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diff ratios

Started by _DVS LH_ , Dec 07 2010 08:56 PM

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#1 _DVS LH_

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Posted 07 December 2010 - 08:56 PM

Hi there just wondering if someone could advise me on any gains from changing my diff ratio from 278 to 336?

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#2 S pack

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Posted 07 December 2010 - 10:40 PM

Hi there just wondering if someone could advise me on any gains from changing my diff ratio from 278 to 336?

Faster acceleration but lower maximum top speed and engine will rev higher increasing fuel consumption.

Rough estimation to give you some idea, so don't take these figures as gospel, they are not accurate.
eg: 2.78:1 at 100kph engine revs 2600rpm.
3.36:1 at 100kph engine revs 2850rpm.

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#3 _TorYoda_

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Posted 07 December 2010 - 11:52 PM

I had an experience some years ago with a well modified holden 6 where we took out a 2.78 and replaced it with a 3.55. The car struggled to make 100mph with the 2.78 at around 4800rpm.... it just wouldn't pull any higher! With the 3.55 it happily made 120mph at closer to 7000rpm! It also accelerated to that speed more rapidly. The explanation appears to be the engine lacked the torque to push the taller ratio, when we let it rev more freely with the lower gear it improved in all areas. Fuel consumption increased very slightly on the highway but improved in traffic due to the reduced effort to get the car moving. Hope this helps with your decision.

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#4 S pack

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Posted 08 December 2010 - 12:12 AM

I had an experience some years ago with a well modified holden 6 where we took out a 2.78 and replaced it with a 3.55. The car struggled to make 100mph with the 2.78 at around 4800rpm.... it just wouldn't pull any higher! With the 3.55 it happily made 120mph at closer to 7000rpm! It also accelerated to that speed more rapidly. The explanation appears to be the engine lacked the torque to push the taller ratio, when we let it rev more freely with the lower gear it improved in all areas. Fuel consumption increased very slightly on the highway but improved in traffic due to the reduced effort to get the car moving. Hope this helps with your decision.

Very good point Craig. The same thing can occur with overdrive gearboxes. Higher top speed can be reached in 4th gear than in 5th (based on a 5spd box).

Ultimately the choice of diff ratio will come down to the intended use of the car.

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#5 _73LJWhiteSL_

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Posted 08 December 2010 - 01:02 AM

Hi there just wondering if someone could advise me on any gains from changing my diff ratio from 278 to 336?

Faster acceleration but lower maximum top speed and engine will rev higher increasing fuel consumption.

Rough estimation to give you some idea, so don't take these figures as gospel, they are not accurate.
eg: 2.78:1 at 100kph engine revs 2600rpm.
3.36:1 at 100kph engine revs 2850rpm.

My LJ SL was a factory auto so had the 2.78 diff in it. When I changed to M20 it was sitting on about 2550RPM at 100ks, running Sprintmasters with 205/60R13 rubber. With the 3.36 it was sitting on about 3100RPM with same setup. It actually caused me to replace the M20 with a Celica 5 speed which from memory dropped the revs to about 2900RPM in 5th at 100kph.

Food for thought.

Steve

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#6 _threeblindmice_

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Posted 08 December 2010 - 07:51 AM

It would depend on where you drive ,open fast roads high diff , mountains or slower roads low diff . the benefit of a low diff is you are more likely not to get caught speeding as much , as you drive to the engine noise.

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#7 _DVS LH_

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Posted 08 December 2010 - 11:41 AM

Hi there just wondering if someone could advise me on any gains from changing my diff ratio from 278 to 336?

Faster acceleration but lower maximum top speed and engine will rev higher increasing fuel consumption.

Rough estimation to give you some idea, so don't take these figures as gospel, they are not accurate.
eg: 2.78:1 at 100kph engine revs 2600rpm.
3.36:1 at 100kph engine revs 2850rpm.

Thanks there for the helpful info this forum stuff is ALL new to me so still trying to work it all out.

