How Do You Make Your T Go So Fast?
Part one: Mechanical
Henry Ford said that the top speed of Model T Fords was 45 miles per hour. A lot of them now days are hard pressed to achieve that speed. Sometimes after someone has taken a ride in my car, they will ask me: "How do you make your T go so fast?" My stock, off-the-cuff reply is that there are only two things to make a T go fast - compression and aspiration. This is essentially true, but is an oversimplification. There are really THREE things! The three things are: 1. Mechanical efficiency 2. Thermodynamic efficiency and 3. Volumetric efficiency. This article will deal with the first issue, mechanical efficiency.
What are we talking about when we say mechanical, thermodynamic and volumetric efficiency? Efficiency is getting as much work done with as little energy (or fuel) spent as possible. Does this mean the best gas mileage possible? It can, but what we are looking for in the Montana 500 is the most power possible given the obvious limitations of the Model T motor. Mechanical efficiency (henceforth M.E.) deals with things such as friction, vibration and wind resistance. Thermodynamic efficiency (henceforth T.E.) deals with things that make the bang of the power stroke
stronger. Volumetric efficiency (henceforth V.E.) deals with getting the
biggest and best charge of fuel into the combustion chamber.
What are some ways to maximize M.E.? The number one thing to help here is
to reduce friction whenever and wherever you can. Let's start at the front
and work to the back. Wheels. Bad or overly tight wheel bearings can
cause extra friction. Imbalanced, or under-inflated and poorly aligned
tires use more energy, as do bent rims. Engine. Within the engine
there are many areas where M.E. can be increased. Friction of all moving
parts can be reduced by using the best lubricant possible. Some people use
S.T.P. and such like that to help reduce friction. Having proper
clearances on your bearings, wrist pins and cylinder walls helps. Boring
your engine to the maximum size also increases M.E. Wait a minute! I
thought that increasing the bang of the power stroke (which a large bore does)
fell under the heading of T.E.! Yes, it does. But it also falls
under the heading of M.E. It increases T.E. because it causes the
compression to rise. It increases M.E., because all other things being
equal, a larger piston will apply more force to the crankshaft than a smaller
one. It is sort of like a wheel cylinder, the larger wheel cylinders on
the front wheels apply more pressure to the brake shoes than the smaller rear
cylinders, even though they are both supplied with the same amount of pressure
from the master cylinder. Bigger pistons also slightly decrease M.E due to the
added weight and added surface area. These factors are negligible though, and
more than made up for by the increase to M.E. that bigger pistons cause.
Increasing the stroke of the crankshaft increases M.E. Again it also increases
T.E., because it causes bigger displacement and more compression, but it also
increases M.E. because it increases the rod angularity, which gives you a
mechanical advantage (more leverage from piston to crank pin). This is not
allowed on the Montana 500 though and is merely mentioned for the sake of
illustration. I presented this article to Steve Coniff to check for errors and
omissions. He pointed out that the optimum rod length for a piston engine is
twice the stroke of the crank. Since the crank stroke is four inches on a T, the
theoretical optimum rod length would be eight inches. The stock rod length is
seven inches. The rules allow for increasing the rod length to 7.030".
Reciprocating weight always decreases M.E. Rotating weight decreases M.E. only
on acceleration. Vibration and imbalance always decrease M.E. Misalignment
increases friction therefore decreases M.E. There is one more mechanical
thing that the engine does that I'm not going to go into right now other than to
mention it. It really should be under the heading of M.E., but I'm going to go
into it in depth in the article on V.E. After the
power stroke the engine has a major job yet to perform to which we don't give
much thought. That is, pumping the burnt gasses out of the cylinder. A good deal
of effort is required to do this, and there are ways to make it easier on the
poor engine. Rear end. Poorly set-up rearend gears decrease M.E., as do bad axle
shafts and bad axle bearings. Wind resistance. There isn't much a guy can do for
this other than run a 26-7 body, which is a little more streamlined. It also
helps to use proper 26-7 springs, which have a lower crown. This sets the car
lower for better aerodynamics and helps the ride and rearend alignment too.
Another important consideration is weight. On the level, a heavier car takes
just a bit more energy to propel at any given speed than a lighter one, probably
not a significant amount. Where weight really matters is going up a hill. Of
course going down hill weight helps a little, although you never get back the
energy that you spent going up the hill. Most driving time is spent on fairly
level ground. Therefore I'd say that weight was less important than wind
resistance if you had the choice of trading one for the other.
This article doesn't give a lot of specifics, but it should give some food
for thought. The article in the next newsletter will be about T.E.