Why is my MPG so low?

Question:

I have a GM 60-degree 2.8-liter engine with a 700R4 transmission installed in a 1976 MGB. The engine has been bored .020 inches and has forged flat top pistons. I’ve installed an Edelbrock Performer manifold with matching cam, headers, stock HEI ignition, roller tip rockers, and a Holley 4160 390 cfm car-buretor. The rear end is a narrowed GM 7.5” with 3.42:1 gears.

The engine starts and runs great. It turns 1850 rpm at 60 mph and 2200 at 70 mph. Timing is advanced 14 degrees. Idle vacuum is 13 inches Hg and while cruising it is 13-15 inches.

Now for the problem, I’m only averaging 16 mpg with very conservative city/highway driving. I’ve installed an oxygen sensor that verifies that the engine is running rich at all speeds. The electric choke operates properly and the power valve is not blown. I’ve downsized the carburetor jets to the point that the engine would get up to speed but had no power. And even with the smallest jets, once up to speed the engine ran rich. I went back up in jet size to #56.

Do you know of any way to lean out a Holley carburetor other than changing the jets, or do you think that this is the best that I will get with this setup? My gut tells me that I should be able to get better MPG than this. I have a ’33 Ford street rod with a Chevy 350/700R-4 that gets 20 mpg with similar driving.

Answer:

I agree that you should be able to achieve considerably better fuel economy given the weight of your car and the engine and transmission that are in it.

The vacuum figures you cite are not particularly good and this may have something to do with your poor fuel consumption. At idle your engine should generate at least 15 inches Hg, and 17-21 inches Hg is usually indicative of a healthy engine.

The low vacuum can be indicative of a variety of problems that would reduce fuel economy, including incorrect ignition timing, bad valve timing, low engine compression, poor valve sealing, or a vacuum leak somewhere in the engine.

It is also possible that the engine is perfectly healthy and the low vacuum reflects the presence of a somewhat “radical” camshaft profile and this, too, will negatively impact your fuel economy.

Something else that may be causing or at least contributing to the fuel consumption is your ignition timing. You state that the timing is advanced 14 degrees but you don’t say at what rpm, and you also don’t say what the total advance is. As a general rule of thumb, initial advance should be about 10-12 degrees when your camshaft duration is less than 220 degrees @.050-inches of valve lift. Initial timing should be advanced to 14-16 degrees when cam duration is between 220 and 240 degrees @.050-inches of valve lift, and 18-20 degrees when cam duration is 240-260 degrees @.050inches of valve lift. Total advance and the distributor’s advance curve should be tailored to a host of variables, including the engine’s compression ratio, cam profile, engine speed range, mass of the vehicle, driving style, and so on.