DeTomaso P70

Why wasn’t the Shelby – DeTomaso P70 ever finished?

In my book on this car, I stated I never knew why De Tomaso didn’t finish the 7-litre engine he had promised Shelby for the P70. Wonderfully, someone who read my book sent me the answer. He is absolutely correct and I thought I’d share his GREAT letter with you!

Dear Mr. Brock:

I just read "The Road to Modena." What an incredible book about an incredible car! Thank you for writing it and including all the other biographical and historical information. It now occupies a treasured place on my bookshelf. I do believe that I can unravel a mystery for you.

You refer, several times, as to being mystified at why DeTomaso never completed, or even initiated, the 7 liter version of the 289 Ford. The reason is simple - it can't be done. There's no way to get 427 cubic inches into the 260/289/302 block. There's not room for an overbore (max is 4.030) or the long stroke that would be needed, due to its low deck height of 8.2". Today, there are small-block Fords built at 427 cubic inches, but they are based on (normally) the 351 Windsor block (9.5" deck height) or, less commonly, the 351 Cleveland block (9.2" deck height). Neither of these designs existed in 1964, and that extra inch or more of deck height, and its attendant relocated camshaft, makes all the difference in the world. Those 427s have a 4" bore (stock, all of these are thin wall castings and can't take more) with a 4.125" stroke. That stroke wouldn't even begin to fit in a 289 block; the connecting rods and crank throws would hit the camshaft - and those rods would have to be so short that the pistons would hit the crankshaft counterweights. To this day, even with modern metallurgy, the biggest low-deck (289 block) Ford stroker combination that has been run with any practicality comes out at 363 cubic inches, and this requires very high dollar billet parts of the type that would have been nearly impossible to manufacture in 1964.

Whether DeTomaso knew at the start that he was selling Shelby on an impossible dream, I can't say - but if he didn't, he knew about 10 minutes after taking a 289 apart and doing some rudimentary measurements. Since the entire project was based on the 7 liter engine, your car was doomed from the start, unfortunately. And it's a damn shame, because the car was beautiful and no doubt would have been extremely effective. History shows that the big-block McLarens, Chaparrals, and Lolas didn't take center stage in CanAm until 1966, so had the car been completed, you might have had a good season in 1965 in the USRRC. We'll never know. As you noted, DeTomaso was the better of the two con men.

In any case, this is but a minor nitpick in a tour de force of a wonderful book, documenting a great career and a great car. My compliments as well on the high production qualities. I hope my email wasn't offensive; I have the utmost respect for you. Heck, mine might be the 2,000th email telling you this!”

This is a GREAT letter!  And he is absolutely correct in defining why the P70 was never completed...It simply could not be done!  As he says, DeTomaso probably understood this from the moment he did a thorough analysis of what was possible with the 289 block, but never mentioned it to Shel’ as that would have prevented the money stream from continuing! :0)  Having only the cursory experience of being around the later Windsor blocks and making the assumption that these were still  “small block” Fords rather than an entirely new architecture I missed the obvious. I had experience with both the original 289s and later engines (in replica Cobras with 427 “small blocks) but honestly had never gone “inside” the later engine to look and measure.  This reader really clarified this and I wish I’d had his fine explanation in hand when writing the book.

I agree, the P70 might have had a chance in ’65, but by ’66 and beyond the DeTomaso chassis would have been obsolete. That whole era was changing so fast you had to be able to create “improvements” from race to race….a really fun, innovative era. Many thanks to this reader for taking the time to clue me in!  And if you’d like to read the book yourself, you can get it from us here.

10 thoughts on “Why wasn’t the Shelby – DeTomaso P70 ever finished?

  1. DeTomaso was notorious for announcing far out concepts with great fanfare, and never delivering even an ounce of what he promised.
    Shelby’s engine people I’m sure would haven’t known that a 289 could not be stretched that far. That’s why Ford only could reliably produce a 325 for the big sports cars.

  2. The book was a very enjoyable read. Finished reading a couple of weeks after SAAC43. The car is beautiful and the book is great!

  3. I haven’t read the book yet, though I will soon, so perhaps I’m missing something. I am unclear has to why one would imagine a 289 could become a 427, or even need to, as Ford already had a 427. So why not just install an actual 427? Was it a fitment issue?

    1. It was a matter of weight and size. Ford did have a 427 (and even a “390” in an all-aluminum version) but these were NASCAR engines and too physically large to fit in the mid-engined protos of the time. Read the book and you’ll learn even more! Thanks for your interest. Peter Brock

  4. Outstanding!

    I often wondered how De Tomaso could get 427 out of a 289, now I know why!

    I believe I have most,if not all of your literary works Pete. And have read them at least two times!
    All of your writings area wonderful, educational and enjoyable read!

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