How low can you lug a carbureted motor?

I'm looking for a Maverick and trying to plan what I'm going to do with it.

I'm looking for a 4 door with a 250 I6 or a 302. I want to put an AOD behind it. The goal is maximum miles per gallon.

I'm trying to decide on on gearing. With 3.00 rear gears, .67 o.d., and 24.7" tall tires, I get 1914 rpm at 70 mph.

Can a stock carbureted 250 or 302 cruise at that low of an rpm?

With 3.25 gears the rpms at 70 go up to 2073.

With 3.40 gears the rpms at 70 go up to 2169.

with 3.55 gears the rpms at 70 go up to 2265.

What would give the best fuel economy without lugging the engine down too low?

Thanks for any thoughts,

paulie

 

buying a 2.3l turbo 4cyl motor with a t5 and building motor mounts and run a fuel injection system.

If you are looking at one of these cars for just fuel economy you might be better off looking at something else. The fox body turbo mustangs are quick little cars with good fuel economy.

Several of the guys average about 21+ mpg depending on the mods to their cars, but in the same respect some of the guys are lucky to get 9 mpg it is all depending on what you are looking for, if you go with too high of a gear ratio 3.00 2.73 ect. and a 4V carb you can loose fuel economy because the 4V keeps kicking in just to keep the car in motion. If you note the stats, the guys that achieve some of the best fuel economy are running 3.54 gears or 3.83 with an overdrive transmission because it is better suited to the powerband of the motor.

 

I guess my question is not clear. I'm not looking to get maximum fuel economy period. I wouldn't use a maverick at all if that were the goal. I have a Saturn which gets 29/38 mpg. Let me try again.

I am looking at getting maximum mpg out of either a 250 I6 with an AOD or a 302 with an AOD.

paulie

 

I cruise right at 1,600 @70ish with an AOD 2.79 gear and a 225/70/14 even though mine is fuel injected I wouldn't imagine it would be lugged to hard at cruise depending on what is done to your motor.

 

Thank you. That's a super low cruise rpm. Which engine do you have? I'm guessing a 302? Have you checked mpg? I was thinking a carbureted engine might have trouble running under load at very low rpm. I don't know that for sure though.

paulie

 

yes it is a 302 bone stock except for a set of headers and exhaust.

last time i calculated i was getting 23 with it burning alot of oil I have since fixed the issues but havent went through a tank of fuel yet Ill probly have a good figure by the end of this coming week if the weather holds out

 

I'd love to hear what mpg you get when you check it again.

Thanks!

paulie

 

It also depends on how the engine is built, not just which one you pick. The reason a carbed V8 is not known for being efficient at low RPM is people typically modify them for higher HP in a higher powerband. Dual-plane or stock type intake manifold, modest sized 4v or 2v carb, low stall and/or lockup converter, sane exhaust, and a cam intended for low-end torque, such as an RV cam, will all help you keep the power at the low end of the tach where you want it. You'll have all the grunt you want when you take off, and still be about as efficient as these cars can get.

 

you shouldnt be concered about luging. the trans. should down shift when the load becomes to much for the motor. thats how the manifactures did it in the 80s and 90s. they put these overdrive transmissions in the car with 2.73 gears. the cars would be in overdrive and when you had to give it some gas to get over a hill or pass anther car the trans. will kick down and you have the power to go around. a v8 will probly get you the better gas milage. i would use the smallest vac. secondary 4bbl carb you can get. it will get you the best milage. as long as you dont floor and allow the secondarys to open it will get really good milage.

 

That's kind of what I was thinking. If I had a 302 for example; I was thinking 8.5:1 - 9:1 compression and a tiny hydraulic flat tappet cam. Lunati has one I like with 195/205 degrees @ 0.050". I'd use stock heads except for some pocket porting and a good valve job. I was thinking small runner dual plane intake like an Edelbrock Performer or a Weiand Action Plus. A 500 cfm Edelbrock 4 bbl. Full length headers with 1-1/2" primaries. Trying to get the rpm range as low as possible. I just don't know how low is too low.

You think that'd work okay with stock 3.00 rear gears? Is 27 mpg possible on the highway with a well tuned combination? I'd be really happy with that.

I'm at more of a loss with a 250 cid build. I don't know much about them and aftermarket parts are harder to find so I would probably end up with something close to a stock rebuild.

Thanks for the reply,

paulie

 

Sounds good to me. I would think if you tune it right and drive it right, 27mpg wouldn't be far off for a combo like that.

 

That makes sense. Good advice. Thanks.

I remember people used to say "for best mpg, gear the car to cruise at the torque peak rpm". Heck, even on a stock motor that might be 3000-3500 rpm. I think better mpg will be had at rpms below the torque peak.

paulie

 

You need to run the engine where it's most efficient at, not necessarily the lowest RPM. You need to do modifications to the engine aimed at improving efficiency. I have an internally stock 200 and a C4 with a shift kit, 2.79 gear out back. I have a header, MSD ignition, electric cooling fan and I've tuned the mechanical advance to the point that I don't even use the vacuum advance anymore. I consistently get mid to high 20's at 70 mph.

 

Thank you, that is exactly what I was trying to say here

If you stay within that band you are well within achieving a good fuel economy, otherwise you can be counterproductive and actually hurt fuel economy.

Oh and I was serious about the turbo 2.3l motors there is one of these floating around on the forums now, they are reputed at getting good fuel economy and good performance and are for the most part bullet proof.

Also as a note if you are using an edelbrock carb it would be to your advantage to use one of the mid range cfm carbs like a 600cfm the 500cfm is a little too small. The secondarys are actuated off of the vacuum of the motor and with the small venturi's there is a definite possibility that the secondaries will keep engaging too allow the engine to breathe, thus this will kill fuel economy.

 

For the best fuel economy you want to run the engine at the rpm where it produces twice the hp required to keep your car at that speed. The problem with doing that is that you will never have any acceleration! In all things there are compromises. I ran 41 mpg in a 62 falcon station wagon for a year. Top speed? 55mph!
If your rear gears are too low then you will have to spin the engine too fast, If they are too high then your power valve will be opening and closing all the time to compensate for the engine load. It gets worse if the secondaries open and close constantly too so selecting the right gear for a carbureted engine with an AOD is just as important as choosing a locking or non locking converter for your AOD.
Talk to your transmission builder and your converter supplier to get the numbers that fit your specific application. I haven't seen enough information about your car, its use or your driving habits to recommend anything of importance.