Lotus Elan

3.9 diff

PostPost by: Dieschelan » Wed Jan 02, 2019 5:35 pm

Hi

Someone has in his elan a 3.9 diff with a close ratio gearbox and a tuned engine?
I have in my S3 a 3,9 diff. And I want to increase the power of my stock engine (105 hp) to 140-150 hp and also to put a close ratio gearbox. Is This a good combination?

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PostPost by: bitsobrits » Wed Jan 02, 2019 6:59 pm

I originally ran my 135bhp Elan with 3.9 and close ratio gearbox, but later changed it to 3.55 and semi close box when I reconfigured the engine for 1.7L 160bhp. Am much happier with the combination now, but I'm sure you will get many different opinions on this topic :D
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PostPost by: 1owner69Elan » Wed Jan 02, 2019 7:17 pm

I have a 3.77 diff but with low profile tires (185/60) it is effectively a 3.94 diff. I have the stock semi-close gearbox with a broad torque band so I don't have the issue of trying to stay in a smaller power band with a close ratio box. My observations with my setup are:

1. The 1st gear is good for launching. I wouldn't want to sacrifice the initial acceleration by having the taller close ratio first gear (2.972 vs 2.51).
2. The gear ratios that you eventually choose should fit the power curve for your engine. You can see below the dyno curve and gearing/shift points for my setup. The broad torque curve allows for larger rpm drops between gears and less need for the close ratios.
3. Are you trying to setup a street or race car? The semi-close I would say is generally better suited for the street. A race car where you don't have to worry much about standing starts and with a peaky torque curve would be better off with a close ratio.
4. The one factor that is negative, in my opinion, with my setup is the gearing for the highway (motorway). Revs are just too high at 75/80 mph to be comfortable. Thus, I am considering a 5 speed box with o/d.

gear-ratios.jpg and


dyno-lotus.jpg and


Thus, your choice primarily depends on the characteristics of your motor and your intended usage.
The above is my amateur, simple analysis. I am sure there are others on the board with more experience and knowledge better able to comment.
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PostPost by: msd1107 » Wed Jan 02, 2019 9:09 pm

I ran my Elan with both a stock engine and modified engine with 3.55 diff and CR transmission, even in LA's mobile parking lot commuter traffic. No problems and much more enjoyable than when I had the original semi-close transmission.

I had originally ordered the car with the CR transmission, but Bob Challman, being the persnickety person he was, delivered the car with the semi-close transmission and I eventually retrofitted the car with the proper CR transmission.

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David
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PostPost by: prezoom » Thu Jan 03, 2019 12:28 am

Here are three different combinations of gear ratios and different sizes of tires.

My S2 has an engine I would estimate at somewhere between 105 and 115 horse power. 3.9 diff, semi close gear box and Sumitomo 200HTR 175/70-13 tires. Good for acceleration from a start, but needs a taller 1st for my liking. High speed cruising sucks.

My Plus2 has had three different setups. All have been with the 2L Zetec, 45 DCOE equipped, horsepower estimated around 160, torque unknown, lightweight Fidanza aluminum flywheel, Vredestine 165-13 tires. Started with a 3.77 and the semi close ratio. Enough torque to easily spin the tires on a quick takeoff, but ran out of gear way too soon. Changed to a 3.54 diff, everything else stayed the same. Less of a chance at spinning the tires on takeoff, but still ran out of gear early. Cruising rpm better, but acceleration rate dropped off, but still acceptable. Changed to a close ratio, stayed with the 3.54 diff. This combination is a bit more difficult to get off the line, but with the torque, it is totally manageable. Gets out of my steep driveway up to the road with no difficulty, even when the engine is cold. Love the long legged 1st gear, 6500 rpm is 52 mph.

My Sabra GT has a 100 horsepower engine with a very heavy stock iron flywheel, close ratio ZF (same ratios as CR 2000E), Vredestine 165-15 tires, and a 3.55 diff, about 150 lbs lighter than the Plus2. The maximum rpm of this engine is 5000 rpm, so even thought it has more mph per rpm in 1st gear, it will not stretch as far. This car is at its limit with the standard combination of gearing. Doesn't like climbing out of my driveway, or getting started going up hill from a stop, Takes a bunch of rpm and quick action with the accelerator/brake/clutch action. Without the heavy flywheel, it would be nearly impossible. The E brake is handy and is used on an uphill start. Almost all slow driving is in 2nd or 3rd gear. Not enough hp or torque for easy driving.

If I had a choice (read money), I would have a 5-speed with the close ratios in both, along with a .82 fifth. Would put the 3.77 in the S2 and leave the 3.54 in the P2. The Sabra will stay the same, and it has a more relaxed highway cruising. Eighty mph is quite pleasant. The larger diameter tires make it like being in overdrive all the time.
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PostPost by: Mazzini » Thu Jan 03, 2019 9:07 am

I have a CR box and 3.5 diff in my S3, I think it's the best combination for modern road conditions, that said if you read the Brooklands road test books several contemporary articles say the same.

