quote:Originally posted by captaintobeys:
What are the casting numbers for the Boss Engine with the 11.1 CR and specs if you know them.
The 1971 R code motor (aka Boss 351 or 351 H.O.) used D1AE heads (66cc chambers) and pistons with small pop-up domes. Although advertised as having 11:1 compression its true compression ratio was about 10:1.
The 1971 M code motor (aka 351C 4V) used the same heads and flat top pistons. It was advertised as having 10.7:1 compression, the true compression was 9.7:1 at best.
The 1970 & 1971 351C 4V & Boss engines were all advertised as having about 1 whole number more compression than they really had. I'm sure the 4V Clevelands were originally intended to have 11:1 compression, but by the time the Cleveland went into production the compression ratios had been reduced; adjusted no doubt because of the impending changes in gasoline octane.
In the late 1960s the highest octane pump gas had octane in the mid to high 90s, but when they "took the lead out" the highest octane gasoline you could buy here in So Cal was 91 octane low lead or 92 octane unleaded.
I've never seen an explanation why Ford continued to quote the inflated compression ratio numbers instead of choosing to be truthful. The 8.6:1 compression ratio quoted for the Q code (351 Cobra Jet) motors was honest however.
The earliest 351C block castings (D0AE-J castings) had slightly thicker cylinder walls because they were intended to have 11:1 compression, when the compression ratio was dropped the cylinder walls were cast thinner in order save a little bit of money on the castings.
-G