Before I get to my question, let me reveal what I've already learned here so far.... After reading several old posts, I see that HP ratings were changed to "Net HP" somewhere around 1971 or 1972. So, that explains why the HP ratings in 1973 and later Mav's and Comets were rated significantly lower, on top of all the smog/emission add-on's. Okay, I get that. I've also found that the HP ratings on a 1970 Maverick with a 250ci 4.1 was rated at 155 HP, and that the 1970 Mav had 9:1 compression, as opposed to the 8:1 compression in our 1974 Comet. Now, recently I've come upon a head from a 1970 Mustang with a 250/4.1 and what I'd like to know is this... Was the 1970 head milled .030 to .040 in order to boost the compression?... or, did the higher compression come from the bottom end of the motor? We're looking to buy a separate head for our Comet and have it machined for a 2 barrel carburetor while we still drive the car (stock) as we wait for the work to be done. Having a head that doesn't also need to be milled would be a plus. I've also read that the later 250 heads (late 70's) have better flow due to a larger bore of the intake log. But we like the idea of getting a larger increase in HP by boosting compression without having to go into the bottom end of our motor. So, does anyone know if the 1970 head would accomplish that?
What I read the 250 head was the same 69-76. The 77-80 head has a larger intake log...1345cc vs 1240cc. Some '80 heads had 1420cc log, they hade a hex shape log instead of the flat top log. The 77-80 head also has larger intake valves...1.75" vs 1.649" Starting in '73 Ford retarded the 250's valve timing twice for better emission numbers and this changed resulted in less horse power. All 250 heads have 62cc combustion chambers.
Both early and late 250 heads had a chamber volume of 62cc. Looks like you're best option is to find a 77-up head with the larger runners and valves and mill it for more compression. There may be thinner head gaskets available to help out too.
Those two replies answered my questions perfectly!! Looks like we'll be looking for a '77 or later head. Thanks!!
The factory head gasket were .027". Modern replacement gaskets come in the .045"-.055" range. You will need to shave your head using todays gasket. The head on my 250 was shaved .080". I'm guessing the CR is around 9.2 to 1 and runs fine on regular unleaded gasoline.
The Feds required the change to net HP numbers beginning with 1972 models, that's also year Ford cut compression and retarded timing to help comply with emissions std... Not that it's scientific comparison, GM cut compression for the '71 models... Interestingly Chevy's 250 that was rated at 155Hp with 8.5 was down to 145 for '71(still 8.5 comp)... With net figures required for '72, rating was down to 110Hp, engine still at 8.5 compression ratio... Basically before 1972, real world figures were inflated by 20-30%...
