[TCSGTO]

Got to put some miles on the 517 this weekend and was able to record some mileage figures. Over a 200 mile loop I got 10.3 on Citgo 93. This included a few full throttle blasts, some tire smoke and a mix of some 70Mph highway running and mostly stop and go city driving. Pretty much just cruising around and not trying to “economize”.

Currently running a 950 QFT Frankencarb with 76/86 jets,25/28 MAB’s,73 IAB’s, .130” BLP bottom feed needle and seats, and old school Holley 850 metering blocks with 2 .028” emulsion bleeds per side. 36 degrees total timing. No vacuum advance at the moment but I plan to get it hooked up and dialed in.

With the VA working and taking it easy it should get into the teens. I’m good with that. I know economy is not a priority with these cars, and most don’t check or really care, but I’m curious as to what others are getting with their set ups.

[Chief of the 60's]

MPG's and 517? Now there's an oxymoron

[TCSGTO]

Ain’t it though?

[redhawk44]

I'm nowhere near that stage yet but I've heard others mention their 461 getting around 20ish. I believe an overdrive was part of the equation.

[77 TRASHCAN]

What bore and stroke?
I think you will do much better MPG wise.

[TCSGTO]

Originally Posted by 77 TRASHCAN

What bore and stroke?
I think you will do much better MPG wise.

4.350x4.350. I might be able to do better but I know I could do worse. The biggest influence on mileage is the right foot.

[Formulajones]

Pretty good. Vacuum advance should help some.

When dad cruises his somewhat normally, we've done 300 miles on the RT66 cruise, and several 200 mile round trips to Phoenix and back, he's gotten a best of 10.1 MPG and a worst of 9.8 MPG. That's traversing mountain terrain and elevation changes of 1500 feet to 6,000+ feet.

His is a 571 ci with a 950HP that I modified. Runs a vacuum advance that I also modified. But what is killing his is his very loose driving converter. It's just mushy and pretty much just goes up to 3,000 and stays there until the car catches up under very normal throttle application. Couple that with 3.73 gears and 28" tire, he's buzzing 3100 rpm on these trips to go about 62 mph, which is about 7 mph off of where it should be. So the combo isn't really as efficient as it can be and the converter isn't well coupled.

Currently now switching to a more efficient converter from TSP and installing a 3.42 gear as well. If all works as I think it should, it should drop his cruise rpm and pick up his cruise speed, where the car should drive around easily at 65 mph at a more reasonable 27-2800 rpm and the car should drive even better around town. I expect it'll pick up another 1-2 mpg.

Actually MPG has always intrigued me with these classics, since we drive them daily it's always been a goal of mine to make them as efficient as we can. I have all kinds of MPG figures for our fleet of classics here and they are all respectable mpg wise, but I never share much of that info as it doesn't seem to interest many. The way I see it, the more efficient you can make them the more we can enjoy them without putting as much of a dent in the pocket book, and the less excuses can be made about not driving them. Funny just about every week at the gas station I get at least one person that makes a comment about gas mileage when I'm filling up, and then are in disbelief when they find out it's doing as well as the SUV they are driving LOL.

[Chief of the 60's]

I wonder what Larson or Force gets?

[ta man]

Originally Posted by Chief of the 60's

I wonder what Larson or Force gets?

Lutz was getting 12 mpg during dragweek a few years ago. Dave Schroeder from up here that won a few years ago with the nitrous 66?Vette pulls down about 10mpg.

[Cliff R]

My biggest problem back when I did some fuel economy testing was making it thru a full tank without some full throttle blasts, nearly impossible!

I'd start out by adding 10 degrees from the VA. Well thought out engine combo's don't need much.

Mine could care less if you hook it to ported or manifold vacuum and that has next to ZERO effect on the fuel economy anyhow as it's always "all-in" when you are driving at light load, that's an argument for the trolls and those that don't understand engine function.

A good way to know if the engine is going to like VA is to get it running steady about 1800-2000rpms and pull the hose to the advance and see how many RPM's the engine drops. If you have one of the good adjustable units that add timing by turning the screw inside the can you can add more and more to see where it starts to "protest". I'm guessing your engine woln't like, want or need much more than 12-14 degrees.

The VA will be good for a MPG or two no other changes. It also allows for a leaner mixture to be burned at light load, so in conjunction with taming that circuit a bit you will find some decent gains if you spend time in both areas........Cliff

[Formulajones]

Originally Posted by Cliff R

My biggest problem back when I did some fuel economy testing was making it thru a full tank without some full throttle blasts, nearly impossible!

.......

[77 TRASHCAN]

[QUOTE=Cliff R;6058108]My biggest problem back when I did some fuel economy testing was making it thru a full tank without some full throttle blasts, nearly impossible!

I'd start out by adding 10 degrees from the VA. Well thought out engine combo's don't need much.

Mine could care less if you hook it to ported or manifold vacuum and that has next to ZERO effect on the fuel economy anyhow as it's always "all-in" when you are driving at light load, that's an argument for the trolls and those that don't understand engine function.

We're probably OK for the moment, it's past their bed time.

[ta man]

I did a few mpg tests last year..close to 16mpg. Mostly highway with quite a few stops. 60 mph average.
It really shocked me it was that high, I did try and lean the cruise up and my timing is all in at cruise rpm along with ported vacuum advance.
Maybe it's the 15 x 4's in on the front that help?

I also ran 10.8 at the track the day I tested..Same tune.

