What differential ratio is needed?
Posted: Sun Nov 05, 2006 11:08 pm
There have been questions as what final drive ratio is necessary under some conditions. This post addresses what final drive ratio is necessary for top speed. Other questions pertaining to gearing for race conditions are best addressed in a different post.
The question of what R&P may be needed is dependent on the car, engine power, and tire size. As opposed to the original Elite that was designed to a rather high theoretical standard, the Elan is very much a car of compromise, and was substantially under geared in all configurations produced.
For instance, the original Elan was geared for 90 mph on 145-13 tires, 3.9 R&P, and 95hp at 5,500 as introduced, although it would easily pass 6,000. Contemporary road tests either ran up to the 6,500 rev limiter, or to 7,000 with the fastest being 128 mph on 155-13 tires, 3.55 R&P, and a 115@6,000 SE engine (at which speed the engine was probably barely producing 100hp).
Often, a car is geared so that maximum speed is achieved at over, but close to, the peak power RPM. The tables below will give a sampling of what could be top speeds achievable for various configurations of engines, and the closest R&P to go with different tires to achieve that speed. The RPM chosen is 200 over the power peak. (For those who want to experiment more, I have a spreadsheet that eases this what if scenario considerably. Enter tire info, engine RPM info, R&P, and gearbox ratios, and it generates enough tables and graphs to satisfy the most extreme figure maven.)
Note that the calculated figures will be for a FHC, a DHC will be slower due to the poorer CD of the convertible top. Speeds will be slower if you run large side view mirrors, engine driven fan, wide tires, flares, etc. Speeds will be faster if: You do not have side view mirrors. No engine driven fan. Use narrow tires and no flares. Fair in the windshield. Go to a nose mounted scirocco or other nose mounted aluminum radiator. Use an electric water pump. Install 036B 1145 as per page K9 of the workshop manual. Improve the air venting from the engine compartment as per page K10 of the workshop manual or other methods to vent air from the engine compartment.
Common tire sizes are 155-13, 165-13, 185/70-13, 185/60-13 and the Spyder (Minilite) 185/60-14. Engines will be 95 at 5,500 (standard or Stromberg), 115 at 6,000 (SE), 126 at 6,500 (Sprint Big Valve), 160 at 7,000 (well modified road engine) and 240 at 8,200 (BD series 2.0L).
......................Calc.................diff ratios for various tire sizes
.......................top
.HP......RPM speed 155-13..165-13..185/70-1..185/60-13..185/60-14
095 at 5,500..123....3.03......3.11.........3.09..........2.89...........3.02
115 at 6,000..131....3.09 .....3.18.........3.16..........2.95...........3.08
126 at 6,500..136....3.24......3.33.........3.30..........3.09...........3.23
160 at 7,000..147....3.22......3.31.........3.28..........3.07...........3.21
240 at 8,200..168....3.28......3.37.........3.35..........3.13...........3.27
Interesting, eh? Shows we have been running around in virtual 3rd gear all the time. And a well modified Elan is a modern day 150 mph car.
But look at the speeds in 1st gear. Even with the 2.97 wide ratio (which should be pitched over the first cliff and replaced with the proper 2.51 set) you get 41, 44, 46, 50, and 57 for the different horsepower figures. With the 2.51 1st gear, you get 49, 52, 54, 59, and 67 for the different horsepower figures. Now, I have run a 2.51 1st with 3.55 and 165-13s, and it is possible although challenging in San Francisco, but I think few others would like to do so. (I also ran a motorcycle with a 5 speed with a 1.9 1st, over 60 mph in 1st, that was challenging also)
What are remotely achievable R&P ratios? Well, there were pinion gears with 7, 9, and 11 teeth. And ring gears with 31, 32, 34, 37, 39, 41, and 43 teeth. 11:31 is 2.82, 11:32 is 2.91, 11:34 is 3.09, 11:37 is 3.36, and it goes away from there. Does any one know if the 11 tooth pinion can be mated with the smaller ring gears?
