Hi all.

I've got a '72 XJ6 with a 350/700R4 conversion. I bought the car in November last year with a dodgy gear box. I replaced the tranny last saturday with a freshly rebuilt 700R4, but the RPM's I get are a bit confusing to me. There is offcourse a possibility that the tachometer is inacurate as it has been adapted to fit the 350 obviously. Anyway, being a series 1 XJ6 I presume I should have a 3,54 diff. I get 2200 RPM cruising along in 80 km/h (50 mph) in OD with the converter locked up. According to this calculator TH700R4 calc. I should be getting just shy of 1700. As I mentioned, the tach may be inaccurate, but judging by ear I do feel it runs a bit high on RPM's as well. Especially running around town in 3rd. Could I have a custom built rear diff With lower gears?? It does take off like a bat out of H..l when I floor it. Is there a way of identifying the gears without opening the diff. If I do have lower gears, where would be a good place to get the orignal 3.54 or even 3.31 gears? I'm planning to rebuild my rear diff next winter, so I guess that would be a good time to replace them. Oh, and before you ask, yes I'm sure it's in 4th, I've counted the shifts more than once.

Any input would be greatly apreciated!

Cheers
Tor

 

Mmmm, 3.54 is the preferred ratio down here in the XJ-S cars with that trans behind the V12.

3.31 is common on S2 XJ12, again, down here. Some early XJ-S also had that ratio. Mainly those with the BW12 trans.

3.07 is quite common, S2 6 cylinder, of which there are many in all sorts of dismantle. PreHE XJ-S with the TH400 trans also use this ratio.

Being as it is a standard "Salisbury" type diff unit, other ratios would be out there. A Diff (Spiral gear) specialist should be able to sort that very easily.

 

Besides the tachometer, are you sure that the *speedometer* is accurate?

Cheers
DD

 

Well no, but judging by other traffic and the speed limits it must be in the ballpark. Unless offcourse everybody is speeding terribly. I'll jack it up, spin the wheel and count the revs on the shaft. Should get me a rough idea of the ratio.

 

I'm betting the tach is off because it seems like you've got it all verified except the the tach and speedo. If you have an Iphone or Android phone there are apps you can download that will use the GPS in your phone to calculate your road speed. Carefully setup your phone on the dash, run the app and wait for it to register your speed on a flat stretch and compare it to your speedometer.

As for the tachometer...A 6 cyl tach will be appear very close to normal at idle but as the RPM'S increase so will the disparity. If we multiply 2200 RPM (what you have at 50 MPH) by .75 (8cyl instead of 6) you get....1650RPM which is real close to the "just shy of 1700 RPM" you expect with your rear end and transmission combo. This suggests whoever did the conversion didn't convert the tachometer...check it out.

BTW...looks like a nice setup with TPI, the 3.54 and a 700R4 I bet it gets up and goes very well. Should get good fuel economy if its got the stock cam.

 

I spun the Wheel and counted the revs on the shaft. I worked it out to be 3,5:1. That pretty much comfirms that I have 3,54:1 gears. Thanks icsamerica, your theory seems very probable!
I thought of getting a speedo app for my phone, but there are not many long and straight roads around here, so I doubdt I'd get a reliable reading. I'll look into it.


Thanks again

 

Two ideas or so.

An enhanced strobe will read engine rpm quite decently. Compare it's reading to the fascia tach. If you don't have one, they are a neat thing to do a lot with.

The speedo probably reads KM's as well as miles. Your roads may have KM markers.
Try a test run. Fascia odometer vs road posts.

Around here, the police departments have trailers that they spot along roads with speed issues. They are radar trailer and flash an advisory reading.

My speedo is nuts. when the trailer radar reads 25 mph, the speedo says 40!!

As my tach is calibrated, I merely drive by it. Stay under 2K on the freeway and all should be well. That keeps me in the range of most traffic, so probably OK.

My car has it's original 2.88 to 1 rear. So, with the 4L60E trans' .70 OD, it's long legged. But, the LT1 has the torque to pull it with alacrity.

Carl

 

The drive vs driven gears in the 700r4 should mimic the original Jag trans output factor

On my 86 Jaguar XJ6, the BW trans output for the speedo ( my car has the 2.88 diff gear set) is .441 .....

So that means the gearset in the "replacement 700r4" needs a 15 tooth driven and a 34 tooth drive gear to achieve .441 (15 / 34 = .441) Now the speedo should read as it did when the BW was in the car.

