Engine Designed for Long Distance

It's time for a new motor in my XJ. For a while I was drawn in by the idea of a stroker and the power....but before I bit the bullet cooler thoughts came into my thoughts. Besides, it's not a race car and a lack of power isn't why I'm rebuilding now. But rather than go stock, is there a formula to have better fuel economy.

I'm not asking just because gas is hitting $4/gal but also for extending the range of a tank of gas. If I get 15mpg and a 20gal tank that's 300 miles. If I get 25mpg and a 20gal tank that's 500 miles. There's a big difference and I like that idea better.

So, anyone ever build a more fuel efficient 4.0?

I'm by no means an expert on AMC/Jeep engines, but typically, a lot of the same techniques that get you more power also get you more fuel economy. Concentrate on breathing - head porting, bigger valves if possible, ported intake, better exhaust. Higher compression will also help economy but may make you go to premium fuel which will hurt your wallet. The things that are typical go-fast tricks that will hurt your economy are the obvious ones like more displacement, longer-duration camshafts, etc.

If you want every last little bit of efficiency, does anyone make a mild-grind roller cam which could be combined with roller rockers? Gains might be minimal, but then you could cut the top off a valve cover and put a Plexiglas panel on it so everyone could admire your awesomeness.

good luck!

With a stroker, you will use less throttle and potentially get better gas mileage.

I get 17 mpg on the highway, with an exo cage, 37's and a stroker.

less throttle generally does equal better gas mileage. less air going into the engine = less fuel going in, too.

Hell, that's how those old vacuum-based fuel economy gauges worked, iirc.

I got 23mpg out of a 200k+ tired 4.0, 200k+ tired AW4, 3/4 ton of cargo, stock 225 tires, and stock rearend gears (3.55s.) How? I drove like a granny, running 55-65mph the entire way, for a trip that went from a gas station just off the highway to another gas station just off the highway.

with all the performance goodies the power will be there but if you do not use the power the engine is working less because it has more power so with all the performance goodies and less throttle you would get good gas mileage...

or if you want good gas mileage daily drive a 4-cyl car and drive the jeep when you want

AlabamaDan said:

So, anyone ever build a more fuel efficient 4.0?

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Keep the stock engine in good tune. No lift, stock tires inflated properly, keep all dead weight out, and drive like Grandma. A stock XJ doesn't need a stroker to have far more power than needed.
Of course, who wants a stock XJ? I found it easier to buy a second XJ and leave it alone. Driving like kastein mentioned, netted 24mpg on the highway. But not driving over 65mph and coasting downhill is almost suicidal in Socal.

I have over 100K on my stroker and I went from 15mpg to 21mpg on the highway, lifted on 35's

old_man said:

I have over 100K on my stroker and I went from 15mpg to 21mpg on the highway, lifted on 35's

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AX15? What gears? I'm on 33s, aw4, and 4.56s and get 15 on a good day.

i get 12 on a good day, with 31's. not sure how people can claim twice that.

gearing
how they drive
strokers help.

It also helps if you don't have the correct speedometer gear.

N8N_99xj said:

I'm by no means an expert on AMC/Jeep engines, but typically, a lot of the same techniques that get you more power also get you more fuel economy. Concentrate on breathing - head porting, bigger valves if possible, ported intake, better exhaust. Higher compression will also help economy but may make you go to premium fuel which will hurt your wallet. The things that are typical go-fast tricks that will hurt your economy are the obvious ones like more displacement, longer-duration camshafts, etc.

If you want every last little bit of efficiency, does anyone make a mild-grind roller cam which could be combined with roller rockers? Gains might be minimal, but then you could cut the top off a valve cover and put a Plexiglas panel on it so everyone could admire your awesomeness.

good luck!

Click to expand...

This is going in the right direction. Higher compression to improve efficiency and a stock like roller cam and roller rockers to reduce friction. Going from regular to premium fuel is only a couple dollar a tank difference, 20 cents x 10 gallons = 2 bucks. Not much of a fuel cost increase.

