A previous post on the 6-speed thread had a comparison of the various ratios available for the MX-5 gearbox, plus the Lotus 4-speeds. A private communication expanded that to include many of the ratios available for the T9 gearbox. Now that this thread is active again, the most suitable ratios for the T5 are included. There are over 200 T5 gearbox part numbers. Many of them are duplicate ratio sets, and many are much too low geared to be of interest. For instance, a 1st gear ratio of 4.03 wouldn?t be very satisfactory. The highest geared 1st speed is 2.95, not very close. It is available with several 5th gear ratios. The next best 1st gear is 3.35, not very suitable, again with several intermediate and 5th gear ratios.

For each gearbox, the information extracted is as follows: "Gb ratio" is the gearbox ratio, in 1st through top from top to bottom. "% diff" is the percentage difference between the current gear and the next gear. "speed" is the speed achieved at 6,500 rpm. "diff" is the difference in speeds achieved in the current gear and the next higher gear. "err" is the error term for this gear and the next gear when the program calculates the Figure of Merit (FoM). Zero is best, low single digit figures OK, larger figures not OK. The math is simple, the results illuminating. "FoM" is the Figure of Merit. This is a mathematical calculation as to how well the gear ratios are staged. The maximum is 100, and there theoretically is no minimum. You will perceive a gearbox with a FoM above 80 as being a well-staged set of gears. By the time the FoM gets down to 0, there will be awkward gaps between one of more ratios and either too wide or too narrow a gap.

Now, let?s take a pretend drive in 19 cars with these 19 different gearboxes. Back-to-back drives allow for the evaluation of subtle differences between gearboxes that appear to be close together and that might not be so noticeable individually. But the wide range of gearboxes makes differences quite noticeable. I will proceed from the highest geared 1st speed to the lowest geared 1st speed. All the data assumes a car with 155-13 tires, 3.55 differential ratio, and 6,500 rpm. If you want to see results for other configurations, download the spreadsheet (20080411z.xls) at the bottom of the first page of posts at

viewtopic.php?t=16413&view=next .

Lotus CR 2.51 1st, 4th direct.

Gb ratio % diff speed...diff...err....FoM=99.8

2.510....53.4...047.9...25.6...00.0...51.9 at 7,000

1.636....33.0...073.5...24.2...00.0

1.231....23.1...097.7...22.6...00.0

1.000...........120.3

On startup, the acceleration feels soft due to the high gearing (50+ if you go to 7000). 1st to 2nd feels medium close, basically because the gearing is high (in many cars, this is similar to 2nd). 2nd to 3rd feels a little closer, and 3rd to 4th feels closer still. Looking at the speed difference between gears shows why. The speed difference decreases for each gear, giving the impression of closer gearing. Notice the FoM of 99.8, as close to perfect as you are likely to see.

Bill?s hypothetical 2.65 1st, 4th direct, OD on 5th, 6th.

Gb ratio % diff speed..diff..err...gears..FoM=94.2

2.654....56.9...045.3..25.8..00.6..13:30

1.691....33.7...071.1..24.0..01.6..17:25

1.265....26.5...095.1..25.2..00.1..20:22

1.000....20.8...120.3..25.0..00.0..23:20

0.828....17.0...145.3..24.7..00.0..25:18

0.708...........169.9..............26:16

Startup is easier, due to the lower 2.65 1st. 1st to 2nd and 2nd to 3rd feel quite similar, because the mph drops between gears are quite similar. 3rd to 4th feels a little wider, since the CR has a decreasing mph gap between gears and this gearbox has a constant gap. 4th to 5th feels the same, and a car with the proper differential ratio and engine tune should be able to pull max revs in 5th. 5th to 6th feels the same drop, and 6th is a true overdrive. Many of you are running around 4800 rpm at 80 mph. This setup shows just over 3000 rpm

Well, it is not too likely we will get a new gear set cut, and even less likely re-engineer the shift linkage for 4th direct. So I generated a new gearbox with the same overall ratio between 1st and 6th, but with 5th direct. This came out with 3.205, 2.042, 1.528, 1.208, 1.000, and 0.855 ratios and a FoM of 94.2. Using a pinion and ring gear of 11:33 produces speeds in gear very close to the previous gear set and a 9:32 3.55 differential. 10:32 would be equivalent to a 3.77, and 10:33 would be equivalent to 3.90.

BGH E7 2.66 1st, 0.82 5th

Gb ratio % diff speed...diff...err..FoM=72.8

2.660....52.0...045.2...23.5...01.2

1.750....38.9...068.7...26.7...05.5

1.260....26.0...095.5...24.8...01.5

1.000....22.0...120.3...26.4...00.0

0.820...........146.7

The BGH E7 looks quite similar to Bill?s proposed set in 1st through 5th. The differences are primarily in 2nd and 3rd, and although appearing minor, result in a decrease in the FoM from 90+ to 70+. If you drove these two gearboxes back to back, you would be able to sense the differences as you shifted up through 1st through 4th.

