I love my JT5

abowie · Post by **abowie** » Fri Jan 30, 2015 8:47 am

Finally. Today I got to drive my 3.8. The JT5 gearbox is simply brilliant.

The box is virtually silent. The shift is shorter than the E box, but not annoyingly so. The shift is solid and mechanical and precise, but not notchy.

I am running a 3.54 diff. The first 4 ratios are perfect with the 3.8 motor. I sometimes find first a little long on the all synchro E box with a 3.07 diff. By comparison the JT5 is silky smooth off the lights.

And 5th! At 100kph the engine is noisy in 4th, but slip into 5th and the car lopes along. If you put your foot down in 5th you do notice the lesser torque of the 3.8 but it does accelerate.

I made no modifications at all to the tunnel of my car when installing the box and the clearances were tight; under 5mm in places. However I didn't feel any vibration or knocking of the transmission against the body. This may be in part because of the uprated rear gearbox mount Medatronics supply.

I have driven a 3.8 coupe with a Driven Man Tremec box and the two are chalk and cheese. The JT5 is much, much better.

In my 3.8 OTS replacing the Moss box with the JT5 is a magical transformation. No noise, smooth positive shifting and a 5th gear for cruising. 10/10.

Simonpfhc · Post by **Simonpfhc** » Fri Jan 30, 2015 10:18 am

That's great feedback, really appreciated.

I too am considering swapping my moss box for a 5-speed. I was planning on going for the ETF box, but that may now be NLA??!

Consequently, I have made no modifications to the trans tunnel at all and the car is off to get painted in 4 weeks time, so great to hear that it can fit(just).

Did you also use any heat protection inside the tunnel?

Cheers.

paydase · Post by **paydase** » Fri Jan 30, 2015 5:28 pm

Andrew,

I thought that the benefit of the JT5 was to lower the revs when already having a 3.31 or a 3.07, the four first gears of the JT5 being similar to the gears of the four gearbox and only the fifth being in overdrive by a 0.72 ratio.

A 3.54 is usually considered well too short to cruise in final gear when using the four gearbox. So why didn't you keep your 3.07 and did shift to the 3.54?

abowie · Post by **abowie** » Fri Jan 30, 2015 9:03 pm

paydase wrote:. So why didn't you keep your 3.07 and did shift to the 3.54?

This car is a 1964 US 3.8 and came with a 3.54 diff.
My thinking was that you get the best of both worlds and it seems to have turned out that way.
I have 3.07 diffs in both my 4.2's. The 3.8 has less torque, so the shorter diff's advantage is that you can get off the line more easily. The obvious disadvantage is high revs in 4th to cruise, which the 0.73 OD fixes. I considered a 0.63 OD and Paul C talked me out of it. I'm glad that he did; the car will just pull 5th at 100kmh as it is but wouldn't do it with a 0.63; you'd have to change down to overtake every time.

abowie · Post by **abowie** » Fri Jan 30, 2015 9:04 pm

Simonpfhc wrote:

Did you also use any heat protection inside the tunnel?

Cheers.

No only "Koolmat" inside the cab under the carpets.
I looked at the ETF box as well, but my concern was that it was a small run bespoke item designed by someone who hasn't made gearboxes before.
The Tremec box Paul uses is known to be a strong reliable unit and Paul says there are hundreds if not thousands of his boxes in service both on road and on the racetrack. I have found Paul very helpful, both on the phone and by email. Fitting the box was harder than fitting a standard all synchro box ( see my blog post at the URL below, but having done it once it would be a lot easier next time.
If anyone wants feedback on the Driven Man boxes please search this forum or contact me via PM.

PeterCrespin · Post by **PeterCrespin** » Fri Jan 30, 2015 9:07 pm

You can get Tremecs/JT5 with 0.63 5th. That's what I have for a 3.54 diff, or eventually possibly the 4.10:1 in the D-type axle.

abowie · Post by **abowie** » Fri Jan 30, 2015 9:17 pm

PeterCrespin wrote:You can get Tremecs/JT5 with 0.63 5th. That's what I have for a 3.54 diff, or eventually possibly the 4.10:1 in the D-type axle.

See above. I only have 29 miles on this car so far and my other cars are 4.2s. My initial feeling is that 0.63 might be too long for a 3.8 although I reckon it would be fine with a 4.2.
Mind you if the D produces 300BHP you'll be fine

rfs1957 · Post by **rfs1957** » Wed Feb 04, 2015 11:36 am

My 3.8 is on 3.54 and has 0.73 in its JT5 too, I've about 2.000km or so on it now.

Brilliant combination, the jump to 0.63 would be enormous and would be like me changing to a 3.05 rear axle.

(0.63 / 0.73) x 3.54 = 3.05

I think the car would pull a higher 5th comfortably, but not 14%.

(0.73 - 0.63) / 0.73 = 0.14

Something like 3.30 at the back woulld be more what I would aim for. Could the Almanachs amongst us confirm what is nearest ?

