Hi everyone, I had a few questions I wanted to clear up. First of all; if my ECM (someone in the past took it out) is no longer in the vehicle, how much does it impact MPG? I swapped out my electronic 81 carb for the '80. I've been calculating my last 5 trips and after filling up with 14.5 gallons each time (it shows that its near empty on my fuel gauge); I am getting about 110 miles; so a little over 7.5 miles to the gallon. If the ECM does not really play much of a factor in fuel efficiency then my next question is; how can I improve my fuel economy? That may seem like a silly question and I'm not expecting anything tremendous like a new car, but surely it can be better than what I am getting currently. I've been doing a lot of upgrades to the '81, so I am not shy when it comes to doing my own vehicle work, any suggestions are more than welcome.

 

Did you replace the ESC distributor with mechanical/vacuum advance distributor?

 

alkyhol is , to me, the worse thing for mileage.

7.5 is crazy, even for a vette.

My experience is, gearing sets the gas use.
of course mph is a major contributor.

as far as the 81 ecm,, at 55mph it does run efficient.

 

+1.

 

I'm surprised you're not on here asking why it's running bad and fouling spark plugs at 7.5 mpg's ? My 80 gets around 17 average with some spirited driving mixed in , x4 on the distributor if you haven't changed it

 

Thanks for the replies, I replaced the distributor with a Chevy HEI with vacuum advance that I bought from another member on this forum sometime ago. It sounds just fine and I replaced the spark plugs with NGK iridiums when I first bought the car a year ago, I would very surprised if they were fouled since I haven't felt anything or noticed any misfires; I ran Seafoam through it a few months ago as well. This is also 7.5 MPG city driving as well; lots of stopping at lights but even still, not good at all.

 

No, Not good at all.
2 things.
1. How is this "Chevy". HEI curved?.
Any idea?
2. How is this carb jetted?
This is fairly poor economy. And I would be willing to bet once you get the timing correct and the carb properly dialed in. You will also find a great deal more performance.
If for any reason all of this is way over your head, find a dyno tuning shop that works on vintage hotrods.
If your a hands on guy. Contact Lars for his tuning papers and learn to recurve your distributor. Or contact Davis Unified Ignitions and order a custom curved distributor for your car.
Then get to work and get your carb dialed in.

 

Well that changes everything. I drove a 2010 F-250 around the streets of Brooklyn and Queens and that thing use to get 4.5 MPG. Another example when I was working in a Chrysler dealer in 1980 a owner of a Chrysler 5th Ave with a slant 6 complained so much about poor gas mileage the factory got involved. His car was running 100 percent perfectly. The factory representative had me connect a device that went between the fuel pump and carburator to accurately measure MPG. The results was 10 MPG in the city. Now I know 7.5 doesn't sound good but city driving can really kill gas mileage. Do you drive another car the route and what mileage does it get? Are there hills where you drive because that's a gas mileage killer too. Hard to say if this is normal or not. Try throwing in a set of fresh set of copper tip spark plugs and reading them. Make sure both your mechanical and vacuum advance are working properly. Other than that I don't know what else to tell you.

 

In a Corvette its not miles per gallon its smiles per mile

 

Wow, with that kind of mileage you can pay for an air fuel ratio tester and see what you get? Or pay to have checked once to see?
im all city 4 sp l82 3.70 gears all city hard and never went below 11mpg of course i never get above 12.5

pic from Lars rebuild and super happy still!!


Lars af

 

To be fair I haven't even tuned my carb or anything like that, the carb was freshly rebuilt so I assumed it would be fine. However since I don't really have time anymore to experiment and try to do a tune up myself I know a dyno-tuning place nearby that would probably do it for me. Any other advice that you guys would like to add before I take it there? I appreciate everyones input. And I do get plenty of smiles and questions at work about the car!

 

If this dyno place really knows 60's and 70's Cars. they will be able to get her dialed in just fine.
If all they know is tuning fuel injected modern cars.
Keep looking.

 

Set your timing first.

Email @lars for ALL of his papers, especially timing, HEI, vacuum advance, and Q-Jet. You'll save enough in a few tanks of gas to pay for a nice timing light, like an Innova 5568.

My 79 with a Lars-style tune gets about 15 mpg. My 80 does close to that.

Who rebuilt your carb? Can you post some photos?

 

That is atrocious for a 350 engine's fuel mileage. My 427 gets better mileage than that. The original carburetor was computer controlled and it was tuned to get more mileage and run more efficiently. At least if they removed the ECM if the original carburetor is not on the car anymore. Your Corvette was at the early end of the switch to Fuel Injection where the car is able to adjust the air/fuel ratio to get the best performance and efficiency on the fly. I have a 1988 C4 which is fuel injected and if you pulled the ECM out of the car it would be totally useless and not go anywhere.

Depending on your plans for the car you might want to go back and rebuild the factory style system or completely replace the system so there is no need for any ECM controls anymore. In Virginia where I am they would be expecting you to replace the components to make it legal to drive on the roads as a daily driver. It is old enough to qualify for Antique Plates but they are cracking down on people driving on weekdays with Antique plates here.

Another option that might seem a bit expensive would be to pull the entire system off the car and switch it over to an aftermarket Electronic Fuel Injection system like the Holley Sniper system I have on my 427. Your Corvette has an electric fuel pump I believe and it makes more pressure than most carburetors are designed for. There are plenty of things that could cause your mileage to be so bad. Going with a EFI system from Holley cost me about $1700 for the complete system and it has four fuel injectors that are able to work on up to 650 hp normally aspirated. This system is a self-learning EFI system and very easy to install, setup and use. The newer systems are light years ahead of the systems from 5 years ago even. The one I have controls the Air/Fuel ratio and the timing of the engine as well. I simply tell the computer how big the engine is and how many cylinders it has and then it starts the learning process. I bought my system from a company in Florida called EFISYSTEMPRO.COM and they are really good at the Holley Equipment and provide you with 2 years of Post sale tech support to ensure that your engine is running great after installation of the system.

My 427 is tricked out and it is able to get closer to 15 MPG in my 1968 C3 with a 4 speed. I have a 3.36 rear which helps a bit. From your numbers I would guess you have a 4.11 or higher rear axle ratio. There are lots of ways to make your engine work better and make more power and get mileage as well. I wish you the very best in getting your engine working better!

Best regards,
Chris

 

No electric fuel pump in 81.

 

Thanks Bike, I set the timing on it about 2 months ago and it runs very well at least in my opinion when I set the timing; I still own the Innova timing light. I might have pictures of it, but the place was Rod's Carburetor Repair out of Phoenix Arizona. I bought the 1980 Rochester Quad from a member on the forums for a very fair price along with the HEI from him too. I will try to find some photos.

 

To my knowledge most of the ECM related stuff is no longer functioning or has been "half" removed. When I bought the car someone did a bubba job of working on it (Example; safety pin holding the carb choke in place.) So most stuff is either deleted or is on there but no longer functioning. I looked into the Holley option a while ago but never followed up on it, but I would like to look into it again.

 

I've run both the quadrajet and a Holley. The Holley is understood by far more than the quadrajet. Thus finding people who can tune it may be easier.
But apples to apples. Both tuned correctly. The quad will come on top.
But I really liked the low end performance of the Holley.

 

you set timing at idle? or you know you have enough centrifugal and vacuum advance and you have it set to 36 degrees total at 3000 rpm or higher and the vacuum advance kicks in at idle?