Hot Rods 283 sweet spot

The 283 in my 1950 GMC is getting terrible gas mileage. 10 mpg on a basically stock 283 with a fresh rebuild.I know I'm going to have to change gear ratio currently has 3.73. I'm turning right at 3000 rpm at 60 mph.I need to know what seem's to be the best rpm to run this engine at to get decent gas mileage at cruising speed. Let's hear from you guy's who have actual experience with a 283.
thank's Gregg

 

What transmission in the GMC

 

Gregg
Changing to a lower gear may actually improve your mileage.
Another thing many people fail to do when swapping a motor into a heavy vehicle whether trucks or otherwise is getting the distributors' advance mechanism set up for the environment it will be operating in.
Open a (60's) service manual and compare all the various timing/advance specs a**** cars (light weight) and heavier cars and trucks.
An hour of labor on a distributor machine can be money well spent.

 

Something does not add up with your numbers, unless you have really, really short tires on the back.
I would think you would be around 2000 RPM @ 60 with your gear.
Example, my 53 F100 3.92 stock rearend was 3000 RPM @ 60 MPH with a 1:1 high gear.
10 MPG is really off, is your Speedo Correct?
Give us some more info on your Truck.
Oh.... Pics Too!

 

Depending on tire diameter, 3000 rpm at 60 mph sounds more like a rear gear in the 3.70 to 3.90 range and not a 2.73.

 

My worn out '57 283 in my '34 gets about the same. That's with a t-5, '39 rear (3.73?) and three Rochesters at about 80mph on 7.50-16" bias plies.

 

The tires are 27" ,The trans is a thm350 non lockup converter.My bad on the gear meant to say 3.73. I am aware that fine tuning the dizzy will benifit mileage. Clearly my main problem at this point t is excessive RPM. Thanks for the reply.

 

If that's in town, stop and go driving, then the mileage is pretty typical of old trucks. If that's on the highway, you should be doing a little better.

You probably want about 34 degrees total ignition advance (initial plus mechanical), plus whatever the vacuum advance gives you. Check it and let us know what you have.

Driving style can make a difference too, although with an automatic you're kind of stuck with the shift points. Plan ahead, don't accelerate hard, don't brake hard, take it easy, see if that helps.

Changing the gear ratio usually helps most on the highway. But it helps more in smaller, lighter cars than it does in trucks. So don't expect miracles...15 mpg on the highway would be excellent (although I'm sure a few guys will chime in about how they're getting 28 mpg with a 283 and no overdrive and 4.11 rearend gears)

 

Only if they swapped it into a Prius.

 

You really should be in the 15-18 mpg range with that combo. That number would be based on a 2bb. driven "normal". A 4bbl will be real close driven with care. Tune is important. Timing is a start. Exhaust gas should be checked as well. It is very easy to run too rich a mixture and not even notice. Those engines were very forgiving as far as fuel mixture.

 

Jim thank's for responding. First while this is an old truck it's only an old body. Truck is on a non HAMB approved S-10 ch***is. I expect it doesn't weigh much if anything over 3000 labs . I'm not looking for Honda Prius gas mileage. Frankly 15/16 mpg would be just fine. The truck run's great just bad mpg. While I appreciate all the responses no one has answered my question. What rpm doe's the 283 like to run at at cruising speed. For instance the 350 in my 1951 Chevy coupe turns 2200 at 70 mph and gets 15/16 mpg . I read All the time about stock early 60s full size Chevy's getting 18/19 mpg with a 283 and a powerglide . Again I'm not expecting 18/19 mpg

 

I suspect that the big difference maker is going to be what Squirrel said about the Mechanical and Vacuum advance.
Vacuum advance hooked to manifold Vacuum not ported vacuum.
This should yield the improvement you are looking for.
Good Luck with it.

 

actually ported or manifold vacuum should not really make any difference for mileage...it will make a difference in idle and off idle feel and possibly running temperature.

The "sweet spot" depends on lots of things...engine load, cam, manifold, etc.

fwiw I get 17 on the highway with my old Suburban with a stock 350 with a Qjet carb, 3.08 gears and overdrive. It gets 15 without overdrive.

 

Generally speaking, an engine will be at its most efficient right around, or slightly below, its peak torque speed. For a typical 283 this will be 2200 (2 bbl) or 3000 (4bbl) RPM.

