It goes without saying, that the components (cam, head, gears, etc.) you select for your car will affect its performance.  However, many enthusiasts select the wrong components because they do not fully understand the interrelationship of power, torque and vehicle performance.  The information presented here will help you make better selections for your car.  

What do the power and torque curves mean, and what do they tell you about your cars performance?  For example, two performance curves are shown in the figure below.  One was measured by Ford in 1918.  The other is from a simulation (ref. 1) for a popular reground camshaft.  

Which Cam is Better?  

The reground cam obviously makes more power, but is it a better cam for your car?  The answer depends on your objectives for your cars performance. You must have a good understanding about the type of driving you plan to do.  

Combining the performance curves with a little high school physics can give a much clearer picture.  Given some basic information about the car (weight, gears, drag coefficient, etc.), the power requirements can be calculated (see ref. 2 and Physics101.pdf) and compared to the power available from the engine.  By knowing the power required and the power available we can determine how the car will perform.  During the past five years, several power curves have been measured on the Cam Project Tudor (see Dyno Tests under Model T Heads and Cam Project).  We also ran a dyno test on Steve Coniff’s Montana 500 car.  Most of these tests have been incorporated into a spreadsheet to perform the calculations.  You may download the spreadsheet if you would like to do your own calculations. 

Before we get started, we need to say a few words about dyno tests.  The two basic types of dynos are an engine dyno and a chassis dyno.  An engine dyno normally measures power at the flywheel, but on a Model T it would be at the output of the transmission.  A chassis dyno (see photo) measures the power at the rear wheels.  The two should differ by the drive train friction losses, which are normally approximated by a simple multiplying factor.  It appears that 20 percent is a reasonable estimate for friction losses in a Model T.  We have used two different types of chassis dynos in our testing.  We were told that the two would not produce the same results, but we were not given a satisfactory explanation for the reasons why.  In our calculations, we’ve tried to make the results comparable by using different scaling factors for the two dynos.  The accuracy of the dyno results are questionable at low (less than 700 RPM) and high (greater than 2000 RPM) speeds.  You should take these uncertainties into consideration when evaluating the results presented here. 

Are the Calculations Valid?

First, let’s establish that the calculations produce meaningful results.  We are aware of one road test for a Model T (ref. 3).  The car used was a 1912 roadster with a laden weight of 1875 lbs and a standard 3.64 rear axle.  We’ve made calculations using a drag coefficient of 0.8 and frontal area of 25 sq ft, giving the same drag as a Model A (ref.4).  A value of 0.01 was used for the coefficient of rolling resistance (ref. 2).  The two figures below compare the calculations to two dyno test results:  the 1918 test from Ford and a chassis dyno test of our Tudor in 1999.  At that time, the Tudor had a rebuilt engine with approximately 2000 miles of use.  Other than aluminum pistons and modern valves, it was completely stock.

The first figure is in terms of horsepower while the second shows the same calculations in terms of torque.  The light dashed red lines are the power (or torque) required to climb various grades at a given speed.  Of course, the power required increases with both the grade and speed.  Since the curves are based on rear wheel power, the dyno curves are plotted after adjustment for drive train friction losses.  This adjustment is not necessary for chassis dyno curves.  The curves cross at points where the available power equals the required power, so the car can climb that particular grade at the speed shown on the x axis.  Notice that neither power curve crosses the line for a 10% grade, because there is not sufficient power in high gear.  On a long 10% grade, the car would slow down until you would be forced to shift to a lower gear.  The rate of deceleration is proportional to the difference between the available power and the required power.  Correspondingly, if the car were traveling at 20 mph on a 4% grade, the rate it could accelerate would be proportional to the difference between available and required power.

The road test (ref. 3) listed top speed (0% grade) and speeds on various grades.  Values obtained from the graphs are compared to the road test values in the table below. Considering all the uncertainties, the agreement is excellent.  Incidentally, the top speed for the Tudor was about 44 to 45 mph, which also agrees with the calculations.  Now that we have established the validity of the calculations, let’s see what they can tell us about how we should equip our car.

Which Cam is Best?

Let’s now revisit the question originally posed.  Which of the two camshafts is better?  The graphs below combine the two power and torque curves with the calculations.  In this case, the total weight is 2200 pounds, since that is more representative of a typical Model T with two passengers.  Note that the 325 lb weight difference has decreased the maximum grade from 8.1% to 6.9% based on the 1918 curves.  We all know that a light roadster performs better than a sedan.  

By comparing the results, we can get a good idea of the performance tradeoffs for these two camshafts.  The greater horsepower increases the top speed by about 1.5 mph.  However, due to the reduced torque, the maximum grade in high gear drops from 6.9% to about 5.4%.  The reground cam is a good choice if you are primarily interested in top speed and you encounter few hills when driving.  The reground cam would perform well with a 4:1 rear axle ratio.  We will discuss gears later.

The results above are based on performance curves from a computer simulation.  What about real data?  In 1999, we removed a 290 lift reground camshaft from our Tudor and replaced it with a NOS cam.  The car was dyno tested before and after the swap.  The dyno curves are compared with the calculations in the two figures below.  Again, there was little difference in top speed, but the maximum grade dropped from 8% to 7%.  The feel of the car agreed well with the calculations. The top speed did not change appreciably. The performance was noticeably better with the stock cam, similar to the difference with two passengers rather than four.

