XJ6 & XJ12 Series I, II & III 1968-1992

Highway RPM

 
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Old 08-22-2016, 11:20 AM
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Default Highway RPM

Hi everyone,
Took the car on the highway today for the first time. Other than having a tire out of balance all went well. I did nice notice the rpm seems really high. These cars don't have an electric overdrive do they?
 
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Old 08-22-2016, 11:56 AM
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Originally Posted by jordanmc View Post
These cars don't have an electric overdrive do they?
None of the automatic transmission series I-II-III cars had overdrive.

Cheers
DD
 
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Old 08-22-2016, 10:21 PM
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But you can install a 700r4 using a Johns cars adaptor
 
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Old 08-23-2016, 01:06 PM
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yes, the rpm's are high, but you are also dealing with a 3,800 pound car that when at high speeds, remains a lot more stable and controllable under high rpm's than a 3,800 pound loose car in a higher gear. I think they call that Engine Stopping Power, or something like that.

It was recently explained to me by a friend who races at Sebring Florida that he prefers the engine at high rpm's even at high speeds. Simply put he said, I can stop the car faster with the engine at high rpm's than I can in a high gear.

Sure you can add a GM transmission with a 4th speed or Overdrive and that will relax the rpm's during cruising speeds when you're driving relaxed at say, 80 or 90 mph or heavens forbid, 100 mph.
 
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Old 08-23-2016, 04:47 PM
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What ????????????????
 
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Old 08-23-2016, 06:15 PM
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1st there is a big difference between a car for racing and a smooth comfortable,quiet, street car!

but which one will be reasonable MPG, and which would last the longest, with few troubles!

for some information on how things may be with no overdrive, try cruising along in 2nd gear at 60/70 MPH, lot of noise, course it would accelerate quicker!

i use a GM 700R4 and running along i shift from 4th down to 3rd ,it makes it lot more annoying, (unless someone pulls up beside me,LOL).
 
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Old 08-23-2016, 06:57 PM
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Originally Posted by Jose View Post
yes, the rpm's are high, but you are also dealing with a 3,800 pound car that when at high speeds, remains a lot more stable and controllable under high rpm's than a 3,800 pound loose car in a higher gear. I think they call that Engine Stopping Power, or something like that.

It was recently explained to me by a friend who races at Sebring Florida that he prefers the engine at high rpm's even at high speeds. Simply put he said, I can stop the car faster with the engine at high rpm's than I can in a high gear.

I think what your pal was referring to was an increased compression braking effect in lower gears....or with a lower-geared differential.

If you're going 60 mph in overdrive (let's say 1500 rpm) and release the throttle you'll feel less engine braking than if releasing the throttle in 2nd gear (let's say 3500 rpm).

Similarly, in a car with a 2.88 differential (high geared) you'll feel less engine braking effect compared to a car with a 3.54 (low geared) differential.

Not quite sure about the "loose" versus "stable and controllable" thing.

Cheers
DD
 
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Old 08-24-2016, 09:19 AM
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One huge difference. Engine retardation is effective with a manual transmission. Almost non existant with automatic.


I've forgotten at what RPM's my car cruised on the highway at about 70 mph, when it was still 4.2/3.88. Not objectionable at all.


The past:


1. lumped 56 ****** Jeep 4x4 pickup. 5.0 rear ratio. T98 three speed stick. Howled at 55 MPG!!! Warner made a slick Od for them. In my plans, but didn't happen. But, a great stump puller. I don't like Oleander bushes. I removed three. The Jeep yanked the stumps out.... A tractor in first gear, front axle engaged, transfer case in low range...


2. The Jeep gave way to a 69 Toyota FJ40 land cruiser. Better on the highway, but still 60 Mph at 2750 was buzzing....


And, now, the Jaguar loafs in OD at 70. LT1 torque allows that...


