: Pulley changes and thier configuration

08-01-09, 11:30 AM
Hi guys, here is a list of what RATIOS you will be at with when you run certain pulley size configurations.

STOCK PULLEY UPPER SIZE IS 2 7/8. Stock lower is 8 inch.

Ratios are simple to see differences of. You just take the lower pully and divide it by the upper.

So the stock with 2.87 upper pulley makes a supercharger ratio of 2.79.

With Just a 2.55 upper the ratio only changes it to 3.13.

A upper 2.55 pulley and 8.5 lower makes a ratio of 3.33

You get better grip with less belt slip in this configuration and less parasitic loss.. Also sets you up for future boost as parts become available to adjust boost levels.

A 9.5 LOWER and STOCK upper makes a supercharger ratio of 3.3

Works well, Almost as much horsepower as just a 2.55 upper pulley.

A 10inch lower and stock upper makes a ratio of 3.48

Slips the belt when cold, Working on a fix for more spring tension and better idler placement now...

08-01-09, 12:03 PM
Thanks for clearing that up, makes a lot more sense now.

Boy do I feel like an idiot. When I measured the upper stock pulley it was late at night and dark, if you know it, i was measuring the wrong pulley!


08-01-09, 04:00 PM
thanks :) i am planning something with the info you gave :lildevil:

08-01-09, 05:50 PM

Thanks for the informative post!

A few questions:

(1) Can you provide the (approximate) rwhp we might expect with each of the configurations you list, or at least the relative increases in rwhp we might expect as one moves up each step in the supercharger ratios?

(2) You note that a 9.5 lower-pulley-only mod gives about the same supercharger ratio as the combination of an upper 2.55 pulley and 8.5 lower pulley. You also note that the 9.5 “works well.” But in your comments regarding the 2.55/8.5 combo, you also state that “You get better grip with less belt slip in this configuration and less parasitic loss. Also sets you up for future boost as parts become available to adjust boost levels.” So does this mean that you prefer this combo to the 9.5 lower-pulley-only mod, and if so, is this because one will see less “belt pulley slippage” or because there is some other performance advantage in combining the upper and lower pulley mods? For example, in a recent post, CadV reported that he thought you had told him that a combo pulley provided some performance advantages over a crank (lower) pulley only – in which CadV seemed to imply that the advantage was the combo pulley allowed the boost to come on more quickly when at a lower r.p.m. than does the modification with the lower pulley only. Possibly you could clarify whether this might be true.

(3) In a post yesterday, radix raised concerns regarding spinning the blower at such a high r.p.m. that one reached a point of diminishing returns regarding any horsepower increases (see http://www.cadillacforums.com/forums/cadillac-cts-v-series-forum-2009/176043-w4m-2-55-8-5-pulley.html). And on a different thread (“Am I the only one”), d3Mark raised reliability concerns when boosting much more than 3 lbs. or so extra pressure. So when you appear to suggest a “sweet spot” based on using either the 2.55/8.5 combo or the 9.5 lower only (both of which provide about a 3.3 supercharger ratio), are you suggesting that you would not recommend that persons go above the supercharger ratio of about 3.3 – possibly for the reasons radix and d3Mark raise in their posts?

(4) In some of your much earlier posts, I got the impression that you greatly preferred doing crank (lower) pulley mods only. You seemed to suggest that blower (upper) pulley mods can just create a lot of problems (as well as make it more likely that your warranty is voided) – a viewpoint that some other moders seemed to share with you. As you’ve gained more experience implementing the various pulley configurations, have you now changed your opinion regarding this issue?