Just a bit of back ground on my LH for your info. Any iformation much appreciated.
Torana goes very well don't get me wrong but as I have a 278 diff in it single spinner with a trimatic not quiet getting onto the cam nicely just a bit doghy off the mark. I bought a 336 diff centre to put in so just wondering what to expect?
I had an original 2850 six with 153,000 kms on it. COMPLETELY STOCK. Trimatic auto.
Now I have bought a 308 and FULLY rebuilt it.
Fully balanced everything, blue printed, o deck height, 9.5:1 comp, Hypertek pistons, custom grind cam good from 2500 to 4750rpm. 650 holley vac secondrys, extractors,

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#8 _hp179block_

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Posted 08 December 2010 - 12:11 PM

I had a 2.78 in my LJ and after the first engine rebuild I did it would do 155kmh in only 2nd gear (yes, a bit slow off the mark).

I would recommend going to 3.36 gears, it was a good thing for me as without it I would have the speedo needle pointing to the 3rd digit of the odometer from memory which was a bit unusable. I think you'd need to change the stall convertor though

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#9 _Mike73_

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Posted 08 December 2010 - 02:11 PM

I had a 2.78 in my LJ and after the first engine rebuild I did it would do 155kmh in only 2nd gear (yes, a bit slow off the mark).

I would recommend going to 3.36 gears, it was a good thing for me as without it I would have the speedo needle pointing to the 3rd digit of the odometer from memory which was a bit unusable. I think you'd need to change the stall convertor though

I run a 2.78 in my GTR Torana with no problems other than more gear changes required in hilly country, the fuel consumption is identicle to the old 3.08 due to the slightly increased load.

This ratio is a 186 with a stock bottom end, big valve head and extractors, the carbie is the same as will be on your Torana.
If your Torana has a stock 202 it was not uncommon for those when they are getting on a bit to not rev out as would be expected, the 202 had inherent problems with the crankshaft counterbalancing and just would not go over 100 MPH no matter how hard you tried!
Holden eventually got around to putting a decent crank in them in the 1980's Commodores, but those were a bit heaver than needed, so they had to put a lighter flywheel on to compensate.

I have not seen a Auto LC or LJ, 173 or 202 with any other ratio than 2.78, Holden obviously had a reason for doing this.
In a hilly or surburban situation perhaps there may be some argument to drop to 3.08, but 3.55 and 3.36 would be very low for an Auto.

I agree with a previous comment, your diff ration depends on; type of terrain, power of engine STD or modified ),
size of wheels and old or new engine, a resultant increase in reves can not only increase oil consumption but also break the rings if it is an old engine that has not been reved much before.

Also remember that a 3.9, 3.55 and 3.36 diff centres will not fit the 3.08/ 2.78 ratio axle housing without modification as they are different set ups with changed lower bolts to give clearance for the larger pinion gear.

Mike

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#10 rexy

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Posted 08 December 2010 - 04:12 PM

"Also remember that a 3.9, 3.55 and 3.36 diff centres will not fit the 3.08/ 2.78 ratio axle housing without modification as they are different set ups with changed lower bolts to give clearance for the larger pinion gear."

Is that a lc/lj problem?
I changed nothing but the centre in the LX when going from 3.08 to 3.36 banjo centres. Straight forward remove old/refit new. No mods required.
R

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#11 _DVS LH_

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Posted 08 December 2010 - 04:53 PM

I had an experience some years ago with a well modified holden 6 where we took out a 2.78 and replaced it with a 3.55. The car struggled to make 100mph with the 2.78 at around 4800rpm.... it just wouldn't pull any higher! With the 3.55 it happily made 120mph at closer to 7000rpm! It also accelerated to that speed more rapidly. The explanation appears to be the engine lacked the torque to push the taller ratio, when we let it rev more freely with the lower gear it improved in all areas. Fuel consumption increased very slightly on the highway but improved in traffic due to the reduced effort to get the car moving. Hope this helps with your decision.

Thankx craig this is exactly the information I am chasing pros and cons thank you

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#12 _DVS LH_

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Posted 08 December 2010 - 05:00 PM

I had a 2.78 in my LJ and after the first engine rebuild I did it would do 155kmh in only 2nd gear (yes, a bit slow off the mark).

I would recommend going to 3.36 gears, it was a good thing for me as without it I would have the speedo needle pointing to the 3rd digit of the odometer from memory which was a bit unusable. I think you'd need to change the stall convertor though

I run a 2.78 in my GTR Torana with no problems other than more gear changes required in hilly country, the fuel consumption is identicle to the old 3.08 due to the slightly increased load.

