Diffrences between '74 250 and '76 250?

[Jamie Miles]

After well over 200,000 miles of service the 1974 250 in my '72 Maverick has developed a small knock in the bottom end. I am going to retire it (at least for now). I am cleaning up a spare lower mileage 250 from a '76 Maverick. Are there any diffrences between the '74 long block and the '76 long block that would effect performance, such as cam diffrences or anything of that nature?

I currently have the head off the '76 250 to have it freshened up and do some minor porting. I was also thinking of doing the '70 timeing chain. The car this is going in is a daily driver. Right now just crossing my fingers that the knock in the '74 engine dosen't get any worse before the '76 engine is ready to go in. :|

Thanks.

 

[MustangSix]

The only major difference that I have noted is a change from forged rods (C9 castings) to cast rods (D6 castings) sometime during that year. If the earlier engine has forged rods, I would stick with them if possible. It's not a necessity, just a preference.

 

[CZLN6]

Howdy Jamie:

The '74 & '76 long blocks are essentially the same- same specs on the cam. Changing the cam timing to a pre '72 set will help.

Both engines were rated at 8:1 CR- not good! The problem is in the huge deck height of about .125" to .150". This much deck height ruins the quench effect, adding to lazy, incomplete combustion. The new, thicker, composite style head gaskets add to the problem. I hope you are planning to mill the head to compensate for the thicker composite head gasket and to gain some static compression. What brand of head gasket will you be using? I'd shoot for 8.5 to 8.7:1 CR.

Don't forget to add a three-angle valve job and a 30 degree back-cut to the valves. There was a good flow bench article in this months Car Craft on the benefits of back-cutting the valves. Results were especially helpful for low cam lift- That's us! And I'd take the time to polish the chambers and chamfer any sharp edges. Both will help in fighting pre-ignition by minimizing carbon build-up and hot spots.

That's my two cents, for what it's worth.

Adios, David

 

[Jamie Miles]

Don't really have time or money to tear into the bottom end of the '76 engine right now, I'll have to stick with whatever rods it has.

I was planning to run a Fel-Pro head gasket. That is what I have on my '74 250. The head has not been milled on it.

 

[Hot 6t Falcon]

Sounds like you're after reliability more than power.

You probably want to change the timing chain on the engine. They wear and get loose. Worn chains can jump a tooth or just break. Use a chain and gears from a 1969-71 250. The NAPA numbers are S414 for the cam gear, B415 for the crank gear, and 9-363 for the chain. Using the earlier chain and gears will add reliability and a little power.

The other thing to do while you have the head off is to change the oil seals on the valve stems. They are little plastic umbrellas that stop oil from running down the valve stems into the combustion chamber and making the engine smoke. No extra HP, but necessary for a clean burning engine.

The seals are very in-expensive. You can usually borrow a valve spring compressor from some of the big national auto parts stores. Or you can buy one at Sears for $19.95 - which is less than a mechanic would charge for changing the seals - and you get to do it yourself.

http://www.sears.com/sr/javasr/prod...otive+Specialty+Tools&vertical=TOOL&ihtoken=1

Good Luck

 

[Jamie Miles]

Exactly, I'm after reliability. I am pretty happy with the power my current stock 250 already puts out, keeps up with traffic just fine (and dosen't have much of a problem passing when necessary). I just wanted to make sure I wasn't going to loose any power with the later smog engine (I am doing away with all the smog related stuff, none of it is required on pre-1982 cars in Georgia.

I have changed many sets of valve seals in the past and own several types of valve spring compressors. That's no problem and was already in the plans.

Thanks for the help!

In the future I do plan to build a hot little six for my '69 1/2 Maverick, but right now getting back and forth to work in the daily driver '72 is more important.