The Stovebolt Site Forums The Shop Area Engine Shop General Question Y? BTDC

What is the purpose of setting the timing x number of degrees Before Top Dead Center?

What engine/year?

"timing x degrees BTDC"
"timing times degrees BTDC"

Are you asking a general question about timing an engine?

Because you want the engine to fire the plugs at exactly the sweet spot for the particular engine. Influences that affect timing are camshaft profile, compression ratio, type of fuel used and so on. Ignition lead is required in most case because the firing event much occur right before as the cylinder reaches top dead center, so the energy produced during the firing event will push the piston down the cylinder with the most power. Because there is time involved for the fuel to ignite and burn it must happen in advance of the piston reaching top dead center. Some slow cranking engines like the 6 volt stovebolts could have timing specs at top dead center or after top dead center, but as soon as the engine starts mechanical advance is built in to advance the timing at idle speed.

After ignition, the flame front needs time to develop and propagate in order to create the pressure to push the piston down. The time this takes is accounted for by how much spark advance is needed (expressed in degrees BTDC) so the 'explosion' occurs as the piston turns around and heads down the cylinder. As mentioned above, the engine parameters determine the optimum advance, and as the motor spins faster more advance is needed. Thats why there is an RPM dependent mechanical/vacuum advance as well.

What engine, Tom?

sstock's reply is excellent, but it should be noted that the sweet spot (timing mark) is not necessarily the the same for same engine in different years.

And, the "sweet" spot is not the same every year for the same engine (at least for Stovebolt six engines: 216, 235, 261).

And, using an original flywheel for the engine might show "THE sweet spot", but you often cannot be sure that the flywheel that you have is the original flywheel that came with the engine to which it is now attached.

Is that confusing enough?

As explained if fired at TDC the piston would be a ways down the cylinder before maximum combustion pressure was reached and one needs max pressure at or near TDC to get the most power out of an engine.

Makes sense. So, this tells me that if these 60-70 and 80 year old engines have been moved around and flywheels changed out perhaps (as they say up here in Maine - "hard tellin not knowin") , that timing these old engines may best be done using a test other than with timing marks? Timing marks on a flywheel will get the engine close, and if the same flywheel almost right on or on the sweet spot.

Generally speaking, (assuming that the engine has been through several owners and the history is unknown and flywheels changed out (example: finding a 12V flywheel on what should have been a 6V engine)) what would be the best tool and test to determine optimum timing and firing?

If it fires too late, the mixture is still burned, but if the piston is not in the right position of mechanical advantage (leverage) more of this energy is not converted to cylinder pressure (to rotate the crankshaft) but instead increases engine temperature.

Just to confuse things a bit more, factors like fuel mixture, combustion chamber design, the offset of the piston pin, and the length of the connecting rod can also have an effect on where the ignition timing needs to be set. In general, the flame front that travels across the combustion chamber needs to meet the top of the piston just as the crankshaft rocks over center and the piston starts down the cylinder. Since the flame speed doesn't change much, but piston speed does, the spark must happen sooner as engine speed increases. That's why most engines need 35-37 degrees of spark advance at cruise speed, and only 0-10 degrees at idle. Then there's vacuum advance- - - -the spark can be advanced another 8-10 crankshaft degrees at light throttle cruise for better fuel economy. This advance goes away as soon as the throttle is opened for hill pulling, towing, or passing, leaving only initial and centrifugal advance when the engine is working hard.

After 40-something years of doing dyno runs, I've discovered that even ONE degree of over-advance once maximum torque is achieved at full throttle and heavy load robs more power than 5 degrees of spark retard from the ideal setting. Always err on the side of "not enough" spark advance- - - - -firing the spark plugs too early is trying to run the engine backwards! By the time you can hear a "ping" on an uphill pull, you're WAY over-advanced on the spark!

Tom, the best (only) tool to determine optimum spark (and/or valve timing) on any engine is a dyno. Unfortunately, very few people have access to one. I'm thinking about equipping a trailer with a bellhousing dyno and the equipment to make it portable and attending a few events like Homecoming or the KC gathering with it. Of course, a chassis dyno would be the best setup, but a portable one of them gets into the $100K+ range!
Jerry

Good thoughts. Jerry: You should build the trailer, and travel the Stovebolt world with your Dynotrailer and help neophytes like me understand mechanics. I barely understand how bearings work .

I appreciate this discussion - it is helping me understand why and how things work... If I understand why, I can anticipate or better problem solve how to fix issues that surface.

Jerry, if we do not have a dyno, what is the best way to time a standard-equipment 216/235/261 (with or without an original flywheel)?

And then there is the manifold vacuum vs ported vacuum debate. DOH!

A dyno may not be as expensive as Jerry may think. A good friend of mine whom I roomed with while we attended school is an engine inventor of sorts. He has patented his own internal combustion engine based off a Wankel. Anyway, he has either purchased or invented a portable dyno that somehow used water as the ‘friction’ unit. I have no knowledge on just how it works. He has offered to bring it by my friends engine building garages to test their motors. I understand it’s on a trailer.

"Manifold VS Ported"- - - -no debate at all unless someone is totally ignorant of how the system was designed, or chooses to willfully ignore facts.

Tim- - - -the simplest way to tinker with timing is to first establish a true TDC reference by using a "positive stop" setup to restrict the piston travel just before it reaches the top of the stroke in both directions and measure exactly halfway between those two points. Then use a delay flash timing light to set various amounts of centrifugal advance followed by a road test and known load like a measured uphill pull where the engine can be run fill throttle for half a mile or so and timed with a stopwatch. There will be a "shortest time" run that will indicate maximum torque. That should translate to the optimum timing setting. Just be sure other factors like ambient temperature, humidity, the phase of the moon, and the driver's jockey shorts size don't change. Tinkering with timing, fuel mixture, and valve timing is at least as much alchemy as it is science! Good luck!
Jerry

I have a "water brake" dyno that was made by a company called "Go-Power" in the 1970's. Fully equipped with all the bells and whistles and accessories back then, it cost about $5,000.00. I've had engines of a little over 700 HP hooked up to it. A Sun chassis dyno cost a little over $35,000.00 at the same basic point in time.
Jerry

Dynos come in multiple flavors. The "water brake" dyno that Jerry has is a style that uses an impeller to absorb the power as water is added into the case. The reaction force (resisting the case of the dyno turning) and the engine RPM are used to calculate torque and horsepower. The same concept is used for a "friction dyno" (aka Prony Brake), basically a brake shoe that's applied to a flywheel, and the force developed measured at the end of an arm. Force x distance = torque. Add in rotational speed, and you have horsepower. Chassis dynos use a big drum that's driven by the wheels, but the concept is the same - measure torque and calculate horsepower. Back in college in the late 60's we ran lab tests on various engines using a couple types of dynos, one being a water brake type.

Clayton chassis dynos used a big electric generator driven by the wheel rollers and a bunch of resistive strips to load the generator down. By passing the current that the generator was producing through more and more circuits, more power was consumed from the vehicle. When the engine was held to a specific speed at full throttle, a calculation was made of how many watts were being consumed and that figure was converted into a horsepower number.
Jerry

Commando it is then.

I usually went to the race track with at least two, or sometimes three sets of clean spark plugs. After practice laps, we would check the color of the plugs and adjust the fuel mixture accordingly. If a second mixture change was needed after more laps, the third set of plugs were used for the night's racing. During the week, all the plugs would get glass bead blasted and used again for the next night of racing. With a few sets of AC commercial plugs with the heavy duty electrodes, the same plugs would usually last half a season or more.

Jerry