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post #91 of 124 Old 03-28-2013, 02:03 PM
comptiger5000
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Originally Posted by ratmonkey View Post
don't compare to a gm or other style vehicle, completely different motor and completely different aerodynamics dictating different power level needs at different speeds.

at 60 these motors do best above 2000rpm. both the v8 and the i6. they're really undergeared from the factory and hate being lugged down.
It might be happier, and more efficient at WOT at higher rpm, but it'll have far more pumping losses at light throttle cruise, leading to worse mileage. With a 4.0 (equally antiquated design), the difference between 3.73 and 3.55 gears appears to be worth a full mpg in favor of the 3.55s due to the 100 less rpm.

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post #92 of 124 Old 03-28-2013, 04:02 PM Thread Starter
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the best cruising mileage is achieved at the highest manifold vacuum levels. a lugging engine is decidedly not at high vacuum.

'97 zj 5.2, some stuff, some other suff, and some things that even work sometimes.

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post #93 of 124 Old 03-28-2013, 04:07 PM
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the best cruising mileage is achieved at the highest manifold vacuum levels. a lugging engine is decidedly not at high vacuum.
Hmmm... Going by pumping losses, the best economy should be at lower vacuum levels, as long as you stay out of fuel enrichment. If you're spinning 2500 rpm with the throttle barely cracked compared to 1500 rpm with the throttle half open, you're wasting far more energy on pulling air through the engine and internal mechanical friction at the higher rpm. To prove this, take a fairly stock ZJ, measure the highway mileage running in OD, and then do it again in 3rd on the same stretch of road with similar wind conditions. I can guarantee that doing around 60 - 65 mph, you'll lose at least 2 - 3 mpg by running in 3rd.

If getting the revs up would help highway mileage, then I would think Jeep would have put 4.10s in ZJs stock, as it would also make them faster.

EDIT: Mind you, when tuning for economy, assuming you're not changing the gearing, then you want to adjust fuel mix, timing, etc. to obtain the highest vacuum at a given road speed. But if vacuum was the only key and changing gearing to get the revs up would help the mileage, then we should be spinning 3000 rpm down the highway.
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post #94 of 124 Old 03-28-2013, 07:52 PM Thread Starter
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I get significantly worse mileage running 2200ish rpm in 5th vs 2600ish rpm in 4th at 70mph.
I've ran this experiment on my junk, many others have too. These motors hate to turn under 2250 when on the interstate.

I do get better economy in 5th at 50mph on a flat though. All about wind resistance.

'97 zj 5.2, some stuff, some other suff, and some things that even work sometimes.

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post #95 of 124 Old 03-28-2013, 09:49 PM
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I get significantly worse mileage running 2200ish rpm in 5th vs 2600ish rpm in 4th at 70mph.
I've ran this experiment on my junk, many others have too. These motors hate to turn under 2250 when on the interstate.

I do get better economy in 5th at 50mph on a flat though. All about wind resistance.
Interesting... I'm sure being lifted and the extra aero drag that results changes that relationship a good bit. Keep in mind, mine is staying stock height on 29.5" tires for the foreseeable future. Also, the 360 is a bit happier at low rpm than the 318, especially if they both still have a kegger.

If I drop mine to 3rd (2600 at 60), I go from getting about 18 mpg at 60 to getting 15 or so. But it somehow got 11.5 pulling another ZJ on a trailer (just over 11k lbs combined) at 55 in 3rd (and it had no trouble pulling 7% grades at 2300 - 2400 rpm with that much weight, never had to drop to 2nd) It's a little strange. I do usually cruise at 60 - 62 on the highway. If I take it to 65+, it's only for shorter runs and I'm not caring much about mileage (it gets about 13 mpg at 80 )
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post #96 of 124 Old 03-29-2013, 09:20 AM Thread Starter
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i used to get 18-19 going 75 and about 17 when going 55 with only the turbo city dual intake on it and stock tires, auto trans.

now, about 16 going 75 in fourth. if i'm in a big rig's slipstream i can get the same mileage in 5th.

'97 zj 5.2, some stuff, some other suff, and some things that even work sometimes.

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post #97 of 124 Old 03-29-2013, 09:51 AM
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i used to get 18-19 going 75 and about 17 when going 55 with only the turbo city dual intake on it and stock tires, auto trans.

now, about 16 going 75 in fourth. if i'm in a big rig's slipstream i can get the same mileage in 5th.
Sounds like it's the aero killing you then. With the revs you're turning in 5th, you're probably putting enough load on the engine to drive it past its efficiency point and possibly into fuel enrichment at times, killing your mileage.

Mine doesn't seem to be loaded enough cruising around 60, as I get better mileage in mild hills than I do on flat ground (alternating between a bit more load and no load, rather than steady light load). Unfortunately, the math only gives an idea of what should be better, but getting the best results requires some experimenting. Heck, I got about 10 mpg on a highway run once, following some friends to the dragstrip at near triple digit speeds
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post #98 of 124 Old 04-23-2013, 12:32 PM
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Im having a hard time finding an ecm, the only two i can find on car part.com are $95, no warranty, and need reprogramming at a dealer? The pick a parts are filled with automatics
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post #99 of 124 Old 04-23-2013, 01:04 PM Thread Starter
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Im having a hard time finding an ecm, the only two i can find on car part.com are $95, no warranty, and need reprogramming at a dealer? The pick a parts are filled with automatics
i got mine from ebay, think it was the third one i tried. they don't come with warranty because they go bad so often just sitting around. luckily the guy i bought from on ebay exchanged my ecu.
first one i got wouldn't run the gauges
second wouldn't connect with obd port
third was the charm so far.

'97 zj 5.2, some stuff, some other suff, and some things that even work sometimes.

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post #100 of 124 Old 04-24-2013, 11:53 AM
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Did yours need any reprogramming
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post #101 of 124 Old 04-24-2013, 11:56 AM Thread Starter
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No

'97 zj 5.2, some stuff, some other suff, and some things that even work sometimes.

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post #102 of 124 Old 04-25-2013, 01:52 PM
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Still no luck finding an ecm. If i understand correctly, it is only needed so that the cel light isnt triggered by auto trans codes, the truck will run with the jeep ecm, right?

If so, i might live with the cel until i can find a dakota ecm, hopefully before my next smog test lol
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post #103 of 124 Old 04-25-2013, 04:59 PM Thread Starter
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Still no luck finding an ecm. If i understand correctly, it is only needed so that the cel light isnt triggered by auto trans codes, the truck will run with the jeep ecm, right?

If so, i might live with the cel until i can find a dakota ecm, hopefully before my next smog test lol
it won't idle correctly when coasting in neutral, it also won't engine brake correctly, it also tends to flare between shifts.
the auto computer just doesn't know what to do about a manual. you can drive it, just won't be as fun.

'97 zj 5.2, some stuff, some other suff, and some things that even work sometimes.

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post #104 of 124 Old 04-25-2013, 08:01 PM
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Ok, bummer, thanks for the info. Around how much should i expect to pay? Ive seen a wide variety of prices
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post #105 of 124 Old 04-25-2013, 08:38 PM Thread Starter
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Ok, bummer, thanks for the info. Around how much should i expect to pay? Ive seen a wide variety of prices
i paid 75 through ebay for the ones i got. had to pay for two and one was a warranty exchange that still didn't work. they're hard to come by used and the manual trans option for rams in the right year range doesn't seem to be in the books for napa, autozone, or advance.

'97 zj 5.2, some stuff, some other suff, and some things that even work sometimes.

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