Diff 3:54 vs 3:31

Erikantwerp · Post by **Erikantwerp** » Fri Mar 07, 2008 5:13 pm

Hi everyone,

silly question but I was wondering how/where you can see if you have a 3:54 or 3:31 diff ? It sound silly but although I believe I have a 3:31, I am not sure and would like to get certainty.

how would I go about ?

As always, many thx for any feedback

cheers
Erikantwerp

Post by **Heuer** » Fri Mar 07, 2008 6:09 pm

There is (should be) a metal tag on one of the diff casing bolts with the ratio stamped into it. If not let me know and I can tell you what speed you should be doing at say 3,000 rpm in top for each style of differential. The tag is easiest though!

David

christopher storey · Post by **christopher storey** » Sun Mar 09, 2008 3:11 pm

Just to be clear about the tag : it does not have the decimal ratio stamped on , but has the number of teeth on each component . Thus it will say 43/14 for 3.07 ; 43/13 for 3.31 ; and ( I think ) 46/13 for 3.54. It is usually attached one of the bolts to the backplate of the final drive unit

Phil · Post by **Phil** » Sun Mar 09, 2008 6:34 pm

Thank you Christopher !

David

,can you please share with us the speed indicated at 3,000 rpm in top for each style of differential?
Do you also have it for manual/automatic? (I think the US/Canada automatic were 3.31)

Post by **Heuer** » Sun Mar 09, 2008 7:00 pm

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

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

Automatics are a grey area as the ratio varied by country and build date. For example the UK autos had a 2.88 diff but US ones sometimes did/did not. I use a 2.88 powerlock diff in my car (from a 1968 2+2) and with a T5 gearbox in top I have 100mph at 3,000 rpm. We have had the car up to 150mph on the Millbrook bowl (equates to about 160mph on the flat where it would probably take off!) with more to come but a point was proved and we backed off.

Incidentally the exact figures in mph/kph by gear are in the Operating Handbook under 'Performance data' but the above are easy to remember.

David

Phil · Post by **Phil** » Sun Mar 09, 2008 7:31 pm

hello David

thanks !

wow 150 MPH (or 160 !) must be very impressive in a E !
And this is more than the initial test performances !

do you race the car?

Post by **Heuer** » Sun Mar 09, 2008 7:48 pm

Phil

No it is a road car for touring Europe. Engine built for torque - hence the high ratio. It is a very quick car though, about on par with my wife's Porsche 993 in all respects - but draws a lot more attention!

David

Phil · Post by **Phil** » Sun Mar 09, 2008 9:07 pm

hello David

wonderfull !

Hope to share some european roads one day !

I get 70 MPH at 3000 RPM with my E (Canadian, still automatic 8) )
and did no try the 150 (not legal on the roads... :)

andyp · Post by **andyp** » Wed Mar 12, 2008 8:07 pm

Chaps

While we are on this subject it might be useful to know the matching speedos.

The turns per mile are printed on the face and are as follows:

3.54 1312
3.31 1216
3.07 1125
2.88 1058

BTW Very pretty E David.

Andy

Moeregaard · Post by **Moeregaard** » Mon Mar 17, 2008 5:47 pm

This is an excellent thread. For the record, the 3.54:1 ratio should give you 60 MPH @ 2,700 RPM, with stock 185-15 rubber. I lived with that ratio for fifteen years, when I owned my '65. In the dark days of the 55-MPH speed limit it probably saved my licence many times, but with freeway speeds of 75 --80 MPH the norm here now, we're seeing many cars in the U.S. getting 3.07:1 gearing or five-speed gearboxes.

My current E-Type will be getting the higher gearing when I overhaul the rear axle. My only concern is how this ratio works in the real world with the original, close-ratio gearbox installed. Southern California traffic is as bad as it can get, so I have to take this into consideration. Any thoughts?

christopher storey · Post by **christopher storey** » Mon Mar 17, 2008 7:37 pm

I have had a 4.2 litre 3.07 with the EJ close ratio box for 15 years. It is quite delightful with the high second gear being wonderful for overtaking. There is no shortage of torque to pull these high ratios with the 4.2, and the 3.8 still should have no difficulty

Moeregaard · Post by **Moeregaard** » Mon Mar 17, 2008 11:29 pm

Thank you for your input, Christopher. 3.07:1 it will be. You guys have always enjoyed higher speed limits that we have, so you seemed like the ones to ask.

It appears that second gear takes the place of third with the 3.07:1 gears. My old FHC, with the 3.54:1 ratio would absolutely fly in third, but I always felt that first and second were used up way too quickly to be much good.

