Wednesday, October 29, 2003

Are you Unbalanced?

By Tom Cole

Several years ago when we were developing our crankcase ventilation system for the Tecumseh Star engine, I got some seat-of -the-pants experience with the value and need of crankshaft balancing. I was accustomed to driving a 3” bore, 3” stroke test engine, which had one of our billet crankshafts in it. The 3x3 crank had been balanced, and ran very smooth considering it was producing about twice as much horsepower as the Star. The Star’s crank/rod/piston setup had not been balanced, and the difference took me by surprise. As I made 8000 rpm, my vision was so blurred from the engine’s vibration that I had to slow down to see the turn. My teeth and ribs felt like they were banging together and after my 15 lap stint at this ¼ mile asphalt oval, I was not interested in driving any more that day. My body was directly reporting to me the increased pain and fatigue that an unbalanced crankshaft can do to an engine and driver.
Clarence Clark is a friend of mine and he is what I would call an engine-building guru. For many years, Clarence’s company rebuilt the engines for the world’s largest fleet of racecars, the United Parcel Service. He has since traveled the country doing seminars for rebuilding supply companies like Goodson and Cobra Products. When I told Clarence about my experience, he went over to his tool box and produced a metal “H” which was made out of five, six inch long 3/8” metal pipes and two 3/8” pipe “T’s”. He handed me this contraption and told me to gently hold the cross pipe in one hand like an axle and spin it. Everything was fairly in balance and it spun easily making five or six revolutions. He then removed one of the four, six-inch uprights from the “H” and said “now it’s out of balance, spin it again.” I did and it only made one revolution! When I tried to spin it real hard, it only made two revolutions. This was an example of the need of both Force and Couple or “Dual Plane” balancing of a crankshaft. He told me one of the most interesting things about an out-of-balance crank or cam is that this tendency to stop (or inertia) increases with RPM so it requires more and more horsepower to obtain the same RPM as a balanced setup! It is hard to believe that we spend so much time and money on carburetors, valves, porting, flow and displacement and many of us ignore or are unaware of such a power robbing aspect of an engine.
Force and Couple balancing are technical terms used that really just mean top-to-bottom and side-to-side balance and are commonly referred to as “Dual Plane Balancing.” They are easily illustrated with a little history in tire balancing. Many years ago, when car tires were balanced, the rim and tire assembly was mounted on a shaft and then placed on a frame with the shaft resting in a ball bearing V fixture. The tire assembly would then rotate around until the heavy part came to rest at 6 o’clock. A wheel weight was then placed at 12 o’clock on one side of the tire. You kept adjusting this weight until the tire would not move regardless of how you repositioned it in the V fixture. This process is called, Static, or Force balancing and it is the method that is used on most kart and Jr. Dragster wheels today. Later, an improvement was made in this process by splitting the weight and putting half on the inside of the rim and half on the outside. This was the first form of Couple balancing in the tire industry. As years went by the advent of much wider tires came into being, so the need for more accurate Couple balancing increased because the part of the tire that was out of balance was often further from the centerline of the tire. And since one side of the tire could weigh more than the other, it became necessary to be more precise than to just split the weight in half to achieve optimum balance. We now have electronic tire balancers that spin the tire and wheel and calculate the amount of weight needed to Force balance. Then it calculates what amount goes on the inside of the rim and what goes on the outside. This is Dual Plane balancing of a tire.
The crankshaft of an engine has counterweights to dynamically offset the effect that the movement of the reciprocating mass has on the rotating mass of the crankshaft. The reciprocating mass is a percentage of the total mass of the top half of the connecting rod, the piston, wristpin, rings and circlips. The percentage used comes from a chart, which is calculated from expected RPM and stroke length. The rotating mass is the total mass of the bottom half of the connecting rod, the rod bolts, washers and bearings. To balance a single cylinder crankshaft, a bob weight is attached to the journal of the crankshaft that represents 100 % of the rotating mass and the percentage of the reciprocating mass from the chart. This assembly is then placed on the ball bearing V’s of our Stewart-Warner crankshaft-balancing machine and spun up to the expected operating RPM setting. The balancer can read the change in weight on both sides of the crankshaft and precisely tell the operator the amount of weight that needs to be added or removed from the left or right counterweights to achieve optimum dual plane balancing.
Balancing a crankshaft in itself does not produce more horsepower; it improves output potential by eliminating or greatly reducing a non-productive use of horsepower. It also helps prolong the life of an engine by reducing damaging vibration. In any situation where some grinding of the crankshaft or modification of the piston is permitted, it is a largely untapped source of performance gain.

