Late to the party, but I don't want this forgotten. There are two methods of failure that we are concerned with. First is the grenade where the engine lets go in what is usually a spectacular fashion. The second is consistent pounding at high rpm.
The high rpm one isn't an enormous concern, IMO. At 7500 rpm, you are putting a good deal of stress on the motor. However, you are probably at that RPM for a very brief time. For your car Murph, you would have to be doing 125 to be at that RPM in fourth gear. My hat is off to you if you get to that number very often on track. So, you will most likely only get to that RPM in third and lower. Since we can confidently say that the engine will never let go at 6750 (factory redline), you can actually measure how long you are in that 'unknown' RPM zone. Multiply that time by the number of sections on track where you shift into fourth or wind all the way out in third, and you have a per lap 'unknown' time factor. Multiply by number of laps you have run, and you can then calculate the precise amount of time you have exposed the engine to the unknown. My guess is that it is a very small amount of time, and since each occurance is realtively small, you are probably not running a huge risk.
The second mode of failure is the more dangerous. Mainly because you will find that as you start to beat things down, the higher the liklihood of a categrory one failure. Your factory fresh motor may happily sing to 7500rpm and not think about shearing a rod bolt, but add a couple million cycles on that same bolt, through in some extra clearance on the bearing to increase load, and you might have a problem. The best way, in my opinion, to measure just how hard you are beating down the engine, is to compute the mean piston speed during hard use. Chart a lap at you favorite track, and record the RPM for points you know. You can then extrapolate the rest and come up with a pretty reasonable estimate. Then multiple that number by the magic conversion constant of .575, (based on 88mm stroke and unit conversion, if you must know) and you will have your mean piston speed in feet per second. The general rule of thumb is that any motor that has a 3500fps mean piston speed or higher should think about getting some forged internals. On our trusty Zetec, that is approximately 6100 rpm. I know that you have a screamer of a motor, and you might be able to get there on some courses, but with stock gearing, you are really conserving momentum if you hit that consistently. A shift at 7500 rpm in third will put you about 5300 rpm in fourth. The mid point is 6500 rpm, but since the rpm will climb slower in fourth and never get as high, you will really have to be on a hot lap (or at a very third gear intensive track) to average the 6100 rpm danger line.
BTW, how do you only have 15,000 miles on the car? I do that in 9 months? You need to drive to VIR so we can change that!