1e13902 · Post by **1e13902** » Tue Apr 01, 2008 4:00 am

You will love the 3:07.
My 67 ots came stock from Jag with this rear end. The 4.2 has all the torque and rev to run through the gears as fast as you can press in the clutch and gas. I had mine @ 140 mph on a long inner city down hill run and still had a 4th more pedal range till I hit the floor. When shes a solid car agian I can't wait to red line it.
I street raced many American muscle cars in this. Off the line aginst the V8 455s was no sweat.
I've been wondering what a 5 speed would do for me. Maybe a waste of $$, but high way cruise may be a plus. Then again I avoid the highways for our local winedy hilly roads.
To tell what you're driving, Jack up the rear IRS and remove a rear wheel. Mark the propeller shaft as it enters the front of the differential.
As I rotate a 360 Degree turn of the tire hub, I get 3.07 complete turns of the propeller shaft. Like wise 3.54 complete rotations with a 3:54 Dif.
And for the Jag that needs a truck to push it off in first gear, he gets 2.88 complete propeller shaft rotations per single tire rotations.
Its a neat way to relate engine rotation to gear box to differential as its final response to tire rotation and speed. Simple stuff and makes the rear end less of a noman's land.

Moeregaard · Post by **Moeregaard** » Sat Apr 05, 2008 1:54 pm

I was torn between fitting 3.07 gears versus retaining the 3.54 set and fitting a five-speed gearbox. Cost factored into my decision as overhauling the final-drive unit is substantially less expensive than buying a new gearbox, and the stock 'box in my car is in perfect condition. I figured that the money I save by not buying a five speed can go towards bodywork.

Out of respect for the clutch and my wire-wheel splines, I leave the tire-smoking stuff for the Mustangs, Camaros and various pieces of hot-rodded Japanese stuff found in this area.

1e13902 · Post by **1e13902** » Sun Apr 06, 2008 12:51 am

I'm 47 now and those few Hot Rod days were back when I was 17.
I just wonder how old I'll let my 4 year old be before I let him have these keys.
Its a tough car. The rear end can handle all the muscle ya can give it.
No burn outs and drag raceing for me either, Just a seasonal day driver.

Erikantwerp · Post by **Erikantwerp** » Thu Oct 23, 2008 8:55 pm

Hi guys,

A question/comment somewhat related to this title: It seem that a lot of discussions on this forum deal with 2.88 vs. 3.07 and 3.54. There is not a lot said on the 3.31 (you guessed it, I have that one). Just wondering, is there a particular reason for ths ? How is this diff (3.31) compare versus others in your opinion ?

Many thx as always for any feedback/insights
cheers
Erik

Moeregaard · Post by **Moeregaard** » Thu Oct 23, 2008 11:27 pm

Denis Jenkinson commented on the 2.88:1 diff in his book. He had the opportunity to drive a friend's car fitted with this ratio and decided to stay with the 3.07:1 diff. His complaint was that it was more difficult to pull away from rest, and that it put the engine on the wrong part of the torque curve at 105 MPH.

Post by **Heuer** » Fri Oct 24, 2008 10:16 am

Moe

I can maybe agree with Jenks on the pulling away from rest but the comment on the torque curve is completely at odds with my experiences and the maths. At 105mph in fourth I would expect the engine to be turning at:

2.88 = 3900
3.07 = 4200
3.31 = 4500
3.54 = 4800

This assumes 1:1 top gear and 185 tyres with a 13" radius. The E-Type's torque curve kicks in at about 1,500 rpm and starts to drop off at 4,500 so the 2.88 puts the engine in the middle of the curve at 105mph, better than the 3.07. Add a five speed box with a 0.8 top gear and you are even better off with revs dropping to 3,100.

The compromise with a 2.88 diff is at low speeds where first is good for 40mph so you do need to change down fairly often in town. This goes against everything I was taught (first is to get the car moving etc) but once you get the hang of it, not a problem. You also need to be circumspect with the throttle to maintain smooth progress. Really depends on what you are using the car for and you can always down shift, something Jenks seems to have overlooked. I reckon the best possible combination is a 3.07 with a 5 speed box for modern driving conditions. But then I would probably miss my 2.88 once I hit the open road!

Have a play with this little calculator: http://wahiduddin.net/calc/calc_speed_rpm.htm - should give you all the information you need.

Moeregaard · Post by **Moeregaard** » Fri Oct 24, 2008 6:31 pm

Jenk's quote is as follows: "It was that little bit trickier to get away from rest, but, more important, it put you too low down the torque curve at 90--100 MPH, and you lost that wonderful surge forward with the nose rising up, that you got with the 3.07:1 axle. You needed to change down into third gear to get the best performance, and that removed one of the charms of the E-Type for long-distance motoring. I therefore decided that the 2.9:1 ratio was not for me."

Maybe after "rowing" his Porsche 356 all those years turned him off of constant shifting.

rossbraithwaite · Post by **rossbraithwaite** » Wed Feb 02, 2011 4:07 pm

andyp wrote:Chaps

While we are on this subject it might be useful to know the matching speedos.

The turns per mile are printed on the face and are as follows:

3.54 1312
3.31 1216
3.07 1125
2.88 1058

Andy

Does that therefore mean that if one changes the diff ratio one should also change the speedo?
Thanks,
Ross

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