Quote:
Originally Posted by jimepting
I'm beginning to see the logic of all this, but I would approach it a bit differently, particularly as regards available wheel horsepower. Most speed events these days have portable chassis dynos available in the immediate area. I'd be very tempted to figure out when my runs were going to occure and have a dyno run done either before or after my top speed testing. Deducing power is a lot less accurate than measuring power.
Perhaps I've been spoiled by having a devoted old race buddy who tests my race car for free, but it certainly would eliminate a lot of guessing(i.e. deducing).  |
The thing about dynos is that one needs a priori rolling resistance data for the track you're running on,as well as the CdA info,to properly load the dyno.If the loads aren't represented up through the driveline to the flywheel,the engine can go to a place which won't reflect the real world.
To do an end run,you'd have to pull the engine and use a engine-dyno and you'd need to know your exact powertrain mechanical efficiency near your anticipated velocity,as it varies as a function of rpm.Ditto for tires.
As an air-pump,a very close power estimation can be figured from terminal velocity rpm vs rated power rpm.
If say,I top out at 4500 rpm (300 rpm short of rated power rpm) then under
'standard 'atmospheric conditions my Road Load bhp at top speed would be 140.625 bhp.
With a driveline mechanical efficiency of 92%(SAE),the load at the drive wheel would be 129.375 horsepower.Of course,this would be adjusted for ambient air density which can be had at the local airport after 'un-correcting' the barometric pressure.
Hucho's relationship would tell me my Cd,and I'd know if there was any wiggle room left with gearing or tire diameters to optimize performance.
"Air drag,that poison of speed!" HOT ROD Magazine,1962
