Hello, I have sort of a roadblock in building my 71 Maverick...As in I can't seem to get any information on whether I should build a 250 inline...Or a 5.0 from a 86-94 mustang and run it with a carb (for now) after reading plenty of topics on the 250 a lot seem to mention it tends to use more or the same amount of fuel than a 302?? I plan to drive this daily so if a 302 really is capable of getting better fuel consumption than a 250, but what are your thoughts? I plan to use a t-5 and 3:55 gears if that helps? and by this thought a 250 would be rebuilt and built for mild performance (same with the 5.0..or stock) Any help is aprreciated!!
There are many who believe that mileage is directly proportionate to the displacement and power capability of an engine.. BUT.. that is only part of the entire total equation and only a part truth. What they don't fully understand is that the relationship with those two things only plays a small part in the outright mileage capability of any vehicle. It's like this. Torque production matched to gear ratio is a major key to mileage gains. Why do you think many of the newer cars, many that weigh nearly 1,000 lbs more than ours, can get near 30 mpg these days? It's not just about direct injection or EFI and primarily because low rpm torque from bigger displacement engines and double overdrive transmissions lets them putt along at 1,800-1,900 rpm. That's why. So, let's elaborate more with some real world examples. If you have a moderately built up(carb/manifold/mild cam/headers/free flowing exhaust) 250 with 9-1 compression ratio(which also takes some additional machine work/money to achieve) and it makes, let's say.. 130 ft/lbs of torque(obviously not it's peak number) while cruising down the highway at light throttle angles of maybe 3/16" inch throttle blade opening to maintain a fairly constant speed, you'll get xx amount of mpg out of that deal. And getting up to speed takes noticeably longer with higher throttle angles required while going up longer grades and/or carrying extra weight such as passengers and luggage forces you to lean on the pedal harder to maintain those same speeds. As a result, the average comes down a bit more. Assuming you spent the time and money to sweat all the necessary details, this little motor has a peak power potential of around 190 - 200 horsepower. Not all that bad considering the baseline power level from which you started. Now take an even bigger motor, we'll even use a 347 stroker which is nearly 100 cubic inches larger to better illustrate and drive the point home, that is around 11-1 compression ratio with a RV style cam and dual plane manifold which emphasizes low rpm torque production. Let's assume this motor cruises at the same road speed as above but now does it at only 1/16" throttle opening because this bigger engine produces well over 200 lb/ft at this much smaller throttle angle. This bigger higher compression motor makes so much torque at idle and right above the carbs transition circuit that you can also now cruise up hills with ease with barely any additional throttle input to maintain the same speed. This helps to noticeably slow the loss of average mpg in the process. Another potential bonus is that the bigger engine with higher torque capability also frees up the option to use taller rear gears without major loss in acceleration capabilities to offset its tendency to consume more fuel than a smaller motor. An easier way to imagine the affect I'm trying to get across here is that it would be very similar to going down a slight grade with the smaller 250 cubic inch motor in that you can now cruise at the same speed with the larger higher torque engine combo while letting up on the throttle a bit to avoid increasing speed substantially due to the now higher torque production. This is what cubic inches does by way of its inherent and much improved torque production. The bigger issue with this larger more powerful 300 horsepower engine is that generally speaking, "the more you have.. the more you'll tend to use" and any potential net mileage gains go down the toilet in direct proportion with the lack of self control. Either way you go here, increasing static compression ratio to the maximum level attainable with pump gas(10.5- 1 to about 11-1) will pay big dividends in off-idle torque production and overall efficiency. Very tough to even get close to those numbers on the 250 cid engine combo without much machine work and custom pistons. Also keep in mind that building the 200 horsepower 250 cid motor will cost almost the exact same price as a mild 302 would cost you in the long run.
I was getting about 17-18 mpg with a factory 74 250 with a C4 I got over 25 mpg city+highway last week without trying driving at 85-90 mph climbing mountains with the AC blasting in 5th gear w/3.40 gear. My 250 has a pre-emission timing set, header, shaved late model head, custom curved distributor and factory cam. If I had a better cam and acted my age I believe I could had got 28-30
Small Block Six Fords' are my favorite. I run two 250's - one stock in a 74 Maverick and another Mav 250 in a street/strip Comet ('61). The stroked 200 to 250 is a torque monster and enabled the Torquey' little sixes to move the 70's cars with higher gearing for modern 70's interstate speeds like the RPM easy - V8's. BUT, the 250 is pretty thirsty even in stock trim and the 70's smog-stranglers don't help. I would guesstimate' the 250 and 302 are fairly close in MPG figures in the real world. Most small block sixes can be substantially upgraded by a careful rebuild which often is simply better tuning than factory with a few available upgrades with bolt-on parts ( ign/fuel) IF MPG is a concern, the T5 OD tranny with 3.55 gears is perfect match for the 250 which uses SBF FW/Clutch/Bellhouse etc... I rn a T5/3.50 rear in a 63 Falcon with a Maverick 170 CID small block six, it got over 24MPG on trip to Falcon Nationals in Rhode Island cruising at @ 70MPH at @ 2000 RPM. THe stock block Mav 170 has 2Bbl carb , Headers, Points Ign, T5, 3.50 OEM rear and re-worked cyl head ( @ 9.5:1 SCR) . For fun and simple you can not beat the inline six. . This 250 is going in the 74 Maverick: . have fun
Outstanding reply!! Some years back one of the Mustang/Ford rags published a article by amateur road racer that wanted fuel injection so him and another guy(who I believe wanted to drag race) swapped systems from one car to other... The non EFI engine wasn't roller and had the std firing order so he had to repl cam something fairly mild, I believe RV was mentioned... After swap he listed all the virtues of having EFI, but to one thing that stood out was he gained SEVEN MPG cursing at highway speeds(I could get 26/27mpg from my 5-speed 2.73 geared '86 GT and similar from my AOD/3.73 geared T-Bird)... Soooo basically if you want max MPGs and your are thinking about a EFI swap, do it sooner than later...
Krazy, You are convincing me of the benefits (MPG) of cursing at highway speeds. Hope I don't get pulled over.
LOL as long as you don't also use the middle finger, you'll probably be OK... Mebby I'll learn to spell cruising one of these days...