The drive gears final factor Borg Warner = (.441) and the 700r4 should match this original trans output ratio.

I would not mess with the rear diff gears or the speedo - DON'T MATTER - but I WOULD choose drive/driven gears to match the original BW trans speedo output for what was in the car originally (the gearset that is in the 700r4 can be made to match the BW- remove tail shaft section (4 bolts and the tailshaft is off ))

Does not matter what the speedo interior gears are.
Does not matter what the differential gears are.

What matters is that the driven/drive gear ratio match what was in the original Jag trans. Simply match them in the 700r4 trans. Example below:

As I said in my car the magic number was / is .441 and I got close with a 15/34 gearset in the GM trans. Stock speedo gauge & stock diff ratio
Everybody's happy happy happy

With an electric speedo and GM 4L60e trans you may need a SGI5 black box ( or get a DRAC and speed buffer) and then set the jumpers to mimic the original Jag output signal when the car had the original Jag trans. It used to use a transducer and interface with the on board computer that sent speed signals to the speedometer with the old BW.

The important thing is the initial value from the new trans should match the initial value from the old trans as it is presented to the "speedo circuit".

 

Series 1 cars had to have the correct speedo head installed if the rear end
gears were changed... plus - there were no 2:88 ratios available for these
cars so no proper speedos are available. You could pay several hundred dollars to have the work done to your speedo by most shops. Or, just get
a GPS and forget RPM's and the speeds on your instruments.. you could then
make a paper template of the corrected speeds and tape on your speedo.

I bought a Series III speedo and adapted the odometer (it is separate and was off as well) and the speedo parts to make my S1 speedo accurate with 2:88's. It is within 3% or the same as most cars with mechanical speedos.

I had the speedo converted to a V8 pulse and new internals put in by Red Line Gauge works in CA... they did both of my Series 1 tachs...

It would be helpful for you to compare your info to mine on the '72 S1 with 700R but it has 4:09 LSD... would need your tire diameter or at least the size to get even close... my '71 383/200R Jag has 2:88 LSD but the better 200R trans with a different 4th gear (OD) gear set that is lower... at 65 mph with stock sized tires it turns an accurate 1750 rpm.... the 700R with the different rear gears turns about 2450 at the same speed in OD. I has that helpful first gear ratio to get it moving... but much less HP and torque so the "race is over" after
first gear...

 

Fooling around with diff ratios / speedo gearheads will only confuse the issue..
my point was to keep the diff ratio and the speedometer that originally came with the car and only "adjust" the trans "drive and driven gear set" to match the original trans to speedo output( which I misstated - I wrote .441 should have been .447 )

Adjust the gear set ratio, by purchasing the plastic gears from a GM dealer. If you are using a 700r4 with Series 3 stock speedometer and matching 2.88 stock diff gears, 215 70 15 tire size - the gearset should be 17/38 which will match the original BW output and be .447. In my case I am using a 700r4 with "cable drive" output that goes to 90 degree adapter attached to the trans then the VSS signal then the transducer. My gear set is - 17 / 38
17 divided by 38 = 0.4473684

I am simply changing the value at the source - drive/driven gearset in the 700r4 trans using the older style cable drive output. Everything else is stock. Speedometer gearhead is still a matched set with the rear diff ratio that came with the car. Only the tire diameter (if it is changed) will throw the speedo off, then a new plastic gearset will need to be computed to correct the small error.

Electronic trans (4l60E) you can use the SGI-5 from Dakota and set the jumpers correctly so the speedo/interface/trip computer/and EFI computer get the proper signals and the speedo reads correctly.

 

Your S3 speedo uses the normal "TPM" turns by mile - our older stuff does not. They are specific to the rear end ratio to make the speedo and odometer work properly. THERE ARE NO SPEEDOS FOR 2;88 ratio for the Series 1!

He does not have a "E" trans... no computer just a 700R... also different trans.

He will still need the proper two trans gears that are closest to the 2:88 ratio. That is where the error gets worse.. most only only change the "easy" to get to
outer driven gear and make things even worse.

Or, take the easy way and get a GPS that shows MPH... get the tach converted
properly and test with an outside one for accuracy. Then make a paper
speedo needle chart...

 

I would suggest you go back to the beginning of this thread again and read it.

I realize he does not have an E trans - that was only an example I gave -

Older speedos (in fact any speedo works on a turns per mile basis) with electrics using a pulse per mile- i.e. VSS

Yes the JAG speedos were matched to the JAG rear diff ratio-
some were 1000 tpm some were 997 etc.DEPENDS ON DIFF RATIO

I realize he has a 700r4 trans... never said he did not..

