Lotus Elan

Gear Set choice for Voigt 5 speed

PostPost by: 1owner69Elan » Mon Feb 24, 2020 2:27 am

It appears that I may be getting closer to getting my Voigt T9 conversion (waiting since Dec 2018).

Alan Voigt has proposed three options for the gearset:

1. Long 1st @ 2.9(7?). Gives the 5 speed box similar ratios (1 - 4) to the semi-close stock gearbox I have now but with an O/D.
2. A set with a 3.3 1st. This is of no interest to me as I think this is too low to be practical
3. A close ratio box with 2.66, 1.75, 1.28, 1.0, 0.84

Options 2 and 3 come with a heavy duty layshaft bearing. Not with #1. Also a significant premium (additional ~1000 pounds) for #3 over #1.

I like the idea of the HD bearing since I have a high output Twink (181 hp, 143 lb-ft).

I also have a wide and flat torque band from 4500 to 7000 such that the existing semi-close ratios stay within the peak band, (1700 cc, stroked) when shifting at redline (I use 6900). I also have a 3.77 diff. As well as 185/60 low profile tires that equate to an equivalent 3.94 diff. I will say that I am happy with the current ratios (except for lack of an overdrive 5th). I don't find myself burning out in 1st gear with the sticky tires. Car is quick.

The CR ratios also stay in the torque band through shifts but, from calculation, a large portion of the lower rpm peak torque band (4500 - 5500) is missed with the shift to 3rd and to 4th. The rpms stay too high which is perhaps better for a peaky engine.

My question is whether to stay with the 2.9 first (as stock SCR) or opt for the close ratio set. Given my wide torque band it seems that I don't have to sacrifice the potential better acceleration of the SCR ratios while still maintaining peak torque. It seems that the CR ratios are better suited to a more peaky engine, once you get off the line, where you have to stay in a narrower torque band.

Be interested to hear from some of the gurus on here if my "analysis" makes sense. I have been focused on maximizing acceleration but perhaps there are other issues to be considered. The car is for the road and not raced. So not tuning for a particular race course.

Right now, I am inclined to go with the 2.9 SCR but without the HD bearing. I tried to get Alan Voigt to do a SCR with the BGH HD bearing - which they offer on the BGH website but Alan said only the 3 options presented are available for his 5-speed T9 conversion.

Here is the plot of the SCR ratios shift points (stays in torque band and utilizes lower rpm torque):
SCR shift points.jpg and


Here is the Voigt CR ratios (misses lower portion of torque band in 3rd and 4th, and presumably slower off the line):
Close ratio shift points.jpg and



Twincam Dyno:
Vegher twincam dyno.jpg
Vegher twincam dyno.jpg (43.3 KiB) Viewed 2462 times
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PostPost by: Davidb » Mon Feb 24, 2020 4:22 am

I think it is going to depend on how and where you drive it. You are not going to race I think?
I agree the 3.3 first gear is a non starter... (Sorry).

The extra 1000 quid is a hefty hit for the close ratio set and do you really need it?
I have used a 2.9 first gear with a moderately tuned engine and it was fine. I currently have a 2.5:1 first with a 1720cc engine in slightly lower tune than yours and it is fine with 3.55 final drive.

Not much help I know but it is going to be a very personal choice. If cost is no object go with the close ratio!
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PostPost by: nmauduit » Mon Feb 24, 2020 9:02 am

Davidb wrote:I think it is going to depend on how and where you drive it. You are not going to race I think?
I agree the 3.3 first gear is a non starter... (Sorry).

The extra 1000 quid is a hefty hit for the close ratio set and do you really need it?
I have used a 2.9 first gear with a moderately tuned engine and it was fine. I currently have a 2.5:1 first with a 1720cc engine in slightly lower tune than yours and it is fine with 3.55 final drive.

Not much help I know but it is going to be a very personal choice. If cost is no object go with the close ratio!


