nitro-nige wrote:Doc
I'd be careful about your assumptions. When Chris was running his roadster at Bonneville they were have trouble getting over 200 mph with the tall gearing. They actually dropped the diff ratio and have now run the record up to 229 mph. The problem they were facing was that the step between gears with the tall diff gears was pushing the engine revs too low. The motor was struggling to pull the revs back up to where it made big power.
Obviously what happens in an 800hp gas roadster won't be the same for a more slippery car like yours.
Absolutely,
ideally we all want a close ratio box, a push to 80mph and 800hp......not gonna happen for us. With a dyno sheet and a little calculation we could plot the shift points against speed , it would give us the step between each gear then the power curve would show us what power we have available at those points, the unknown would be the exact Cd so that's when the guesswork would start. We can't even afford that.
The roadster has a larger frontal area and a vastly different rear end treatment and so we can safely assume ( there's that word again) that they're going to have something like twice the drag.On basic terms that means half the (useable)power, but more importantly it means the shift time becomes more critical( the car slows more quickly when rolling) , AND the "powerband" crisis is more severe( higher wind-resistance when the motor is under the cam and bogging down).
This is yet another reminder that aero is the key to going faster, the adage " double the speed by cubing the power" is for a given shape, the better that shape the lower the starting price is for power.
In a perfect world we would have a motor that had a torque peak at the point where the revs drop to AFTER a shift, and a power curve that kept going up, then after carefully estimating the total drag we would calculate the force applied to the front of the vehicle at the speed we need to run to break the record, we then find the area on the dyno chart ABOVE that power rating and then choose a diff ratio that puts the revs there AT THAT SPEED.
Seems dead simple doesn't it.
The reality is that we can't afford any of that .
What we can afford to do is buy ring and pinion sets, change our rear bulkhead so we can use a different housing AND have an intermediate step by subbing smaller rear tyres.......between that and the range of ratios available for the series 2 7.5 inch 10 bolt we just MIGHT be able to make the car go as fast as possible and avoid the ratio "hump" of which you speak......
there'll be more.........
...few understand what I'm trying to do , but they vastly outnumber those who understand why..
