Diff ratio help needed.
#1
01-16-2014, 12:35 PM
Diff ratio help needed.
G'day Guys,
I have a 1970 S1 XJ6 with a chevy 350 TH400 conversion, She is great around town but on the open road she revs way too high.
Am i correct in thinking she has the standard 2.88 diff ratio?
I would like for her to become a highway cruiser & am just wondering what ratio you guys would work best?
which year models are compatible?
Any help would be appreciated.
cheers,
DCB.
I have a 1970 S1 XJ6 with a chevy 350 TH400 conversion, She is great around town but on the open road she revs way too high.
Am i correct in thinking she has the standard 2.88 diff ratio?
I would like for her to become a highway cruiser & am just wondering what ratio you guys would work best?
which year models are compatible?
Any help would be appreciated.
cheers,
DCB.
#2
01-16-2014, 01:38 PM
Veteran Member
2:88's did not come in Series 1 XJ
What is the RPM at 65-70 MPH? Do you have stock sized tires?
If so, we can go backwards and figure the ratio (assuming your tach has been converted properly). If not you will have to properly jack up the car
and turn the RR wheel and watch the turns of the drive shaft... or look on the rear end for the ratio tag = FIRST.
My S1 shows 1750 RPM at 65-70 with stock size diameter tires - but it had 3:31 LSD before changing.
To do this change to 2:88's you will have to deal with a speedo that will never be accurate unless you pay big bucks to have it changed and the odometer. Since Jags solution was to change the speedo head to match the rear end you/we are in trouble since that ratio was never offered.
The '72 Jag, with OD, shows 2300 at 65-70 MPH but it has one of the normal higher gear ratios offered in a S1.
You do not have overdrive, it would be much better to change out your 400 trans for a 200R and get a fourth gear of .67:1 or the 700R with .70 (has a much lower first gear but loses too many RPMs in the 1-2 shift for my taste - but with only a 350 engine; the gear multiplication in first gear would help you take off better) and then evaluate your rear end change....
If so, we can go backwards and figure the ratio (assuming your tach has been converted properly). If not you will have to properly jack up the car
and turn the RR wheel and watch the turns of the drive shaft... or look on the rear end for the ratio tag = FIRST.
My S1 shows 1750 RPM at 65-70 with stock size diameter tires - but it had 3:31 LSD before changing.
To do this change to 2:88's you will have to deal with a speedo that will never be accurate unless you pay big bucks to have it changed and the odometer. Since Jags solution was to change the speedo head to match the rear end you/we are in trouble since that ratio was never offered.
The '72 Jag, with OD, shows 2300 at 65-70 MPH but it has one of the normal higher gear ratios offered in a S1.
You do not have overdrive, it would be much better to change out your 400 trans for a 200R and get a fourth gear of .67:1 or the 700R with .70 (has a much lower first gear but loses too many RPMs in the 1-2 shift for my taste - but with only a 350 engine; the gear multiplication in first gear would help you take off better) and then evaluate your rear end change....
Last edited by Roger Mabry; 01-16-2014 at 06:55 PM.
#3
01-16-2014, 03:12 PM
Veteran Member
consider this...
You probably have a 3.31:1 diff so you have two practical options as I see it.
1. switch out to a 2:88 diff from a mid 80's automatic XJS or Xj6. Here in the USA almost all XJS or XJ6 after 1982 came with a 2.88:1 diff. I have no idea if that was true too in your part of the marble. In any case a 2.88:1 is not ideal because it will make your car feel lazy sluggish off the line and will only modestly decrease your highway RPM's.
2. swap your TH400 for a 700R4 or 200R4. This will give you a lower 1st gear and a (.070:1) over drive 4th gear with lockup. This will afford you an improved off the line zippy feel and a very sedate highway cruise. You will also improve you fuel economy on the highway by as much as 6mpg. This would be ideal in every way and would cost much more than a differential swap.
Other things to consider.... What GM conversion kit do you have? Some of them use dodge suspension mounts for the engine that transfer quite-a-bit of harshness in to the chassis when the engine revs get high. Some use normal Chevy mounts that offer much better isolation.