I'm inclined to think that as well : to me the necessity of a CR gearset comes from the requirement of being as competitive as possible, that is using all gears spread on the whole torque band. The decision to fully race the car is usually one best made early on, as many decisions are not really compatible with a road use. I'm not talking about having a lot of fun at track days and being quicker than all or most cars on the tarmak on these occasions : this can be achieved with a nicely prepared car such as yours already.

With an OD 5th, in the event you wanted to use the 5th while racing, that pushes the requirement of a shorter rear end quite far, so as to be able to reach 6900-7000 rpm in fifth with your typical tires (even in 4th).

My blue S4 is not as powerful (~130, redline 6800) but similar in some other respects : 3.77 diff (Quaife) and 5 speed Lotus gearbox, : I don't gear down into 1st and this does not bother me, the car is still at its best at late braking and slow corners thanks to its balance and low weight.

And I don't use the 5th on the track, I don't even reach my readline in 4th (that is 196kph on the Yokohama A048 175x60x13 I use for track days, in fifth that would take me to 244kph with a long downhill and lots of rear wind ... )
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PostPost by: gjz30075 » Mon Feb 24, 2020 10:19 am

With your horsepower and torque, diff ratio, and tire diameter, the close ratio box makes most
sense to me, except 5th gear. Too short. That's only a 15% drop and not worth it. My Sprint
has 165/70 tires, 1720cc engine, 3.77 diff, with the Lotus 5 speed, which has a .80 fifth, a 20% drop
and to me, that's the minimum to make it worthwhile. I thnk there's still room to take the revs further
down and still be in the power band and I have much less hp/tq than you.

The 2.66 first makes sense to me. A longer first gear helps keep up with today's traffic and you have
the torque to overcome the 'long' gear. Think of the muscle cars of the 60s that had Ford Toploaders,
T-10s and Muncies and they all had a less than 2.50 first gear, many of them down in the 2.30 range.
They did not have problems taking off due to the power and torque to overcome it, and made 1st
gear usable. Granted, many of them had 4.xx diffs but our cars are quite light with an excellent
power to weight ratio.
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PostPost by: Frogelan » Mon Feb 24, 2020 2:00 pm

Interesting question and replies. I would add driving technique to the list.

Personally I would opt for 3 as engine wear is much more expensive than the additional cost of the gearbox.

Your car is a road car, it makes sense to be able to keep revs down as much as possible on "freeways" but to have close ratio use for twisty sections.

Does this help?
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PostPost by: mbell » Mon Feb 24, 2020 2:49 pm

For me the question would be more about what speeds and situations I expected to need acceleration or cruising gears than all out fastest gear set etc. So I'd be looking for the gear sets that would put me that best revs for taking corners and other driving thing I'd expect to do, e.g. overtaking cars.

There nothing worse than being at very top of revs or out of the power band going around bends or trying to overtake.

As a different kind of example of this, I have the lotus 5 speed and 3.55 in my car and I like it every where apart from the work car park. 2nd gear is too tall for the bends. So I have to clutch in and out for the bends or drop to first, with the number of turn I have to take to get out I find it quite annoying....
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PostPost by: prezoom » Mon Feb 24, 2020 6:31 pm

While not exactly the same, but close enough. On my Zetec Plus2, I am currently running an aluminum flywheel, the close ratio (2.5) gear box, a 3.55 diff and 165-13 Vredestine tires. Horsepower and torque close to your specs, but carrying approximately 400 more pounds. I was apprehensive about both gear choices when coupled with the flywheel, but my concerns were quickly reduced. First, I have a fairly steep driveway to get up to the street, and with the horsepower and torque, however the Plus2 gets up the drive easier than the S2 with a semi-close and a 3.9, or my Sabra with a ZF 2.5 gearbox and its 3.5 diff on 165-15 tires.

