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5th Gear ratio change/upgrade


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10 replies to this topic

#1 AlpineRaven

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Posted 28 June 2006 - 05:07 AM

G'day fellas,
I'm hoping you could correct/discuss with me with this as no one really wants to talk to me about this and its driving me crazy!...

The question I have been asking is -

CAN I install 0.738 or 0.780 5th gear ratio in 5MT Dual Range box replacing 0.871.

Why? Because the revs is 3050rpm at 100kmh (62.13mph) and its 3400rpm at 110kmh (68.35mph) and I drive for a hour in 110kmh while revving at 3400 rpm..

Currently the 5th Gear is 0.871 in my Liberty (legacy) has 3.9 final drive ratio in front and rear diffs. Its EJ22 engine.

I am hoping to drop revs for highway use, if I used 3.9 diff with 0.738 5th gear at 100kmh the revs would be approx 2250rpm or.... if i used 0.780 5th gear, i would be looking around 2450 rpm at 100kmh.

The reason I want to do that is to drop revs and save fuel on flat highways, if the engine is complaining ie. uphills, i'll select the dual range in 5th to increase approx 500rpm.

Can it be done, to take the gear out from other gearbox and put it in mine?

Thanks
AP

#2 Legacy777

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Posted 28 June 2006 - 11:29 AM

I don't see why you can't do this. I believe some of the forestor guys have done this.

Which box are you wanting to take it out of?

#3 skizix

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Posted 28 June 2006 - 12:11 PM

Well, Scorpion sells an ultra-low crawler gear, that is intended to replace 5th (the idea is also: you'll be putting bigger tires on as well, so don't need as big a top gear anyway). So I would say it is definitely do-able.

However, it is not guaranteed that all that hassle will net you significantly better mileage, or even better at all. Throttle position has a greater impact on fuel efficiency than rpm's. E.g. if you you have to floor it in 5th to maintain speed on a given hill, but you only have to bury it slightly to get up the same hill at speed in 4th...your mileage in 4th on that hill will certainly be better, despite higher rpm's.

I realize you're most likely talking about cruising on the flats. And you probably would see some benefit. My point is that it's probably not going to make as big a difference as you're thinking.

So I guess the question is: how long are you going to have to drive, with slightly improved mileage, to see a positive return on the time and/or $$$ it will take to accomplish your goal (significant, IME)?

Geez, I wish they offered dual-range trannys here in the States! Don't think they have since the '80's.

#4 grossgary

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Posted 28 June 2006 - 12:22 PM

in an AWD XT6 the auto and manual trans... they are both capable of the same highway gas mileage at cruising speed. in this case the XT6 cruising RPM is reduced at the same speed between auto and manual trans. in the auto you're doing 750 less RPM (roughly) for the same speed at highway cruising speeds. this makes for a much quieter ride as well. the AWD auto and manual transmissions end up having nearly identical highway gas mileage figures. i've read that peak torque is optimum cruising RPM for best gas mileage. i don't know where peak torque is on the XT6 but i do know that the auto and manuals get the same gas mileage and i'm guessing it's due to the different gearing.

#5 AlpineRaven

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Posted 28 June 2006 - 03:20 PM

Fair enough, I might go over to forrester forums and see what they say.

I was planning to take out the 0.780 5th gear from RS Legacy or 0.738 from late 1990s WRX models.


But from what I have been thinking and researching about the 0.738 is a bit too low for EJ22. I think 0.780 would be perfect.

Thanks guys for your answers. Do you know anyone might have done this kind of thing apart from forrester guys?
Cheers
AP

#6 nipper

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Posted 28 June 2006 - 03:46 PM

forester guys are twisted :banana: .
This should be easy enough to do, as people do ti all the time in other cars (or used to) Going to be a lot of work. As long your tearing into the transmission inspect the synchros and make sure they are good. That would be the time to change them


nipper

#7 AlpineRaven

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Posted 29 June 2006 - 06:52 AM

Can you pass me a link for Subaru Forrester forum if there is one, only one I could find was - Subaru Forrester Owners Forum - http://www.subarufor...ulletin/forums/

Is that the one?
Thanks guys
AP

#8 1 Lucky Texan

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Posted 29 June 2006 - 08:37 AM

Hey guys, is there 'rule of thumb' or something about this? Like 'best gas mileage is peak torque RPM minus 10%' or something? Would it be worthwhile to find an AWD dyno first and get the chart for his car to find peak torque/hp ?

just wondering


Carl

#9 Legacy777

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Posted 29 June 2006 - 09:23 AM

I dug up some info here.

The fuel efficiency "island" or (3d type graph) as it's called is typically determined from test data...at least that's to my knowledge.

The two axis are BMEP (Brake Mean Effective Pressure) & Engine RPM.

The island or z axis is BSFC (Brake Specific Fuel Consumption)

Here are two pictures showing the graph
http://www.main.expe...sfc_island1.jpg
http://www.main.expe...sfc_island2.jpg

I took those graphs from this document
http://www.cleancarc.../pdfs/MPG2K.PDF

Back to the torque comment. BMEP is pretty much a direct relationship to engine torque.

BMEP = 150.8 x TORQUE (lb-ft) / DISPLACEMENT (ci)

Got that from here
http://www.epi-eng.com/ET-BMEP.htm

So, with moving the equation around, and solving for torque, you can input the BMEP values from the two graphs, and have the graph in values of engine torque.

What this is more or less saying is that IF you wanted to map your engine on a dyno to find out where your max torque spots are, those would probably be your most fuel efficient spots to run your engine.

This of course is not taking into account wind resistance/drag, rolling resistance, etc.

Josh

#10 1 Lucky Texan

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Posted 29 June 2006 - 09:34 AM

I dug up some info here.

The fuel efficiency "island" or (3d type graph) as it's called is typically determined from test data...at least that's to my knowledge.

The two axis are BMEP (Brake Mean Effective Pressure) & Engine RPM.

The island or z axis is BSFC (Brake Specific Fuel Consumption)

Here are two pictures showing the graph
http://www.main.expe...sfc_island1.jpg
http://www.main.expe...sfc_island2.jpg

I took those graphs from this document
http://www.cleancarc.../pdfs/MPG2K.PDF

Back to the torque comment. BMEP is pretty much a direct relationship to engine torque.

BMEP = 150.8 x TORQUE (lb-ft) / DISPLACEMENT (ci)

Got that from here
http://www.epi-eng.com/ET-BMEP.htm

So, with moving the equation around, and solving for torque, you can input the BMEP values from the two graphs, and have the graph in values of engine torque.

What this is more or less saying is that IF you wanted to map your engine on a dyno to find out where your max torque spots are, those would probably be your most fuel efficient spots to run your engine.

This of course is not taking into account wind resistance/drag, rolling resistance, etc.

Josh


Wow - you are good! It kinda confirms my gut insticnt that you probably would want your 'target' speed (in your selected gear ratio) to be at or just before peak torque rpm - certainly not after because there are times you need a little xtra power to -say- pass a semi before settling back down to cruise speed. Plus, there are problems 'lugging' an engine if the rpms are too low for the speed/ratio in use -overheating,etc. I dunno - good work finding all that stuff I don't understand!


Carl

#11 nipper

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Posted 29 June 2006 - 10:02 AM

generally you want to be at as low an rpm as possible and as close to the HP torque begining as you can get. GM s shich tend to have decent highwya milage at speed i seem to remeber are below 2000 rpm.
In fith gear you want the rpm to get as low as possible with out the engine bucking when you stomp on it at 55 mph a good reference point)

nipper




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