slsxz@cc.usu.edu wrote: I have read some interesting things posted about nitro engines.  Problem is they have left me confused and curious. Let me first explain what I think I read.

: There was an article in which somebody mentioned exhaust port timing. I believe that he said you could increase RPMs by increasing the exhaust timing. I assumed that this was accomplished by grinding with a Dremel tool. I later read an article in which somebody else mentioned shimming the cylinder sleeve. I assumed that this was accomplished by using shim stock to lift the sleeve higher 'out' of the block, effectively increasing exhaust timing from BDC. This is where I began to get confused. By shiming the sleeve, the intake timing would also increase (BDC) and compression would decrease (TDC), assuming the crank and con-rod are left alone.

: Is there someone out there who *REALLY* understands how to modify an engine? I am asking because my stock 4 port engine just does not thrill me anymore. I would like to try modification before I spend a grundle of bucks on a high performance exotic. Mabey one of you knows of a good book that explains in detail the whys and hows of mods.

Ok in general terms...

First power is made by "burning" a fuel/air mixture.  The more mixture you burn the more power you generate. So as you burn more over time you make more power which is like using a leveraged gear ratio and is often not that interesting. What *is* interesting is burning more over the *same* amount of time. Thus we want to pack as much mixture into the cylinder every time as we can or have more "packs" per time.

So any given port is like a pipe of a certain diameter and can only pass so much mixture per unit of time. To get more mixture through we either need more time, a faster passage through the pipe, or a larger pipe.

So here is how timing comes into play: as the engine runs faster there is less and less time to get mixture through the port. So for higher speed motors we increase port open duration timing. This is not free however as, especially with piston ported 2-cycles, slower speed performance suffers since there gets to be enough time for the mixture to slow, stop, and finally reverse direction and exit the cylinder (not what we want...).

So now for another potential parameter: faster passage through the pipe. As it works out the speed of mixture passage is governed by relative pressure differential. More pressure on one side (eg: supercharging) and/or more vacuum on the other side equates to a higher flow rate. There is also a pressure loss affect dependent on the amount of turbulence. These pressure affects are most noticeable on the intake where there is only atmospheric pressure to push mixture through the pipe. The relatively high exhaust pressure minimizes this affect. This is why it is almost criminal what engine manufacturers do to the intake manifold on 4-cycle model engines - the right angle turn is a real performance killer.

The last listed parameters of pipe size is pretty obvious. This does have limits however. If we increase the carburetor size too much we reduce vacuum and kill fuel draw. There is also only so much head to fill with valves in a 4-cycle (which is why 4-valve heads are desireable) or so much cylinder in a 2-cycle (or crankshaft/bearing).

Now for 2-cycle engines there is another critical factor and that is scavenging. What happens with scavenging is that we rely on the incoming mixture to blow out the old burnt mixture. This happens based on A) opening the exhaust some amount of time before the transfer ports and B) very carefully controlling the direction of flow out of the transfer ports. In todays schnuerle engines there is precise machining of the transfer ports to achieve optimum cavenging. It is not as simple as just hogging out the port with the closest Dremel or jewelers file... in fact this is actually a good way to *decrease* engine performance.

These basics all get complicated further by the fact that in a 2-cycle there are at least 3 different parts that have to work together overall the intake, the transfer, and the exhaust. To see a real benefit from modifying one of these you must also modify the others.

The point of this long exposition is that it is very complicated to modify an engine to improve it's performance. For the layman there should be an expectation that a couple of engines may very well be ruined before an improvement is arrived at. Usually an individual is better off just buying a different engine. Unless you want to get into engines as a hobby too (:

Now one last contradiction... depending on the quality of the original manufacture you may be able to improve things by doing what is called "blueprinting". All manufacturing is never exact but has certain tolerances plus or minus. Blueprinting is going through and matching the plus or the minus to be closer to even. For example if there are casting webs left in ports these can/should be cleaned out. Or if the top of the piston is *below* the bottom of the exhaust port at BDC the port can be cut on the bottom to match. And so forth. This is how the experts often get better results from the same engine than everybody else.

Please! questions, suggestions, etc. to the author of this document: Steve Jahr