This ratio is a 186 with a stock bottom end, big valve head and extractors, the carbie is the same as will be on your Torana.
If your Torana has a stock 202 it was not uncommon for those when they are getting on a bit to not rev out as would be expected, the 202 had inherent problems with the crankshaft counterbalancing and just would not go over 100 MPH no matter how hard you tried!
Holden eventually got around to putting a decent crank in them in the 1980's Commodores, but those were a bit heaver than needed, so they had to put a lighter flywheel on to compensate.

I have not seen a Auto LC or LJ, 173 or 202 with any other ratio than 2.78, Holden obviously had a reason for doing this.
In a hilly or surburban situation perhaps there may be some argument to drop to 3.08, but 3.55 and 3.36 would be very low for an Auto.

I agree with a previous comment, your diff ration depends on; type of terrain, power of engine STD or modified ),
size of wheels and old or new engine, a resultant increase in reves can not only increase oil consumption but also break the rings if it is an old engine that has not been reved much before.

Also remember that a 3.9, 3.55 and 3.36 diff centres will not fit the 3.08/ 2.78 ratio axle housing without modification as they are different set ups with changed lower bolts to give clearance for the larger pinion gear.

Mike

Hi Mike thankx for your post. Could you give me a bit more information on the modifercation I will need to do to get the 336 centre to fit current 278. I am pretty handy on the tools I am a fabricator. Thankx

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#13 _DVS LH_

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Posted 08 December 2010 - 05:30 PM

I had an experience some years ago with a well modified holden 6 where we took out a 2.78 and replaced it with a 3.55. The car struggled to make 100mph with the 2.78 at around 4800rpm.... it just wouldn't pull any higher! With the 3.55 it happily made 120mph at closer to 7000rpm! It also accelerated to that speed more rapidly. The explanation appears to be the engine lacked the torque to push the taller ratio, when we let it rev more freely with the lower gear it improved in all areas. Fuel consumption increased very slightly on the highway but improved in traffic due to the reduced effort to get the car moving. Hope this helps with your decision.

Very good point Craig. The same thing can occur with overdrive gearboxes. Higher top speed can be reached in 4th gear than in 5th (based on a 5spd box).

Ultimately the choice of diff ratio will come down to the intended use of the car.

Yeah your right. I am keeping it a weekender/street driven ride. Its a sleeper. Looks like a nanna goat rig still have 2850 badges on front quarters!! I drive it around town often and also on country roads regularly 100kph + I think the rear wheels alter the ratio a bit 225/65 14s

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#14 caterham2

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Posted 08 December 2010 - 06:11 PM

"Also remember that a 3.9, 3.55 and 3.36 diff centres will not fit the 3.08/ 2.78 ratio axle housing without modification as they are different set ups with changed lower bolts to give clearance for the larger pinion gear."

Is that a lc/lj problem?
I changed nothing but the centre in the LX when going from 3.08 to 3.36 banjo centres. Straight forward remove old/refit new. No mods required.
R

Rexy, I have had a 3.08, 3.36,3.55,3.89 with Detroit locker in my LJ XU1 also with no mods required

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#15 S pack

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Posted 08 December 2010 - 07:13 PM

Also remember that a 3.9, 3.55 and 3.36 diff centres will not fit the 3.08/ 2.78 ratio axle housing without modification as they are different set ups with changed lower bolts to give clearance for the larger pinion gear.

Mike

Hi Mike thankx for your post. Could you give me a bit more information on the modifercation I will need to do to get the 336 centre to fit current 278. I am pretty handy on the tools I am a fabricator. Thankx

3.36/3.55 will go straight into a 2.78/3.08 diff housing without any mods but I think not so easy the other way around.

From memory the 3.36/3.55 housing has 7 mounting studs and one drain bolt, the 3.08/2.78 housing has 6 studs, one bolt and one drain bolt.
The drain bolt on the 3.36/3.55 is 7/8" long and the 2.78/3.08 drain bolt is 3" long.

Edited by S pack, 08 December 2010 - 07:14 PM.

