Understanding Acceleration - Torque

In Part II, we talked about RPM in the horsepower equation.

Looking at the torque factor, we see that the following methods can be used to increase torque in a normally aspirated engine:

• Larger engine size
• Increasing stroke
• Increased volumetric efficiency
• Higher compression

Larger Engine Size
I hate to sound colloquial but the old racer's adage that "there's no substitute for cubic inches" couldn't be truer.
We have stated before that torque is directly related to cubic inches. Whatever we've increased the size of the engine by, we can count on for our torque gain. Say we have a 3 liter engine and we up the displacement to 3.5 liters, the torque will also increase by approximately 15%.

Increased Stroke
I think this is one of the most overlooked methods for increasing torque. By increasing the stroke, we add more leverage to the crankshaft. Generally speaking, if we go from an 80 to 86 mm stroke, we have added roughly 7.5% more leverage to the crank and torque goes up proportionately. The only problem with longer strokes is that piston speed will be equally increased and engine vibration can increase. Both these effects can be solved with lighter piston rings and reduced reciprocating mass (lighter piston and rod assembly).

Increasing Volumetric Efficiency
Volumetric efficiency is a measure of the percentage of the cylinder that fills during the intake cycle. The key here is to get as much mixture as possible past the intake valve while it's open. There are three ways to do this;

• Camming
  • increasing valve lift
  • increasing duration (how long the valve stays open)
• Porting
  • increasing the air flow at the port and streamline air flow around the valve.
    

By reading between the charts below, you can get an idea as to just how much air an engine can take in during the intake cycle. Follow these steps in this example:
d. Reading the Graphs
i. Look on Graph A - Cam Profile of the Intake Cycle
See that at .200 inches of valve lift, a stock cam is at 42° of crankshaft rotation.
ii. Then look on Table B - Intake Flow
See that at .200 inches of valve lift, the airflow on a stock head will be at 74.5 CFM (cubic feet per minute).
iii. Then look on Graph C- Air Flow During Intake Cycle
Notice that the vertical axis now reads in CFM. At 42° crankshaft rotation the valve is at .200 lift and the head is flowing 74.5 CFM.
iv. We extrapolate the .200 from Graph A & Table
v. Graph C gives meaning to Graph A and Table B
in that it shows just how significantly air intake is increased into the engine by a well-ported head. The effective power increase of a correctly ported head is much like that of a high lift cam.

Higher Compression
With the quality of todays pump gas, a compression ratio of 9:0:1 is about the maximum safe upper limit using BMW factory cast pistons. At 9.2:1 to 9.3:1 you'll start having trouble with 92-93 octane gas and at 9.5:1 you'll be buying your gas at the airport or needing octane booster. By using 8.0:1 compression ratio as a base line, here's the kind of power increase can you expect to gain by altering compression ratio.

Captain Says
Forced Induction
The most effective ways to produce large torque gains are through turbo charging or supercharging. Turbo charging rates at about 90% - 95% efficient whereas supercharging is only around 60% - 65% efficient.

Turbo
Turbo engines have somewhat of a Jeckle & Hyde personality. Any time a turbo is below boost (there is manifold vacuum) the engine will put out less torque than a normally aspirated engine, so the engine will feel a bit sluggish. Once the rpm's are high enough, (usually 3,000 - 4,500 rpms) with as little as 7.5 lbs. (1/2 bar) boost, cylinders fill will be roughly 50% greater and torque will instantly jump up about 50%. Nothing can put out torque like a turbo engine.

Supercharging
Supercharging, since it is mechanically driven off the crankshaft, produces boost as RPM goes up. This has a more normal feeling because low RPM torque is up and boost is more gradual. The big draw back is the sacrifice of 35% - 40% of the potential power gain to drive the supercharger and this means a lot more mechanical stress is put on the engine. Also, superchargers are rather noisy.
For me, I'd go with the split personality and higher efficiency of the turbo.