My goal is to get 25+ mpg from my 99 XJ with 30 x 9.5 R15 ATs - Page 22 - JeepForum.com
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Unread 08-06-2013, 01:31 PM   #316
Charley3
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Quote:
Originally Posted by 1997xj View Post
What rpm did you find was most efficient and at what speed?
I'd like to know that too.

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Unread 08-06-2013, 01:34 PM   #317
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OK, I'm still around the forum a little this week, but much less free time. So any posts I make this week will be brief questions or comments related to other peoples' posts.

When short of time, I still enjoy breaks when I can lurk and read other peoples' posts, even if I don't have time to type up much in return.

Thanks each person for your information, questions, and participation. You make this thread interesting and useful.
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Unread 08-07-2013, 09:17 AM   #318
tbart16
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great thread you have inspired me to give up my subaru and possibly purchase a more reliable cherokee I found one locally for a great price and was concerned about losing MPGs subie avgs 20-23. If I can get a cherokee with minimal mods up to 22 or so it seems worthwhile. Thanks again and keep us posted
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Unread 08-07-2013, 10:59 AM   #319
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Has anyone applied any of this to a 2.5 with good results?
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Unread 08-07-2013, 12:43 PM   #320
Charley3
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Originally Posted by tbart16 View Post
great thread you have inspired me to give up my subaru and possibly purchase a more reliable cherokee I found one locally for a great price and was concerned about losing MPGs subie avgs 20-23. If I can get a cherokee with minimal mods up to 22 or so it seems worthwhile. Thanks again and keep us posted
You should be able to mod a Jeep Cherokee XJ to 25 mpg with a tune up and a few cheap mods. My first XJ got 21 mpg completely stock.

However, you could do similar mods to your Subaru and get 27 or 28 mpg, IMO.

Though a Jeep XJ is way more cool.
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Unread 08-07-2013, 01:20 PM   #321
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Charley3 really wanted me to post up about the AC Delco thermostat I installed, but it hasn't even been 100 miles yet so I can't tell you much by the way of results. These are my preliminary results, but when I know more I will post up.

Quote:
The following questions and your answers will help me get a better understanding of the ACDelco stat so I can begin to compare it to the Gates stat I have.

Many people are asking me which stat is better. I don't know yet. I like my Gates, but need more info on ACDelco.

You are the first person to try the ACDelco.

With 205F ACDelco stat:

What is the op temp (dash guage) your engine runs most of time?
With the original 195F stat I sat right at about 200 most of the time, on occasion it would heat up to 210F but never above it. With the AC Delco 205F stat she stays rock steady at 210F, Haven't seen it get above that yet.

Quote:
At what temp does your electric fan come on?
Best guess according to the dash gauge is it comes on at about 215F, but I run without the A/C on a lot so it comes on all the time with the compressor.

Quote:
How often does your electric fan come on?
With the A/C off it rarely comes on, if sitting at a light it will on occasion, but not a constant.

Quote:
At what temp does your electric fan go off?
Best guess is when it reaches 210-212F, again we are going by a dash guage which is not exactly known for their accuracy.

Quote:
Have you noticed a change in your gas mileage?
Not really, actually it seems a little bit worse going by the gauge. If I don't see a positive change in the next few tanks I'm changing back to a 195F thermostat.
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Unread 08-07-2013, 01:42 PM   #322
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The subaru is great its just that when I need to buy parts they cost an arm and a leg and the 2.5 in my opinon is underpowered for my taste. I have owned plenty of wranglers,CJ's, and grands only one XJ. they are easy to work on and parts are readily available
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Unread 08-07-2013, 05:35 PM   #323
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Thanks for posting Sikentneko.

Your info is almost identical to my experiences, except for a few things.

My 205F Gates stat has me running 208F most days. It runs 210F only on hottest days.

My gas mileage improved with the warmer stat.