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#16 _DVS LH_

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Posted 08 December 2010 - 08:06 PM

Also remember that a 3.9, 3.55 and 3.36 diff centres will not fit the 3.08/ 2.78 ratio axle housing without modification as they are different set ups with changed lower bolts to give clearance for the larger pinion gear.

Mike

Hi Mike thankx for your post. Could you give me a bit more information on the modifercation I will need to do to get the 336 centre to fit current 278. I am pretty handy on the tools I am a fabricator. Thankx

3.36/3.55 will go straight into a 2.78/3.08 diff housing without any mods but I think not so easy the other way around.

From memory the 3.36/3.55 housing has 7 mounting studs and one drain bolt, the 3.08/2.78 housing has 6 studs, one bolt and one drain bolt.
The drain bolt on the 3.36/3.55 is 7/8" long and the 2.78/3.08 drain bolt is 3" long.

Yeah right so it sounds like I should be able to bang in the 336 centre pretty easily??

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#17 TerrA LX

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Posted 08 December 2010 - 08:26 PM

I would be careful using the word bang.

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#18 _DVS LH_

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Posted 09 December 2010 - 04:56 AM

I would be careful using the word bang.

Oh ok will make a note of this. Thanks

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#19 _niterida_

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Posted 09 December 2010 - 05:23 AM

I had an experience some years ago with a well modified holden 6 where we took out a 2.78 and replaced it with a 3.55. The car struggled to make 100mph with the 2.78 at around 4800rpm.... it just wouldn't pull any higher! With the 3.55 it happily made 120mph at closer to 7000rpm! It also accelerated to that speed more rapidly. The explanation appears to be the engine lacked the torque to push the taller ratio, when we let it rev more freely with the lower gear it improved in all areas. Fuel consumption increased very slightly on the highway but improved in traffic due to the reduced effort to get the car moving. Hope this helps with your decision.

Thankx craig this is exactly the information I am chasing pros and cons thank you

Thats dead right right.

The diff ratio should be matched to the nature of the engine as well as the driving environment and driving style. A low revving torque monster of an engine (say a 454) needs a high ratio diff(2.78). A high revving power house with no torque (say a turbo rotary) needs a low diff (4.444). In between we have motors that may be a bit of both and then driving style or environment will come into effect. If you have a motor that is not overly torquey you would want a low diff (say 3.9 or 4.11), but if it is just a street cruiser in suburbia you would probably go higher (3.54 or 3.08). You have to work out where it makes it power and torque, what sort of driving you do and what sort of roads you are on (hilly or flat, suburbia or country etc) and work out what would be best by using a speed / diff ratio / tyre size calculator (find them on the net).

I had a Turbo Liberty a while back and it used to get about 10km/l under normal driving. It had a 3.54 diff ratio and ran about 2500rpm at 100kmh.. I then swapped the motor and box for a more modifed motor running ultra short ratio gears and a 3.9 diff ratio. This new setup ran at about 3300rpm at 100kmh. And guess what - fuel consumption didn't change. This was because both engines were running in their peak TORQUE range at 100kmh. So going to a low diff ratio will not necessarily increase your fuel consumption. NB: the ultra short gearbox was mental and gave the car hilarious accelaration and was just an absolute ball to drive - so much so that I am possibly baout to buy that very car back :-)

Conversely going to a high diff ratio MAY INCREASE the fuel consumption if the engine is not torquey enough to pull that ratio. (as has been stated here already).

personally I think a low diff ratio that improves accelaration and response is a much better idea and the slight trade off in fuel consumption that may arise is worth it.

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#20 _torbirdie_

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Posted 09 December 2010 - 08:21 AM

Basically all toranas came with too low a final diff ratio. Mindset of the day was that the car should be able to pull strongly from 40km/h in top gear, the basic three on the tree ratios picked for suburban running so the thing would be in top gear in the 40-60km/h range. The motors buzzing at 3000+rpm on the highway makes them fatiguing and thirsty.
The LC and LJ autos have the taller diffs because they are lighter cars and ran slightly lower diameter tyres than the later ones.
If the motor is modded than all bets are off on to what is best, but for the standard motor, get/keep the tallest ratio if you intend to drive it on the highway more than once in a blue moon.

