Originally Posted by Injuneer

I figured the tire growth by realizing I was scraping rubber off the slicks with the leading edge of the fender when trap speeds went over 125MPH. I figured about 1/2" growth on radius, which would be the 1" that Rich (I believe) is talking on diameter. Also moved the rear axle backward a bit

I figured the convertor slip by comparing engine RPM with rear wheel RPM calculated from the MPH, on the chassis dyno, which involves an assumption on tire diameter (and of course reflecting the rear axle ratio). That makes it a bit difficult to calculate, because its hard to figure the actual tire growth on the dyno. They should "grow" exactly the same way they do on the track, except for the fact that the rear of the car is tied down to the floor. You can see the back end of the car rise up as MPH goes up. Does it grow as much, or do the sidewalls bulge out more as the growth tries to occur, because the chains are holding the rear of the car down, due to the angle they make with the floor. I chose to ignore that issue, and assumed the tires were growing about the same as they would on the track.

After playing with a bunch of numbers, I built a spreadsheet that includes "growth" by entering it as a constant of 0.50" (radius) at 120MPH, then prorating the growth to any other MPH using the ratio of the square of the MPH. For the converter slip, I got 11% using this method. All of this jives with actual track performance, although that involves using the trap speed, allowing for the fact that the true finish line speed is about 1MPH above the trap speeds, and then matching up the finish line to the engine RPM from the data logger, based on the number of seconds from the time the throttle went to 100% + ET to get the correct frame of data representing the rpm at the finish line.

As you can see, its possible to totally over complicate the whole issue. Its easiest to just run the car down the track, and see what happens. But then you might find you bought the wrong set of gears, so its a crap shoot. Sometimes I think if I spent as much time running the car down the track as I spend doing calculations, I'd have a lot more fun.

And, even in my own case, I no longer have the right gears. With the 125-shot the car trapped 128mph at about 6,750rpm, with a 7,000rpm nitrous cut-off. But now that its set up for a 300-shot, its obvious (fingers crossed) I'm going to run out of rpm short of the finish line. Rather than swap in a set of 3.90's, I'll just hit the button on the Gear Vendor O/D and let it roll through the last 100-ft at 5,400-5,600rpm.

Originally Posted by 30696bird

I did a search yesterday. The main reasons "I" saw was the 12 bolt is strong enought to handle power up to 900-1000HP. The 9" is stronger BUT, it's alot heavier and as I seen in one thread it's simply overkill..........extra weight for not much to gain. That's just my take on what I read, please correct me if I took the post wrong.