I stated he needed the proper trans speedo gear combination that matched the original output from his OLD BW trans - and that factor is ???

My guess is it was the same as mine was - .447 (that is why the factory used different speedmeters matched to the different diff ratios) and the JAG trans speedometer gearsets were all the same.

GM used 1000 tpm speedos and provided different drive/driven trans gearsets to match the diff ratio being used in the vehicle, thereby allowing the speedo to read accurately. Two different approaches. Jag vs GM

So use the same number of teeth on each of the new GM gears in the 700 trans (drive /driven) to match the old JAG gearset (drive vs driven)

To determine the factor number: (old BW)

Divide the number of teeth on the driven gear (the small one) by the number of teeth on the drive gear (the big one) that is the factor number you need to end up with when using the newly purchased gearset for the 700r4 trans regardless of the other items - speedo diff gears etc. (as long as the original JAG speedo/diff ratio combination has not been modified - the speedo that matches the diff ratio is still in place. S1 or otherwise

Why go off changing the tach? Why go to GPS?

Jack up the car, remove the drive shaft, remove the tailshaft section and replace both gears

(replace rear seal while you are at it to prevent leaks in the future)

( if you are using the cable speedo drive -which he is)
change the speedo gears in the 700 trans to match the same output as original BW trans. (.447)

Simple stuff - no magic

- you can not just change one of the gears and hope for the best, of course you will have an error

Does not matter if the car is S1 S2 S3 - or what the speedo/diff matched set is...What matters is what drive/driven ratio factor is (that is being used)

ITS BOTH THE TRANS SPEEDO GEARS THAT NEED CHANGING...

EXAMPLE ONLY:

(MY CAR - not his car) is a 86 with 2.88 diff ratio - my old trans was a BW - the factor was .447

(MY CAR) has a new 700r4 gear set that I selected and produces a .447 factor
My new gearset in my 700r4 is 17/38 = .447 NO ERRORS

 

Hi all, and thanks for your interest in this thread. Lot's off ideas and suggestions flying around here. My speedo seems pretty correct. The car had a bad th700r4 in it when I bought it. I replaced the tranny with a freshly built one, but I put the gears and the 90 deg adapter from the old one into the new one. Apparently somenone did their home work and figured out the right speedometer gear ratio when the car was first converted, because it's pretty much dead on. Now, there is no doubdt I have the original 3.54 rear diff, and my initial question was why I had higher RPM's displayed on my tach than I should according to data for the 700r4 and the above mentioned diff ratio. Icsamerica solved that mystery in an earlier post as the RPM's are correct when multiplied by a factor of 0.75. My tach has not been converted for a V8, so it still thinks it's counting the revs of a straight 6. Case closed.
I had another issue since this one though, it seemed the milage I was getting was horrible. It turns out that while my speedo reads km/h, the trip counter and odometer still counts miles!! My guess is someone replaced the faceplate and left the rest of the speedo stay original. My car is originally a California car so it has undoubtedly had a mph speedo originally. Anyhuu, I had my first "long" trip with the Jag today. 117 miles and 23,5 MPG. Not bad i think! It handled beautifully as well!

 

Nice mileage. That GM EFI stuff is good and reliable technology even by today's standards. Must be a sweet ride, quick from the start and liesurly on the cruise especially when locked up. That TPI fuel injection with it torque promoting long runners was a revolutionary design when it came out and most manufactures have reverted back to long runners. Here in the USA the TPI is known for running out of breath by 4500 RPM and many people modify them to breath deeper. I have a TPI derivative called Stealth Ram atop my 400SBC bored out to 408 with a 700R4 with the MAF based TPI fuel injection.
Do you have an speed parts installed like a bigger cam or full roller rockers?

 

I don't know all the details of the engine unfortunately, it is however an old 010 casting out of a '70's Corvette. The TPI system, which is MAP based, has been retrofit to the older 350. The TPI system is probably from around 90-92. The stock EFI has been replaced with a Holley Commander 950 fuel delivery system. The engine has been rebuilt at some stage with 10:1 Edelbrock alu heads and a mild cam. According to the previous owner it should put out about 300HP. I believe it, because it seems pretty quick. Certainly fast enough for an old lady! I've installed the Holley Commander software on an old XP laptop now, and will try to optimise the tune. Who knows, maybe the mileage will get even better!


Thanks again for your interest!
Tor