”Engine Timing -Technical Notes - 2/28/96"
"The Engine Analysis Program"
Everyone who races in any RC hobby has had this discussion with his buddies (“Bench Racing”): What is the perfect timing for
the RC engine? I must admit that I have participated in more than one such session!
What then IS the perfect timing for a 2 cycle engine?
First we must analyze several things which will materially affect the answer to this question:
- 1. What pipe are we using?
- 2. What is the size of the engine?
- 3. What is the maximum RPM we want to operate the engine ?
- 4. What is the load? (Prop, gearing, etc.)
- 5. What percentage nitro are we going to use?
- 6. What compression ratio are we using?
- 7. What is our driving style ?(Tight inside, Fast outside, Middle of the Road)
- 8. What type timing are we talking about? (Exhaust, Boost, Transfers, Rotor or Crank Assembly)
Lets analyze these things in reverse order.
Most often, Exhaust Timing is thought of as the magic number responsible for the big part of performance. This is really not
the case. All of the components are about equal in the total equation. First let me state my PERSONAL opinion about what
the perfect timing is, for those items that I believe are the same for ANY engine.
I believe that the Intakes (Boost and Transfers) operate most efficiently with a total open duration of 124 to 126
degrees. The Very First thing I do to my engines is to set the intakes to this range. I usually do this by cutting
the UNDERSIDE of the liner and dropping the liner until I get 124-126 degrees of duration. An important note
here is that you MUST be very sure that if you drop the liner you DON’T allow the bottom of the piston skirt, at
TDC, to be above the bottom of the exhaust port thus allowing the engine to have “Sub-Piston Induction”. Why
do I think that this is the magic number for the intakes. I have done considerable testing both at the pond and
on the dyno during all times of the year, at all temperatures. Most of you will verify that as the temperature
gets really hot in the summer over 90 degrees, your engine looses a lot of power. This can be almost totally
eliminated, by using lower intake duration’s. After you have dropped the liner to the 124-126 degrees you can
start to think about the exhaust.
Exhaust timing is like a tuned pipe, the higher you make it the higher the RPM band. If you want to use an
engine in the higher RPM ranges you will want to use a higher total exhaust duration. This will be at the
expense of some loss in torque. If you want to use the engine to LUG a high pitch prop or tall gear, you will
want to use lower exhaust timing. I personally think that the way to heat race most effectively is to use the
higher RPM area of the engine. It seems to have a much broader range of power. The people who use the
LUGGER concept have great success at the timed events since they are relying on a very narrow HP band
and can control it for use in the timed event. If you accept my opinion, that it is better to use the higher
RPM area of the engines capability for heat racing, I think that an ideal exhaust open duration can be defined
with some degree of accuracy. I believe that the smaller the engine, the lower the total Exhaust duration
you should use. As an example, I use 183 degrees of exhaust duration on my own heat racing .21 size engines,
185 degrees for my .45 engines, 188 degrees for my .67 engines, and 192 degrees for my .80/.90 engines.
I usually run a smaller area propeller, with high pitch in order to utilize the RPM area of the engines capability.
Rotor or crank timing is one of the easier areas to deal with. Since this is the “MAIN VALVE” of this air pump
(rc engine) it is very important that it seals closed VERY WELL. You have no doubt had an engine that
performed well and would launch almost any propeller and pull very strong off the turns. After running the
engine for a few months, it lost most of its power and launchability. This can be traced “in almost every
example” to a poor sealing rotor assembly or crank assembly. This unit should be surfaced to insure a
perfect seal at least 2 time per season for a rotary valve engine. JFA Custom firstname.lastname@example.org can do this
for you at a VERY reasonable price if you don’t feel confident in doing it. On the front induction engine
(Crank Induction) you should change the bearings AT LEAST 2 times per year (Once before the season
and once in the middle of the season) with the best bearings you can buy (Boca). As for the timing of the
rotor/crank assembly, I believe that the rotor/crank should close 65 degrees after top dead center. I think
that the total open duration should be 215 degrees, thus automatically making the intake open at 210
degrees after top dead center. I believe that this is the most efficient rotor/crank timing for all size, high
RPM 2 cycle engines, using high nitro.
