Gas mileage improvements?

[foggybottombob]

OK I'm done with my 4.6L Low compression stroker on a '91 XJ. I have a SCR of around 8.9. My gas mileage with the 4.0 at 225K miles was about 16mpg. My current mileage with the stroker is around 14mpg. I live at 7000 ft. I used the 24lb/hr ford injectors called for in the recipe but by all other indications these are a bit too much for a 240hp 4.6 Liter. I wondered if I should reset the computer again now that I am past break in on the engine. I plan to do that anyway. I tried to search and the search function would not run. Will I likely have to change the computer to make my EFI fit the new engine displacement? Any tips about getting my mileage back to normal?

[lafrad]

Going to be very tough to do with the low compression. Mine stayed the same on the hiway (or even increased a bit) when bumping to 10:1 SCR (and a small cam), but around town its still down quite a bit... its too fun to stab the throttle and jump around

the extra displacement will naturally consume a bit more fuel due to more heat loss to the cyl walls, and increased ring friction from the increased stroke... Best thing to do is just keep your foot off the gas, at this point...

[Cheromaniac]

I'm using the stock computer, stock fuel pressure regulator, and Ford 24lb injectors on my 9.25:1 compression 4.6 "poor man's" stroker (248hp/311lbft flywheel). I have a homebrew MAP adjuster set at 5.2v to keep the A/F ratios optimal. Without it the engine would run too lean.
Since I swapped in an '01 XJ intake manifold last year, I gained ~1mpg so now my gas mileage is about the same from my stroker overall as it was from my old 4.0. Around town it's only slightly worse but it's better on the highway. Getting 18mpg (22mpg Imperial) in mixed driving is as easy as pie.

[foggybottombob]

So do you think an adjustable MAP sensor would be worth trying?

How would I know if the engine were running rich or lean other than looking at a spark plug?

[SilverXJ]

Wideband O2 sensor

[Cheromaniac]

Wideband O2 sensor

In my case it was the Autometer A/F gauge running off the standard O2 sensor that was telling me the engine ran lean at the stock 5.0v MAP voltage. The engine also had a slight miss at low rpm at light throttle. Turning the MAP voltage up to 5.2v corrected the lean condition, got rid of the miss, and the engine runs like a raped ape.

[foggybottombob]

I had my exhaust analyzed and got this back.

@2600 RPM
CO = 0.21%
HC = 30 PPM
CO2 = 18.32%
O2 = 0.0%

@Idle
CO = 0.28%
HC = 35 PPM
CO2 = 14.98%
O2 = 0.0%

Does anyone have any comment about these numbers? The mechanic says this is all good. I'm still getting 14.5 mpg which seems low to me.

[foggybottombob]

I got 18mpg on a trip this weekend driving generally about 65mph in the mountains. So it looks like I have 14.5 in town and 18 highway unless I change gear ratios. I could probably get 1mpg if I drop from the 3.92's I have to 3.73's. But I can't do that unless I put a 4:1 in the NP231 since my off road low range gearing is nothing to write home about already.

[Cheromaniac]

I got 18mpg on a trip this weekend driving generally about 65mph in the mountains. So it looks like I have 14.5 in town and 18 highway unless I change gear ratios.

Those figures aren't bad at all if your Jeep has a lift and bigger tires than stock.

[kf_chris]

I got horrible mileage before the motor build, but that was due to issues (and the lift and tires on the stock gearing)
Now I get a solid 15 mpg hwy, and significantly less when I have fun in stoplight traffic. I have a set of axles to put in with LSD and 4.10 gears, hope to go up on the city mileage and stay the same or better on the road. 4.5" lift and 31" AT's.
I like the ability to roast the tires with stock 3.55 gears