Sure - my point was will the temp decrease compensate for the fact that the piston, con rod, crank etc will all experience greater forces both at normal RPMs and at any possible higher rpms generated?
The theory and physics of muffler mods, and their practical effects
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Sure - my point was will the temp decrease compensate for the fact that the piston, con rod, crank etc will all experience greater forces both at normal RPMs and at any possible higher rpms generated?
I was thinking mainly about the piston being the weak link, because it seems to be the most common source of trouble (but that could be due poor tuning or other maintenance issues). It is possible a lower temp would compensate for the increased forces on the piston itself.
I'm certain you're right about the crank, rod and bearings taking more of a beating though, and having a shorter service life as a result.
Log Hogger
ArboristSite Operative
I believe that's actually 85% of the exhaust port, as measured on the cylinder side. No matter, the 85% is just a rough guideline, gains are reported with larger openings, but the returns are diminished and loudness increases substantially.
neutral4x4, that side gapping method is interesting, I wonder if you've ever quantified the gains or know of any testing that does so. I'm not doubting you, it's just that I've found most changes in spark plug electrodes produce very subtle power increases that wouldn't be significant on a small two stroke. There's even plugs on the market that are the opposite of the side gap mod, they use enormous forked ground straps that surround the electrode on three sides to produce a stronger spark. It seems that in at least some instances, spark intensity is more crucial than ground strap shielding effect.
Log Hogger
ArboristSite Operative
On second thought
More ground strap area would probably make for less spark intensity. Maybe they need to be gapped less often?
Anyone ever try the side gap electrode, with the addition of filing the ground strap to a point? Would that further increase spark intensity?
More ground strap area would probably make for less spark intensity. Maybe they need to be gapped less often?
Anyone ever try the side gap electrode, with the addition of filing the ground strap to a point? Would that further increase spark intensity?
bitzer
******** Timber Expert
Muffler modding gives back the power that the epa has stolen. Period. Mufflers were never as restricted in the past as they are now. The exhaust port holes on the 390 are so damn small its ridiculous. The exhasut also has to travel through a swiss cheesed baffle to get there. With such a restriction on the muffler the saw is able to be tuned leaner in order to make the epa happy and to give Stihl some emissions credits. By opening the muff you can give the engine more fuel, which allows the engine to create the power it should. Anything to increase flow is what modding and porting is all about. Cutting out the corners. The straightest smoothest path wins. The 390 is about the easiest mod to do. A1 you are overthinking this tremendously. The indentation below the front cover is meant to be open. Drill a 1/2" hole in it or larger. The spark screen looses 40-50% of the port area. Pull off your limiter caps and richen it up to a good four stroke. The saw will produce the power it should be producing. Muffler modding will only get you what the saw should be running at before epa restrictions. The 290/310/390 series are all so restricitve that opening the muff a little causes big changes. Porting on the other hand will change things quite a bit in the way hp.
timberwolf
Addicted to ArboristSite
Edisto, This I was surprised to learn is not always the case as it would appear to apply to muffler outlets.
The coefficient of discharge from smaller holes can be higher than larger holes as the thickness of the orifice is greater in relation to the diamiter (given the same thickness of metal and total area). Also if small holes are placed close together the air streams align better as they enter the orifices, entrain less surrounding air as they exit and don't force into the center as much as they would with a single hole. This reduces the true point of restriction (vena contracta) which is formed on the discharge side downstream of the orifice.
A simple round hole in thin metal only discharges about 60% of the ideal for the area. A circular arrangement of smaller holes of the same area in the bit of flowbench testing I have done flowed up to about 70% of ideal.
timberwolf
Addicted to ArboristSite
A simple way to test the flow of a muffler is to place a pail of water a height above the muffler and pipe it to the muffler. Then simply measure the length of time it takes to empty the pail.
This will give a pretty good read on how much increase in flow capacity is provided by any given muffler mod. If you vary the height of the pail above the muffler so that it provides the same discharge time for the stock and modified mufflers you can get an idea of how much change in "backpressure" will result. water puts about 0.43 PSI pressure per foot. For example if it take 4 feet of head to push the water through a stock muffler in 30 seconds and 3 feet of head to push the same amount of water through a modified muffler it will likely be producing about 25% less resistance to flow or "backpressure" (a bit more complicated as muffler flow is dynamic but it should be in the ball park).