Thursday, May 8, 2003

Setting Ignition Timing

By Tom Cole
Almost every day someone calls or emails us asking how to set the ignition timing on their engine. It is an important topic because as little as one degree can be the difference between an engine that runs up front and an engine that sputters and pops its way to last place. In this article, I am going to describe what I believe to be the most accurate and reliable method to set the timing on a Briggs and Stratton™ Engine. If you are using an ARC adjustable hub flywheel, begin by setting the hub index mark in the middle of the degree marks on the aluminum body. This will give you the maximum amount of adjustability after you set the timing based on the cam manufacturer’s specifications. The adjustable hub gives you an “at the track” advantage, because it allows you to easily advance or retard the ignition timing to tune for variable conditions.
The first thing you must do to set the timing is to identify the exact position of the flywheel’s magnet in relation to the coil when ignition occurs. To do this right, you need a plain old induction timing light and a car battery. Some folks will tell you about aligning the trailing edge of the magnet with the center of the little button just in front of the left leg of the coil, and for the most part, they are correct. But there is no such button on the Animal coil and factors such as coil gap make this method only a close approximation.

Below is the technique I use to find the exact trigger point using a timing light:

  1. Install only the crank, its bearings and its timing gear in the engine block and put on the side cover.
  2. Install the sheet metal guard that goes behind the flywheel.
  3. Install the flywheel with a standard key on the crank and snug it up with the starter nut. (No need to torque it, you are going to take it back off)
  4. Install the coil with the proper gap. (It will not be removed so tighten it up)
  5. Attach a spark plug to the plug wire and tape it to the block so as to ground it and create a spark.
  6. With the magnet at 12 o’clock under the coil put a white line on the outside rim of the flywheel at about 3 o’clock. This line needs to be plainly visible when looking at the block from the front.
  7. Attach the timing light to the battery and clamp the induction lead on the plug wire. Be careful that all wires are away from the flywheel.
  8. With the timing light pointing at the front of the block, turn the crankshaft clockwise with a drill or starter and you will see the timing strobe light up the white line. You need to spin it faster and more consistently than possible with a pull starter because a magneto has a retarding effect at higher rpm, and you want to compensate.
  9. While the strobe is flashing and the flywheel is spinning, make a white mark on the sheet metal guard or block that aligns with the white mark shown by the timing light on the flywheel. You can now place the magnet exactly where the spark is triggered when and if you ever remove the flywheel. BUT, if you move the coil or the metal guard, you have to start all over again.
  10. Remove the flywheel, side cover and crank, and install the piston, rod, crank, cam, etc (leaving off the cylinder head) getting to the point where you are ready to install the flywheel and set the timing.

You are now ready to set the timing. Truthfully, it is more accurate to set the timing by fixing the piston at a measured distance before it reaches its highest point on the compression stroke. But, although everyone knows the “in the hole” distance for a pure stock setup, (30deg is .2115”) it is difficult to calculate the distance needed by different length rods since the distance traveled by the piston per degree of rotation varies with rod length and/or stroke. You can calculate it, but it just really isn’t worth the effort.

So, since the cam manufacturers generally provide you with a recommended ignition timing expressed in degrees before top dead center (BTDC) of the compression stroke, it is going to be best to set your timing using a degree wheel. (This is a good time to degree your cam too) On to the dreaded degree wheel…
  1. Using coarse grit sand paper, rough up the tapered part of the crank and then make sure it is clean. If you are un-willing to lick it, it isn’t clean enough!
  2. Similarly, rough up and clean the inside of the hole in the flywheel hub.
  3. Set the crank so, according to the degree wheel, you are at the cam manufacturers specified degrees of ignition timing before the piston reaches the highest point of its compression stroke BTDC.
  4. Put a few drops of Loctite™ on the tapered part of the crank and install the flywheel with the white timing lines aligned. DO NOT USE A KEY! Keys do very little to prevent a flywheel from spinning, and they will hinder your accuracy.
  5. Carefully tighten down the starter nut. Check and recheck the timing several times as you tighten to be sure that the white timing lines are still aligned and that the degree wheel is still where it is supposed to be. Then tighten the starter nut, A LOT. You are shooting for tight enough to hold it together, but not so tight as to split the flywheel.
  6. If you have an ARC adjustable hub flywheel, you can fine-tune your ignition timing using a dyno or a stopwatch and a Digatron CHT/Tach at the track.
That’s it! Once you have the timing set and are done with any track adjustments, measure the distance from the deck to the top of the piston (with the white timing lines aligned) and record the measurement. As long as you refresh the engine with the same parts, you can just set it in the hole and go when you refresh. If you re-deck the block, just subtract the amount removed from your recorded depth figure.