What an airflter can be doing

[drf255]

As many of you know, we got a chance to run some of our saws on the Dyno that Malk315 and Wcorey created.

My curiosity was more about what made a gain or loss than if my saw had more HP than others.

I had a big surprise on my MS360, my latest ported saw.

 

[drf255]

First up was my 036. The first saw I ported.

Flat transfers with maybe 1/2* higher opening on the intake side.

This saw had one of the coveted old style OEM metal mesh aitfilter on it. Almost no restriction to airflow, you can breathe air from your mouth through it with no difficulty.

I was pretty happy with the numbers. The saw has always been a bit of a bruiser. Always strong.


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[drf255]

Next up, my MS360.

On this saw, I raised the uppers but copied the factory angle on them. The factory actually sets the transfers sloping up 4* towards the intake.

I put a new OEM flocked air filter on the saw. I noticed it was tough to breath air though, but I figured that the surface area would make up for the difference. I was a bit worried when it made 1-1.5hp less than my 036.

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[drf255]

That night, the filter dawned on me. The next day we pulled the filter and the saw did lean out. The Dyno results were quite suprising

It was lean, but it made more power than some of the big dogs there. The piston is just fine. The saw revved to 16k on the runs.


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[drf255]

This saw will now get a custom air filter with a 100 micron screen. No metal 360 filters are available to the best of my knowledge.

 

[pioneerguy600]

Air/fuel mix in, exhaust out makes the most power, you can mess around with the port timing numbers but you have to get the fuel in and out to get the most out of the engines capability.

 

[Shagbark]

Good information! Are there any opinions on how large an airborne particle a two-stroke engine can ingest on a regular basis and not adversely affect the designed service life of the engine? (100 microns is about .004" or about 1/2 the diameter of a human hair)

 

[Chris-PA]

The next day we pulled the filter and the saw did lean out.

Not arguing with the test results, but this conclusion is backwards. If less filter restriction increases the air flow, it will get richer, not leaner. Was it re-tuned for the run without the filter?

I've been thinking about changing the filter on one of my ported Poulans, as I think the foam is pretty restrictive.

EDIT: Thinking about it further, if the filter was so restrictive it was acting as a choke then removing it might indeed be leaning the mixture. And I suspect it was, as the fist plot of the ported engine with the flocked filter looks kind of strange. It has no peak, rather just progressively drops off from lo rpm.

 

[drf255]

Didn't retune.

To some degree and regardless of tune, the results are suprising. At least in my opinion.

From 5.5 HP to 7.5 and peaks hitting 8 are very hard to believe.

I'm not arguing true horsepower here. Maybe me saw was only making 3hp. But a 40-50% increase without the airfilter is still unbelievable.

I don't think the change in tune alone would make that much change.

 

[Chris-PA]

I don't know anything about these saws - does it have a conventional choke plate in the carb, or one of those chokes that is integrated into the filter?

 

[Mad Professor]

I'll still run a flocked over a mesh as the mesh lets lots of fines in which is not good.

If it runs 1/3 better but lasts less than 1/2 as long..........

I'd imagine a big boxy efficient filter would be the best, I've seen KNs on some modded saws but exposed with no cover. Too bad OEM manufacturers don't address this

 

[drf255]

I don't know anything about these saws - does it have a conventional choke plate in the carb, or one of those chokes that is integrated into the filter?

Choke in carb Chris

 

[Chris-PA]

Choke in carb Chris

It must be a seriously blocked up filter. Do you see what I meant about the shape of the plot with the flocked filter? Even though the rpm scale is not the same, looking just at 8000rpm and up it's pretty much peaked at the bottom end of the plot and just falls off from there - and at 8k it's a whole hp lower. But the no filter plot rises another hp beyond that and holds it to 13.5k.

A minor restriction would have reduced the air flow a bit, which would have leaned it out some and also reduced top end power. Removing the filter would increase the air flow, which would make the mixture richer (like lifting in the cut causes 4-stroking). A major restriction would be like putting on the choke, where now the engine is sucking fuel directly out of the jet, which is not the mode a carb normally runs in.

 

[tacomatrd98]

It must be a seriously blocked up filter. Do you see what I meant about the shape of the plot with the flocked filter? Even though the rpm scale is not the same, looking just at 8000rpm and up it's pretty much peaked at the bottom end of the plot and just falls off from there - and at 8k it's a whole hp lower. But the no filter plot rises another hp beyond that and holds it to 13.5k.

A minor restriction would have reduced the air flow a bit, which would have leaned it out some and also reduced top end power. Removing the filter would increase the air flow, which would make the mixture richer (like lifting in the cut causes 4-stroking). A major restriction would be like putting on the choke, where now the engine is sucking fuel directly out of the jet, which is not the mode a carb normally runs in.