The following T5 gearboxes, the Lotus ?Mid-close?, and the BGH E2 have virtually the same 1st gear ratio. However, the intermediate gears and, where present, 5th gear provide different driving experiences.

T5 1352-184 1988 Sierra 2L Turbo 2.95 1st, .80 5th.

Gb ratio % diff speed...diff...err..FoM=28.0

2.950....52.1...040.8...21.2...05.8

1.940....44.8...062.0...27.8...05.8

1.340....34.0...089.8...30.5...05.0

1.000....25.0...120.3...30.1...05.0

0.800...........150.3

The first 4 ratios produce a FoM of 67.1, so 5th gear is not optimal but would work well on the road. In comparison with the Lotus ?mid-close? and the BGH E2, the slightly greater 1st-2nd drop and slightly closer 3rd-4th drop would feel better on the road.

T5 1352-028 1983 Camaro/Firebird 5L V8 2.95 1st, .73 5th.

Gb ratio % diff speed...diff...err..FoM=-23.1

2.950....52.1...040.8...21.2...02.0

1.940....44.8...062.0...27.8...00.3

1.340....34.0...089.8...30.5...25.1

1.000....37.0...120.3...44.5...09.5

0.730...........150.3

Changing 5th gear from 0.80 to 0.73 decreases the FoM from 28.0 to ?23.1, passing the optimum 5th gear ratio for the FoM of 0.77. On the road, you would notice the wider ratio drop when shifting into 5th.

T5 1352-065 1984 Mustang/Capri 5L V8 2.95 1st, .63 5th.

Gb ratio % diff speed...diff...err..FoM=-1241.2

2.950....52.1...040.8...21.2...44.1

1.940....44.8...062.0...27.8...05.4

1.340....34.0...089.8...30.5..232.9

1.000....58.7...120.3...70.6..119.9

0.630...........190.9

Changing 5th gear from 0.80 to 0.63 decreases the FoM from 28.0 to ?1241.2, passing the optimum 5th gear ratio for the FoM of 0.77. On the road, you would notice the much wider ratio drop when shifting into 5th. The only reason to choose this ratio set is if you want a very tall OD 5th for relaxed cruising, but the TC probably cannot pull this gear. Note that there is 1352-115 for the Ford SVO 5L V8 with a 0.68 5th gear ratio. Neither of these 5th gear ratios is very suitable for our cars.

Lotus ?Mid-close? ratio 2.97 1st, 4th direct.

Gb ratio % diff speed...diff...err..FoM=97.5

2.972....47.9...040.5...19.5...00.1

2.010....43.9...059.8...26.3...00.3

1.397....39.7...086.1...34.2...00.0

1.000...........120.3

The so-called ?mid-close? ratio is actually a sedan ratio set. Startup is much easier in 1st since the Elan is quite a bit lighter than the corresponding sedan. 1st to 2nd feels closer than the Lotus CR and Bills hypothetical 6-speed gear sets. It is in an absolute sense (% drop of 47.9% vs 56.9% and 53.4%) but the speeds are lower, so the mph difference is less also (19.5 mph vs 25.8 and 25.6). 2nd to 3rd feels a little wider (26.3 mph difference vs 24.6 and 24.2). The 3rd to 4th shift feels quite wide, both percentage wise (39.7% vs 26.5% and 23.1%) and speed difference (34.2 mph vs 25.2 and 22.6). The FoM is an excellent 97.5.

BGH E2 2.98 1st, .82 5th.

Gb ratio % diff speed...diff...err..FoM=-169.0

2.980....51.3...040.4...20.7...11.0

1.970....43.8...061.1...26.7...03.2

1.370....37.0...087.8...32.5...42.0

1.000....22.0...120.3...26.4...24.5

0.820...........146.7

The first 4 ratios look quite similar to the Lotus mid-close ratio set and the T5 gear sets. But driving the ratio sets back to back would show the BGH set a little better in driveability compared to the Lotus mid-close ratio set, and not quite as good as the T5. 2nd and 3rd are a little further from 1st and closer to 4th, which gives a better on road experience. The 5th gear ratio of .82 is too low, contributing to the FoM of -169. A 5th gear of 0.76 would give a FoM of 99.6 or 2nd gear of 1.795, a 3rd gear of 1.285 would give a correspondingly high FoM, and the first 4 ratios have a FoM of 99.8. These also look similar to the 2.95 1st gear T5 ratio sets, but the T5, with its slightly greater 1st-2nd drop and closer 3rd-4th drop, would feel better on the road.

