All Small Six 250 head on a 200 block ??

[Spadgesa]

Hey all, I have been digging into my project car (68 Mustang, I6).
I have determined that my block is out of a 68 Falcon (C8DE-6015-b). My head is out a 77 Maverick (D7BE-6090-AB(8C14)), large log .
I am going to improve its performance this winter. That is, a T5 trans conversion and some head work (porting and new 2v carb) and header.
This engine was rebuilt 30 yrs ago by a much much younger me. The car has been in storage for the past 23 yrs (family and babies came along ).
When I rebuilt it 30 yrs ago, I assumed it was a 'Mustang' 200 I6. Now, after my investigating, it looks like I have a 250 head on top of a 200 block.
I just checked the compression of all six cylinders. To my surprise, I only have compression of 136.5 to 138.1 psi. on all six cylinders. Wow, very consistent but low.
The Ford book says it should be between 150 and 200 psi. The engine runs and drives great. I just rebuilt the Autolite 1100 about 1200 miles ago and changed the plugs. Today, the spark plugs look great.
So, after all that, my question is.. Is it possible that my compression is low because I have 250 head on top of a 200 block? I suspect that way back when I rebuilt the engine, I just had the head flattened prior to installing. Therefore, my cylinder volumes are larger than they should be.
Am I on the right track here?

 

[Kritas]

I also have a '77 250 head on a '78 200 block. I believe the combustion chambers are 62cc compared to 52cc for the 200 head. That should lower compression. My compression is also very consistent across the cylinders and slightly lower than your measurements.

 

[bubba22349]

X2 yes all most all the Large Log Heads before any Milling will have 62 CC Combustion Chambers also there is no real difference between the 200 Head or 250 Head especially on the later ones around 1973 and up. The Stock 1963 1/2 to 1968 200 Six with an average of a 52 CC Head that also used a Thin Steel Shim Head Gasket giving them a 8.7 to 8.8 to 1 C.R. and had a cranking Compression of 175 PSI. Your right depending on the other parts that you used your engine could have a Static Compression Ratio of as low as 7.66 to 1 and explains your low Cranking Compression. You are giving up quite a bit right there and yes having that even of a cranking compression on all the Cylinders is a good thing. Best Wishes.

 

[Spadgesa]

Wow. Thank you guys, for the quick responses.
That is what I was hoping as an answer. I will leave the lower end alone. For now. I will spend my time and money on improving the top of this engine. I will get the head chambers cc'd next. Do you think I will be able to take enough off the head alone to get the C.R. into the high 8s?
I am excited to see what this car can become. Right now, my wife and I call it our Mary Tyler Moore car. It drives and is cute. But that is about it.
With a 5 speed and modified top, it should be much more snappy and fun to drive.

 

[B RON CO]

Hi, showing 135 psi from a compression test is really not bad. Many engines get rebuilt because they are burning oil, low power, and a few cylinders have low or no compression. I can't believe how many guys pull the engine apart before they think of a compression test. Part of the head work should include CCing the combustion chambers to decide how much to mill the head. Good luck

 

[drag-200stang]

A hotter cam will effect and lower cranking compression.
Compression test should be done with the carb. fully open when cranking and turned over about four compression strokes making the gauge go to the highest number after the engine has been warmed up.

 

[Spadgesa]

I am pretty sure that I have a stock cam. When I rebuilt this engine 30 yrs ago, I couldn't afford any upgraded parts.
When I did the compression test, I had the throttle wired wide open. Also, engine was hot.

Oh, by the way. I did poke a camera into the cylinders. I have the stock dished piston heads in there.

Thank you for the tips.

 

[bubba22349]

Most of the Stock Dished Pistons are 6.5 CC and the OEM Standard Ford 200 Pistons were a 1.511 Piston Pin Height this puts the Piston at .019 down the Bore for most engines. With the early 200's small Log Head Combustion Chamber at 52 CC's and their stock Steel Shim Head Gaskets this gave a Quench Distance of .041 (right in the meddle of the Ideal .035 to .050) this was a very decent combo, but there is also room for improvement. With the typical Over Size replacement Pistons the Pin Height is 1.500 this drops the piston down the Bore even more to .030 below the deck plus with the .050 Composition type Head Gaskets that are being used now the Quench Distance is at .080 not ideal, this may also be a part of why your engine's C. R. is so low. A 200 Six with a good Quench Distance can use up to a 9.1 to 9.2 to 1 Static C.R. at Sea Level Altitudes and still be able to use 87 Reg. Fuel. Milling the Head and Block is the best way to get it right though you can still get real good results with just Milling the Head. Best wishes

 

[78_200_C4]

I have a D7 78 Fairmont 200 head on my 78 200. I had it milled .060.
Large log 200's had a longer duration 256 cam.
The ENGINETECH ES448 and the MELLING FOV9 are 256 degree cams.
Both can be purchased at Rockauto
A 5 speed, quick advance curve and a header will make your Mustang fun to drive.
Next you could enlarge the intake bore then install a 2V with the adapter matched to the enlarged intake bore.