I don't pretend to be a carb guru but, removing a restrictive filter most definitely leans out the saw. Think about it this way, The carb is tuned to deliver a set amount of fuel at the maximum airflow available at WOT. If you remove the filter or change to a freer flowing unit, you take away an air restriction, increasing available air. If you do not retune the carb accordingly, it becomes a lean mixture and rpms will go up as the carb is only capable of providing a set amount of fuel. Saw carbs have a very narrow window where they're happy to start with so making a significant change on the amount of airflow will make a pretty noticeable difference in the way it runs.

I will agree that more airflow through the carb will pull more fuel, that is the basic concept behind how a carb functions. However, it can only provide as much fuel as the carb is set to provide at a specific amount of airflow.

 

[Chris-PA]

I don't pretend to be a carb guru but, removing a restrictive filter most definitely leans out the saw. Think about it this way, The carb is tuned to deliver a set amount of fuel at the maximum airflow available at WOT. If you remove the filter or change to a freer flowing unit, you take away an air restriction, increasing available air. If you do not retune the carb accordingly, it becomes a lean mixture and rpms will go up as the carb is only capable of providing a set amount of fuel. Saw carbs have a very narrow window where they're happy to start with so making a significant change on the amount of airflow will make a pretty noticeable difference in the way it runs.

I will agree that more airflow through the carb will pull more fuel, that is the basic concept behind how a carb functions. However, it can only provide as much fuel as the carb is set to provide at a specific amount of airflow.

That's not how they work though. If it were a normal carb that could hold a constant fuel/air mixture then an increase in air flow would pick up proportionately more fuel, so a 20% increase in airflow would pick up 20% more fuel and the mixture would stay constant, not lean out. But in these carbs a 20% increase in air flow will pick up much, much more than 20% more fuel (this is why your saw 4-strokes when you lift).

So it would be reasonable to expect that maybe a flocked filter would be a little restrictive compared to none at all, and that no filter would increase the air flow some (20%? 30%?). This should both make the mixture richer as well as allowing the engine to breath better - with a re-tune it would make much more power. But here it leaned the thing out and made a lot more power - the flocked filter must have been acting like a choke, which caused the H to be set lean to try to compensate. Then with the restriction removed, but not re-tuned, it's no longer on partial choke and it's quite lean (at 16k) and breathing well. Maybe not good to run it that way too long though.

The root issue may be that the porting was very effective, and that the flocked filter is no longer able to flow enough - it might be OK on the stock saw.

 

[drf255]

The flocking got cut out of the filter and metal mesh was added last night. Ugly but effective.

My only regret on the saw was not swapping the 036 filter onto the saw. It would've fit sans cover.

There was a lot of spitback that was not contained by the filter when we popped it. It's conceivable that more got sucked back in and richened it up with the filter connected.

I believe the entire episode just shows how a low restriction air filter can really wake the right engine up.

If my saw really hits 8's with a metal filter, and if the hp reading are real, I'm making ported 066 numbers with a 62cc saw.

 

[Chris-PA]

The flocking got cut out of the filter and metal mesh was added last night. Ugly but effective.

My only regret on the saw was not swapping the 036 filter onto the saw. It would've fit sans cover.

There was a lot of spitback that was not contained by the filter when we popped it. It's conceivable that more got sucked back in and richened it up with the filter connected.

I believe the entire episode just shows how a low restriction air filter can really wake the right engine up.

If my saw really hits 8's with a metal filter, and if the hp reading are real, I'm making ported 066 numbers with a 62cc saw.

Or the spit back was just keeping the filter wet and making it more restrictive?

To compare to another saw you'd have to test it on the same dyno, unless someone can do an absolute calibration of the dyno (which would probably be a bit difficult). Otherwise the dyno will be more useful for doing relative measurements.

 

[Mad Professor]

Should do a couple of runs with a dirty flocked filter.

I bet it would make a few sawyers keep things clean and/or bring a clean spare filter into the woods

 

[pioneerguy600]

The flocking got cut out of the filter and metal mesh was added last night. Ugly but effective.

My only regret on the saw was not swapping the 036 filter onto the saw. It would've fit sans cover.

There was a lot of spitback that was not contained by the filter when we popped it. It's conceivable that more got sucked back in and richened it up with the filter connected.

I believe the entire episode just shows how a low restriction air filter can really wake the right engine up.

If my saw really hits 8's with a metal filter, and if the hp reading are real, I'm making ported 066 numbers with a 62cc saw.

Just what I have been saying in our other threads for a long time now, get the air to the carb throat, it will make a big boost in power.

 

[Mad Professor]

Which makes me think, was the flocked filter REALLY cleaned or a dirty take off? If clean, my bad and I'm humble

Now clean is: shake it, use a tooth brush while on saw, take it off and clean by hand with detergent, just use 150 psi air, buy a new saw screw the cruddy filter, Filter? it has one?, there is a carb in there?, I planted three blueberry patches each year with the saw dust......


I'll go with # 3