Thursday, September 8, 2016

Porting Chainsaw Cylinders

Porting a Chainsaw Cylinder is "Cool"

When it comes to porting a saw, there's a lot to know. First of all, it's important to realize the differentiation between porting work saws and ones for racing. You port a chainsaw cylinder for racing differently than you port a chainsaw cylinder for regular work saws.

Regardless, the idea is to get more air/fuel into the engine.

The reason for porting a cylinder of a chainsaw for a work scenario is to
gain power so that you can lean on the saw more, and hopefully cut denser wood, quicker. The byproduct is that the increased fuel/oil mixture passing through the engine, and increased exhaust cause the engine of a work saw to run cooler. That's right, ported chainsaws used for regular work, should actually run cooler. Not to mention that a good worksaw port job ads 25-40% to the cut speed without sacrificing engine life. Read about porting a muffler for increased exhaust here. (Semi-polishing the exhaust is an easy way to get some cheap power.) The goal is to get more air and fuel moving through the engine. A moderate level of porting can help a saw run cooler, and give one a faster throttle response.

For chainsaw competitions, cylinders are ported much more aggressively, and engine durability is sacrificed to gain 40% or more speed.

How to Port a Chainsaw Cylinder

The easiest port job is just to remove any casting flaws from any factory ports with a file. You see, factory cylinders are made by melted aluminum poured into molds. For a glimpse of how small engine cylinders are made, go to cross performance cylinder website. Jugs can have flaws that are overlooked. A file is easier to use in these tight spaces than a dremel. Later, you can try expanding port holes for greater air and fuel flow.

After removing a cylinder, you can look inside the jug through all of the openings (ports). These disclose hints about ways to get more air/fuel into the engine.

The next simplest step would be to take the oval shaped exhaust port and make it more square by only expanding it by adding corners. The corners do need to be rounded and its edges need a symmetrical slope to match. Push the square profile all the way out to the muffler can, and open up the muffler to match the port as closely as possible. After expanding the exhaust port, you'll want to tune the muffler some.

Next you could consider that the carburetor breathes air into the crankcase where air gets forced up through the pistons and combustion area. More porting ideas.

Caution When Porting Small Engine Cylinders

If you take too much metal from the bottom of the exhaust port, then the piston can expose the exhaust port to the crankcase when it travels to top dead center. That will allow the fuel/air mixture to vent into the exhaust and not into the combustion area though the transfer ports. That's bad.

Same goes for other openings. Don't cause air leaks by porting the cylinder holes too much.

Another reason not to port too much is that if you open a port too wide, your piston ring can go into the port and break.

Conversation About Porting Chainsaw Cylinders

I recently had a conversation with a reputed chainsaw mod guru about porting a chainsaw. Maybe you've heard of @Mastermind on Randy does loads of mods for racers and workers all around. He's a busy monkey, and his saws are kind of a big thing.

Mastermind Worksaws will port a cylinder for your chainsaw

I was wondering why people did chainsaw modifications instead of just buying another saw with bigger displacement.

@Excanuck: What kinds of performance boosts do your customers ask for?
Are they looking for faster Cutting speed?
Ability to cut through denser wood?

@Mastermind: The people I do work for are looking for more power along with reliability. Faster? Maybe......but they mostly want to be able to run a more aggressive chain, and/or lean on it harder.

@Excanuck: Now, when I think about more power, I think about cars. Souped up cars are just not good on gas. What about souped up chainsaws? Will chainsaw mods hinder the gas consumption?

@Mastermind: In some cases fuel economy is improved.....but the opposite is true on some saws. No pat answer.

@Excanuck: Where does porting fall in the steps one should take to increase all of the above? Should they port the cylinder before a muffler mod, or something else, or a better chain?

@Mastermind: Modifying an engine is not any one step. It's a combination of several. Machine work to improve squish velocity and increase compression, port work, timing tweaks, muffler work, and in some cases carb mods and air filter changes. The chain is most important in all this. No matter how strong the saw is, if the chain sucks, you've got nothing impressive.

@Excanuck: How long does it take to port one saw?

@Mastermind: I spend a day on each saw normally.

@Excanuck: So I'm picturing Randy standing over a cylinder, gouging away at its innards for a whole day. Removing bits at a time, reassembling the engine, and testing it in the saw. Repeatedly. Yeah, I can see that taking even more than a day. Is it magic? Is porting a cylinder common practice with standard steps? What are the steps in porting?

@Mastermind: I've posted a lot of threads..... (Randy loves the ellipses)
Read the Steps to Porting a Cylinder on

@Excanuck: What measurements are documented to prove successful enhancements?

@Mastermind: We've used an increase in cut speed mostly, but there are dyno runs out there too. Here's an example of a dyno test after work done on a chainsaw.

@Excanuck: Back to my introduction to this interview. I was wondering why people did chainsaw modifications instead of just buying another saw with bigger displacement. Is it cost effective to port a saw instead of going and buying another one that has a bigger cc engine?

@Mastermind: Yes. Bigger displacement saws are heavy, and in many cases much slower. They are build to work hour after hour without overheating, but speed is not that important to the manufacturers. It's about pulling a long bar, and living through it day after day. If I can make a 70cc saw 40% faster......

Hope this helps. If it doesn't, join for all your small engine repair questions.

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