What effect does compression ratio have on the timing you use?
COMPRESSION RATIO’S EFFECT ON TIMING
The higher the compression ratio you use, the more horsepower you are going to generate! This will have
a huge effect on the durability of your engine parts, and the heat the engine generates. You should use
the highest compression ratio you can and still allow the engine to be reliable. It depends to a large
degree on the nitro content that you use as to how high a compression ratio you can run. The higher the
nitro content the more heat which is generated, thus a somewhat lower required compression ratio. If
you try to fool the engine into thinking that it is being cooled very well, you can get away with a higher
compression ratio. I started running a special head which cools the glow plug with water, and am able
to run 1 to 2 point higher compression ratio, thus making maybe 10% more horsepower.
NITRO’S EFFECT ON TIMING & PERFORMANCE
By this time you already know that the higher the nitro content the higher the heat level you will generate,
but also the higher the horsepower! I use high nitro fuel in ALL my boats! Why do I do this? Simple, I can
“throttle down” and effectively have low nitro. BUT, when I need the extra power, it is always ready to serve
me with my throttle. You don’t have to use high throttle all the time you are racing, but as you NEED it!
I suggest you set your model up initially to use high nitro and use the throttle to lower the nitro. What is
high nitro? I use 65% in my .21 engines, 60% in my .45 engines, and 55% in my .67 engines, 50% in
my .80/.90 engines.
THE LOAD - WEIGHT OF THE MODEL
A large consideration in the application of all the data presented here is the basic concept of Physics
that a LIGHTER object is much easier to push than a HEAVIER object! In constructing your model,
I believe that the lighter you can make it the better! Look for boats, cars, planes that are constructed
with integrity, yet are very light. Most of the “HIGH END” competition models will advertise lightness.
These people KNOW the importance of lightness.
THE RPM BAND WE WANT TO OPERATE THE ENGINE IN
After numerous tests on the dynamometer, I have a very good feel for the RPM range which allows
the engine to operate in its most efficient mode. I believe that a .21 engine with a fairly “square” bore
stroke ratio (the bore is almost exactly the same as the stroke measurement) should be run in the
26500 to 30000 RPM range, a .45 24500-28000, a .67 22000-25000, a .80/.90 20000-24500. You will
find that the engines which have a larger bore than stroke will operate in a higher RPM range by about
1000 to 1500 RPM.
WHAT TUNED PIPE ARE WE USING
The tuned pipe is a tremendously important item to the total equation. I will spend a complete technical
paper on this subject sometime in the future. The pipe MUST be integrated into the total equation by
testing on the water, in the air, or on the track unless you have a dynamometer at your disposal. I have
a dynamometer in combination with John Ackerman at JFA custom. We test year round on many items.
One of the first items we tested was tuned pipes. Why, because the largest gain by bolting on a
component is the tuned pipe! In a typical test we would see a variance of 5000+ RPM under load on a
typical engine. This translates to more than you can gain by engine modification! You must get the
proper pipe for your model before starting to modify the engine. What is the best pipe? I won’t state
that in this paper.
As you can see, the timing of an engine takes into account MANY items of importance. There are a
few items which are almost cut and dried, but MOST require some variation, dependent upon model
& operator variables. Your analysis of the items discussed above are what directed me to develop
my Engine Analysis Program. I found the mathematics and analysis cumbersome to the point
of making me dread working on engines. I don’t dread it any longer since I have a tool I use
constantly to analyze data. I might state also, that I found a tool anyone working on engines
MUST HAVE. It is a depth measuring gauge developed and made by Bill McGraw owner of Rossi
Sales of America (901-396-7485) This tool will ACCURATELY measure head clearance and piston depth.