I started out with a 351W and a c-6 with 3.25 gears running a 600 Holley. I got an average of 15.5 mpg on the interstate.
I've since went to an AOD, added full MSD, compcams Extreme 4x4 cam and 3.7 gears, I figured the overdive would more than offset the change to 3.70's I was wrong as now I'm luckey to get 12 mpg.
I'm thinking something just is'nt working right with something else but I can't figure it out.
The combination runs great except a constant battle with spark knock.
I ran a vacuum gaude into the cab and went for a ride. Found out in OD going up a grade I was dropping down real close if not past the 6.5 of my powervalve so i replaced it with a 5.5 but that did'nt seem to help.
This is one reason I am trying to figure out a good water injection method.
Anyone see anything that I've missed.
I have 70 jets in the front now. I've run it with 65 and 67. It runs best with the 70's and helps with the sparkknock
I just feel like I should be able to get at leat 6 with this combo. Heck when I just has a 351W with a holly 600 and 3 speed on the column I regularly got 18 mpg taking it easy.
Clint

 

I think you're in the same boat I am, you need to back the vacuum advance off a good bit. Try this:

if your vacuum advance is hooked up to the ported vacuum signal from the carb, then it will kick in around 1200-1500 RPM. Back your base timing down to around 6 DBTDC, or until there is a slight puff of smoke from the tailpipe at idle. With the smoke visible from the side view mirror, apply brake and throttle until smoke goes away. If it does this at around 1200-1500 RPM, then it's your vacuum advance pushing your spark advance too far. You can't really compensate for too much vacuum advance by dropping your initial timing because the vacuum can push it up about 15-22 degrees in some cases, and there is a much tighter tolerance for initial timing. You need to find out how to tone down your vacuum advance until it no longer pings. In fact, you could also try capping the advance and see what happens then. It certianly won't help your fuel milage issue by capping the vacuum advance, but if it gets rid of the pinging then at least you know who the culprit is. The problem we both have is that we built our engines to very tight specs, and they run very healthy and make a strong vacuum signal. The MSD distributor that you and I have is tuned for kids driving Camaros and Mustangs that make little, erratic vacuum because they have way too much cam in them. The Comp Xtreme cam you have will actually increase your vacuum siganl, so you can only imagine what that does to your timing when you hook up a vacuum advance. Vacuum advance is still a great thing, no doubt it saves a good deal of fuel for street driving, but it must be tamed and tailored to your engine. When I figure out what the heck I'm doing with mine, I'll let you know. Until then, I'm just like you, all these wonderful ideas that may or may not work, and driving a truck that pings like a power stroke and sucks fuel *almost* as bad as a Triton. TK

*edit* If your pinging starts at part-throttle, then it's definately a vacuum advance issue. If you can mash the throttle and get rid of it, then you're pinpointed the problem. There is very little or no vacuum at WOT, so that in effect should stop the ping if it's a vacuum issue. Mine starts under light load/high vacuum situations, so that's why I'm somewhat sure my issue is of vac. advance.

 

Yes, sounds like we are in the same boat. I have an adjustable vacuum advance on my MSD that I tinker with often. If you don't have one let me know and I'll tell you how to get one.
I just can't figure why my mileage sucks so bad. At 70 mph I'm turning 1900-2000 around in there. Man that aint very high. That 351w is just loafing up the road and should'nt be sucking it down so fast
Clint

 

Lucky to get 12mpg? I dream of 12mpg!
I have an 86 F-150. It's got a 351w. C6 with 4bbl Holley, all rebuilt last year. I get 9 in town and 14 hwy if i'm lucky.and the power isn't there. Could the problem be the vaccum advance?

 

Jeff, it's also got a big cam in it. I get 15 in town and on the highway runnin 70.