Compression Ratio question on a 79 200CI

[WhitePony]

OK, well I'm picking up a 79 200CI from a Fairmont to use as the starting point for my engine build. I have a couple questions for all you CR gurus out there.

I understand that the heads on 77 and up are the same on both the 200 and the 250. They have a 62cc combustion chamber. OK I can understand why you would need a larger combustion chamber to compensate for CR on the 250 with it's larger displacement, but why on the later 200's? It seems that the larger chamber will drastically lower the CR. I have the Falcon Performance Handbook, and it really doesn't go much into it. Although it does say that 73 and up pistons had a 13cc dish in the pistons, which seems like it would make the CR problem worse.

Are there any fundamental differences between the early and late 200's? I know that in 79 Ford started switching to the low mount starter, but the one I'm getting has the high mount.

Did the late 70's 200's have a much lower CR? If so, is there any problems milling the head to bring it back up? Should I use a piston with a smaller dish or a flat-top?

I just don't want to go off half cocked and break something due to ignorance.

Kris

 

[CZLN6]

Howdy Chris;

Yes, the compression on the later engines was reduce. It is officially stated by FoMoCo as an 8.2:1, but typically measures less.

The difference in the 200 and the 250 compression is in the excessive deck height on the 250s- somewhere between .100" and .150"- not the pistons. The pistons and the head of the 200/250 are identical.

The later heads can be milled safely up to .090" without much concern. Beyond that it is a good idea to do sonic testing to make sure there are no thin spots.

You'd be better off to deck the top of the block to zero, then measure everything and then mill the head to achieve you CR goal. Be sure to reread the sticky at the top of this forum for details about head gasket thicknesses and using washers under the head bolts on reassembly. This will get you a better quench, even with the thicker, aftermarket, composite head gaskets.

Piston dish on 200s and 250s have been standard at a 7cc dish since it's beginning. The exception is the larger 13cc dished pistons which 1st appeared on the California Emissions (C/E) engines beginning in 1967. They were used in an effort to reduce emissions by reducing CR.

Thanks to your letter, we checkied your question and we have found the error on page 38. It will soon be corrected. The only stock, OEM engines, that we know of, to use the 13cc pistons are the C/E. Compression begin to be reduced beginning in '69 with the "M" heads with the larger, 62 cc chambers, but the big, across-the-board CR reduction came after '72, with the D3 casting codes which have 65cc chambers.

I would use the small dish pistons as they have an advantage in minimizing knock/pre-ignition and seem to have a power edge over flat-topped or domed pistons in a wedge-shaped combustion chamber engine.

Obviously, the reference to piston dish in the Handbook will be updated and corrected for future editions. We sure don't want to confuse the puzzle anymore then Ford has already done.

Another difference in the '79 engine is retarded cam timing. FoMoCo did this with altering the location of the keyway on the sprockets. You would do well to get a pre-'72 timing set. Or, better yet, get a double roller set from AZ Mike for much more durable and reliable cam timing.

Thanks again for the question. It sent us scrambling to see what we said wrong. Sorry for the confusion.

Adios, David

 

[WhitePony]

Awsome! Thanks David! That's the info I needed. I'll proceed like you have it written in the handbook. I plan on doing the "Full Monty" to the head. Unshrouding the valves, porting, and polishing the combustion chamber, and adding the port divider. I have already ordered a cam and timing set from Mike, and will probably go the full ARP fastener route as well.

One question though, will I need to shim the top of the rocker towers to get the proper push rod to rocker length? I'd like to order the roller tipped rocker set, but will probably have to hold off for a bit. Daughter got married three weeks ago, and that took such a big bite out of my wallet that I think half of my right butt cheek is missing! I'd hate to put the rockers back together and find that they bind.

Thanks

Kris

 

[CZLN6]

Howdy back Chris;

Congratulations on your new son-in-law. Your payback comes when you get grand children.

No, there is no need to shim the rocker stancheons. The hydraulic lifters will more then take up the slack. The adjustible, roller tipped rockers are really nice though and the trick way to make sure that you are getting the proper pre-load on the lifters. I believe they include the appropiate pushrods too. If not Mike will have what you need.

Enjoy the journey.

Adios, David