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#21 _DVS LH_

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Posted 09 December 2010 - 07:56 PM

I had an experience some years ago with a well modified holden 6 where we took out a 2.78 and replaced it with a 3.55. The car struggled to make 100mph with the 2.78 at around 4800rpm.... it just wouldn't pull any higher! With the 3.55 it happily made 120mph at closer to 7000rpm! It also accelerated to that speed more rapidly. The explanation appears to be the engine lacked the torque to push the taller ratio, when we let it rev more freely with the lower gear it improved in all areas. Fuel consumption increased very slightly on the highway but improved in traffic due to the reduced effort to get the car moving. Hope this helps with your decision.

Thankx craig this is exactly the information I am chasing pros and cons thank you

Thats dead right right.

The diff ratio should be matched to the nature of the engine as well as the driving environment and driving style. A low revving torque monster of an engine (say a 454) needs a high ratio diff(2.78). A high revving power house with no torque (say a turbo rotary) needs a low diff (4.444). In between we have motors that may be a bit of both and then driving style or environment will come into effect. If you have a motor that is not overly torquey you would want a low diff (say 3.9 or 4.11), but if it is just a street cruiser in suburbia you would probably go higher (3.54 or 3.08). You have to work out where it makes it power and torque, what sort of driving you do and what sort of roads you are on (hilly or flat, suburbia or country etc) and work out what would be best by using a speed / diff ratio / tyre size calculator (find them on the net).

I had a Turbo Liberty a while back and it used to get about 10km/l under normal driving. It had a 3.54 diff ratio and ran about 2500rpm at 100kmh.. I then swapped the motor and box for a more modifed motor running ultra short ratio gears and a 3.9 diff ratio. This new setup ran at about 3300rpm at 100kmh. And guess what - fuel consumption didn't change. This was because both engines were running in their peak TORQUE range at 100kmh. So going to a low diff ratio will not necessarily increase your fuel consumption. NB: the ultra short gearbox was mental and gave the car hilarious accelaration and was just an absolute ball to drive - so much so that I am possibly baout to buy that very car back :-)

Conversely going to a high diff ratio MAY INCREASE the fuel consumption if the engine is not torquey enough to pull that ratio. (as has been stated here already).

personally I think a low diff ratio that improves accelaration and response is a much better idea and the slight trade off in fuel consumption that may arise is worth it.

Thanks for your post here. I reckon I am on the right track with the diff will fit it in the next 2 weeks hope it all works out

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#22 _TorYoda_

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Posted 09 December 2010 - 10:14 PM

All you need to do when swapping centers from the short bolt version to the long bolt version is to unscrew the short stud where the long bolt needs to go in..... or if going the other way fit the short stud where it is needed or use a second drain bolt instead of the short stud.

I have never encountered a housing that needs modification to fit a different center into it (other than the bolt/stud swap) however the hub of the center itself has a different offset for the 2.78 and 3.08 ratios. This is because the pinion gears on these two ratios are larger in diameter which requires the crown wheel to be offset further to one side. The difference is about 1/4". This only becomes an issue if putting (for example) a 3.55 crown and pinion into a 2.78 center section. The 2.78 carrier can be used but the hemisphere from the 3.55 would need to be fitted to the 2.78 carrier to make the swap successful.

If you are changing a complete center none of this will matter.

Edited by TorYoda, 09 December 2010 - 10:15 PM.

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#23 Heath

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Posted 10 December 2010 - 05:12 PM

As long as you have two complete centres then yeah you're right, but as above, swapping things from the 3.36-3.9 range into the 2.78-3.08 range can make you run into problems.

I went from 2.78 to 3.36 and it was basically the best thing I ever did to the car, but yours will still be shocking with the standard auto. Either get a converter with some slip and tighten the rest up or close your wallet and just put a four speed in it - THAT will liven the car up.

It will rev hard on the highway regardless, but it's well and truly worthwhile sacrifice.

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#24 _Squarepants_

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Posted 11 December 2010 - 09:05 PM

I went from 2.78 to 3.45 (Borgy) in my LH with a mild 202 and Celica 5 speed. Best thing I ever did to the car! It sits on about 2500 rpm at 100kph but I don't do much hiway driving. The difference in acceleration makes it all worthwhile.