A previous post
elan-f14/twin-cam-bellhousing-anyone-t15954-120.html had a comparison of the various ratios available for the MX-5 gearbox, plus the Lotus 4-speeds, many of the ratios available for the T9 gearbox, and the most suitable ratios for the T5. I had forgotten about the Isuzu, but Maurice?s post outlines the advantages of this gearbox. Since it is raining (in SoCA?) I decided to extract the same technical data for Isuzu gearboxes and splice in some of the other gearboxes for comparison.
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 9 cars with 9 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.
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 (see link above). 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.
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, which shows that you can generate a set of ratios that measures well, but does not drive so well.
Isuzu Gemini Twincam 3.174 1st, .855 5th,
Gb ratio % diff speed...diff...err..FoM=-406.6
3.174....61.7...037.9...23.4...17.2
1.963....43.9...061.3...26.9...02.1
1.364....36.4...088.2...32.1...88.1
1.000....17.0...120.3...20.4...44.6
0.855...........140.7
The 3.174 1st gear is lower than desirable but will work acceptably with the 3.55 differential. The first four ratios are spaced well, with an increasing mph differential between gears. Indeed, ignoring 5th gear produces a FoM of 93.6. In this case, the gearbox designer may have wanted a close gap from 4th to 5th. Using the Isuzu Piazza Turbo 5th gear ratio of 0.775 improves the FoM to 94.8, so that if the gears can be swapped, this is the way to go.
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.
Isuzu Piazza Turbo 3.431 1st, .775 5th,
Gb ratio % diff speed...diff...err..FoM=80.6
3.431....74.8...035.1...26.2...01.1
1.963....43.9...061.3...26.9...04.1
1.364....36.4...088.2...32.1...00.4
1.000....29.0...120.3...34.9...00.1
0.775...........155.2
The 3.431 1st gear is lower than desirable and the 1st to 2nd gear drop is much larger than desirable. The top four ratios are spaced well, with an increasing mph differential between gears. Indeed, ignoring 1st gear produces a FoM of 92.2. In this case, the gearbox designer may have wanted a lower 1st gear for startup ease. Using the Isuzu Gemini Twincam 1st gear ratio of 3.174 improves the FoM to 94.8, so that if the gears can be swapped, this is the way to go.
Isuzu diesel 5-speed 3.48 1st, 0.86 5th,
Gb ratio % diff speed...diff...err..FoM=-360.5
3.480....74.9...034.6...25.9...14.9
1.990....46.3...060.4...28.0...01.0
1.360....36.0...088.4...31.8...80.3
1.000....16.3...120.3...19.6...41.3
0.860...........139.9
The diesel gearbox has been mentioned before. The difference between the diesel ratios and the Piazza Turbo is fairly small, and the FoM of the 1st 4 ratios only is 93.2. Using the Piazza 5th gear ratio of 0.775 would bring the FoM up to 97.0. The 3.48 1st gear is lower than desirable and the 1st to 2nd gear drop is much larger than desirable. So none of the Isuzu ratios sets is desirable, if you could mix and match the best 1st or 5th gears, the whole gearbox would be close to ideal. This gearbox is lighter than a T9 or T5 gearbox, and the cost of conversion is probably less that an equivalent T9.
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.
Maurice?s Isuzu 3.77 1st, .81 5th,
Gb ratio % diff speed...diff...err..FoM=-507.3
3.770....63.2...031.9...20.2...49.7
2.310....65.0...052.1...33.8...64.2
1.400....40.0...085.9...34.4...28.6
1.000....23.5...120.3...28.2...39.8
0.810...........148.5
The 3.77 1st gear again is really just a startup gear. The 1st to 2nd shift is reasonable at 63.2%. The rest is pretty much a muddle. The 2nd to 3rd shift at 65.0% is greater than the 1st to 2nd shift, which makes the shift feel wider than the 1st to 2nd shift. The MPH drops between 2nd and 3rd and between 3rd and 4th are much greater than the drops between 1st and 2nd and 4th to 5th. The FoM of ?507.3 reflects the difficulty in matching a reasonable shift progression with the actual gear ratios. You can see this in the err column where each shift has a substantial error compared with better staged ratios. Actually, if we exclude 1st gear (which would be normal operating procedure in real life) the FoM increases to 32.7. Sometimes on these wide ratio sedan based gearboxes, I find the top 4 ratios to be rather reasonably staged.
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.
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. If you can mix and match the appropriate 1st or 5th gears with the corresponding Isuzu Twincam, Piazza Turbo, or diesel you would wind up with near ideal ratios and a gearbox that is lighter than the other candidates. A properly engineered conversion like Maurice?s at a reasonable price would be a real winner. As usual, your particular situation will determine what solution you choose.
David
1968 36/7988