I'm running 35's with 4.56's. A correctly designed and built stroker can run with regular gas. Those numbers were with an AX 15 and a calibrated speedo. My numbers are maybe a mile or two lower with the AW4 since most of the time on the road the TQ is locked anyway.

bmpweXJ said:

AX15? What gears? I'm on 33s, aw4, and 4.56s and get 15 on a good day.

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Depending on how much power you need, you might be better off not stroking, staying with the stock displacement but otherwise building it like a "performance" stroker (head/valve work, etc).

Even if the extra power of a stroker means you can be easier on the throttle, you're still having to supply an extra .5-.9 liters of volume with an adequate amount of fuel to burn safely.

I think it would be interesting to build a 4.0 with a zero decked block, dished pistons, opened up combustion chambers and ports, large valves, ported/polished head & intake, cam (possibly a full roller set up), and a good tune. I would imagine you could get some pretty decent mileage :dunno:

I've been keeping track of mileage on my 90 for 10 years. When it was stock, and relatively new and the speed limit was 55, using real gas, no ethanol or that other crap they used to add in the winter, I could get 21 or 22 mpg. It has an AX-15 and still had the low altitude CPS. Later on, when I put in the high altitude CPS, on good gas around town with some exhaust and intake work, I was getting 16-18 mpg.

I think if you want mpg on an XJ, stick to 30" tires or less, gear properly, keep it tuned. Mpg goes down with the mileage of the engine, at least mine did.

I also think that a good air dam that is removable for off roading would help. I had one of those bug shields for awhile, but the mounts broke off. I duct taped it to the stock air dam. I only have a few mpg readings from the one trip in the mountains, but I got around 20 mpg with a loaded XJ pulling a camping trailer over Mt. passes. It's not the best data point in the world, but it is suggestive.

Recently, I got around 16 mpg on the 90 after taking off the front driveshaft. It's so worn now that it's lucky to get 14 mpg with the front driveshaft on.

I'll add that the boy's 97 with 175k miles, an AW-4, and all new electronics, practically, gets around 12 MPG in town.

The 99 WJ with a 4.0 will still get 20 mpg in CO, and 21-22mpg at sea level on the highway.

My DD 90 XJ gets a solid 16 in town and 21 highway. I'm geared 4.10 on 30's, run the Spectre cowl intake, other than that it is stock.

My uncle was in management at a GM motor plant, He'd tell me how they'd pull a motor off the line and rework it by hand. Port, polish, tolerances to bring everything as close to perfect as they could and get good increases in HP and MPGs. Just to labor intensive and time consuming for mass production.

Milford Cubicle II said:

Depending on how much power you need, you might be better off not stroking, staying with the stock displacement but otherwise building it like a "performance" stroker (head/valve work, etc).

Even if the extra power of a stroker means you can be easier on the throttle, you're still having to supply an extra .5-.9 liters of volume with an adequate amount of fuel to burn safely.

I think it would be interesting to build a 4.0 with a zero decked block, dished pistons, opened up combustion chambers and ports, large valves, ported/polished head & intake, cam (possibly a full roller set up), and a good tune. I would imagine you could get some pretty decent mileage :dunno:

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Its been done. The stroker is still better.

Well I would prefer the stroker for the increase in power. But for a stock rig and for someone that doesn't want/need the extra power, I can't imagine a stroker getting better mileage. Unless you know of a stroker design that can beat 21-22 mpg highway :dunno:

Jes got 25 mpg with his stroker, 5.38's and 35's.

I get 17 mpg with my stroker, 5.13's, 37's, 8" lift and an exo cage.

Find me those numbers on a stock 4.0, and I'll be impressed. The 4.0 is a great motor, but it doesnt have the torque not to require throttle when under load.

Stroke it, and you use a lot less throttle all the time. I know it sounds wrong, but economy increases (assuming everything is running right, etc. A lot of people use old junkyard sensors and report 9 mpg..).