Port Timing

[marinetech51@earthlink.net]

How is it accomplished and does it increase power at top end or hole shot?
Why isn't it done at the factory to start with?

[Bill Van Steenwyk]

To help you to understand porting in 2-cycle engines, and the various port heights, locations, dimensions, shapes, numbers of same, etc., etc., and what effect all these different types/shapes/heights, in relationship to the piston top and skirt opening and closing of same, and the effect this would all have on hole shot versus top end, would require more time and energy than most of the posters on this site have available.

The ports, exhaust and intake are arranged in the typical 2 cycle engine to give either top end and a very narrow power band, or hole shot/low end performance. There are many good books and even web sites, computer programs, etc., to help you understand the answer to the question you are asking, BUT you must have a working theory of 2-cycle engines first, to be able to understand which arrangement of porting helps in the areas you ask about.

Similar to the question, which comes first the chicken or the egg, you must first understand the organs of the chicken and the reasons for them, to figure out the answer to the question. The same thing is true about porting and what type do you need for top end versus hole shot. You have to understand the basic workings of the engine first. Unless I am mistaken (very possible) the way you ask the question leaves me with doubt you understand basic two cycle operation.

Not trying to be a wise a$$ at all, but based on the way the question is posed, you need more information to be able to understand the answer.

Try some search engines with the phrase "basic two cycle engine operation" or something similar. Once you understand the basics, the rest will be much clearer as to specific porting arrangements and what they do or not do for performance.

[Tomtall]

How is it accomplished and does it increase power at top end or hole shot?
Why isn't it done at the factory to start with?

Bill is correct.

Let me add to what he said however. Port timing is done at the factory to start with. All two stroke engines go thru design mock ups at the factory for port timing configuration depending on the intent of use of that particular engine. All factors of the engine are considered in selecting the best port timing for that engines application.

What I think you ment to ask was how is performance porting and port timing accomplished and why is it not done at the factory. Is this correct?

[marinetech51@earthlink.net]

Bill is correct.

Let me add to what he said however. Port timing is done at the factory to start with. All two stroke engines go thru design mock ups at the factory for port timing configuration depending on the intent of use of that particular engine. All factors of the engine are considered in selecting the best port timing for that engines application.

What I think you ment to ask was how is performance porting and port timing accomplished and why is it not done at the factory. Is this correct?

Yes, that is correct. I understand two stroke operation, I don't have any experience with modification of the ports and thats what I was looking for.

[Bill Van Steenwyk]

If you use the "search" function here on BRF, and type in, for example, porting, high performance porting, exhaust and or intake porting, etc., there have been references to other web sites with computer programs available. There is also a large amount of information just on the Technical Forum on this site.

As Tom so well put it, all engines are designed with ports arranged so as to provide performance in the range that the manufacturer wanted for the purpose that the engine was intended for. You can change that RPM/POWER range by, in some cases, raising the exhaust ports by re-machining them so that the piston uncovers them sooner in the downward stroke after ignition, allowing the exhaust gas to exit the cylinder sooner, enhancing more complete evacuation of the burnt charge so that more of a fresh charge comes in to replace it. Along with modification of the exhaust ports, work on the intakes, or changing/enhancing the "blowdown time" is also sometimes beneficial. A lot depends on what you are starting with, as to how far you can go with modifications. Once the exhaust ports have been changed or modified, then the exhaust system itself may benefit from some changes to take advantage of whatever you have done in changing the exhaust port timing in relationship to other parts of the intake/exhaust tract.

As a further example, it might not do you a lot of good to go hog wild on porting to get more fuel in to the cylinders thru modifying the intake ports if your intake tract from the carb/fuel intake mechanism is small and restricted. You might not benefit until you "unrestricted" the intake tract by less restrictive reed valves, if you are choked down in that area.

Not trying to make this sound more difficult than it is, but usually when you change something on a 2-stroke to gain in one area, you lose in another.

Something else to take into consideration is the gear ratio in the lower unit. If you are handicapped there with prop selection, then a lot of mods on the ports might not gain you much.

What type engine do you plan on trying this type modification on, and what type boat, and what exactly are you trying to accomplish?? You have not provided much specific information as to what you are wanting out of the motor, and it is difficult to know how to advise you, or where to point you to get the info you want.

[zul8tr]

How is it accomplished and does it increase power at top end or hole shot?
Why isn't it done at the factory to start with?

Here is a text that might help. Gordon Jennings has a whole chapter about 2 cycle ports.

http://www.datafan.com/TunersHandbook/2-strokefiltered.html

Not easy to find stuff since the pages are not numbered but Table of Contents and scrolling does it

[Mark75H]

The Gordon Jennings suggestion is very good. That is where I came to understand porting. I had to read and reread it many times before it sunk in. Jennings is a good writer and a ton of info is presented very concisely

[marinetech51@earthlink.net]

Thanks, Sam, Tom, Bill and Zul8tr. This is the info I was looking for.
The powerhead I would like to mod is a 40hp Nissan. I'd like to see
what kind of speed I can get out of a Famous Craft Sport C boat.

[ice_spy]

I know this is out of date, but I was hoping for some results in porting the 2 cyclinder 40hp nissan/ tohastu ?
I too wish to play with a spare block i have.