MTFCA Forum

Hello all i need help with an accessory T Ward head i would like to find the compression ratio of this head. I could only work with Mils & it worked out to be aprox 283 mlis= 283 cc late original head comp ratio 3.98:1 & accessory head 262 mls=262 cc. Can anyone tell me the compression ratio of the acc head PLS cheers Warwick.

It looks like about a 7 % increase, or about 4.3 : 1. While that's not much of an increase, if my SOTP math is correct, it would help make more power. The combustion chamber design may offer as much or more benefit as the modest increase in compression. Improvements in chamber shape can be quite beneficial.

"SOTP" math is estimated, seat-of-the-pants math. I compared the two combustion chamber volume measurements and estimated the difference at about 7%. Pictures of the head would be helpful. As far as shaving the head to further increase compression, I don't how how much, if any, you could remove without causing problems. The safest course would be to clean it up and use it as-is, making sure that you have enough piston clearance.

I presume Pat's 7% is a comparison of the volume of the two combustion chambers. I was under the impression that the compression ratio also involves the swept volume of the piston. If that is the case, the gain may well work out to be considerably less than 7%. Others will know how to make the calculations.

Allan from down under.

My estimate was based on my assumption that a 7% decrease in TDC chamber volume would yield about a 7% increase in compression pressure, which is what we're after. As far as I know, all Model Ts have the same (stock) bore and stroke, and thus the same (stock) swept volume. But actual compression varies with chamber volume. I assume that compression ratio is the difference between the volume of the cylinder AND the combustion chamber at BDC, and the volume of the chamber, including any volume in a "dished" piston crown, or one with valve reliefs, at TDC. A T with stock pistons has no significant "dish" or relief in the piston crown, and the top ring comes very near to the top of the bore at TDC, so I'd think that a specific reduction the the Model T head chamber volume would yield an increase in compression, and the compression ratio, at about 1:1 percentage wise.

WE THOUGHT YOU OUGHT TO KNOW ... - Ford Motor Company
[Search domain performanceparts.ford.com] https://performanceparts.ford.com › download › pdfs › EngineBuildingTipsSpecs.pdf
Henry Ford's Model "T" has a CR of 3.6:1. High-performance engines operate in the area .. ////// SELECT: "Engine Building Tips" for compression ratio definition and discussion (PDF)

Using your direction to the Ford site, I found the directions to the compression ratio calculator. The minimal contribution of the difference in combustion chamber volume made very little difference to the compression ratio when added to the not inconsiderable swept volume of the piston. I could be way wrong in my calculations though, especialy having to convert 3.75" bore and 4" stroke to centimeters.

Allan from down under.

Just the same, many manufacturers offered a range of compression ratios from around 8.5 to around 11.5 in the same engine family at the same bore and stroke. The difference was in the head chamber and/or piston compression height and piston head design. That's a very broad range. Ford, for instance, offered a range of compression ratios similar to the example above in the FE V8 engine family, all at 352 CID.

THANKS everyone for your help cheers Warwick.