Building a motor for mileage.
 

In our quest for mileage, we cover many things. Gasoline, tune up, tire pressure, fluids, all of these have an impact. But consider the average enthusiast. They want power. So when building a motor, they go with a bigger cam, among other things. Then they get bad mileage. "Oh this motor sucks," is the saying. It isn't the motor, it's what they built it for. It cant help but get bad mileage.

I'm surprised that more people aren't considering mileage when they build their motor. I know I didn't, but it still came out ok. Not great, but ok. But think of the gains that could be had if you had mileage in mind when building a motor. It'd be quite a different buildup than most performance builds you see, wouldn't it? Your compression ratio would have to be different, the cam would be different, the heads would be different, valves, headers, and so on.

You would use a smaller cam, probably smaller than stock. For an example for the FE engine, go to Crane's site and look at the 343971. 248 / 260 degree dual pattern. For the heads, you'd probably want to run just stock heads. You'd raise the compression ratio a little, to get more efficiency. Valves would be kept stock. Intake manifold would be small, like the Edelbrock Performer series. Headers with small primary tubes would provide efficiency and torque. A small carburetor (if EFI is not used) like a Holley 390cfm 4 barrel, should help efficiency. Holley fuel injection systems are available if you want to spend the money.

I dont see why we cant significantly improve on factory mileage if we're going to open up a motor. Most people say "Rebuild it stock." We can do better. I say, if you really want good mileage, invest some money and start at the source of the problem!

Just trying to get the idea out there. :-wink

:-blah :-blah :-blah

we can make him better...we have the technology!!!

i agree, but disagree.
outside of the obvious building stuff (best bearings, lots of balancing and blueprinting) i wouldn't necesarily run factory heads, I would find the most effecient heads i could afford, for example, gt40p heads from exploders, they were concieved so that the 5.0 could do better on mpg and emmisions in its last couple years. i would go for at least a 9.5:1 comp ratio. and would go for a high torque cam, size for your application, other than that i agree with you.

its all in the details... :-X22

that is how i built my 390. i used stock heads bumped the compression to 8.9 i am running a clevite stock cam.edelbrock intake and motorcraft 2150 carb. i pulls a nice steady vacuum of between 19 and 24. it does decent on gas,it would do better if i had a manual,but the 3.00 gears in the back keep the rpms down.

Lots of folks build for mileage- I was reading about back in '77 and I'm in the middle of doing it now on a '77. Some motors are better at building for economy than others. A lot of info is on this site as well.

Back in the late 1970's when fuel prices were rising due to the Iran situation, my Chevy truck needed a rebuild and I was going for mileage in addition to power. I dumped the original 250 six, which was averaging 13 mpg and picked up a junk 283 to build up for power and mileage and it worked, better power and 17 mpg. Here is what I did. This was a low cost project, so I changed as little as possible. I spent less than $500.

Ordinary valve job on the stock heads, no porting.
High compression pistons compared to early 1970's SBC stock.
Stock Rochester 2-bbl carb on stock cast iron two-plane intake.
High-lift short-duration "RV" cam and matching hydraulic lifters.
Tubular headers and true dual exhaust with Cadillac mufflers (cheap, free flowing, quiet)
Stock GM point ignition with silicone wires (due to the headers).

Good low end power and excellent cruising mileage was the result. Gearing was 3.73 with three-on-the-tree and G78-15 bias-ply tires.

I built a 350 for a '73 Chevelle this way when I was about 17. Stock rebuild on the shortblock. Small chamber heads from a 300hp 350(compression estimated at 9.5).
CompCams 260H (.447 lift). Stock cast iron intake/q-jet, stock exh manifolds with true duals/no cats/glass packs. HEI ignition. TH350 auto/2.73 rearend. Got consistent 18mpgs in a 4200 lb car when I kept my foot out of it, and almost all of that was city mileage. Not GREAT off the line, but pretty good, and I had the speedo pegged about an inch past the 120 mark (approx 140mph) more than once. One time with 4 passengers and both windows down. (You're only young and invincible once).
My brother also had a 55 F100 with a bone stock 2bbl 351W, FMX, 2.79 rear that got 23mpgs on the interstate on the way to shows. Very do-able these days with OD trannies. That'll let you have some decent bottom end and still high highway gearing for MPGs. Low RPM torque mods and high gearing are the ticket, although it's not just the engine. The whole package is the thing. Realize that you need to keep cruise RPM at or near the torque peak, but cruise speed low enough so that aerodynamics don't kill it.

Back when I had my Mustang, I was considering a 302 to build. It would have had AFR165 heads, which are fairly large heads for a 302, but I decided I wanted to use a roller 5.0 block and keep a stock cam in there. With 3.5 rear end gears, 5-speed, 9.5:1 comp. and only about 2900lbs to lug around (including me), I was hoping for 25-30mpg on the open road, although that's perhaps a little optimistic.

In other words, I prefer medium-size heads with a small cam to make good all-around power while preserving gas mileage.

As it was, that 302 could hold its own. C4 tranny, 2.73 rear end, mostly stock except for a stocker 351 cam, ported heads, edelbrock RPM intake with Edelbrock 600cfm carb and Pertronix electronic ignition. At a steady 60mph it could touch 23mpg, although it normally got more like 20 on the open road.

My 86 Mustang GT 5.0HO got 25-27 mpg over a 700 mile trip (three tanks of gas). heads on the 86 are very 'closed'. If I were building another 5.0 (and I will be in the next year or so), I'd use the F7TE closed chamber heads, flat-top no notch pistons, zero decked. I'd use the stock Mustang intake or the Holley SysteMax I lower intake and stock upper intake, and 1 1/2" long tube headers. Stock cam, stock SD SEFI.

I've got the parts to do this, I just need to giterdone.

are you going to burn platonium or what with that compression. are the valves going to complain about the no notch and zero deck?

The 86 has flat-top, no-notch pistons. Admittedly, the chambers are larger on the E6SE than on the F7TE heads, but I live at 3000 feet of elevation and can tolerate higher compression.

F7TE? wasn't the only head on 302's in 97 the gt40p's on the explorer?

I agree with you rusty70f100! We can do better...and we should!

:-tap

The main points of concern are:
-Camshaft overlap.
-Matching the engine to the vehicle/driveline to keep the RPMs down.

Some performance mods that are typically used in racing might help. Roller lifters and rockers to reduce power losses in the valve train for example. I don't know how useful a lightened flywheel might be, but it could help some.

There was a trick I learned about formula vee engines at one point - a groove is cut around the cam gears to allow oil an escape channel so that the pressure between the gears did not result in a power loss.

Anything that reduces power losses in the engine itself.

I had an experience some years ago with a chevy 454 gas hog trailer tower. The factory cam had rubbed some of the bumps off itself and the torque converter turned itself into metal puree about the same time.

Most people recommended an "RV" cam for replacement but I chose a super tiny, barely open the valves economy cam. I also chose a very low stall speed replacement converter.

The differences were amazing, fuel mileage improved but the performance of the engine under heavy load conditions greatly improved. It got more "diesel like" powerful low end torque and gorgeous pulling power with 30,000 lb trailer.

Ken