The International Cessna^® 170 Association

Every engineering design is a trade off and performance tends to get better with experience. On later models Continental figured out they could get more hp and run cooler by making the change.

I have brought this up before. When i my partner and i purchased our 170 i got into this forum and found out about the different mag timing setting. I adjustedthe mags 2 deg different . The eng run perfect no roughness , but during a mag ck there was a 200 mag drop on one and 50 on the other. I talked to my friend the I/A and we all agreed that was not normal. That one mag was removed ck internal ad found timing to be correct. After all back together same thing. That has been over 200 hr ago and everything is still the same. But engine runs perfect..

Dave,i did encounter the same roughness on a pacer one time. After ck everything from plugs to mags and even a compression ck. An old mechanic said did you ck your primer. I found my primer was by/passing just enough fuel to cause the problem. R/R the 0'ring solved the problem and double ck the primer lock as part of my ck list ..

DAIRYdr wrote:DOES ANYONE KNOW WHY THE C145 IS SET UP TO FIRE RIGHT MAG TOPS &LEFT MAG BOTTOMS?????

The lower plugs are fired 28 degrees before top-dead-center and the upper plugs are fired 26-degrees because..... of flame propagation patterns. Lower plugs foul more readily than upper plugs. Firing them first keeps them cleaner.

Normal mag drop should be not more than 100 rpm each, and not more than 50 difference. (TCM Ovhl Manual and OM)

75 to 80 years ago all they had was low octane; unleaded fuel was the design basis for these early low compression engines. They did not have lead fouling problems when they decided on the mag timing, it was based on minimizing the mag drop when operating on one or the other. On later engines they swapped, each mag fires top on one side bottom plugs on the other, timing is the same for both. The result is cooler operation and a little more hp for the same displacement.

If you advance the timing it tends to run hotter. The burn time is a little longer resulting in more heat transfer to the cylinder wall, thus a hotter running engine.

N1277D wrote:... unleaded fuel was the design basis for these early low compression engines. They did not have lead fouling problems when they decided on the mag timing,.....

Let's be a bit clearer on this. This engine was designed for and it's mfr has always insisted it be run on leaded fuel, and as long as leaded fuel has been used it has caused lead fouling. (Lead is not the only type of fouling either. Significant oil fouling occurs on lower plugs also, even on healthy engines.)
Timing is advanced in order to develop maximum power within the confines of engine design and operating speed. One cannot arbitrarily decide to advance the timing to create more power, or to retard the timing in order to run cooler without a loss of power. Too much spark advance can also damage the engine from excessive cylinder pressures.
Setting magnetos to fire half-bottom and half-top plugs requires that both magnetos be set to the same timing at a loss of power and therefore a slightly cooler running engine. (The one converted from firing only the lower plugs must be retarded. The overall effect is an engine whose combustion event occurs later and therefore marginally cooler with marginally less power.)
Spark plugs are specified to be rotated at regular intervals to equalize wear on either scheme.
Many mechanics feel that two degrees of timing is not worth the trouble and for their own convenience average the timing of the two magnetos by splitting the difference and sharing the top/bottom plugs between the mags. This is very curious to me because offtimes it's these same mechanics who will argue endlessly regarding details of "fine-tuning" for increasing engine-power in other areas, while ignoring the late timing they have chosen in the "sharing" scheme.
These same cylinders are utilized in the O-200 engine which, under certain circumstances, are allowed both magnetos to fire 28 degrees BTC. This is not authorized in C-145/O-300 engines.

It is commonly said here and elsewhere words to the effect that "these are old engine designs" and it is commonly suggested that more "modern" specifications, data, fuels, oils, etc. are "better" or somehow going to improve things. An example of how it can be harmful to accept such comments without scrutiny is TCM MSB 94-8C "Magneto to Engine Timing". Note 5 deals with cylinder design and the various changes that have occured over the decades. In certain circumstances (such as the O-200 event mentioned above) such changes allow advanced magneto timing. BUT WHICH CYLINDERS DO YOU HAVE on your engine that was last overhauled 18 years or more ago? Without ALL the information, you can damage your engine and possibly lose your airplane or worse.

BEWARE of using data that has not been specifically authorized by the manufacturer. Do not make changes or adjustments to your engine except in accordance with the manufacturer's specifications.

Swiss paper work with Timex tolerances & precision; it makes an Engineers day and keeps the regulators happy.