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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.

hp graph

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.

hp graph

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)

hp graph

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

hp graph

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

hp graph

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

hp graph

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

hp graph

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

hp graph

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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