MPG Improvement
Thread Starter
Instructor
Joined: Oct 2009
Posts: 165
Likes: 1
From: Stanley NC
MPG Improvement
I have a 1981 stock vette, 350 engine, 4 bbl carb w/auto trans and get about 10 to 10.5 mpg. Are there any thoughts on improving mpg that I might consider? I will entertain all, low cost and high cost options but wanted to start here for others of you may have solved this prior. Thanks in advance for your responses and comments.
Team Owner
Joined: Jul 1999
Posts: 65,492
Likes: 230
From: Orange Park Florida
TPI done out of junkyard parts, some wiring talent, and a aftermarket Cal Pak remains the most simple mod possible for fuel economy...
whole thing costing less than 700 bux these daze....BUT you have to wire your own harness...otherwise some clowns will want over 300 bux for a few wires with crimp connections and chunks of plastic...
whole thing costing less than 700 bux these daze....BUT you have to wire your own harness...otherwise some clowns will want over 300 bux for a few wires with crimp connections and chunks of plastic...
Melting Slicks
Joined: Mar 2005
Posts: 2,095
Likes: 299
From: Midlothian VA
I have a 1981 stock vette, 350 engine, 4 bbl carb w/auto trans and get about 10 to 10.5 mpg. Are there any thoughts on improving mpg that I might consider? I will entertain all, low cost and high cost options but wanted to start here for others of you may have solved this prior. Thanks in advance for your responses and comments.
When's the last time you tuned it up?
You should be getting 15/21 city/hwy with an auto.
I get 16-17 on the hwy with my 72.
Pro
Joined: Nov 2002
Posts: 599
Likes: 16
From: vista CA
I agree with previous posters that you should be getting better mpg. When my 79 had the stock 350 / TH350 I got about 15 mpg combined driving.
Tune up w/carb overhall should help out before major mods....
Tune up w/carb overhall should help out before major mods....
Le Mans Master
Joined: Jan 2006
Posts: 6,506
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From: Butler Pa
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Melting Slicks
Joined: Oct 2002
Posts: 2,317
Likes: 111
From: St Louis MO
Just checked mine after a recent road trip - I got 16.1 mpg with my '78. Sounds like you might have a bad carburetor idle/transition mixture, lack of vacuum advance, dirty plugs or bad ignition, retarded spark, high rpm (low rear end gears), or just poor driving technique.
__________________
1978 Corvette
406 SBC AFR heads Lunati roller Quadrajet TH-350 3.08
11.59 sec / 118.9 mph 1/4 mile
__________________
1978 Corvette
406 SBC AFR heads Lunati roller Quadrajet TH-350 3.08
11.59 sec / 118.9 mph 1/4 mile
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
An '81 with the stock CCC computer/dist/carb system [which is working correctly] should get 20 mpg on the road and about 15 in town. If that equipment is still on the car, get a copy of the Chevrolet Chassis Service Manual and follow the steps for check-out and set-up of the CCC fuel management system. If the CCC stuff has been stripped off the engine, but you still have the original carb, that is your problem. The fuel enrichment system isn't getting any feedback from the computer and it stays at "full-rich" position all the time. In that case, swap out the stock '81 carb for an all-mechanical carb and tune it properly. Lastly, if you have a standard carb on the engine, tune it up [and/or rebuild it], adjust timing per Lars Grimsrud's papers (CF Archives) and you should be in good shape.
Thread Starter
Instructor
Joined: Oct 2009
Posts: 165
Likes: 1
From: Stanley NC
An '81 with the stock CCC computer/dist/carb system [which is working correctly] should get 20 mpg on the road and about 15 in town. If that equipment is still on the car, get a copy of the Chevrolet Chassis Service Manual and follow the steps for check-out and set-up of the CCC fuel management system. If the CCC stuff has been stripped off the engine, but you still have the original carb, that is your problem. The fuel enrichment system isn't getting any feedback from the computer and it stays at "full-rich" position all the time. In that case, swap out the stock '81 carb for an all-mechanical carb and tune it properly. Lastly, if you have a standard carb on the engine, tune it up [and/or rebuild it], adjust timing per Lars Grimsrud's papers (CF Archives) and you should be in good shape.
One thing I do know is the "check engine" light does not come on at start up so may be an issue there.
There is no smoke from tailpipes and car will run strong. One thing different is that it has a Jasper motor installed instead of the original one so something may not have gotten hook up right.
I own the vetter about 2 months now so going thru it slowly.
Le Mans Master
Joined: Apr 2007
Posts: 7,353
Likes: 72
From: Graceland in a Not Correctly Restored Stingray
FWIW, the tri-power 427 BB w/manual trans & 3.55:1 rear combination formerly in my '78 was capable of ~14 mpg, which was a significant improvement over anything I ever got with the original L82/auto & 3.55:1 combo...
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
Tommy...
The '81 CCC system is an OBD-I system which has the capability of displaying "trouble codes" via a flashing Check Engine (CE) light. There may be info on how to get a "dump" of stored trouble codes in the Chassis Service Manual, or you can search the web for that info. It must be specific for the '81 year, though; no computer on '80s and a new OBD-II system for '82 and later.
