blw

My Feedback: (3)

Cam wear

The engine’s power is enhanced by the maximum open duration, meaning the least valve clearance we can use. This can affect the idle, but normally the effect is minor at most.

Wear? Let’s suppose you have a landing strip with a 20 foot hill right in the middle. On take off, with the plane accelerating along the ground it will roll up the hill with no problem, and if not yet to flying speed will roll smoothly down the back side. This can be likened to the tappets with minimum clearance. When you land, if you touch down on the flat before the hill you still have no problem. On the other hand, if you come in at a four foot altitude your touch down on the slope of the hill will test your landing gear at best, at ten feet altitude you will probably have to get out your glue before flying again. This is an analogue to loose valve clearance, and these early cam grinds work best with no more than ½ thousandth inch clearance.

Now let’s suppose someone has built up a four foot rise from the flat part, with a few feet of level runway between the four foot rise and the rest of the upslope of the hill. Now, if you come in with the four feet altitude you’ll touch on that short level area, and again roll gently up the hill. This is like the later cams that have a quieting ramp, and should be set at 0.04 mm clearance.

All current Saito cams have quieting ramps. Sad to say, the only way to tell the old and new grinds apart is to degree the cam lift, the ramp will show in the profile graph.

Now we’ll forget about airstrips with hills in the middle, and get to the cams and valve gear.

With the clearance set to the minimum the tappets are accelerated smoothly up the ramp, with minimum forces on the face of the tappet. With looser clearance the tappet will be hitting the side of the lobe, with much higher contact pressure. Granted, the tappet may be touching the base circle of the cam, but the clearance in the valve train still has to be taken up before the valve opens, any excess clearance will cause much higher contact pressure when the valve does start opening. This is like landing on the up slope of our hill.

The problem is made worse by the increasing use of pure synthetic oils, their film strength is not sufficient to prevent penetration of the oil film, an amount metal is scoured from the face of the cam lobe with every operating cycle. The end result is large pits in the surface of the cam. Castor oil has a much higher film strength, using castor oil (or a castor blend) will decrease the rate of damage, but it will still happen over a period of time.
So, keep the valves set at 0.04 mm for best power and engine life.


Cylinder color and intake stacks:

The dark colors radiate heat better than lighter colors, the best for radiation is a flat black. Since a flat finish is difficult to maintain, and also has to be thicker than a gloss coat, I just use the shiny paint. Not a great difference between the matte and gloss finishes in radiation anyway.

The intake stacks help a lot, but not in power. I'm sure you've noticed the drop;lets of fuel shooting back out of the carb when the engine is running - the stack keeps them in the air flow of the carb, they get drawn back in, and used by the engine.

This gives two immediate benefits. One, the inside of your engine compartment doesn't get all that raw fuel sprayed around. And Two, you can generally lean the needles a bit, giving better fuel economy. As much as 15-20% longer flying time on the same amount of fuel. Really.

A few months ago we had a thread in Glow Engines about stacks, many posts told of people having the same result.

A third benefit, if you like, is being able to mount a Bru-Line air filter on the engine.

All my Saito engines in service have stacks with the air filters mounted.

Would you like a 20 minute flight from 14 ounces with a 120 engine? Truly, I don't know if I could make 20 minutes, but with 14 ounces my UltraStick has fuel left after 15.