In reading some of the forums, it looks to me that most glow engines are made and timed for a certain RPM. Can the crankshaft be modified or one built to change this to increase rpm for a certain prop size for planes? And if this is possible would it be to retard or advance the timing. The next question would be would I have to change compression, nitro or such to also change the time along with it? This would be a general question for any engine.

....The "timing" has to do with the port timing. That is to say....the ports cut into the
cylinder liner. The port cut into the crankshaft, under the carb mount has nothing
really to do with the ultimate rpm of the engine., as long as it is not restrictive.

FBD.

It can be done, but you are far better off buying an engine that is already timed to fit your requirements, be it C/L stunt, pylon racing, sport R/C, combat, or whatever. The crank timing works with the bypass, boost, and exhaust port timing. Then there is compression ratio, plug heat range, ntro %, and exhaust tuning to consider. They all work together and the effects of one change do overlap others, making it even more confusing.
You can experiment if you want to, just don't expect to improve on the designs already out there, especially if you are just trying this for the first time.

I assume you are asking about two strokes. The short answer is no. The crank has little to do with the overall timing, but the piston and cyliner lining is what provides the real timing. You could play with opening up the ports, but it likely wouldn't do much for you. Todays engines will run a range of prop sizes and across a range of RPMs. Few of us fly, or drive with a full open throttle all the time. The engine has to be able to idle down and also has to run well over a wide range of RPMs. The prop/rpm references is what you can expect from the engine.

If it's all out speed you are after, you can go with a tuned pipe to ring out every bit of power your engine has. You can also play with different fuels to gain a bit more. All of this results in an engine that isn't going to last as long.

Now, if you are a skilled machinest, you can make new sleeves with different port configurations and you can play with differen piston skirts and dome shapes. It would be a lot of expense and time for questionable results though.

On the four strokes, you would have to grind different cams. there isn't a way to make fine adjustments to the cam timing, and if you go off by one tooth, the engine runs poorly.

Don

A crankshaft port that closes early will restrict the top RPM's of a engine that is otherwise timed to really spin up. My skyward .40 (picco) is a good example of a engine that was tamed this way for "sport" use.

Speak for yourself

The volume of the boost ports and crank case also play into the equation. The engine book put out by MAN is very comprehensive in explaining how all these systems work with each other. I just finished reading it and while being a pretty decent hand in a machine shop have decided to hold off on whittling away at a motor just yet without fully understanding which mod will give me the biggest bang for my work.

On the tuned pipe, If it was Lets say a os 46 ax what would be gained In average on the rpms from stock to a tuned pipe.

Well, someone was writing something I fully agree:
"There is no replacement for displacement" ... the rest is a kind of self-torture exercise
and hope to do better job than the manufacturer could do. Surely that is possible mostly
on cheap engines but they do not last becuase of using bad materials. Thus it seems
better to buy good engine which fits the needs and forget machining it.

RSO,


The various port timing numbers of an engine determine its behavior.

I.e. longer induction port, intake by-pass(es) and exhaust port timing (more degrees) will allow an engine to make more high RPM HP.
More modest numbers in these ports will make an engine more docile and easier to start, but lower on high-RPM power.

A longer blow-down period; the difference between the exhausts timing and the intake-bypass timing halved, if longer, will allow the engine to get more power boost ftom installing a tuned exhaust system...


Changing any of these numbers can be achieved only by machining the parts... Using a Dremel/other grinder on them, raising/dropping the sleeve, Etc..

The ignition timing in a glow engine is a function of several things; the compression ratio, the percentage of nitro in the fuel, the heat-range of the glow-plug and the load on the engine, are the less immediately changed contributing factors.


With all the above aspects considered given; the immediate control that the user has over ignition timing, is the mixture strength... Yes, the needles are the ignition timing control of the glow engine...

A leaner mixture will ignite faster and more easily than a colder, richer mixture.
Also, the flame propagation in a leaner mixture will be faster than in a richer mixture, so a leaner mixture will attain maximum combustion pressure, sooner than a richer mixture.


So, by turning the needles (both of them, each in its effective range), you are doing something that is akin to turning the distributor, in an older spark-ignition engine.

Closing the needle too far will not really cause the engine to self destruct due directly to a lack of lubrication, but rather from detonation and over-heating, caused by an over-advanced ignition (and maybe even a resultant lack of lubrication - burnt oil isn't a good lube...).

The damages to a lean-run glow engine will be alike to your car's engine after running with the distributor turned too far against its rotor's direction of rotation.

I've measured the crankshaft and port timings on 34 of my engines and most sport type engines have the exhaust opening at about 75 degrees BBDC with the transfer at around the 60 degree BBDC mark (about 15 degrees of blow down). But this is just in general because there are some surprises. The Rossi 45 for instance is actually very mildly timed at 75/66 while an early ringed ST S45 is 80/60. Engines timed for CL stunt are generally around the 70/60 mark which, surprisingly, is what the venerable OS 40/46VF is. One engine I have (actually 2 different versions of the same make) have virtually no blow down period at all yet they're quite powerful.

Crankshaft timings though are all over the place and I haven't been able to find any rhyme or reason with them. Opening times vary between 32 to 50 degrees ABDC and closing from between 30 and 58 degrees ATDC but most have a total open time of about 190 degrees.

I have modified one engine. It was K&B Series 75 35 used for CL Midwest Sport Race. I opened the crank timing to the same numbers as the K&B 5.8, Raised the top of the exhaust ports, Cut away the part of the piston skirt which covered the bottom of the transfer passage at BDC. Using same prop, fuel, etc, my airplane went 10 MPH faster. I am now retired from the engine modification business--quit while I am ahead.

Hey Downunder and Dar!
crankshaft induction timing is very-very important, though tuning is also very difficult. The cylinder porting is the rpm range mainstay, but if the crank induction angle does not match the rpm range, you will have one sick engine bugger at hand. I use carb blowback, visible as a slight fuel mist, to judge if I can let the induction last longer.
The induction range closing can be from 40° ATDC upwards. This is engine design dependant, so no hard figures can be given. If you goof, and increase the angle ATDC too much, low end torque will be gone, and you will end up with a lot of fuel blowback!

Thanks for all the wonderful information and help. I can see this is something that I dont want to get into at this junction in my life as free time is valuable and there is not enough of it for me. Ill spend the extra cash for a engine that just does it to start with.

Hi Pe
That's pretty much what I'd always thought until I started measuring and collecting some figures which is why I said I can't find any rhyme or reason.

As a for instance, compare my Moki M5 with my Enya 60-IIIB. Both are 60's, both baffled piston, both designed for low down torque and both have identical cylinder timing. But the crankshaft timings are radically different. The Moki (and the Magnum GPA 53) has by far the most conservative timing of any of my engines at 45/30 while the Enya is 40/55. The Enya has identical crank timing to my Rossi 45. So there's 2 engines ideally suited for CL stunt yet one has the crank timing of a high speed engine.

Another comparison in the RC sport type where the Rossi can be used as the bench mark at 40/55. The Magnum 46XL is 35/50 (same duration but 5 degrees earlier open/close) but with far more aggressive cylinder timing. The ST G51 though has a very mild crank at 43/40, far less duration than either of the others but it's certainly no slouch.

I'm confused .