OutdoorKing Repair Forum Forums Restoring, Customising and Modifying Tech Talk Self propelled Victa power plus

She's a Beaut mate!

Really nice job!


Kori

I am sure you have read it elsewhere on here but do not let ethanol fuel touch that fuel cap, it will melt it, along with the other seals, but the fuel caps like that are harder to find unbroken, even harder to find at tiny's after I have been shopping.....

Any chance of grinding the rim around the edge of the old power plus muffler to open it up so we can see the internals of the power plus type muffler? It might clear up some recent conjecture. (I reckon it might have an extra hole/s inside or bigger than STD holes, if any different)

I have a rusty std one I can cut open and post pic here for a comparison. What you guy's think?

Edit: Like so.....

The 3 Holes are 3/8" in Dia. you can see how the dimple would lower the flow/increase backpressure.

Inside HI-COMPESSION MUFFLER.
Do the extra holes mean more flow?
I'm not sure I understand back pressure?



To finish this mower, I just have to sort out the self propelled arrangement with the idler wheel and how the cable makes it tension the drive belt.
If someone could put up an image, that would be much appreciated.
Thanks from Mark

Thanks for the photo Mark, good stuff!

I guess the best way to describe back pressure is to do a little experiment: take a deep breath, close your mouth, close one nostril and breath out as fast as you can. Then repeat with your mouth and both nostrils open..... a self evident answer to both your questions (and make sure the bushmans blow doesn't make a mess)

Mark, two of the three steel stampings are the two halves of the outer shell of the muffler. I'll call them the back (which has the hole for the exhaust port near the bottom, and also has the rectangular exit port at the top left) and the front cover (which has no external holes in it). The third, flat piece sandwiched between the back and the front cover is the baffle. The exhaust gas enters through the single large round hole at the bottom, which is clamped across the exhaust port. On the inside of the muffler, the perimeter of that hole is pressed against the matching hole in the baffle, and probably welded to it. The exhaust gas that enters the muffler is then on the wrong side of the baffle: there is no external port in that side of the muffler, because there is no hole in the front cover. To reach the only exit (the elongated exit hole at the side of the top of the back pressing) the exhaust gas has to go through the small holes in the baffle. On the standard muffler, those holes are positioned as far away from the exit hole as possible, so the gas will have to bounce around a lot before it leaves. The bouncing around breaks up its pressure pulses (which are sound waves).

On the high performance muffler, a whole bunch of additional holes have been punched or drilled right near the exit, so the gas doesn't have to bounce around much getting there.

Back pressure is a crude measure of how restrictive a muffler is. If you have a simple muffler - say an old jam tin stuffed with steel wool - the back pressure will increase as you stuff more steel wool in there, and the exhaust sound will get quieter as well. More sophisticated mufflers use "interference effects", not back pressure, to damp sound waves. The Victa muffler effectively consists of two expansion chambers, with a restrictive damper (the baffle) in between them. Simplistically, if you make the expansion chambers bigger, you can use a less restrictive damper and get the same sound level. In practice you usually find tricky ways to make the expansion chambers effectively bigger without making them physically bigger, because of the space requirements. If you crawl under a dual exhaust V8 from the 1960s or 70s, you are pretty sure to find a very short "balance pipe" approximately at the transmission support mount, joining the two pipes (which come close to each other at that point). That way, each of the two exhaust systems is able to use the whole of the other exhaust system as an expansion chamber. (It only works because there cannot be two pulses arriving at the balance pipe from opposite ends at the same time, since the engine fires each cylinder at a different time from all the other cylinders). The system has to be tuned to work properly: the engine's output power changes slightly as you vary the position and length of the balance pipe.

Designing exhaust systems is an interesting blend of science, experience, and experimentation.

Good stuff guy's...Here is one more muffler off an old 125 I got but not sure what its off..Ive not seen one like this before so had to have a look in side.