I had a Voigt 5 speed in my S2, it had a 3.7 diff and even with .82 top it still felt under geared. I think the set up would have worked better with a 3.5.

I'm putting my S2 back to standard - 3.9 diff and CR box, it will be interesting.
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PostPost by: Foxie » Thu Jan 03, 2019 1:41 pm

Mazzini wrote:I have a CR box and 3.5 diff in my S3, I think it's the best combination for modern road conditions, that said if you read the Brooklands road test books several contemporary articles say the same.

I had a Voigt 5 speed in my S2, it had a 3.7 diff and even with .82 top it still felt under geared. I think the set up would have worked better with a 3.5.

I'm putting my S2 back to standard - 3.9 diff and CR box, it will be interesting.


I'm using a 3.55 with Voigts 5-Speed with 2.98 long first in the Plus 2. Perfect on the road. But I'll be putting in the 4.55 for Hillclimbing this year. :)
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PostPost by: Mazzini » Thu Jan 03, 2019 2:39 pm

Foxie wrote:
Mazzini wrote:I have a CR box and 3.5 diff in my S3, I think it's the best combination for modern road conditions, that said if you read the Brooklands road test books several contemporary articles say the same.

I had a Voigt 5 speed in my S2, it had a 3.7 diff and even with .82 top it still felt under geared. I think the set up would have worked better with a 3.5.

I'm putting my S2 back to standard - 3.9 diff and CR box, it will be interesting.


I'm using a 3.55 with Voigts 5-Speed with 2.98 long first in the Plus 2. Perfect on the road. But I'll be putting in the 4.55 for Hillclimbing this year. :)


I had the tall first gear too. Not a bad box at all, thought I didn't like the gate between 1st and reverse, it was a bit too easy to select reverse, though it could have been just my box.
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PostPost by: Quart Meg Miles » Thu Jan 03, 2019 5:17 pm

Unless you're a traffic-light dragster the 3.9 and CR box is a great road combination, and with more power it will be excellent, but the revs are about 4,200 rpm at 70 mph. With the semi-CR box you'll curse the low first gear, 37 mph at 6500, and probably do already.

I ran my CR box with a 3.55 for ten years (1st gear 52 mph) and hated it, it bogged down, but it would be fine with Sprint power or more. Five speed and 3.9 is the solution but how original do you want to stay?
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PostPost by: Mazzini » Thu Jan 03, 2019 8:14 pm

Quart Meg Miles wrote:Unless you're a traffic-light dragster the 3.9 and CR box is a great road combination, and with more power it will be excellent, but the revs are about 4,200 rpm at 70 mph. With the semi-CR box you'll curse the low first gear, 37 mph at 6500, and probably do already.

I ran my CR box with a 3.55 for ten years (1st gear 52 mph) and hated it, it bogged down, but it would be fine with Sprint power or more. Five speed and 3.9 is the solution but how original do you want to stay?


Is your engine tuned? I have a Q420 spec engine in my S3 and yes it can bog down in first, but on country roads it's fabulous.

I have a similar engine in my S2, but as I mentioned even with the Voigt box and a 3.7 it felt under geared.

I hope to fit a Lotus 5 speed box to my Sprint (3.5 diff) before the summer (2019)...I'll let you know how I get on.
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PostPost by: Quart Meg Miles » Thu Jan 03, 2019 8:43 pm

Mazzini wrote:
Quart Meg Miles wrote:I ran my CR box with a 3.55 for ten years (1st gear 52 mph) and hated it, it bogged down, but it would be fine with Sprint power or more. Five speed and 3.9 is the solution but how original do you want to stay?

Is your engine tuned? I have a Q420 spec engine in my S3 and yes it can bog down in first, but on country roads it's fabulous.

No, dead standard apart from a tubular 4-branch manifold.
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PostPost by: Davidb » Fri Jan 04, 2019 12:44 am

I run a 3.55 diff with CR gearbox and about 150 bhp and aluminium flywheel-no problem. (175/70x13 tires)
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PostPost by: vstibbard » Fri Jan 04, 2019 11:40 am

Nicest road combination I've owned was S3 Coupe which had a CR box, 3.55 diff and 125bhp (Super SE D type cams) with 155x13 tyres, this car now resides in Tasmania, I missed it a few years ago as I'd have bought it back if I've known, and would today.

Compared to my first Elan S4SE which had (Stromberg with OEM D types cams and head ported by Ed Winter back in the 80's and semi close with 3.55, the longer first gear and closer ratio made for easy modern driving personally. I've never suffered from bogging down, both were healthy and well tuned.

I've recently completed a S1 with CR and 3.77 155x13 tyres with QED420's and lots of compression it was a terrific combination for sprints and track days, the Coupe was much nicer to drive distances.

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