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Due to the poor quality of gasoline during the Model T era, the compression ratio of the Model T engine was intentionally held at about 4:1. Since good quality gasoline is now available, an easy way to improve the performance of your engine is to increase its compression ratio. Compression can be increased by milling a stock head, installing pop up pistons, or by installing a high compression head. The increased compression will not only improve the power of your car, but also increase its fuel economy.

Theoretical Effects of Compression Ratio

Compression ratio is strictly a ratio of volumes, i.e. the volume in the cylinder and head when the piston is at the bottom of its stroke divided by the volume when the piston is at the top of its stroke. Many factors effect engine performance, however, if air flow, volumetric efficiency and other factors are equal, a theoretical relationship (see Huntington*, p. 67) can be used to estimate the effect of compression ratio on engine power. For the Model T engine, this relationship is plotted on the following graph. An engine simulator can also be used to estimate the power from increased compression (see Cams/Simulation). The results of two computer simulations are plotted below to show the effect of compression ratio on the horsepower and torque curves. A nice feature of this modification is that unlike other modification (cams, carbs, etc.) it increases power and torque for all engine speeds.

Combustion Chamber Design

A key phrase in the preceding discussion is the one, "if air flow, volumetric efficiency and other factors are equal". Generally, these factors are not equal. Huntington* states that increased compression is usually accompanied by poorer air flow, so that one generally does not achieve the theoretical power increase. However, the Model T combustion chamber is antiquated even compared to other flathead engines, e.g. Model A or flathead V-8. These later engines employed squish style combustion chambers patterned after the work of Harry Ricardo. The Waukesha-Ricardo head produces a significant power increase with only a small increase in compression. The photo and sketch below show a stock high head and a Waukesha-Ricardo head. The crossection sketch of the combustion chambers compares the shape of a stock head (dashed lines) and Ricardo head. The Ricardo design employs a small clearance over the piston and a larger cavern around the valves for good air flow. For best results, the clearance above the piston should be 0.050 to 0.080 inches (Huntington, p. 70).

We have started a photo gallery of Model T heads (see Photos).  Most of the aftermarket heads have a squish style combustion chamber. The Reeder head is a notable exception. The combustion chambers for the Haibe, Giant and Simmons heads appear to be identical. The combustion chamber for the Z head is quite similar. Unlike the Ricardo head, all of these heads have a "valley" in the combustion chamber.

Compression Ratios for Model T Heads

The photo at left (click to enlarge) shows the method we use to determine head volume.  A Plexiglas plate with a small hole is clamped onto the head with a little silicon sealer.  The head is filled with water using a 100cc syringe.  The compression ratios can be determined from the measured volume. A stock high head has a volume of about 294 cc or 17.9 cu in. The volume is reduced by 2.8 cu in because the piston rises 5/16 inch above the deck. The volume is increased about 0.8 cu in due to the head gasket.  For a stock head, the combustion chamber volume is 17.9 - 2.8 + 0.8 = 15.9 cu in. An engine with stock bore and stroke has 44.2 cu in displacement per cylinder, so the total volume when the piston is at the bottom of its stroke is 44.2 + 15.9, and the compression ratio is (44.2 + 15.9)/15.9 = 3.8. The values determined for other heads are shown in the table below. The volumes for milled heads were calculated using the open area of 18.3 sq in and the thickness of material removed. This works out to about 3 cc for each 0.010 in milled.  For example, milling a head 1/8 inch reduces the volume by 18.3(0.125) = 2.3 cu in or 37.5 cc. The compression ratio calculations can be modified for other cases. The table shows compression ratios for a engine with stock bore and stroke and values for one modified with an 0.060 overbore and a Model A crank (48.5 cu in per cylinder).  The rare antique heads (Green and Riley) could have been modified at some time in their past, so the head volumes may differ from their original values.

*Huntington, Roger , Souping the Stock Engine, Fisher Books, 1950