The design of that one seems pretty awful. It relies on the packing material, which has migrated to the exit hole and become choked up with carbon. In the end, there would have been lots of back pressure: it had turned into a jam tin full of fibrous material, caked up with crud.

Mark for painting the barrel and muffler plain old matt black out of a spray can works great and will last a long time no need for heat paint...The power plus looks great I will have to keep my eye out for one Ive not got a self propelled mower....

Your right there grumpy awful looking thing that muffler..When I open it up and seen that fibrous material I wet it down it could even be Asbestos for all I know....

It doesn't look like asbestos, Chris: I'm old enough to have handled the stuff a number of times. It looks like animal hair in the picture, but that seems insane.

mmmm Yeah I was thinking heavy gauge glass fibre, I think paint shops/hardware stores or local shire councils offer a free fibre identification service.

The baffle plate seems to separate the wadding from the outlet, from my veiwing of the pics.

Thanks for the input Joe for ethanol fuel.
Chris for the paint.
FAST GRASS for the muffler.
Grumpy for the muffler theory. Thanks alot.

I tend to think of myself as a cleaner that can pull stuff apart, clean & put back together. It gets most engines working for me, I am slowly learning the engine theory, there is more than you think to small engines.

What I am trying to get my head around: is, I had a 2 stroke power torque engine that would not rev above idle due to a blocked muffler.
Once I cleaned it out, the engine then ran as expected.
So the back pressure was greater and the flow through the engine restricted, giving slow speed.

Now with this power plus engine, the head is only a small bowl compared to a normal one, so the fuel air mix would get compressed more.
This would give better detonation, therefore the baffle has to have more holes to accomodate this.

I have run 2 strokes with the muffler off. The still run fine, just alot of noise.

For some reason I thought the engine needed some back pressure to run correctly.

First, you meant combustion, not detonation. Detonation refers to the situation where petrol-air mixture burns much faster than usual. The ignition is timed so that the peak pressure normally occurs slightly after Top Dead Center, when the available space is increasing rapidly as the piston descends. Detonation brings forward the time when peak pressure occurs to approximately TDC, which results in an abnormally high peak pressure in the combustion chamber, accompanied by a knocking sound, which strangely enough is called a knock. In severe cases this sudden ultra-high pressure is prone to breaking piston crowns. Detonation is a severely destructive phenomenon, which at high speed can destroy pistons in less than 5 seconds from its onset.

Crankcase compression two stroke engines rely on a rather marginal process to scavenge the burned gas from the chamber after combustion. The momentum of moving columns of gas plays a key part, and at light throttle, when there isn't much incoming charge, scavenging is very poor and the engine will need a two or more successive attempts at scavenging before a combustible mixture will be achieved (called 4 stroking, or 6 stroking, or 8 stroking depending on how many tries it takes). One of the methods used to improve scavenging is to tune the exhaust system to produce pressure pulses timed so that there is a momentary low pressure at the exhaust port late in the exhaust process: this low pressure helps suck out the burned gas. Having a heavily baffled muffler right up against the exhaust port has the opposite effect: it messes up scavenging by causing a sustained positive pressure at the exhaust port (back pressure). This matters a lot more to a crankcase induction two stroke than it does to a four stroke. So, to get power from a crankcase induction two stroke what you would like to have is a pulse-tuned pipe (preferably tapered, megaphone-style). This is easily accommodated on a motorcycle but is a cost and packaging challenge on a lawnmower.

Any petrol engine will produce more power on less fuel if you raise the compression ratio, up to the point where detonation occurs, and provided you maintain the same volumetric efficiency. Power is also increased by improving scavenging efficiency, which in a crude baffled system can be brought about by omitting the baffle or making more or bigger holes in it. Of course this increases noise levels, which is a problem when there are laws about such things.

Thanks grumpy for taking the time to explain this, well written and detailed. I will need some time to digest this information.

Sounds like a well timed balancing act between intake, combustion, scavanging and exhaust. Pretty amazing stuff.