'65 Mustang Hi-po vs '65 Formula S

Dave_J said
So, my 273 4bbl had 235 HP @ 2675 pounds curb weight (0.0879 HP per pound) and his 289 4bbl has 225 HP @ 2650 pounds curb weight (0.0849 HP per pound).

In my above post, our 5 runs, I won the first race by a car and the next 2 by a door, lost the next 2 by a fender as my 9 1/4 inch clutch was slipping each time. Let the clutch cool and it did not slip much.

273's clutch pressure plate had 3 springs in all the 3 places. I had it rebuilt with 4 springs of heaver wire diameter springs. It was HARD to step down on but it did not slip much. Later bellhousings allowed a 10.5 inch clutch.
;)

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My original 273 4-barrel clutch lasted 90,000 miles and a bunch of speed shifts. Finally crapped out. Local parts store replacement clutch was garbage. Couldn't speed shift it even when new without slipping badly. Tried a couple different ones, finally figured out that although the catalogs listed what I was given as a 273 four barrel clutch, they were really six cylinder clutch assemblies. Finally found an actual Commando 273 clutch disc and paired that with a Weber pressure plate. Boy that would wear out your knee if you didn't put the trans in neutral at a long light, but it also didn't slip. Only problem was that after two years, one of the release fingers would bend and the clutch wouldn't fully disengage. After the second one did the same thing, I converted to a 340 clutch. Found that even a local parts store 340 clutch couldn't be speed shifted without slipping. Switched to aftermarket (Zoom, etc) which at the time would actually be in stock at your local speed shop. They worked, but wore out relatively quickly compared to my original clutch. Went through several of them (I've owned the car for over 50 years), then switched to a Centerforce dual friction clutch, and it's lasted longer than anything else I've ever run. Only problem is it bites so hard I've gotten afraid to speed shift it, but I haven't raced anything in a long time so the tenth or two I lose by not speed shifting doesn't matter.