Obviously, we need a 5 or 6 speed transmission. Comments are gratefully accepted. My personal email is david_harralson at hotmail.
The question of what R&P may be needed is dependent on the car, engine power, and tire size. As opposed to the original Elite that was designed to a rather high theoretical standard, the Elan is very much a car of compromise, and was substantially under geared in all configurations produced.
For instance, the original Elan was geared for 90 mph on 145-13 tires, 3.9 R&P, and 95hp at 5,500 as introduced, although it would easily pass 6,000. Contemporary road tests either ran up to the 6,500 rev limiter, or to 7,000 with the fastest being 128 mph on 155-13 tires, 3.55 R&P, and a 115@6,000 SE engine (at which speed the engine was probably barely producing 100hp).
Often, a car is geared so that maximum speed is achieved at over, but close to, the peak power RPM. The tables below will give a sampling of what could be top speeds achievable for various configurations of engines, and the closest R&P to go with different tires to achieve that speed. The RPM chosen is 200 over the power peak. (For those who want to experiment more, I have a spreadsheet that eases this what if scenario considerably. Enter tire info, engine RPM info, R&P, and gearbox ratios, and it generates enough tables and graphs to satisfy the most extreme figure maven.)
Note that the calculated figures will be for a FHC, a DHC will be slower due to the poorer CD of the convertible top. Speeds will be slower if you run large side view mirrors, engine driven fan, wide tires, flares, etc. Speeds will be faster if: You do not have side view mirrors. No engine driven fan. Use narrow tires and no flares. Fair in the windshield. Go to a nose mounted scirocco or other nose mounted aluminum radiator. Use an electric water pump. Install 036B 1145 as per page K9 of the workshop manual. Improve the air venting from the engine compartment as per page K10 of the workshop manual or other methods to vent air from the engine compartment.
Common tire sizes are 155-13, 165-13, 185/70-13, 185/60-13 and the Spyder (Minilite) 185/60-14. Engines will be 95 at 5,500 (standard or Stromberg), 115 at 6,000 (SE), 126 at 6,500 (Sprint Big Valve), 160 at 7,000 (well modified road engine) and 240 at 8,200 (BD series 2.0L).
......................Calc.................diff ratios for various tire sizes
.......................top
.HP......RPM speed 155-13..165-13..185/70-1..185/60-13..185/60-14
095 at 5,500..123....3.03......3.11.........3.09..........2.89...........3.02
115 at 6,000..131....3.09 .....3.18.........3.16..........2.95...........3.08
126 at 6,500..136....3.24......3.33.........3.30..........3.09...........3.23
160 at 7,000..147....3.22......3.31.........3.28..........3.07...........3.21
240 at 8,200..168....3.28......3.37.........3.35..........3.13...........3.27
Interesting, eh? Shows we have been running around in virtual 3rd gear all the time. And a well modified Elan is a modern day 150 mph car.
But look at the speeds in 1st gear. Even with the 2.97 wide ratio (which should be pitched over the first cliff and replaced with the proper 2.51 set) you get 41, 44, 46, 50, and 57 for the different horsepower figures. With the 2.51 1st gear, you get 49, 52, 54, 59, and 67 for the different horsepower figures. Now, I have run a 2.51 1st with 3.55 and 165-13s, and it is possible although challenging in San Francisco, but I think few others would like to do so. (I also ran a motorcycle with a 5 speed with a 1.9 1st, over 60 mph in 1st, that was challenging also)
What are remotely achievable R&P ratios? Well, there were pinion gears with 7, 9, and 11 teeth. And ring gears with 31, 32, 34, 37, 39, 41, and 43 teeth. 11:31 is 2.82, 11:32 is 2.91, 11:34 is 3.09, 11:37 is 3.36, and it goes away from there. Does any one know if the 11 tooth pinion can be mated with the smaller ring gears?
Obviously, we need a 5 or 6 speed transmission. Comments are gratefully accepted. My personal email is david_harralson at hotmail.