 

Depends on the 283, a mostly stock 283 can be 2 bbl, single 4, 2x4, FI, any number of head combinations, any number of cam shafts, anywhere from 8:1 compression to 11.5:1 compression. probably 3 different distributers to choose from maybe 4.

I have had more then my share of 283s in my time and I know that we are talking too many variables here for any of us to throw a target RPM at you.

 

Thank's rusty your the first person that answered my question. I question the 3000rpm figure for a 4bbl. Since at cruising speed you are only running on 2 bbl

 

What size and type of carburetor are you running? I had a 66 C10 when I was in High School, 283, overcammed, with large chamber 350 smog heads,powerglide, 3.73s everything about this combo wrong. With a 750 Holley I got about 8 MPG. Playing with various Carbs, I got it up to 12MPG. Don't remember which, but I had a 600 Holley, a small neck AFB, and a Q jet at the time. I didn't know anything.

 

Cruise speed at 60 Mph with 29 inch tires was 3000 rpm for my truck.

 

I guess I should have mentioned. 600 edelbrock,power pack head's,flat top pistons ,very mild cam, dual exhaust 2"pipe's gl*** pack muffler's ,HEI dizzy

 

I didn't have a truck in the last century that got more than 10 mpg. A 1950 was designed to be driven about 45 mph. Unless you go totally modern you will never get great mpg. I say don't worry about it, not worth the trouble or $ spent. My 2009 Chevy truck only gets 16 or s in the city. Or, a least it does the way I drive.

 

How much effort would it take to change the gearing? how much effort have you put into figuring out what the distributor advance curve looks like? how many miles do you put on the truck every month?

might help you figure out your priorities?

 

don't forget aerodynamics, even though the truck has rounded frontend, etc it is like driving a brick. search for sites like project33.com that has calculators for various transmissions, gear ratios, etc

 

Have you tried changing the air in the tires? If you run genuine GM air your RPM will come down to 2500 and you'll get 30 mpg.

 

What camshaft? How much cranking compression? What jets in the carb? Give me name and #'s. Let me know what you got.

I had a 262 in a Monza that got 33 mpg with an Erson RV cam and 450 Holley back in the 70's. With a 283 you should easily get in the 20's with a little tweaking, especially at sea level, even with a truck.

 

Jim I know the gear need's to be changed. That's why I want to figure what rpm I need to be turning. I have not checked my total timing yet. Will be doing it tomorrow. I am a heavy line guy at a Chevy dealer I will be changing the gear and if needed will install a 700r4. I am aware of the aerodynamic limitation's of a truck. As for mileage per month I plan on this being my daily driver. When I first started with this truck I was only getting 7 mpg. Changing from a 390 Holley and playing with timing got me to 10 mpg. I am more than aware that tune players an important part of the equation. I honestly don't think theres alot left to gain with tuning. Having said that I will check my total timing tomorrow.Jim you yourself said you where getting 17 mpg with your truck so getting so me getting 15/16 shouldn't be to big of a deal. Again what I am looking for is an approximate rpm range the engine should work best at. My 350seem's to workout efficiently at 2200. I suspect that the short stroke 283 might be around 2400 rpm . I'm just asking if anyone has real world experience with a 2

 

Does anyone reading this not think 3000 rpm at 60 mph is excessive ?

 

Gregg I think 3000 at 60 is about in the ballpark with the gear you are running and a stock turbo 350,and yes,to me that is excessive.I have owned a number of cars over time with stock 283s both 195 and 225 hp and likely others besides .Engines seemed "happy" to me in the 2200-2500 rpm range.BTW I have found that the Edelbrock 600s (at least the ones I have owned) are jetted to be safe on a lightly modified 350 chev\351 ford.Likely rich for a 283. If you want to try a leaner setup I live in Samsula and have the complete jet\spring\metering rod kit for those carbs.Dont know why the confuser suddenly changed fonts----

 

I had a 265 in our 56 Wagon. It had a 3.36 and the best it got was 21. That was a 3600 lb car though.

 

My avatar has the same ****** (350) as you and 28" tall rear tires, my rear end is a 3.08 (62 Chevy), @ 2,000 rpm I'm showing 55 mph, 2,500 rpm is 68 mph and 3,000 rpm is 80 mph, this is using my Garmin GPS to verify mph.

 

Steinauge thank's for the offer I might take you up on it. First though I'm gonna take the 500 edelbrock off my 51 coupe and swap it with the 600 in the truck. That should be a better fit for both car's. I'm also gonna go ahead and install a 700 r4.