Back to basics....L vs. HD Pulley Combo's...Explained in Chart/Graph Form

[Harley#356]

the cog isn't going to make any more boost than a non-cog pulley combo of the same size. And it'll actually be closer to the actual values on the chart since it won't lose boost due to belt slip.

8 lower with a stock size 3.25 upper is 15346 blower RPM which is 12.26 theoretical lbs of boost. With the stock manifolds still the high flow cats won't really lose much noticible boost, and you'll pickup an easy 2 lbs from the intake and thundertube, so you're looking at around 14.25-14.5 lbs of boost with an eaton. Now the MP I'm assuming its like the ported eatons where you can pickup boost just from the smooth blower case and improved inlet of the MP, so if that gains 2 lbs you're probably looking at 16.5 lbs

It's doable with a good tune, but it is on the edge of that safety margin. Now that said I know guys with 20 lbs on their stock block with a MP for racing and running perfectly fine for the last few years, but personally its more than I'd feel comfortable with. I'd drop to a 6 lower if you can get one in a cog.

[ryansmithm3]

the cog isn't going to make any more boost than a non-cog pulley combo of the same size. And it'll actually be closer to the actual values on the chart since it won't lose boost due to belt slip.

8 lower with a stock size 3.25 upper is 15346 blower RPM which is 12.26 theoretical lbs of boost. With the stock manifolds still the high flow cats won't really lose much noticible boost, and you'll pickup an easy 2 lbs from the intake and thundertube, so you're looking at around 14.25-14.5 lbs of boost with an eaton. Now the MP I'm assuming its like the ported eatons where you can pickup boost just from the smooth blower case and improved inlet of the MP, so if that gains 2 lbs you're probably looking at 16.5 lbs

It's doable with a good tune, but it is on the edge of that safety margin. Now that said I know guys with 20 lbs on their stock block with a MP for racing and running perfectly fine for the last few years, but personally its more than I'd feel comfortable with. I'd drop to a 6 lower if you can get one in a cog.

Nice chart and makes since...that being said I am putting an 8pound lower on my 03 HD because i have faith in your chart and I'm in florida so its also hot. Am i going to be okay running an 8 pound lower stock upper and CAI with the canned pulley tune from diablosport?????????????????????????

[Spoolx]

Great information. Quick question, would it make sense for someone with a Harley truck to start with upgrading to a lightning upper pulley?

[Black Molly]

Great information. Quick question, would it make sense for someone with a Harley truck to start with upgrading to a lightning upper pulley?

Well yes and no...do you plan to do a lower pelley? If yes i would stick with the larger upper pulley so you have more belt rap and put a 8lb pulley on the bottom to make 14 to 15 pounds of boost which is about as much as you want on stock block and eaton....if i had to do it over again i would just throw a whipple twin screw on and be done