Fuel System: MPG With Demon Carb

[JoeHall]

Today, I finally got a chance to take the 56J out for 175 miles gas mileage check. The trip was about 20 percent rural roads, and 80 percent Interstate. and speed was 70-75 on the interstate. The car got 17 3/4 MPG. On the same trip, which I have made many times, the same car woulda got around 20 MPG, with the AFB/Edelbrock. So gas mileage is down about 10-11 percent with the Demon.

Considering how well the Demon does everything so far, I am not gonna try to squeeze a bit more MPG out of it. Everything is a trade off, and I do not want to mess with an excellent performing carb. Will just have to spring for the extra bucks for gas.

[52-fan]

It would be interesting to see how the mileage was on only rural roads and then only interstate, but that's probably too much trouble.

[brngarage]

Hi Joe. Just for my curiosity, is the 56J an overdrive car or automatic? And stock other than the Demon carb? Rear end ratio?

[Corley]

I just wonder about a couple things... It may run a touch cooler, with the richer mixture? Is it rich enough to cause any cylinder wash down? Being richer, will you need to back off timing a tad? Is your vacuum advance connected to manifold vacuum, or ported vacuum now? While this is not terrible mileage, an overly rich mixture can cause greater engine wear and oil contamination...

[JoeHall]

Hi Joe. Just for my curiosity, is the 56J an overdrive car or automatic? And stock other than the Demon carb? Rear end ratio?

It was an Ultramatic with stock 3.07 rear end, when I first got it in 1986. After a few thousand miles, I converted it to T85/OD and 3.31 rear end. I keep meticulous operation & maintenance logs on all Studes. Per this 56J's log, with the Ultramatic, 3.07, and original WCFB carb got a little over 17 MPG on the Interstate. Once converted to the OD and 3.31 rear end, it has always gotten 20+ MPG on the Interstate. I had another 56J that I set up with the OD and 3.31, and it also got 20+ MPG on the Interstate. I advised my bro-in-law to set his up the same way, and it got 20+ MPG. My two 56Js had AFB/Edelbrock carbs, and my bro-in-law's had a WCFB. So most any 56J, with a healthy motor and proper gearing, will surprise most folks with the MPG they are capable of. With their higher compression (9:1), which they tolerate very well, they are very efficient motors, for their time.

At 17.75 MPG recently, on combined roads, this 56J's MPG is down 10-11 percent, and I attribute that to the Demon. Down the road, I will likely lean the Demon out a bit. But as mentioned, its running so well, I hate to tinker with it. (But that has never stopped me before. LOL)

[brngarage]

Thanks Joe. As you know, mine is a factory T85 w/overdrive and it has a 4.09 rear end. I suspect that will make my mileage a little lower, but it should still be OK.