On the subject of suppliers/manufacturers, it's a pity that no Forum member has been able to go and see Paul C ; we lend instant credibility with pictures of CNC machined cases that are then withdrawn in favour of castings, whereas there would be no substitute for consulting the man who has a solution that probably has more ten-times miles under its belt than all the pretenders added together. And who always answers the ?phone.

Post by **Heuer** » Wed Feb 04, 2015 12:45 pm

I have spoken/emailed to Paul C a few times and I had planned to meet him when he last came over to London but I could not make the dates he was free. Nice guy and very knowledgeable.

My OTS has a T5 box and 2.88 diff which is great for touring Europe - 100mph = 3,000rpm in 5th. Done 50,000 miles with that combo although if I had to do it again I would go for 3.07. The 2.88 is a bit of a pain in towns as 1st is too low and 2nd too high. Also when negotiating Alpine hairpins I find I need a lower gear than 1st so the car bogs down and I may have to slip the clutch. The 3.54 is woefully low though and does not make for pleasant progress. For Europe Jaguar offered 3.31 and 3.07 diffs, the 3.54 was US only as the need for competitive 0-60 times over-rode all other considerations. You need a high torque engine for 2.88 but any E-Type should be able to cope with 3.07 and I doubt you would notice the difference in acceleration. It would make your car far more pleasant to own though.

paydase · Post by **paydase** » Wed Feb 04, 2015 1:22 pm

I am also considering a JT5 to replace my Moss box but only in the medium term because the overall cost is substantial.
An alternative I also consider would be to find an EJ full synchro box with the closer ratios but I understand that finding out such a box in good shape (in particular considering the synchros) and at a good price is not easy.

My diff is a 3.31 LSD (US origin), not as common on this side of the pond.
I am thinking of a JT5 box with 2.95 in 1st and 0.80 in 5th (instead of 0.73). This combination may be sufficient to decrease noise and revs when cruising while fully exploiting the five gears and the 3.8 willingness to rev when required.
Any opinions on this combo instead of other combinations reported here (0.73 with 3.54 or 0.80 with 2.88 )? The alternative being to possibly swap the 3.31 diff for a 3.07 combined with the 0.80 fifth gear.

Also, referring to availability and shipment of the JT5, I understand Paul is based in the US and is not willing to deliver his gearbox in the UK (at least to professionals). There seems to a limited number of authorised dealers in Europe: one in France and one in Nederlands.
For those in Europe who swapped their Jag box with a JT5, how did you proceed: direct procurement from Paul in the US or from an authorised dealer, and then installation in the car by yourself or by a professional (because I understand it may be a challenging task for somebody not familiar with mechanicss and well equipped)?
Of course I will contact Paul in due time when the project matures.

rfs1957 · Post by **rfs1957** » Wed Feb 04, 2015 4:53 pm

What 5th ratio are you using in the JT5 David ?

Post by **Heuer** » Wed Feb 04, 2015 6:14 pm

0.8 IIRC

rfs1957 · Post by **rfs1957** » Thu Feb 05, 2015 9:07 am

That would be the eqivalent of moving to a diff with a ratio of 3.16 for anyone running a 0.73 5th like Paul has advised.

0.8 / 0.73 x 2.88 ?

So going for a 3.31 would only be 5% shorter than what David waxes lyrical about, and maybe pulling that extra 5% is made possible by his breathed-on motor.

3.31 / 3.16 = 1.047 ?

My motor is stock, and maybe even flakey, so the day my 3.54 gives up on me I'll go for a 3.31.

Since it was me that overhauled it last year the jury is currently sitting on its verdict .....

paydase · Post by **paydase** » Thu Feb 05, 2015 10:00 am

Rory, that is exactly what I was thinking.

With 3.31x0.80 you get 2.65 which is only slightly higher than the 2.55 magic number of Paul and which would better fit a used engine.

Additionally, not sure if Paul's figures correspond to the 4.2 engine or to the 3.8 which better likes higher revs.

So that would answer my question about a 3.31 diff with the 2.95/1.95/1.34/1.00/0.80 combo which may give the better of all worlds for our 3.8 engines

PeterCrespin · Post by **PeterCrespin** » Thu Feb 05, 2015 10:54 am

There is no single ideal set of ratios - it depends what you want most.

My logic for choosing the 0.63 /3.54 combo was based on several considerations that don't often apply:

Firstly, the car is a stock 4.2 and I only want 5th for when I've stopped 'driving' and wish to opt out and cruise at sensible speeds and save fuel on long trips. 3.8s feel very different and I'd stir the pot more with one of those and not gear as tall.

Meanwhile, for a car I'd intend doing high miles in, I am not the least interested in high performance in top gear because of the strong probability I'd get caught and lose my licence doing long distances at high speeds attainable if you rev an E-type in fifth. I will have the same attitude to 0.63 top as I do to manual overdrive 4th in an MOD box. I.e. it's the antithesis of a performance gear, it's a gentle cruise ratio that you get out of the minute you want to drive in a sporting manner. If I still lived in Europe I might be more interested in 5th gear acceleration, but not here.