Up hill starts are not a problem, and my 2 day jaunt up to the Lotus Meet in Folsom a year ago through the mountain roads of the Western Sierras, proved my choice in gearing was perfect. An accompanying Sprint found itself between gears, over reving in 2nd and bogging in 3rd, often having to shift up, where a longer gear would have carried to the next curve.

Yes, the longer gearing has taken some of the performance out of the car when it was equipped with its 2.9 first, and its 3.7 diff, but the advantages of having the longer two bottom gears makes the car much more pleasing to me to drive. The 2.6 first offered, is a bit lower, but close enough to what I have, so as to not be that much noticeable.
Go for Option 3.
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PostPost by: Davidb » Mon Feb 24, 2020 6:47 pm

I missed that the fifth gear is 0.84--that is not useful enough as someone else pointed out. I fitted an Alfa Montreal ZF gearbox in my Aston DB2 and it had the 0.85 fifth gear-it was not enough of a drop in rpm to make it worthwhile-which was the whole point in changing the box in the first place! I manage to find a set of 0.80 gears from a Lotus Sunbeam (see, there is Lotus content!) in the UK and fitted them-that made all the difference. However, my DB2 had a low final drive ratio...
The ZF box is the one that was used in the works Escorts, and many other competition cars in the late sixties/early seventies and bellhousings are readily available to fit one to an Elan. Parts are not cheap though.
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PostPost by: RichardHawkins » Mon Feb 24, 2020 8:11 pm

If I remember correctly, it is torque, not power that kills gearboxes. The type 9 was fitted to some heavy saloon cars, so I don’t really understand the advantage of heavy duty bearings for a light car like an Elan. Can Voight explain what the bearings are and how much more durable they are?

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PostPost by: steve lyle » Mon Feb 24, 2020 8:45 pm

Is there a history of hopped up twincs killing T9's with standard layshaft bearings?

If yes, then #3.

If no, then #1.

I'm betting the answer is no, BTW.

#1 comes with a .815 OD, right?
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PostPost by: StressCraxx » Mon Feb 24, 2020 8:57 pm

RichardHawkins wrote:If I remember correctly, it is torque, not power that kills gearboxes.

Richard Hawkins


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You remember correctly.
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PostPost by: Frogelan » Mon Feb 24, 2020 9:39 pm

Richard

Have a look at overall oomph of that engine. It has very fruity torque and Mr Voight seems to have understood that the owner is enthusiastic.

I'm not so sure that the bearings alone justify the price but that is another matter!
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PostPost by: 1owner69Elan » Tue Feb 25, 2020 6:25 pm

Thanks to all for the thoughtful replies. Appears that the consensus is the close ratio gear set.

I had previously thought that the CR would only be suitable for a peaky torque curve, unlike the relatively flat one that I have. But, the CR would also allow me to drive the car through shifts without going to redline each time. One could shift more at 5000-5500 (or lower) instead of 6900 and still stay on the flat part of my torque curve with less of a rev drop with the CR. Conforms to a driving style more in keeping with road car driving as well as less stress on the engine. One could still take it to redline if wanted and the higher speeds in each gear are attractive.

I think the hefty premium for the CR may be due to the fact that it may be a completely new gear set from BGH vs reconditioned T9 gears. Trying to verify with Alan Voigt. Also trying to see if I can get an 0.82 vs 0.84 5th gear.
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PostPost by: Foxie » Wed Feb 26, 2020 12:53 am

steve lyle wrote:Is there a history of hopped up twincs killing T9's with standard layshaft bearings?


I fitted my Alan Voigts 5-speed T9 to to my Plus 2 in 2004.

I've been circuit racing and hillclimbing it for the last 14 years.

Engine is up to 1860cc for the last 3 years, with QED 450s and 45 webers.

Never had a problem with the gearbox.

:)
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PostPost by: msd1107 » Wed Feb 26, 2020 5:19 pm

Some previous posts about 5 speeds

viewtopic.php?f=37&t=13955&start=

viewtopic.php?p=82911

Follow the links for more.

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