TRD 33030-SE100 3.106 1st, 0.869 6th.

Gb ratio % diff speed..diff..err..FoM=-21.9

3.106....42.8...038.7..16.6..13.7

2.175....37.2...055.3..20.6..02.3

1.585....29.6...075.9..22.5..13.3

1.223....22.3...098.3..21.9..05.3

1.000....15.1...120.3..18.1..14.2

0.869...........138.4

1st is lower than the ?mid-close? and BGH E2 sets. Running up and down through the gears you will get the feeling that 1st to 2nd and 5th to 6th are slightly closer than the other ratios and 3rd to 4th is slightly wider. See this in the speed difference column where 1st to 2nd and 5th to 6th are less than the other ratio differences and 3rd to 4th is greater. See this also in the ?err? column, where the high error figure accurately predicts problem in the staging of that ratio pair. Also, the FoM of ?21.9 indicates problems in the staging of the ratios.

The following T5 gearboxes and the T9 2.8L gearbox have the same 1st gear ratio. However, the intermediate gears and 5th gear provide different driving experiences. The 3.35 1st gear is lower enough from the TRD or higher than the S15 1st gear to be noticeable.

T9 2.8 3.35 1st, .82 5th,

Gb ratio % diff speed...diff...err..FoM=-257.8

3.350....85.1...035.9...30.5...00.8

1.810....49.6...066.4...32.9...28.4

1.210....21.0...099.4...20.9...64.9

1.000....22.7...120.3...27.3...13.2

0.815...........147.6

A big problem is in the very wide 85.1% drop between 1st and 2nd and the smaller speed gap between 3rd and 4th . See this in the %diff column and speed diff column. The large figures in the FoM err column also accurately predict problems with these ratios and lead to the ?257.8 FoM.

T5 1352-126 1985 Mustang/Capri 5L V8 3.35 1st, .80 5th and others

Gb ratio % diff speed...diff...err..FoM=27.8

3.350....73.6...035.9...26.4...02.3

1.930....49.6...062.3...30.9...12.5

1.290....29.0...093.2...27.0...06.5

1.000....25.0...120.3...30.1...00.3

0.800...........150.3

The slightly lower 2nd and 3rd gear ratios produce a better FoM of 27.8. The 1st to 2nd % difference is still very wide at 73.6% but would still feel better shifting through the gears. Alternative 5th gear ratios of 0.71 (producing a FoM of ?533.3) and 0.68 (producing a FoM of ?941.8 ) are too tall to consider for our TCs. On the road, you would notice the wider ratio drop when shifting into 5th.

T5 1352-199 1990 Mustang 5L V8 3.35 1st, .83 5th

Gb ratio % diff speed...diff...err..FoM=-80.6

3.350....68.3...035.9...24.5...14.0

1.990....49.6...060.4...30.0...14.3

1.330....33.0...090.4...29.8...12.8

1.000....20.5...120.3...24.6...13.0

0.830...........144.9

This gearbox again has slightly lower 2nd and 3rd gear ratios. The gap between 1st and 2nd gear narrows slightly to 68.3%, almost acceptable. Considering only 1st-4th gears, the FoM is 27.5 (and would increase to 90+ with one tooth more on 3rd gear main shaft). 5th gear ratio at 0.83 is too low, a figure of 0.78 would be optimum. On the road, and using 1st-4th, you would notice a slightly larger perceived gap between 2nd and 3rd than between the other two shifts.

T5 1352-047 1984 Nissan 280ZX 2.8 Turbo 3.35 1st, .75 5th

Gb ratio % diff speed...diff...err..FoM=87.7

3.350....61.1...035.9...21.9...00.4

2.080....50.7...057.8...29.3...01.9

1.380....38.0...087.2...33.1...01.3

1.000....33.3...120.3...40.1...00.1

0.750...........160.4

This gearbox again has slightly lower 2nd and 3rd gear ratios. The gap between 1st and 2nd gear narrows slightly to 61.1%, almost acceptable. Shifting up through the gears, you would get the impression of wider gaps as you go up the gears. See this in that the mph gaps between gears increases by almost 6 mph between each gear. What you are seeing is the tradeoff between minimizing the ratio gap between 1st and 2nd and minimizing the ratio gap on the top gears. A 1st gear of 3.35 is just too much to get suitable ratios and gaps in four speeds. The FoM of 87.7 is excellent. There is another 5th gear of 0.78 available (1352-080). The FoM is not as good, but the on-road experience is probably good.

S15 3.626 1st, 0.767 6th.

