Power outputs

I believe it depends on how you drive your car. The ZF will not take too much shock loading. Most owners don't speed shift or dump the clutch from a dead stop. The higher the torque the engine puts out the more likely it is to cause serious damage to the ZF. My engine is approximately 500 hp. No problems yet but I do drive my car smoothly thereby not shocking the ZF.

Peter, the ZF 5DS-25/2 transaxle has proven dependable on the street at 600+ bhp, and if you shift gently, even at 800+ bhp in semi-race conditions. Jr Wilson's 540+ inch BB Ford). Jr did break a ZF input shaft when he got airborne over a bump at 220 mph in one SS race and did not let off the gas. The shock-landing snapped the 1" shaft in two. Jr always had problems slowing down.... Greg Isakoff broke several ZFs by brutal speed-shifting and his track car 'only' had about 700 bhp.

Ford never did trust the ZF, possibly based on their Le Mans testing in the GT-40s (with a different version). Dealers were advised to order 1 spare ZF for every 5 Panteras sold. 20-30 years later, almost all of those 'spares' were for sale still in the crates.

More horsepower only lets you accelerate faster- it does not let you GO faster- unless you regear the ZF. Geared stock, 600-up bhp will still have a sensible rpm limit of maybe 7000 rpms in 5th and your top-end will not be much higher than those running 400 bhp, assuming the car is not air-drag-limited. So it kind of depends on what plans you have for your street car. Mine runs a true 160 (6000 rpm) with less than 400 bhp at the flywheel and at my age, that's plenty to get my attention.

The big-power guys certainly will be quicker on open-track days and also will be much lighter in the wallet! Jr. once mentioned he emptied a custom 42 gallon fuel cell in 90+ miles of flat-out running, so that's yet another consideration.

Sharkey and I just dyno'd my 1974 Pantera with a 351C overbored by .03 (work done by Mike Creecy in 2004). When he had the car he said he did low 12's at track. Car dyno'd at 337 hp at wheels and at 6,800 rpm's the power was still going up. We didn't want to break it, so we didn't go any higher. Sharkey thinks we could get another 20-30 by using different fuel. All in All it was fun and nothing broke You can see it on youtube https://www.youtube.com/watch?v=eUKEV3Sk3aw