Kenne Bell, FTW.. Decisive Victory

merc123 said:

I was reading on the KB website and they give an equation for figuring S/C RPM's. It's something like (engine RPM * gear ratio) = S/C RPM's. They say you want to keep it under 18,000 for the S/C. So if you want a 5800 redline you have to have a 3.10 gear ratio. With 3.73's by that calculation you can only turn 4825 RPM's safely. I dunno though. They said that 3.73's are the best gear for the KB and that's what most recommend.

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Gearbanger 101 said:

Possibly, but I don't imagine levels would increase that significantly. After all, it's going to have be pullied to spin a little slower than the smaller blower because of it's physically larger size and as a result may not be as effective as the smaller unit at lower speeds.

Here's why.....Traditionally the trade off when employing the use of a larger blower is it’s more prone to the effects of leak down at lower, slower blower speeds due to the increased sealing length and surface area of the longer/larger lobes/rotors. Although over all volume with the larger unit is increases with each revolution, spinning the larger blower at the same RPM that you would the smaller blower would result in lesser initial boost levels than that of the smaller compressor because more air is getting by the surface gaps between the intermeshing screws. In theory….one revolution should be equal to the volume of 1.7…2.1…(or whatever capacity the unit is) cubic liters of air, but because of leakage, friction, different air densities at different temperatures, etc, real world conditions don’t allow this to happen.

So, to counter the effect of the initial “lag” so to speak, one would need to increase the blower RPM of the bigger unit in order to overcome said issues and start making boost figures at lower engine RPM, that are on par with a smaller blower. This is accomplished by switching out to a smaller driven pulley size to alter the blower ratio and make the compressor itself spin up faster at any give engine RPM, which in turn will allow boost levels to come on sooner. The down side to this is that although the pulley swap allows boost to now come on sooner at a lower engine RPM like it did before with the smaller blower, it also as a side effect increases low speed discharge temps due to increased friction from the now faster spinning lobes/rotors. Not only that, but blower speed has also increased all the way across the board....not just down low to overcome the initial leakage. This leads to an increase in over all air flow levels at all RPM right to peek. Great for making power, but not so great when you're trying to keep the stock bottom end in its happy place. This is why it’s not recommended to run a big blower on a small horsepower budget. Running any blower below or past its efficiency range is a waste of potential….with respect to either the blower itself of the rest of the combination. This is the reason one size does not fit all and bigger is not always better.

That being said, Kenne Bell claims a new rotor profile will help increase over all efficiency levels some over their previous design, which will in turn reduce low speek leakage and friction......but only real world testing will determine this for certain.

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