I can vouch for the effectiveness of the J&S as a friend used it in his F3 car (which did not have any knock detection at all). Was very easy to setup and use. Knock was reduced to undetectable trace levels once it was set up. As you say worth it for the insurance alone.
Answering post #21 though: the difference between a Roots eg 8-71 and a Whipple (Lysholm principle) is that the Lysholm has internal compression whereas the Roots is just a moves more air than the engine can take (thus creating boost).
This means that the Lysholm device creates the boost internally rather than the roots which creates the boost in the plenum (as it pumps more than the engine does). This sounds a small difference but in practice it results in improved efficiency. Roots are normally 45-55% efficient whereas a Lysholm device is typically 5-10 points better than that (depending on application obviously!). Better efficiency means lower charge temps and less power required from the belt to make any given amount of boost/massflow.
In a perfect world you'd be able to hookup blower bypass valves to your knock detection so it all went pear shaped, you'd be able to dump the boost back to the blower inlet at the point where the knock detection noticed something was going badly wrong. Of course you'd need to be running port EFI to do that though, I'd have thought.
One of the things to be aware of with blowers is that although your overall mixture may be OK, you can still have problems where the AFR from one cylinder to the next varies a lot. Individual cylinder knock detection will help a lot here as obviously the slightly lean cylinders won't tolerate as much spark as the richer ones.
It depends on the design. A twisted Roots reduces pulsation and consequent noise but does not have an internal CR. The degree of internal compression is the key to the difference rather than just a lobed design with a twist. It is this compression that makes the device more efficient.
From a purely engineering POV you would use the term Roots to describe an older straight vane blower with no internal compression whereas the Lysholm design certainly does feature internal compression which is more efficient.
Newer Eaton devices are twisted but still do not feature internal compression. They still generate pressure at the exit port where the greater flow of the blower exceeds the engine's demands. This is a less efficient thermodynamic process.
OK, I'm with you there.
Let me ask in a different way- How do they create internal compression?
I understand that by the timing of a 3-lobe roots you get sequential pulses each time a meshed lobe pair opens up and that the continuous mesh of a Lysholm gives you a somewhat continuous discharge. The question is still, how does that differ from high-helix?
I've looked on-line before for a better technical description of the theory and benefits of high-helix and have found little- beyond "I's just different" Any sources?
From what I remember, the exit porting of the twisted screw roots type blowers when compared with a Lysholm is quite different.
I've not looked at one of these for a while and things have changed quite a lot recently in the SC industry as Eaton has also started producing Lysholm type SCs as well alongside their older roots designs (just to confuse things).
I'll have to dig out some pics when I get to the office Monday.
Last edited by Ruaraidh; 11-30-2007 at 02:31 PM. Reason: got that bit wrong!
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