This will give a pretty good read on how much increase in flow capacity is provided by any given muffler mod. If you vary the height of the pail above the muffler so that it provides the same discharge time for the stock and modified mufflers you can get an idea of how much change in "backpressure" will result. water puts about 0.43 PSI pressure per foot. For example if it take 4 feet of head to push the water through a stock muffler in 30 seconds and 3 feet of head to push the same amount of water through a modified muffler it will likely be producing about 25% less resistance to flow or "backpressure" (a bit more complicated as muffler flow is dynamic but it should be in the ball park).
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timberwolf
Addicted to ArboristSite
Another challenge to conventional thinking. The total heat developed in an engine is predominatly a product of how much air and fuel is combusted and that equates quite directly into how much mechanical energy the engine produces.
If more fuel and air is being combusted each stroke producing more mechanical energy or power output then also more total heat is being produced. The fan turns the same amount each stroke and actually becomes less efficient at higher RPM so even if the engine is turning higher RPM the cooling system may be loosing ground on the heat produced.
There are much bigger variables though, I was doing some under plug and EGT temperature readings on a saw and found that just the smallest tweak to the HS jet (100-200 RPM WOT change) could bump the EGT 50 degrees and head temperature 10-20 degs. That said how the saw is set to run after a muffler mod may have a lot more to do with the actual engine temperatures than the muffler mod changes them selves.
If more fuel and air is being combusted each stroke producing more mechanical energy or power output then also more total heat is being produced. The fan turns the same amount each stroke and actually becomes less efficient at higher RPM so even if the engine is turning higher RPM the cooling system may be loosing ground on the heat produced.
There are much bigger variables though, I was doing some under plug and EGT temperature readings on a saw and found that just the smallest tweak to the HS jet (100-200 RPM WOT change) could bump the EGT 50 degrees and head temperature 10-20 degs. That said how the saw is set to run after a muffler mod may have a lot more to do with the actual engine temperatures than the muffler mod changes them selves.
neutral4x4
ArboristSite Operative
I'm not really sure as far as gapping the plugs more or less often. I have dyno results from a 2 stroke snowmobile that had the plugs replaced with side gapped plugs. The power curve was slightly higher than a normal plug. I can look for them, but they're in a dark corner of my hard drive. I'll have to look for them.
It does make the saw rev smoother and give a subtle increase in hp.
There are actually plugs made like that for 2 strokes, but they're considerably harder to find at the store than the regular plugs.
They also make a plug that has a little platinum wire. Platinum has a higher melting point than iron, so it can be made into a point. A pointed iron ground will glow red hot and cause dieseling.
Those two prong plugs are a gimmick. Not only do they block the electrode, but you also have to have them gapped exactly the same for them to work. A spark is going to follow the path of least resistance. Unless your grounds are perfect (Which is pretty much impossible) you'll only have a spark on one side and not the other.
Why don't manufacturers use these as OEM plugs in their saws, and sell them to us at a premium through their dealers, if there is a significant advantage?
Philbert
neutral4x4
ArboristSite Operative
I'm not really sure. I've run 2 stroke bikes with and without them and all the people I know from the 2 stroke racing crowd swear by them. All I know is that they're hard to find in the store.
If you don't believe me, fine. It isn't my loss. I always take everything with a grain of salt too.
Whats the worst that could happen if you try it, the laws of physics are different for you, and I'm wrong? You need to pay $2 for a new plug? A. Graham Bell mentions it in his book Two Stroke Performance Tuning. if you don't trust me, maybe you'll trust him. His book is great. While you're at it check out 2 Stroke Tuner's Handbook by Eric Gorr. He might mention it in there too, but don't hold me to it.
All I'm saying is that I do it to all my 2 strokes (saws, motorcycles, mopeds, weed eaters, snowblowers) and I notice a small but noticeable gain in power.
It's not about trust, just a simple question.
Manufacturers like STIHL, Husky, etc. are competitive and always try to wring more out of a saw to get a higher power-to-weight ratio and improve performance. They may have to deal with regulatory constraints (noise, emissions, etc.), and they need to balance performance with longevity, service needs, expected use, etc. of their target customers.
So I get that they design saws to run like a Camry or Taurus, rather than like a hot rod on a drag strip. But if simply replacing a spark plug with a different tip or heat range will make that much difference, I can't imagine that they wouldn't do it. These guys sell enough saws that they can get plugs made to their specs.
So, back to the question if anyone knows: are there any downsides to a side gapped plug (e.g. shorter life), are the improvements just not that significant to make it worthwhile, or . . . ?
