Jeg's is advertising carbs for a supercharged engine that have a "manifold referenced power valve". How is that different from normal carb? I thought the power valve always compared the manifold pressure to the ambient pressure, and started to open at 6.5 or 8.5 inches of vacuum, or whatever the power valve spring was set to. Are they saying there's a tap to feed a line around the roots blower, and get a true intake manifold reference, after the blower? I guess I could see how that might give a little earlier opening of the power valve.

A roots blower will create vacuum under the carb, especially at high rpm's. If you don't reference the vacuum to the manifold, you could end up closing the power valve at high rpm's. We recently had a 40 Willys on the dyno whose main complaint was that his exhaust system was red hot all the time. Turns out he had his custom modified boost referenced power valves incorrectly plumbed, causing the mixture to be extremely rich at everything above an idle. Couple his with retarded timing, and you have some very hot headers.

I would think a lean condition, not rich would cause the exhaust to get red hot?

Dan, your theory about what would happen with the power valve on a roots blown engine for a boost-referenced vs. non-boost-referenced carb lines up exactly with what I've actually expereinced on my mild roots-blown 383.

The engine won't go into boost until manifold vacuum drops below about 5" under the carb, and this cross-over point is pretty constant regarless of RPM or throttle position. At WOT I'm still seeing only negligible vacuum under the carb even at high RPMs (1-2" at most) with an 800 CFM QJet.

If you're pulling enough vacuum under that carb at WOT to close a 6.5" power valve then you went way WAY too small with the carb!

You just want to make sure that the power valve is a high enough number that you never go into boost at any point when the power valve is still closed. So a 6.5 would probably still be OK on most applications, but you might consider going up to an 8.5 just to stay on the conservative side.

I'm sure it would be nice to have a boost-referenced carb, but I just don't feel like I'm missing much by not having one. BTW- my QJet runs a 7" step-up spring (the QJet equivalent of a power valve) and there is no way to boost reference a QJet.

<font face="Arial,Verdana" size="2">Originally posted by theflash:
I would think a lean condition, not rich would cause the exhaust to get red hot?</font>

Not the case here. This was a 355" SBC with a 6-71 and twin 4777 Holleys modified for a blower motor. He had assumed the same lean theory, and moved up 8 jet sizes in the primaries. The exhaust was so hot is melted the poly motor mounts and burned the back off his alternator. After we jetted back down 8 sizes in the primary, added 3 degrees of timing and provided the proper signal to the power valves, the exhaust system cooled down considerably and the power went up considerably.

Interesting. By chance I'm currently working with a supercharged Northstar engine at work, but I've got some software bugs that prevent me from getting up into the high load, high RPM regions. We don't use a pressure sensor between the throttle body and the blower anyway, that's modeled from the MAP sensor and the compressible flow equation. Still, it would be interesting to see, assuming I can get the correct cals, what the pressure under the throttle body is at high RPM and load. Maybe if I can solve my software bugs ..