Gb ratio % diff speed..diff..err..FoM=-308.6

3.626....64.8...033.2..21.5..09.2

2.200....42.8...054.7..23.4..03.4

1.541....27.0...078.0..21.1..21.4

1.213....21.3...099.2..21.1..66.8

1.000....30.4...120.3..36.5..62.7

0.767...........156.8

If 5-6 is not included in FoM calculation, FoM improves to 72.8.

The 3.626 1st gear is markedly lower than the previous gear sets and is hardly more than a startup gear, getting only to 33.2 mph. The 5th to 6th ratio gap is very wide for the top gear pair. This is responsible for the FoM of ?308.6. Considering only the first five ratios produces a FoM of 72.8 and feels pretty good. A 0.85 OD 6th gear would produce a FoM of 85.1

T9 1.6 3.65 1st, .815 5th,

Gb ratio % diff speed...diff...err..FoM=-11.3

3.650....85.6...033.0...28.2...00.0

1.967....43.9...061.1...26.8...05.1

1.367....36.7...088.0...32.3...22.3

1.000....22.7...120.3...27.3...06.0

0.815...........147.6

The 3.65 1st gear is almost the same as the S15 3.626 and noticeably higher than the MX5 1st gear. However, there is a large ratio drop between 1st and 2nd, a small speed gap between 2nd and 3rd, and a large speed gap between 3rd and 4th. This is seen in the speed diff column and the FoM error column.

MX-5 ?99 on 3.76 1st, 0.843 6th

Gb ratio % diff speed..diff..err..FoM=68.5

3.760....65.7...032.0..21.0..00.2

2.269....37.9...053.0..20.1..03.5

1.645....30.9...073.1..22.6..00.4

1.257....25.7...095.7..24.6..06.0

1.000....18.6...120.3..22.4..02.5

0.843...........142.7

1st is even lower than the S15 gear set, getting only to 32.0 mph. The situation is even worse if the differential ratio is a more common 3.77 or 3.90. You would hardly ever use 1st gear after startup. Second gear is not that different from the Lotus CR 1st gear. Shifting up and down, what is noticed is that 2nd to 3rd seems slightly closer and 5th to 6th seems slightly wider than the other gears. See this in the speed difference column, and also in the FoM err column.

RX-8 3.76 1st, 0.843 6th

Gb ratio % diff speed..diff..err..FoM=-156.7

3.760....65.7...032.0..21.0..01.0

2.269....37.9...053.0..20.1..06.6

1.645....38.6...073.1..28.2..72.5

1.187....18.7...101.3..18.9..22.6

1.000....18.6...120.3..22.4..00.0

0.843...........142.7

1st, 2nd, 3rd, 5th, and 6th are the same as the MX-5. 1st again is only a startup gear. Just changing 4th brings the FoM down from 68.5 to ?156.7, indicating some kind of problem in the staging of the ratios. See it in the speed difference column where 3rd to 4th is wide at 28.2 mph, and 4th to 5th is close at 18.9 mph. I wonder why they did this.

SXE-10 3.874 1st, 0.869 6th

Gb ratio % diff speed..diff..err..FoM=-51.6

3.874....78.1...031.0..24.3..00.4

2.175....46.6...055.3..25.7..15.0

1.484....21.3...081.0..17.3..36.4

1.223....22.3...098.3..21.9..08.9

1.000....15.1...120.3..18.1..00.0

0.869...........138.4

Here 1st feels even lower and more useless, getting only to 31.0 mph. To compound the problem, the 1st to 2nd percentage drop of 78.1% is extremely wide. Shifting from 1st to 2nd bogs the engine unless you rev the h... out of the engine in 1st gear. In addition, the 3rd to 4th and 5th to 6th feel closer than the other shifts, which is reflected in the speed difference column and also the FoM error column and also reflected in the FoM of ?51.6.

So there it is, guys. We use an off the shelf sedan gear set and we suffer the consequences. Bolt up an optimum set, the installation goes more smoothly since the shafts hook up to our existing clutch and driveshaft and the driving experience is more pleasurable. For those who are doubtful as to being able to detect these differences, you only need to do a back-to-back once to be convinced. But, hey, it is your car, and your money.

So what is a person to do? As usual, there is no perfect solution. If you can cope with 1st gear startup and do not do long distances on the freeway, then the Lotus CR is as good as you are likely to get, and it is completely standard. The Esposito/Voight/BGH E7/Quaife 5-speed is a possible candidate, provided John can get production quantities and assuming you can stand the price. Although John is in the US, the gearbox can be delivered worldwide. For a DIY ?solution? in the US, a T5 1352-184 1988 Sierra 2L Turbo 2.95 1st, .80 5th would work well, although you suffer a considerable weight penalty and can't use a Spyder chassis. The 6-speed solution being developed by Vincent still needs engineering development to make it competitive with the T9. As usual, your particular situation will determine what solution you choose.

David

1968 36/7988