We’ve also compared a stock cam to a new 280 lift billet camshaft from Bill Stipe.  These new cams are very close in performance to a stock cam, producing essentially the same peak torque and slightly more peak power. See the Dyno Results for a detailed description of our camshaft testing. 

What About High Speed Gears?

Many Model T owners have installed so called “high speed gears” (3:1 rear axle) thinking that it will have a dramatic effect on their top speed.  Let’s take a look at gear ratios and how they impact performance. The figures below show calculations for a stock Model T with various gear ratios: 3.64:1 (standard), 4:1 (10 tooth pinion), 3:1 and 5.6:1 (3.64 in Ruckstell low).  The graphs show that top speed increases just a bit more than one mph with 3:1 gears, i.e. not much.  However, performance in the hills suffers dramatically.  In high gear, the maximum grade drops from 8% to 6%.  For speeds below 30mph, the acceleration will be much worse with 3:1 gears. A 3:1 rear axle makes sense only if you have auxiliary gears, e.g. Ruckstell, a light car with low wind resistance and an engine which has been modified for more power.  Even with a high compression head, a standard Model T with 3:1 gears gains only about 2 mph top speed, but can barely pull an 8% grade.  If you have a heavy car or live in a hilly area, a 4:1 rear axle makes a lot of sense. Ford knew what he was doing when he selected axle ratios.  

Notice that the curves for different axle ratios resemble the curves for different cams.  Consider the combination of a reground cam and 3:1 rear axle.  In this case, the car can barely pull a 5% grade in high gear.  If you run a reground cam, you can get back much of the lost performance by installing a 4:1 rear axle.  When our Tudor was running the reground cam with 3.64 gears, there were some hills on Talimena Drive that it could not pull in Ruckstell low.

How About a High Compression Head?

High compression heads are also a popular modification.  In 2002, several members of our local club tested four different Model T heads.  At that point in time, the Tudor was running a Stipe 280 cam and an antique Bosch 600 distributor equipped with an electronic ignition module.  We have not done a detailed comparison of stock ignition versus a distributor.  One test made with our Cheapo Dyno showed an 8 to 10% improvement with a distributor, and the car did seem to perform better on Talimena Drive.  

The graphs below compare the calculations with performance curves for a stock head and Z head.  The performance improvement with the Z head is truly wonderful.  The top speed increased about 6 mph and agreed very well with actual high speed runs.  The car can easily maintain the minimum speed on most expressways.  The calculations predict an increase in maximum grade from about 8.5% to 11%.  Talimena Drive is reported to have grades of 13%.  It seems that the steepest grades are encountered when traveling east.  When traveling west with the Z head, the car can pull every hill in high gear except for a short stretch on one hill.  That is pretty good performance for a sedan that weighs over 2400 lb with two passengers.  

Do You Want a Good Touring Model T?  

What have we learned by combining the engine power and torque curves with the calculations for power requirements? In summary, the calculations show that if you are interested primarily in top speed then you should concentrate on peak horsepower.  If you are more interested in being able to pull the hills in high gear, then you should look at peak engine torque, gearing and weight.

Here are some guidelines based on the information presented here and other calculations with the spreadsheet.  Obviously, a good cam and high compression head will help your car’s performance.  Either a new Stipe cam or an original stock cam with good lift will do nicely.  In the case of our Tudor, the cam and head accounts for most of the increase in maximum grade from about 7% to more than 11%, a huge improvement.  

Use gears that are appropriate for your car. As a rule of thumb, for every 10 percent you increase or decrease the rear axle ratio, the maximum percent grade will change by about one percentage point. For the example presented the maximum grade with gears of 4:1, 3:64:1 and 3:1 was 9%, 8% and 6%, respectively.  Consequently, a 3:1 rear axle is not recommended unless you have a lightweight speedster with a good strong engine and a Ruckstell rear axle or other auxiliary transmission.   

Weight is very important.  We estimate that for every 150 to 200 pounds the weight is reduced, the maximum percent grade will increase by about one percentage point.  It doesn’t matter where the weight reduction comes from.  Reduced passenger weight, less luggage, fewer spare parts all work just as well as removing weight from the car.  Ruckstell axles and auxiliary gearboxes are heavy.  A car with a good strong engine should not need one.  Since a new billet cam and Z head costs about half as much as a Ruckstell axle, this approach is also cost effective.  

Many Model T owners use distributors instead of the original vibrator coils.  We have only limited and not very systematic data concerning this modification.  In one test, we found that a distributor may give an 8 to 10 percent increase with peak torque improved more than peak horsepower.  We have not considered alternate carburetion either.  Since we can achieve very good performance without this modification, we see no need for it.  Unlike the cam and head, changes to the ignition and carburetion will alter some of the unique features and the original appearance of the engine.  

Click on the link if you would like a copy of the spreadsheet (VehDynamics.zip) so you can make your own calculations.

References:

1. Sigworth, Larry: “Camshaft Testing – Part 2”, Secrets - The Ford Speed and Sport Magazine, Vol. 9, No. 1, pp 1-15, July, 1999.
2. Bosch Automotive Handbook, 4^th edition, pp 330-334, Robert Bosch GmbH, Stuttgart (1996).
3. Model T Times, No. 314, July-Aug, 2001, p.15, reprinted from The Motor.
4. Cannon, Bill: “Effects of Rear Axle Ratio on Antique Car Performance – Part 1”, Skinned Knuckles, pp. 9-13, Sept. 2000