc92 said:

Not if you know what you're doing. The ribs should be the easiest part. I'm actually using a threading bit to do the grooves. The angle in addition to the compound rest, I can get the groove at 40*. The rest is just a boring bar and a turning bit. I'll have to face it off, flip the part and then do the details on the front. Then finish it off in the mill to do the hole pattern. If you have the prints, with the correct datum points, it should go fairly smooth. A DRO helps too...

Click to expand...

How long did it take you to whip up that drawing? I can't wait to get to do stuff like that back at school.

took about an hour, then around 20 min for each additional ratio combination that I wanted. I may modify it again to make it a little easier to machine.