So long as the torquey 4.2 doesn't lose ground up interstate-spec gradients I'll be happy with what I get from fifth. Meanwhile, the stock engine will be OK performance-wise with the 3.54 and standard-ish first four ratios.

The other reason is that the car will probably end up with the 4.7 engine I'm building, which will be extremely torquey but peak at around 4500 due to running out of valve/port area even with 1.92 valves in the S3 XJ head. Playing to that engine's strengths will involve the same sort of logic, only more so. If the fully-counterweighted billet crank and Arrow rods were standard dimensions it would be pulling hard at a safe 6500 and I'd gear it like a 3.8. But even with triple 45 or 50 DCOEs and hairy cams it will run out of gas flow lower down unless I get a wide-angle head, so I'll exploit the fat mid range rather than scream it.

And finally, I already have a T5 with 0.75 top. It's a dog box T5 4-speed (yeah, I know, that's a contradiction in terms) with 1:1 third gear, that sits in the D with a 4.10 axle and tall 6.50x16 wheels. I wanted a different top gear on the five speed to be able to swap them over if the mood takes me.

The only bit I don't quite follow in David's case, is that I am not surprised his 2.88 needs clutch slip on uphill hairpins but don't see how that same bottom gear is too low in town? These seem incompatible characteristics - bottom can be too low or too high but rarely both?

Chacun ? son go?t, I suppose.

Post by **Heuer** » Thu Feb 05, 2015 12:41 pm

Gear ratios have been covered before.
Here: viewtopic.php?t=120&postdays=0&postorde ... io&start=0
Here: viewtopic.php?t=1140&highlight=jt5

Approx figures at 3,000rpm (3.8 & 4.2 cars) in 4th:

2.88 - 81mph
3.07 - 75mph
3.31 - 70mph
3.54 - 65 mph

The various ratios for Jaguar and 5 speed boxes are as follows:

3.97, 2.34, 1.46, 1.0, 0.76 - JT5 (recommended for 2.88 and 3.07 diffs, discontinued)
3.57, 2.07, 1.38, 1.0, 0.76 - Getrag 225
3.37, 1.86, 1.28, 1.0, n/a - Moss box
3.35, 1.99, 1.33, 1.0, .68 - JT5 variant
3.35, 1.99, 1.33, 1.0, .73 - JT5 variant
3.35, 1.99, 1.33, 1.0, .83 - JT5 variant
3.35, 1.99, 1.33, 1.0, .90 - JT5 variant
3.23, 1.90, 1.39, 1.00, n/a - Jaguar KP/KM
3.04, 1.97, 1.32, 1.00, n/a - Jaguar EJS
2.95, 1.94, 1.34, 1.0, 0.8 - Watjag T5/T5
2.95, 1.94, 1.34, 1.0, .63 - JT5 variant
2.95, 1.94, 1.34, 1.0, .73 - JT5 variant
2.95, 1.94, 1.34, 1.0, .80 - JT5 variant
2.93, 1.90, 1.39, 1.0, n/a - Jaguar K* series box
2.68, 1.74, 1.27, 1.0, n/a - Jaguar EJ series box

This online calculator will tell you all you need to know about your choice of ratios: http://wahiduddin.net/calc/calc_speed_rpm.htm

Pete - my lower ratios are 2.95, 1.94. At 20mph it means my revs are either 2,250 or 1,500 - neither is the sweet spot. At 750rpm the car is doing 7mph in first and you need to be doing less than that for uphill hairpins.

PeterCrespin · Post by **PeterCrespin** » Thu Feb 05, 2015 3:46 pm

How many seconds are you in first? 1500 @ 20 mph doesn't sound too low-geared for city work? In fact I would beware Lisbon or Luxembourg! You must have been to Besancon - how did you get on there? I remember in 1981on our return leg from Turkey we rode up the hill to the citadel and Resistance Museum. They took my wife's money, gave her our two tickets and then confirmed the museum was shut that day and they didn't do refunds. :-(!

rfs1957 · Post by **rfs1957** » Thu Feb 05, 2015 3:59 pm

Serge,

Rory, that is exactly what I was thinking.

This thread is ours, those big boys appear to have their own agenda.

I think you would find 3.31 x 0.8 = 2.65 too short on your 3.8.

My current 3.54 x 0.73 = 2.58 is great, as I have said, but for high-speed illegal cruising - and ONLY if I ever had to re-do the diff - I would be tempted to go for 3.31 x 0.73 = 2.42.

Expressed in this way, David's is actually 2.30.

So I think you might want to aim for a "lower" (ie "longer") magic number rather than a higher one ?

I'll just call in the pit crew for a quick diff-swap and let you know which I prefer.

Post by **Heuer** » Thu Feb 05, 2015 4:03 pm

My engine is very camy. If I drive through town in 1st my wife complains about the exhaust noise at 30mph/3,500 rpm; if I drive in 2nd she complains the ride is uncomfortable (i.e. jerky because of the cams) at 20mph/1,500 rpm

Bear in mind that at 5,500rpm the car will be doing 50mph in 1st and 75mph in 2nd. Probably explains why I can leave almost anything standing from the lights - only one gear change to most cars needing two.

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