I suppose that there are a number of things done in racing situations that don't make as much sense in 'normal' usage (cars, bikes, snowmobiles, saws, etc.).
Just curious.
Philbert
Another simple addition would be to index the plug, not that it will help a lot, but it won't hurt.
timberwolf
Addicted to ArboristSite
Which way should a plug be indexed in a 2 stroke piston ported engine? No intake valve to aim the plug at.
Side gaped plugs should not last as long as the surfaces that transfer the arc are smaller and would be expected to wear faster. Also if every saw that went out of the factory cost an extra couple bucks for a performance plug the money could never be recouped in sales. LOL though they would certainly make sure to nail us on parts... a special stihl spark plug (if they had such a thing) clearly would fetch at least 3 times what a generic plug is worth.
There are arguments against side gaping also, it moves the center of the point of ignition up and potentially away from the center of the charge, and for high RPM it also could move the point of ignition away from the exhaust port wich could hurt the the top of the powerband.
Side gaped plugs should not last as long as the surfaces that transfer the arc are smaller and would be expected to wear faster. Also if every saw that went out of the factory cost an extra couple bucks for a performance plug the money could never be recouped in sales. LOL though they would certainly make sure to nail us on parts... a special stihl spark plug (if they had such a thing) clearly would fetch at least 3 times what a generic plug is worth.
There are arguments against side gaping also, it moves the center of the point of ignition up and potentially away from the center of the charge, and for high RPM it also could move the point of ignition away from the exhaust port wich could hurt the the top of the powerband.
neutral4x4
ArboristSite Operative
Some saws come with a plug with too hot of a heat range. Just because it comes from the store like that doesn't mean thats optimum.
Even if manufacturers can make better parts, its cheaper just to buy parts from some other distributer (Champion, NGK, Autolite) than make the plug themselves. Do you honestly believe that a saw manufacturer is going to pay for the tooling to make their own spark plugs when they can easily just add 1mm to the bore of their saw and call it a day? It doesn't matter how many saws they make. Its always cheaper to outsource a generic part than come up with the tooling to make your own.
I haven't heard of any downsides, unless you don't clean all of the metal shavings out before you run it.
A lot of people have the "you can't make it better" attitude. A lot of saws have less than optimum porting. Why is that? Ported saws cut faster than a stock saw. Why didn't the factory make the ports the same as in a ported saw? If one manufacturer was really trying to get an edge over another, don't you think they would open up their ports a little? I'll give you a hint. It doesn't have to do with emissions. Saws made before the EPA cracked down can be ported too.
Most people don't care. I'm probably just :deadhorse:. sorry.
You would have the open end indexed up and to the boost port
Very cool...I didn't consider smaller holes as "longer pipes". Pressure drop does not seem to be affected by changing from continuous to pulsating flow, but discharge coefficients do...I just don't know if the amount of change varies with orifice diameter.
timberwolf
Addicted to ArboristSite
It was a bit of a facicious question, soory.
I don't know achieving both is possible in a typical saw with the plug angled in from the rear intake side of the cylinder. The first senario would place the ground strap in a saw to the bottom and gap up, the second would place the ground strap up with gap facing towards the intake/transfer flow.
In general terms it sounds best to have the gap facing the exhaust but then some say the intake. Also it is best that the ground strap does not block the path to the center of the chamber, that puts the ground strap back to the botom. But then in these tight engines placing the ground strap close to the piston can be a problem.
That leave it maybe best to put the electrode to one side or the other. Problem is at most its a 1-2% change. I have yet to see anyone who can put in three back to back timed cuts that don't vary at least that much, so how would this 1-2% change in performance be detected?
http://www.fordmuscle.com/archives/2000/07/indexplugs/ The second picture shows the example of the ground strap blocking the center of the chamber, the first in a two stroke places the ground strap closer to the piston which can be a heating problem.
ap1
ArboristSite Lurker
Edisto, please see the PM I sent you.
Everyone, I spazzed on Edisto the other night and I want to publically apologize. Not only was I wrong to jump on him, I was wrong on facts. It appears I had the relationship between torque and power backwards.
So, in the interest of truth, I must admit my mistakes.
ap
Everyone, I spazzed on Edisto the other night and I want to publically apologize. Not only was I wrong to jump on him, I was wrong on facts. It appears I had the relationship between torque and power backwards.
So, in the interest of truth, I must admit my mistakes.
ap
