.................Return To Mine & Other Bonneville Car Construction Pages
.........................................-- Rambling Thoughts on --
............................................-- Land Speed Racing --
Previous Page.........................Return to Index..............................Next Page
Gears --Transmission Gearing and Rear End Gearing:The final key to going fast and probably setting a record in a class that already has an existing record is gearing. Get it wrong and you are in trouble.
The last example, I promise, using Hooley's Stude will show you how important this is. The first year we ran we felt we would be lucky running over 150. He got his D (127 mph), C (173 mph), and B (191 mph) licenses in three passes. On the last pass the oil pressure went to zero. One of the things we learned the first year is you need a well thought out oiling system. On the 4 pass he got his A license at a 207 mph with an exit speed of around 219 mph and the oil pressure again was zero and we were done. We were quite lucky to do as well as we did considering we knew nothing about what I've talked about on the last 4 pages. Hooley stuck small spill plates on the car just because he saw them on another car. We had no idea of how fast we could go with the car/HP combination. We just got lucky.
One of the areas we got lucky in was that of gearing. We are using a close ratio Muncie 4 speed with a 2.47 gear in the Ford 9 inch rearend and 28 inch tires. It just so happened that our motor was putting out probably maximum HP at our maximum speed. That was 219 mph at 6500 RPM. So by blind luck we had our maximum speed at the right RPM for the maximum HP. This is a very important point if you are trying to hit everything right on the head. If our maximum HP would have been say at 5500 rpm we would have probably run about 185 to 190 as faster than that and the HP would have fallen off and not been enough to run faster. Likewise if the maximum HP would have been 8000 rpm and would have been the same HP (in the 500-550 HP range) we would have most likely run slower as 8000 rpm is 270 mph and 550 HP would have never gotten us to that point. In third 8000 is 210 mph, so we probably could have run that in third, but with an upshift to 4th it would have fallen on its face and probably not run any faster. (Don't forget you can figure all of these factors -- transmission gears, rearend gears, tire size and speeds at any rpm and the rpm drop shifting from 3rd to 4th with the spread sheet ( HERE )).
One more example using this year. On our record run we averaged 239.7 mph in the 5th mile and our exit speed was 241.2 at 7100 rpm, so we weren't going to run any faster even if the course would have been longer. On one of the previous pages we showed that we needed about 700 HP at the crank to run 240 mph and that is what the motor puts out at 7100 rpm (240 mph). Our motor makes maximum HP at about 7400 rpm, about 740 HP. So why couldn't we run up to 7400 rpm and maximum HP. Seems logical that we should be able to. I gave away the answer in the last paragraph. Our speed at 7400 rpm with the gearing in the car would be 250 mph. The answer lies in the fact that if it took us 700 HP to run 241 it would take 781 HP to run 250 and we didn't have that much so there was no way we were going to get to 250. We might have crept up to 245 or so if we would have had another mile or so and the motor would have stayed together.
So the moral of the story is try and be realistic about your top speed and try and get your gearing set so that you are at or near max HP at that speed/rpm. Use the spread sheets I've listed.
The other option is to use a cam that puts out the HP at the rpm you are forced to run with whatever gearing you have. Build the motor around a RPM range that is good for what you are doing, not just maximum HP. Also don't forget "torque". You need a good "torque" motor at B'ville. Torque will get you up to the speed you are looking for and HP will make the speed happen. Make sure, using the spread sheet, that you stay in the torque curve during your shifts. This is really important with the "tall" rear gears we run on the salt.
I know guys run 2 and 3 speed transmissions on the salt, but personally I'm in favor of having all the gears you can afford. At speeds over 200 it can be a drag race getting up to the speed you want to run, hopefully by near the beginning of the 5th mile. If you are running a 2 or 3 speed it is going to be hard to stay in the torque curve after your shifts with the tall gearing. Also the faster you go the more weight you are going to need in the car and that means accelerating that weight becomes more important and that is what transmission gears do for you (multiply torque). Also on the next page I'll show what a 4 or 5 speed, like a Jerico, can do for you and how it might save the expense of a quickchange.
..................................................................Next Page