[Cliff R]

"I did a few mpg tests last year..close to 16mpg. Mostly highway with quite a few stops. 60 mph average.
It really shocked me it was that high"

Doesn't surprise me in the least. First off a well thought out engine build (optimum compression and cam choice) is going to be fuel efficient without a lot of help. Then there is the law of physics, it simply takes a given amount of BTU's to get a given amount of work done. IF you engine isn't dumping tons of heat into the coolant and exhaust system and making great power at very light load then it's going to be thrifty on fuel. Mine is a similar build and it moves my car effortlessly at highway speeds with very little throttle opening. Hence it just sips fuel rather than guzzle it.

The vacuum advance is a very important piece of the puzzle. To this day there are very capable folks building these engines that eliminate the vacuum advance and try to get the timing "all-in" very early. If questions as to why the vacuum advance isn't being used they say they don't need it. However, there is still a LOT of timing missing at light load so you are going to need more fuel in that range and will see more fuel consumption.

I find that fact interesting, that as smart and capable as some of these engine builders are they don't have a clue as to how a vacuum advance works, or why it is needed on a street driven vehicle. Power level has NOTHING to do with it, all N/A engines are going to like, want, need and respond well to some additional timing at very light engine load. I've questioned a few and they seem to think it will ping their engines and destroy them, or they are getting enough timing with the lighter springs and heavier distributor weights they installed.

Here's a related story that happens to me more than it should. I was hired to restore a 1970 W-30 Olds carb. For decades the owner raced the car but retired it and putting the engine back to "stock" and restoring the vehicle. He even source out an original Olds W-30 cam for it. I restored the carb, calibrated it exactly for the engine, and tested it....FLAWLESS.

I send it to him and immediately I get a call. He says it's working very well, idles fine, heavy and full throttle excellent, but a very slight hesitation right off idle and "flat" at light part throttle with very light "surging". He sends it back. I find nothing wrong, and put it on my 455 and logged about 30 miles with it, seamless right off idle, cruises just fine, no issues anyplace. I send it back, he calls up and says it's still too lean at part throttle.

So then I asked the question, how much timing is the VA adding? VA, I don't use vacuum advance and haven't since Moby Dick was a tiny minnow and my engine builder doesn't either, he put an MSD billet distributor in it, etc, etc..........LOVELY! I tried to get him to send the original distributor here and he wouldn't have it and his engine builder told him using VA would add too much timing and could damage the engine, plus he NEVER uses it and it doesn't need it.

The short ending to a long story was going up a jet size and down about 5 numbers on the primary metering rods to make it happy.

Morel of the story is that ALL street engines at any power level can benefit from some additional timing at light load and at that point effectively manage a leaner mixture. The result is less fuel consumption, less sooting of the exhaust system, less carbon build up in the engine, and fewer byproducts of the combustion process finding it's way into the lube oil. The spark plugs will lighten up as well and usually go from black/grey to light tan to almost white. This is what you should see when you inspect them IF the engine is correctly tuned in the "normal" driving range.......Cliff

[OCMDGTO]

Been getting 10mpg on the new engine w 37 degrees locked out & no VA. Got 10mpg on the old 400 also w no VA. I've only ever checked on road trips with mostly highway driving & agree it's almost impossible to not stomp on it for a whole tank of gas.

[TCSGTO]

I stomped on it more than a few times last weekend. I wanted a real world mileage test. It could have done much better if that was the goal but it was driven without mileage in mind.

When I did the HPP Pavement Pounders a few years ago I drove 2 hours to the track, made 7 or 8 passes then drove it back home thru a bumper-to-bumper rush hour traffic jam on I-480 in Cleveland and got a little over 11.

[Half-Inch Stud]

MPG is strictly a function of Cubic air inhale vs tire rollout, since the fuel:air mixture is locked. In other words get Overdrive.

My 469 with 4L80 drives so much better with the 3.54:1. Highways have opened up as fun.

[Formulas]

I remember in my youth seeing the Shell Oil Co. Commercial the gallon IV. jug in the car test engineering techs in a white lab coat

They would send it off show a glimpse here and there of the bottle being depleted then it rolling to a stop and the sales pitch which I cannot remember

[carbking]

Originally Posted by Half-Inch Stud

MPG is strictly a function of Cubic air inhale vs tire rollout.

And weight!

Jon.

[Formulajones]

Always, ALWAYS, run vacuum advance as already mentioned. Just have to dial it in properly and it will work fine on any engine build at any power level, and it's always worth a couple MPG.

Our 79 1 ton pickup, weighs 5300 lbs. 454/400 with 3.73 gears. These trucks were always known as gas hogs. With careful tuning it knocks down 14 mpg highway spinning 2800 rpm to go 65 mph, and it's shaped like a brick.

Our 70 Formula with 3.31 gear and 400 turbo gets 17 mpg highway.

Our 69 Z with a Muncie 4 speed and 3.55's out back also tickled 17 mpg highway, with a big fat 780 holley on it buzzing along about 2800 rpm.

My 454 Chevelle makes 600hp and runs a 4.10 gear out back with a 400 turbo and it's gotten 12 mpg highway buzzing along at 3,000 rpm, doing about 60 mph, with a very well coupled Continental 10" converter in it. It would get teens for mpg if I took some gear out of it.

All these figures are driving around at 5,000+ feet elevation with varying changes and mild grades. There is no "flat" driving around here.

You don't really need an overdrive to get some respectable mpg numbers out of these cars. They just have to be an efficient running engine with a sharp tune and they will treat you pretty good at the gas pumps.

I do now have an overdrive in the Z, and I fully expect that car to easily top 20 mpg highway. Just haven't been on a trip yet.