1. switch out to a 2:88 diff from a mid 80's automatic XJS or Xj6. Here in the USA almost all XJS or XJ6 after 1982 came with a 2.88:1 diff. I have no idea if that was true too in your part of the marble. In any case a 2.88:1 is not ideal because it will make your car feel lazy sluggish off the line and will only modestly decrease your highway RPM's.
2. swap your TH400 for a 700R4 or 200R4. This will give you a lower 1st gear and a (.070:1) over drive 4th gear with lockup. This will afford you an improved off the line zippy feel and a very sedate highway cruise. You will also improve you fuel economy on the highway by as much as 6mpg. This would be ideal in every way and would cost much more than a differential swap.
Other things to consider.... What GM conversion kit do you have? Some of them use dodge suspension mounts for the engine that transfer quite-a-bit of harshness in to the chassis when the engine revs get high. Some use normal Chevy mounts that offer much better isolation.
Last edited by icsamerica; 01-16-2014 at 09:04 PM.
#4
01-16-2014, 04:49 PM
As said.
The S1 had 3-31:1 standard open diff.
The 2.88:1 is HE XJ-S, and HE V12 sedans up to about 1991.
The crown wheel and pinion will not fit the 3.31 housing, so a rear cradle from a donor car will be needed.
Also as said, the drop in revs will be minimal, a few hundred maybe, been there many times.
The ...R4 spec trans units would be the best, and easily found even down here.
The S1 had 3-31:1 standard open diff.
The 2.88:1 is HE XJ-S, and HE V12 sedans up to about 1991.
The crown wheel and pinion will not fit the 3.31 housing, so a rear cradle from a donor car will be needed.
Also as said, the drop in revs will be minimal, a few hundred maybe, been there many times.
The ...R4 spec trans units would be the best, and easily found even down here.
#5
01-17-2014, 01:24 AM
#6
01-17-2014, 08:29 AM
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Comments:
1. The engine tune including build mean a lot in the off the line performance. Lots of torque low in the rpm scale or is the torque further up?
2. If the speedo is cable driven, converter boxes may yet be available to match the final ratio and read close to real mph. Mine is wacky. i know how to fix it and have the stuff. That enertia thing, and a bit of angst as to screwing up somnething else while redoing a bit of electrics.
3. My car is powered by a "mere" 350. And has OD and the 2.88 ratio. Off the line may not be dragstrip worthy, but quite respectable. And, once in OD it is a freway flyer. I attribute it to the engine torque.
4. My Jeep Grand Cherokee is a box powered by a mere 4.0 (302ci). It doewn't have drag strip off the line power, but is respectable and the torque isd in the 2000 plus or minus band. Just right for most driving. Ok in OD on the freeway. But, over 70 it struggles a bit for breath. Past the torque band and pushing the wind.
To get your car optimum, I'd start with evaluating engine performance.
Just my ideas.
Roger:
Is the Glendora fire threatening you? Nut campers with a fire in this drought!!!
Carl
1. The engine tune including build mean a lot in the off the line performance. Lots of torque low in the rpm scale or is the torque further up?
2. If the speedo is cable driven, converter boxes may yet be available to match the final ratio and read close to real mph. Mine is wacky. i know how to fix it and have the stuff. That enertia thing, and a bit of angst as to screwing up somnething else while redoing a bit of electrics.
3. My car is powered by a "mere" 350. And has OD and the 2.88 ratio. Off the line may not be dragstrip worthy, but quite respectable. And, once in OD it is a freway flyer. I attribute it to the engine torque.
4. My Jeep Grand Cherokee is a box powered by a mere 4.0 (302ci). It doewn't have drag strip off the line power, but is respectable and the torque isd in the 2000 plus or minus band. Just right for most driving. Ok in OD on the freeway. But, over 70 it struggles a bit for breath. Past the torque band and pushing the wind.
To get your car optimum, I'd start with evaluating engine performance.
Just my ideas.
Roger:
Is the Glendora fire threatening you? Nut campers with a fire in this drought!!!
Carl
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