Sorry to post this if this is a faq.. the search function doesn't seem to be turning anything up on this for me.. and sorry for the simple nature of the question, I'm not familiar with the implementation details of FI.

Anyway, I was riding in a friend's supercharged vehicle the other day, and observed the behavior of his boost gauge. If he accelerated moderately through the RPM range, the boost gauge stayed quiet, but if he stomped on the gas, the boost gauge would respond. Am I correct to assume that this means there was a clutch of some sort between the supercharger and the crankshaft? If the supercharger were spinning purely according to crankshaft RPM, wouldn't the boost also be purely a function of RPM, not also throttle position? Or am I missing something?

Thanks for any help you can provide.. and if this question is already answered somewhere, if you could point me to it, I'd appreciate it..

thanks..

 

JRSC has a bypass that does just this... it bypasses air past the rotors directly to the motor..

 

Assuming it's a supercharged Honda with a Jackson racing kit on it, you were observing the bypass valve (relieves the load of building boost pressure when you don't need it). Other vehicles such as the supercharged Mercedes engines also have a clutch to take the load of turnin the SC completely off the engine.

 

cool.. thanks to both of you.. do most turbo implementations have the same thing, then? or are the BOV and Wastegate the only things regulating what boost is generated (except for the usual turbine-compressor dynamics, of course)?

 

The difference is, a turbocharger doesn't start to generate a significant amount of boost until the engine is under load and produces enough exhaust to spin up the turbine. When you're cruising, the turbo is spinning much slower... but by nature, a centrifugal compressor can pass enough air through it for the engine to run even if it was not spinning at all. A roots-type positive displacement supercharger, on the other hand MUST have a bypass valve because if it's spinning, it's generating boost, and if it's not spinning, air simply can't get through it.