I currently have a Mighty Demon 750 right now. It works perfect. Never had one issue with it. However, being the type of guy that loves to tinker, and loves absorbing data, I decided to check how much vacuum I had at full throttle. I did a few runs and surprisingly found the gauge showing 2 - 3 inches of vacuum in 3rd gear at 6500 rpms which is where I cross the stripe. So I borrowed another vacuum gauge and got pretty much the same results.

How true is the rule of thumb that you shouldnt have more than 1- 1.5 inches of vacuum at the end of the track? I just find it hard to believe that I need an 850 with my setup. Could my 750 not be as close to 750 as I think it is? Its an Demon, probably bought it 8 years ago or close to it. I have heard that they typically flow very close to their advertised cfm, but maybe the older ones were not as good?

 

Back a few years ago I talked to a BG Tech. He was telling me that the 750 Mighty Demon (I have the same) would flow 930cfm +/- on a Holley flow bench since Holleys are dry flow rated where Demons are wet flow rated. He also said this is where a lot of bad mouthing came from when guys would go from Holleys to Demons and have tuning issues.

 

Yea Ive heard that the Demons are supposed to flow good. Thats why I cant understand the vacuum restriction. Already tried 2 different gauges like I said. I may be able to borrow an 850 and see what happens. Even if it doesnt improve much, I'm really curious now to see what the vacuum gauge reads with a different carb.

 

I would think you would need some type (amount) of vacuum since an engine is basically an air pump.

Suck, squeeze, bang, blow

 

Found the following info from a SuperChevy article. How much hp difference can a half inch of vacuum really make?? I'd love to test 3 or 4 different sized carbs if I had the money.



http://www.superchevy.com/how-to/eng...t-carburetors/

Starting with the smallest carb option from the above formula, get a vacuum gauge and connect it to your engine’s intake manifold.


Seeing 1.5 inches of vacuum at WOT means you’ve nailed your carb sizing perfectly.

Seeing around 2.5 inches of vacuum shows restriction—and the need for a slightly larger carburetor.

Seeing 1 inch of vacuum or lower means the carb is too big for your engine, and it needs to be smaller.