Two differences between our Jeeps are that I don't use air conditioning, and I use mid grade 89 octane gas. I wonder if either of those differences is important?
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Unread 08-07-2013, 06:26 PM   #324
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Received new injectors with four holes that were recommended for the 1996 4.0. I was wondering what exhaust you were going with? I replace my air filter about every six months and clean out the housing so I will stay with a stock filter. What part of Washington are you located?
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Unread 08-07-2013, 07:04 PM   #325
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Great thread!
Some general comments:
1) AC compressor should only come on when the control knob is pointing to the blue snowflake positions (left hand side of the dial) or the towards the defrost positions. The basic vent and heat positions do not engage the compressor. Experiment with this in your own driveway - you'll hear when the compressor comes on.
2) the compressor only runs to pressurize the system. On cooler days, less heat is being removed from the cabin air and therefore the compressor has to run less to keep up with the demand. Theoretically better gas mileage will occur.
3) opening all the windows hurts gas mileage as speeds increase. Much above 40-50 mph, better to just put on the AC.
4) alternators will only output the amperage (current) your electrical system needs at that point in time. The voltage regulator controls the output by regulating the voltage applied to the field coil of the alternator. Therefore, you can have a big alternator, but if you are driving around with minimal users drawing power, the alternator - any alternatofr - is spinning fairly easily (keep in mind there are base loads you can't control like the fuel pump, injectors, computer).
Vehicles with police packages sometimes have higher output alternators and larger pulleys so they can idle with all the lights on. Go that route if that's what you need but for fuel mileage, not going to do much of anything.
5) aerodynamics are huge - especially when driving a brick like an XJ. Remove roof rack. Don't lift. Smaller narrower tires offer less rolling resistance and present less of a "face" to the wind (remember our tires are fairly exposed to the airflow compared to modern cars). Lower the front end and/or extend the length of the front air dam lower to minimize air getting into that very turbulent area under the vehicle (I had an old Saab many years ago that had basically a flay underside - no doubt the influence of Saab's aircraft engineers on the design).
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Unread 08-07-2013, 07:16 PM   #326
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Quote:
Originally Posted by ECL View Post
Great thread!
Some general comments:
1) AC compressor should only come on when the control knob is pointing to the blue snowflake positions (left hand side of the dial) or the towards the defrost positions. The basic vent and heat positions do not engage the compressor. Experiment with this in your own driveway - you'll hear when the compressor comes on.
2) the compressor only runs to pressurize the system. On cooler days, less heat is being removed from the cabin air and therefore the compressor has to run less to keep up with the demand. Theoretically better gas mileage will occur.
3) opening all the windows hurts gas mileage as speeds increase. Much above 40-50 mph, better to just put on the AC.
4) alternators will only output the amperage (current) your electrical system needs at that point in time. The voltage regulator controls the output by regulating the voltage applied to the field coil of the alternator. Therefore, you can have a big alternator, but if you are driving around with minimal users drawing power, the alternator - any alternatofr - is spinning fairly easily (keep in mind there are base loads you can't control like the fuel pump, injectors, computer).
Vehicles with police packages sometimes have higher output alternators and larger pulleys so they can idle with all the lights on. Go that route if that's what you need but for fuel mileage, not going to do much of anything.
5) aerodynamics are huge - especially when driving a brick like an XJ. Remove roof rack. Don't lift. Smaller narrower tires offer less rolling resistance and present less of a "face" to the wind (remember our tires are fairly exposed to the airflow compared to modern cars). Lower the front end and/or extend the length of the front air dam lower to minimize air getting into that very turbulent area under the vehicle (I had an old Saab many years ago that had basically a flay underside - no doubt the influence of Saab's aircraft engineers on the design).
Perfect comments--every single one.

You're spot-on about the windows vs AC thing, and I'm glad you mentioned it because it's useful info that I hadn't thought to mention. At slower speeds, windows are better. At higher speeds, AC is better. I'm not sure at what speed the line is drawn, and I would think it could vary depending on which windows are open and how far. It is merely anecdotal, so don't jump on me if my impression is dead wrong, but I FEEL like having two diagonally opposed windows open only (like driver's side front and passenger rear) doesn't create as much resistance while still encouraging airflow through the vehicle. Anyhow, at a certain speed, closed windows with AC on is still the way to go.
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Unread 08-07-2013, 11:06 PM   #327
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Maybe it partly depends on the vehicle. How aerodynamic it is, etc.

I tested my Buick Park Ave.

Obviously it gets better gas mileage with windows down around town.

On freeway it gets approx equal gas mileage with windowns down or with AC on.

It does get better gas mileage with windows up and AC off, but I sweat to much.

I haven't tested my XJ. Possibly it's different. The Buick is a lot sleeker than the Jeep.

Also, it depends on how fast you're driving. I think if you drive fast enough (for the given vehicle's aerodynamics) at some point AC is more efficient. This is because (obviously) windows down creates increasingly more (quadratically more) drag as you drive faster. AC power drag on engine is same at all speeds. So at some speed the two are equal. Above that speed the windows down drag has increased above above what AC drag would be.

I'll retest the Buick, and test the Jeep, at a few highway different speeds.
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Unread 08-08-2013, 05:47 AM   #328
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The trick to using AC is to cycle it yourself and imitate the way it works in your house. Run it full blast until you're cold, then switch to fan only / recirculate to swirl the cool air around for a while. When you start to get warm, switch back to AC again.
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Unread 08-08-2013, 06:57 AM   #329
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Quote:
Originally Posted by Kevin108
The trick to using AC is to cycle it yourself and imitate the way it works in your house. Run it full blast until you're cold, then switch to fan only / recirculate to swirl the cool air around for a while. When you start to get warm, switch back to AC again.
Do the 97+ XJs have a toggle recirc? The only way the earlier XJs recirc is on max AC.
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Unread 08-08-2013, 08:09 AM   #330
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Quote:
Originally Posted by Charley3 View Post
So you substantially increased your alternator output. That is opposite of the concept Newtons was talking about.

More alt output means more drag on engine, less horsepower, and less gas mileage.
Very true. I saw this first hand on an engine dyno.
Built an engine (non Jeep, '71 Buick 350 v8) for a client, was doing some dyno pulls to check tune and see the power output. The vehicle had an aftermarket electric fan setup (not cheap stuff). On the second pull, the fan came on and you could see it on the dyno graph when it turned on, that pull was 30hp less than the other pulls.

E-fans are great, but they will cause high parasitic draw when running at high speed, which would be a rare event if your cooling system is in good condition, and probably only at low/idle speeds, at which point, MPG is screwed anyways.

And just because an alt can put out 120A at idle, doesn't mean it won't cause a huge draw. It takes energy to make energy. If MPG was the goal, I'd go with a lower output or the minimum to get the job done.
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