Carl
 
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Old 08-24-2016, 09:38 AM
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Originally Posted by JagCad View Post
One huge difference. Engine retardation is effective with a manual transmission. Almost non existant with automatic.

I disagree. Sort of.

Shift into 2nd at 60mph and see what happens

However.....

Some O'drive automatics do have a (real or perceived) free-wheeling sensation in the 4th gear (or whatever the o'drive gear is...nowadays we have 6 and 8 speed automatics!)

Plus, with any automatic, torque converter slippage enters in as well.....

I've forgotten at what RPM's my car cruised on the highway at about 70 mph, when it was still 4.2/3.88. Not objectionable at all.

I remember 80 mph was 3000 rpm, right on the money. I think 70 was about 2500.

Cheers
DD
 
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Old 08-24-2016, 11:07 AM
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Thanks, Doug. I was thinking of just backing off on the pedal while still in drive. Not considering a down shift.


My auto box experience does not go past 4. Modern units with a lot more not in my lexicon.


But, yeah, down shifting either my Jeep or the 4l60e in my Jaguar
sure does retard!!!!


A technique only used for pre SMOG test prep. Along with some pump gas Techron and canned Techron. Get the cat/s good and hot....


On the bucket list. Drive a stick shift car of any age or genre....


Carl
 
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Old 08-24-2016, 02:52 PM
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Yes now with 6 and 8 speed automatics, we are well on our way to a total variable speed trans - no distinct speeds /gears but rather a smooth slipping transmission with constantly changing ratios that will keep the engine in the " sweet spot " relative to the demand on it ( manifold vacuum etc. ) - unless we all go electric (Tesla) what with current (no pun intended ) battery technology that is happening ...and with Hydrogen fuel cells on the horizon...that will make electricity onboard ...to fuel the electric drives.. whew... - thinking about my old 4 cylinder 1960 Austin Healy "bugeye" sprite,... how simple things were... then.
 
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Old 08-25-2016, 08:17 AM
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I go a bit further back. Before the Jaguar project, I built a replica in the style of a "Hot Rod" of the forties. Old original Ford parts for the most part. A revamped frame from a 30 a ford, cast off by a Street rod builder. First plan, revice the old A. In tough shape. second, a better B from the famed Pleasanton Swap meet. Got carried awy in modifications, mainly converting it's pressure oiling to include the rod bearings. Fraught with peril, using the a transmission meant transmisioion failure almost guaranteed. did that decades ago!!!

Son came up with a cast off Ford V8 and three on the tree transmission from a barn find old Ford. They ID'd the engine as a 37. Wrong, it was a post war refit 59AB. A much better flat head. The engine sound, the transmission not so much. Shucked teeth of the cluster. A common malady from missing a shift while "speed shifting".
Been there as well.

Rustled up a three on the floor 39. Very desireable. Late synchro's. Ugh, same issue. A stand up seller. He sent me a good cluster.

That warmed up V8 in the light a frame was a ball to drive. Not too pleased with the glass 27 roadster body. Just nmissed the makings of a steel one, rough and not quite complete, and a bit over priced. But, I could've used it.

So, succombed to a genrous offer by a Flying Tiger Air freight pilot.
He had a good A rodster pickup body and was looking for a drive train.

Fantasy. Jaguar 4.2, three Webber DHOe 45's, Jaguar "crunch box" four on the floor and a circa 20/30's Ford roadster body. Twin exhaust, three to one side, three to the other. Mufflers????

Carl
 
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Old 08-25-2016, 12:09 PM
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i never thought i would like a car with a 7/8 speed automatic,with sport mode, but after driving one for about a month, i was amazed how well it worked!

4cylinder 2.0T turbo could run with some of the best of them. 2015 Sonata Hyundia .

it always seemed to be in the right gear,wether traffic, or hy-way passing!

sport mode was great for traffic and slowing /retarding speed, YUP they got it figured out quite well!
 

Last edited by ronbros; 08-25-2016 at 12:11 PM.
 
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