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LJ Diff ratio?

Started by _lexa_ , Mar 14 2006 10:19 PM

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15 replies to this topic

#1 _lexa_

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Posted 14 March 2006 - 10:19 PM

hey guys, just wondering wat diff was standard in the 1972 LJ S? also, i have a new engine soon to be put in engine specs towards bottom of this page and was wondering if an upgrade would be advised? this car isnt going to be used for racing or anything (perhaps the occasional drag but nothing full on) and i dont have ALOT of money to throw around. so any help on this topic would be great. thanks alot

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#2 MODNIZD

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Posted 15 March 2006 - 03:14 AM

Pretty sure you will find a 2.78 Open Wheeler.

Put in a taller ratio diff say a 3.36 and it will feel a bit more punchy.

But not recomended your your on the freeway a lot. Unlease you like using lots of fuel.

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#3 _73LJWhiteSL_

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Posted 15 March 2006 - 09:33 AM

If its a manual it should have a 3.08 in it. If its an auto it should have 2.78 in it.

Crawl under the car and have a look on the metal ring thats right behind the uni joint facing the uni on the front of the diff housing. If it hasn't been played with it will be accurate.

Another way to check would be by what reves its doing at 100kph. With a 2.78 at 100kph the rpm should be ~2500, with a 3.08 ~2700, with a 3.36 ~3100 and with a 3.55 ~3300. This is with 205/60R13 tyres, but the standard (175/75R13s) should be simialar.

Steve

Edited by 73LJWhiteSL, 15 March 2006 - 09:34 AM.

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#4 _lexa_

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Posted 15 March 2006 - 11:31 AM

thanks guys, it is a manual with a trimatic badge so i'd say someone has converted previously. so would u suggest an upgrade? if so wat am i looking at spending?

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#5 _73LJWhiteSL_

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Posted 15 March 2006 - 11:40 AM

If it was a auto it should have the 2.78. Great for highway cruising and good economy, but not much good for taking off at the lights, unless you got an 8 infront of it.

If you have a stock 173 or anything smaller I would recommend a 3.36 (which is what i am running 173,m20, 3.36) or if you have a stock 202 or worked 173, 186 or 202 go with the 3.08

But then it depends what the car is used for and if you prefer highway or taking off in a hurry.

Steve

Edited by 73LJWhiteSL, 15 March 2006 - 11:41 AM.

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#6 _lexa_

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Posted 15 March 2006 - 12:14 PM

so with the following specs:
HP block
bore size: 3.665 inches
capacity: 190 cubic inches
stroke: standard 186
compression ratio: 9.2:1
balanced
pistons: ACL flat tops
rings: ACL moly
starfire rods
ARP rod bolts
ACL bearings
LC XU1 spec cam
powerboss hydrolic lifters
street terra roller rockers
ACL gaskets

and a celica 5 speed i shouldnt have any problems with the diff? in the future i will upgrade but for now i'm just trying to get it on the road

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#7 _73LJWhiteSL_

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Posted 15 March 2006 - 12:53 PM

you shouldn't have any problems with the diff. With the 5 speed you could easy run a 3.36. When i suggested a 3.36 with a 173 or smaller motor i was think with a Aussie four speed, but if your running a celica 5 speed i would go with the 3.36. It will give you good launch, and with the 5th overdrive should give you good highway cruising and economy.

Steve

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#8 _lexa_

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Posted 15 March 2006 - 02:03 PM

thanks for the great advice steve. any idea as to wat a 3.36 would set me back>?

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#9 _high_rpm_

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Posted 15 March 2006 - 02:22 PM

wouldnt pay anymore that 50 bucks i got 4 complete diffs for free one a nicely welded 3.36 locker and the rest various ratios with axles

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#10 _lexa_

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Posted 15 March 2006 - 02:42 PM

that would fit into my budget very nicely. lol

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#11 rodomo

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Posted 15 March 2006 - 02:53 PM

I would look at the difference between the 1st gear ratios between the 2 boxes before I chose a diff. If the celica box has a higher ratio (or lower speed) first gear than the aussie you are using you may end up with a go nowhere 1st gear. By the sounds of your engine mods the 3.08 may be O.K. with a celica 1st gear. Hope this helps with the decision.

RACV MAN

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#12 _devilsadvocate_

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Posted 17 March 2006 - 08:12 PM

B4 replacing a diff ratio, do be certain that you know exactly the ratio of your present one. Adjacent diff ratios (2.6? 2.78,3.08.3.35, 3.55,3.9) are within ~10% of each other. If you use speedo, tacho and wheel circumference to establish your diff ratio then errors of 5% each in speedo and tacho could easily get you a value that is 10% out, you could find yourself thinking you had a 3.08, replace it with a 3.35 and find you had a 3.35 all along! Im sure its a job that only needs to be done once if at all.
One can use a formula for 100kmh in top(direct) gear :
diff ratio = (rpm x TC)/1666 or at 60kmh: diff ratio = (rpm x TC)/1000
TC = tyre circumference in metres. How accurate is this with a trimatic? Don�t think they lock up completely at 100kmh?
TC can be found for any tyre size at: http://www.powerdog.com/tiresize.cgi, there are other sites on the net that will do all the calcs for you.
It is more accurate to get your TC by measuring it directly, especially if partially worn etc. A good way to do this is to mark the surface of the tread with some liq paper and roll the car along some smooth ground. Measure the distance bw spots on the ground and you have effective rolling circumference. Possibly rolling circumference increases slightly with speed, but guessing not by much.
An accurate and quick way to establish diff ratio(if unknown) is to get first hand measurements from the axles and tail shaft:
Jack up one of the rear wheels(or both if lsd), release handbrake put in neutral(chock front).
Put an easily seeable marker, perhaps a loop of electrical tape stuck around the tailshaft with a portion hanging off.
Rotate one of the road wheels exactly 10 times and count the corresponding rotations of the driveshaft.
31 rotations = 3.08, 28 rotations = 2.78, 39 rot =3.9 etc.
What sized rear tyre you intend to run. Do factor in that for example a 195/65/14, which could fit on your vehicle is 6% larger than a 205/60/13, over half a diff ratio.

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#13 _lexa_

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Posted 17 March 2006 - 09:50 PM

thankyou very much devilsadvocate, you are a wealth of information, all those suggestions you have made are very helpful and were previously completely unknown to me.

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#14 _draglc_

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Posted 17 March 2006 - 10:07 PM

i was told when i bought my lc that it had a 2.78 in it. so i went and bought a 3.08 to put in it. but when i got the original diff out, it was already a 3.08.. so bit of a waste of time, but it was a better 3.08 i put in it.

A1