Dear Mr. Millar, PE,
Wow, I can’t believe it! A letter from the “Big Cheese” himself. Or should I say from Le Grande Fromage? Minnesota just hasn’t been the same since you retired to Florida. As I have always said, “Antique boaters never die, they just move to Mt. Dora.” However, it is always a pleasure to see you and your
family every year in Red Wing.
No one knows this octane stuff better than you. I agree with everything you said. You have added a new dimension to the complexities of diagnosing people’s problems without actually being there to go through the multitude of steps to actually identify the problem.
For example, as you well know, a specific symptom like pinging can come as a result from many different problems or issues. You have to work through each cause until you find the specific problem.
You may not know this; when I respond to a specific question, I research the potential symptoms for weeks and sometimes months before I actually reply in my article. In this case, I called Lou Brisity at his Speed Merchant and Transmission Shop in Milwaukee. I asked if it is his practice to shave the block and head a bit to increase compression and performance. Like the good technician he is, and I am sure you will agree, he does not like to screw around with the compression on these older engines. Just as you said, they are very sensitive to minor changes. I was pleased to hear that he shaved both surfaces only about .010, just enough to ensure that there was no warp in either face and there was a good seal to the head gasket. Therefore, I was pretty darn sure that a change in compression ratio had not occurred from the engine rebuilding.
I am however, remiss by not going into detail such as you have to the very common cause of pinging. Your detailed explanation adds a great deal to my last article. Thank you very much for your letter. I am flattered to stand among the motor-knowledgeable elite such as
yourself and Lou Brisity.
PS – Been doing any flying lately?
Dear Doctor Motorhead:
As a charter member of BSLOL (LOL) I am a devoted reader of your “Service Tips” articles. Interestingly enough, Steve’s ACBS number is 1206 and mine is 1207. We put our applications in at the same LOL meeting, but Steve beat me out in the alphabet.
In your recent article commenting on the problems Tommy Bums is having with his engine, I agree with everything you said, but reading between the lines in Tommy’s letter I have a strong suspicion that some other factors than those you mention are involved.
Engines knock for only one reason. The octane number requirement of the engine is higher than the octane number of the fuel being used. In an engine running at normal temperatures two factors control octane number requirement: engine compression ratio which the operator can do nothing about, and ignition timing as you have pointed out which the operator can control. Another important fact is that flat head engines (low or no turbulence combustion chambers) have a very narrow range of knock-limited ignition advance and are also extremely sensitive to relatively minor changes in compression ratio, both of which have a major influence on octane number requirement.
Buried in Tommy’s letter is the fact that he had his engine overhauled by a shop that specializes in high performance engines and takes enormous pride in the quality of the work they do. My guess is that during the overhaul process the shop took a light cut off the deck of Tommy’s engine block and did the same to the mating side of the cylinder head. This is common practice in quality shops to guarantee good gasket seal knowing full well that the result is a small but clearly measurable increase in compression ratio with its subsequent increase in octane number requirement.
In the case of Tommy’s engine a 020” cut off the top of the block and a .020” cut off the mating side of the cylinder head to remove pits and irregularities would result in a compression ratio increase from 7.22 – the
published value for the flat head Hercules based engines (MCL) to 7.64 almost a full half ratio change. For our antique flat head engines this is a major change. Engine builders did not control the compression ratio to the
second decimal place because they wanted to make things tough on the manufacturing line. If this half ratio increase in fact exists, you can no longer run the ignition advance at factory specs and still use regular grade fuel. The engine will knock.
All our antique engines were designed to run on unleaded marine fuel which had a motor octane number (MON)(D-357) in the mid 80s. Today, we use a new designation called octane number index (ONI), which is an average of the motor method and the research method (RON)(D-908) of the SAE standard octane number rating. Unfortunately, there is no direct comparison between octane number index and the motor method octane
number of a given fuel, but statistically the octane number index appears to be two or three octane numbers higher, even though our older marine engines were rated on the motor method.
Fortunately, for those of us with these kinds of problems the fuel sold dockside is usually premium with an ONI of 92 or 93. If Tommy is filling up dockside, there is not much he can do to increase the octane number of the fuel in his tank. If, on the other hand, his boat is on a trailer and he fills the tank with low cost automotive fuel of 87 ONI, he can help himself by switching to premium fuel.
If switching to premium fuel does not eliminate his problem, retarding the basic timing of the distributor by probably no more than three or four degrees will eliminate the ping, and you will never notice the small loss in
performance. Just to make sure that octane number is the problem, have some of his pilot friends get five or ten gallons of IOOLL avgas which should clearly eliminate the knock, verify the problem and not do the engine a bit of harm.The worrisome thing is that running the engine under knocking conditions will sooner or later destroy the pistons by either burning a hole in the top, breaking the upper ring lands or breaking the rings. That usually spells the end of the boating season for the owner.
An alternative way to check whether inadvertently the overhaul shop has raised the compression ratio is to measure compression pressures. Cranking compression pressures should be in the range of 125 psi. If the
pressure in Tommy’s engine is in the 130 or low 140 range, you can bet your bottom dollar the shop did a good clean up job on the deck and cylinder head and he is running an engine with higher compression ratio than specified.
Glad to hear you finally got the “Ark” home to Minnetonka, but as I remember, it sank at the dock not long after being launched, or wasn’t that the “Lana Turner” Riva.
Gordon H. Millar, PE
Editor, The Sheerline