Having your head examined.
By Tom Carnegie
I have tested several different Model T cylinder heads on a dynamometer. The motor that I used for these tests is bored .040 oversize with Jahns aluminum pistons. The rods were bored improperly when they were rebabbitted, and are 1/16" short of their correct length, so the engine is a little shy of the amount of compression that it should have. The valves are stock, the porting is stock, the cam is an unreground original that I set up using the piston travel method. The manifolding is stock with a cast iron intake and stock exhaust pipe and muffler system.
Dyno Basics
A dynamometer essentially does one thing. It measures torque at a given speed. If we know torque and speed (rpm's) we can calculate horsepower. For instance, a motor turning 101 rpm's and putting out 52 foot-pounds of torque would be putting out one horsepower. A motor putting out 52 foot-pounds of torque at 1010 rpm's would be putting out ten horsepower, 10100 rpm's 100 horsepower and so on. But there is more to the story than this.
Achieving a Standard
Different atmospheric conditions will effect how much power an engine will put out. In order to have a meaningful comparison between tests done on different days the horsepower ratings need to be corrected for ambient conditions. The folks at the S.A.E. have a whole set of formulas (that I won't go into right now) for correcting power readings to compensate for humidity, air pressure and temperature. Then if you take a reading on a warm muggy day the corrected readings should equal the corrected readings taken on a cool dry day. All of the following charts and graphs have been corrected to standard conditions.

Graph one: This graph compares an unmilled low head to an unmilled high head. Although the maximum horsepower is lower on the low head, it has better torque on both the low and high end of the curve.

Graph two: This graph compares an unmilled high with a head milled .125". The milled head on this engine produced a near textbook power curve which peaks at 20 horsepower. The extra milling no doubt compensated for the short rods. (see text)

Graph three: This graph compares Ford's data from the Service Bulletins to a high head milled .125"

Graph four: This graph compares a 1909 head with a high head milled .125"

Graph five: Waukesha-Ricardo verses a 1909 head. Waukesha wins on the bottom end, Ford on the top.

Graph six: Reeder head and Waukesha-Ricardo. Wow! talk about some easy bolt on power.

Graph seven: The Reeder head is better on low end power, but for speed the Z-head is the champ.

Graph eight compares our first head with the last. We've come a long way.
Horsepower
RPM |
Low head |
High head |
High head .125" |
Waukesha Ricardo |
Reeder Head |
Z-head |
MPH |
600 |
8 |
7 |
9 |
9 |
8 |
7 |
15 |
700 |
10 |
9 |
11 |
12 |
10 |
9 |
17.5 |
800 |
12 |
11 |
14 |
15 |
13 |
11 |
20 |
900 |
14 |
14 |
15 |
16 |
15 |
14 |
22.5 |
1000 |
14 |
14 |
17 |
17 |
17 |
16 |
25 |
1100 |
15 |
14 |
18 |
19 |
18 |
18 |
27.5 |
1200 |
16 |
15 |
19 |
19 |
19 |
20 |
30 |
1300 |
16 |
16 |
19 |
20 |
20 |
20 |
32.5 |
1400 |
16 |
17 |
19 |
20 |
21 |
22 |
35 |
1500 |
16 |
17 |
19 |
21 |
21 |
23 |
37.5 |
1600 |
16 |
17 |
20 |
21 |
22 |
25 |
40 |
1700 |
16 |
18 |
20 |
20 |
22 |
23 |
42.5 |
1800 |
16 |
17 |
20 |
20 |
23 |
24 |
45 |
1900 |
15 |
17 |
18 |
19 |
22 |
24 |
47.5 |
2000 |
15 |
14 |
18 |
19 |
20 |
21 |
50 |
2100 |
13 |
13 |
14 |
18 |
19 |
21 |
52.5 |
2200 |
8 |
8 |
9 |
12 |
13 |
19 |
55 |
This chart shows the horsepower of the various heads. The highlight denotes the point of highest horsepower. For comparison sake I would consider the .125" head as being closest to the "correct" rating of a stock model T motor. The mph figures are for 40:11 ratio rear-end.
Torque
RPM |
Low head |
High head |
High head .125" |
Waukesha Ricardo |
Reeder Head |
Z-head |
MPH |
600 |
71 |
63 |
80 |
83 |
74 |
59 |
15 |
700 |
75 |
69 |
80 |
91 |
78 |
67 |
17.5 |
800 |
78 |
72 |
92 |
98 |
87 |
74 |
20 |
900 |
79 |
79 |
88 |
94 |
89 |
82 |
22.5 |
1000 |
75 |
72 |
88 |
91 |
89 |
83 |
25 |
1100 |
72 |
69 |
85 |
89 |
86 |
85 |
27.5 |
1200 |
69 |
67 |
82 |
85 |
84 |
89 |
30 |
1300 |
65 |
65 |
78 |
83 |
82 |
83 |
32.5 |
1400 |
62 |
63 |
72 |
76 |
80 |
84 |
35 |
1500 |
56 |
59 |
67 |
75 |
74 |
82 |
37.5 |
1600 |
54 |
57 |
65 |
68 |
71 |
84 |
40 |
1700 |
50 |
57 |
60 |
62 |
67 |
72 |
42.5 |
1800 |
45 |
51 |
57 |
59 |
67 |
69 |
45 |
1900 |
41 |
46 |
51 |
53 |
60 |
67 |
47.5 |
2000 |
38 |
36 |
47 |
49 |
54 |
56 |
50 |
2100 |
22 |
32 |
36 |
45 |
48 |
52 |
52.5 |
2200 |
19 |
18 |
22 |
32 |
38 |
45 |
55 |
The highlights denote the points of
maximum torque. The mph figures are for 40:11 ratio
rear-end. In actual use, with the unmilled high head,
this test motor would propel my car about 48 miles per
hour on a flat road in calm wind conditions.
Conclusion: I don't know what to
make of all this. The most surprising thing to me was the
poor performance of the low head. Most model T people
will tell you that low heads run better than high heads.
Maybe the short rods effected the low head
disproportionally.
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