Since you have all of the CCC equipment in place, that is the best way to go. Even if you find some of the equipment is defective, there are working parts 'out there' from those who are dumping that system for EFI or just a basic carb set-up. I would bet that the O2 sensor is kaput and some of the other sensors may be past their useful life. Sensors are relatively cheap compared to swapping out the entire system, however. Good luck.
The '81 CCC system is an OBD-I system which has the capability of displaying "trouble codes" via a flashing Check Engine (CE) light. There may be info on how to get a "dump" of stored trouble codes in the Chassis Service Manual, or you can search the web for that info. It must be specific for the '81 year, though; no computer on '80s and a new OBD-II system for '82 and later.
Since you have all of the CCC equipment in place, that is the best way to go. Even if you find some of the equipment is defective, there are working parts 'out there' from those who are dumping that system for EFI or just a basic carb set-up. I would bet that the O2 sensor is kaput and some of the other sensors may be past their useful life. Sensors are relatively cheap compared to swapping out the entire system, however. Good luck.
Melting Slicks
Joined: Mar 2005
Posts: 2,095
Likes: 299
From: Midlothian VA
Thanks for the info. Yes it still has the CCC compontents all on the car and I have the service manual so will start reading up on checking the system.
One thing I do know is the "check engine" light does not come on at start up so may be an issue there.
There is no smoke from tailpipes and car will run strong. One thing different is that it has a Jasper motor installed instead of the original one so something may not have gotten hook up right.
I own the vetter about 2 months now so going thru it slowly.
One thing I do know is the "check engine" light does not come on at start up so may be an issue there.
There is no smoke from tailpipes and car will run strong. One thing different is that it has a Jasper motor installed instead of the original one so something may not have gotten hook up right.
I own the vetter about 2 months now so going thru it slowly.
If the PO went to the trouble of replacing a Corvette engine with a Jasper motor then there's no end of possible issues you could be facing regarding fuel consumption.
Le Mans Master
Joined: Aug 2006
Posts: 7,431
Likes: 1,561
From: mount holly NC
2025 c3 ('74-'82) of the Year Finalist - Unmodified
2019 C3 of Year Finalist (appearance mods)
Here's the base of a spare carb (7028212) that I use to experiment with. I don't see any holes to block off so I never followed up on this improvement. I hope MG can enlighten me.
The carb on my car is a stock 4MV 7044206 and I get 13 MPG around town (stop & go) at speeds between 35 & 45 MPH. On the highway I get about 17 MPG. My original and stock carb has no extra holds to block either. Stock 74 350 auto th400 with 3:08 rear cast iron manifolds and true dual chambered exhaust.
Better mileage would sure help but I don't seem to be able to follow your information, what am I missing? PG.
The carb on my car is a stock 4MV 7044206 and I get 13 MPG around town (stop & go) at speeds between 35 & 45 MPH. On the highway I get about 17 MPG. My original and stock carb has no extra holds to block either. Stock 74 350 auto th400 with 3:08 rear cast iron manifolds and true dual chambered exhaust.
Better mileage would sure help but I don't seem to be able to follow your information, what am I missing? PG.
Race Director
Joined: Oct 2000
Posts: 15,104
Likes: 79
From: central FL, near the beach.
Here's the base of a spare carb (7028212) that I use to experiment with. I don't see any holes to block off so I never followed up on this improvement. I hope MG can enlighten me.
The carb on my car is a stock 4MV 7044206 and I get 13 MPG around town (stop & go) at speeds between 35 & 45 MPH. On the highway I get about 17 MPG. My original and stock carb has no extra holds to block either. Stock 74 350 auto th400 with 3:08 rear cast iron manifolds and true dual chambered exhaust.
Better mileage would sure help but I don't seem to be able to follow your information, what am I missing? PG.
The carb on my car is a stock 4MV 7044206 and I get 13 MPG around town (stop & go) at speeds between 35 & 45 MPH. On the highway I get about 17 MPG. My original and stock carb has no extra holds to block either. Stock 74 350 auto th400 with 3:08 rear cast iron manifolds and true dual chambered exhaust.
Better mileage would sure help but I don't seem to be able to follow your information, what am I missing? PG.
If others want to see what we are talking about see my article http://community-2.webtv.net/MATTGRU/carb
Le Mans Master
Joined: Aug 2006
Posts: 7,431
Likes: 1,561
From: mount holly NC
2025 c3 ('74-'82) of the Year Finalist - Unmodified
2019 C3 of Year Finalist (appearance mods)
that carb sure needs a 4 hole gasket to work right. an open gasket could mess up the mpg, IF it has a cam with more overlap than stock. nothing wrong w/17 on the highway w/TH400. seems normal to me.
If others want to see what we are talking about see my article http://community-2.webtv.net/MATTGRU/carb
If others want to see what we are talking about see my article http://community-2.webtv.net/MATTGRU/carb
