mopar1rules
ArboristSite Guru
Oh gotcha.
Brad, you get a chance to think about what I wrote in a previous post on the transfer velocity?
Brad, you get a chance to think about what I wrote in a previous post on the transfer velocity?
I don't think of it at that complex of a level. I'm simply looking for flow. The saw needs more fuel to make power on the topend.Brad, you said the main transfers were smaller than the secondaries. I wonder if Solos idea with that was to get a higher velocity stream going up through the mains, so it would come into the cylinder with more force, creating a wall that would help hold the mixture coming in from the wider/slower secondaries, thus to contain more air/fuel mix to make more hp.....but, maybe the air/fuel was coming from the mains a little too hard and starting to really stir up what came from the secondaries and pushing more air/fuel out the exhaust than what was wanted and hence the reason you found good gains when widening the mains and slowing down the air/fuel charge some in that port?? Or maybe it just simply wanted more transfer area all together, or perhaps a mix of both?? What do you think?
Is it done yet?I don't think of it at that complex of a level. I'm simply looking for flow. The saw needs more fuel to make power on the topend.
I'm not prepared to explain why these saws want so much blowdown, but they do. Let's just accept that as a given for this conversation. With that being the case, you can't simply raise the just the transfers to get more fuel. Without using a different piston or moving the ring locating pin, widening the main transfers was the only option. It nicely matches up with the case matching of the base of the cylinder. It's just what my eyes saw when looking for a way to get more transfer flow.
I really think that opening up the transfer ducts is key here too. They're just too small to flow the volume that's needed to make the most power possible on the topend.
I will also add that I'm using a little more intake duration as well. This can help fill the case a little more to make up for any loss of case compression.
I made these changes incrementally and found gains each step of the way. Hopefully it'll work here as well.
I think I'm accomplishing the same thing with this approach that others are by raising the exhaust and transfers, thereby gaining more transfer time/area, but having to raise the exhaust in order to maintain blowdown. THIS IS ONLY A THEORY AT THIS POINT. I very well may be wrong and will end up raising them myself as well. I'm curious though and want to see how it responds to my approach first.
Thoughts?
I don't think of it at that complex of a level. I'm simply looking for flow. The saw needs more fuel to make power on the topend.
I'm not prepared to explain why these saws want so much blowdown, but they do. Let's just accept that as a given for this conversation. With that being the case, you can't simply raise the just the transfers to get more fuel. Without using a different piston or moving the ring locating pin, widening the main transfers was the only option. It nicely matches up with the case matching of the base of the cylinder. It's just what my eyes saw when looking for a way to get more transfer flow.
I really think that opening up the transfer ducts is key here too. They're just too small to flow the volume that's needed to make the most power possible on the topend.
I will also add that I'm using a little more intake duration as well. This can help fill the case a little more to make up for any loss of case compression.
I made these changes incrementally and found gains each step of the way. Hopefully it'll work here as well.
I think I'm accomplishing the same thing with this approach that others are by raising the exhaust and transfers, thereby gaining more transfer time/area, but having to raise the exhaust in order to maintain blowdown. THIS IS ONLY A THEORY AT THIS POINT. I very well may be wrong and will end up raising them myself as well. I'm curious though and want to see how it responds to my approach first.
Thoughts?
Most quad ports have way more case volume than a 7910!It seems to me that saws with 4 transfer ports are more prone to be happy with 22*-26*ish degrees of blow down, while saws with only 2 transfer ports like around 15*-19* of blow down. Maybe the primary crankcase compression has a lot to do with it?? Thoughts??
I'm usually about 18-20 in most quad ports. Even on dual ports I rarely go less, except on the 440, 460, 461, and hybrid.It seems to me that saws with 4 transfer ports are more prone to be happy with 22*-26*ish degrees of blow down, while saws with only 2 transfer ports like around 15*-19* of blow down. Maybe the primary crankcase compression has a lot to do with it?? Thoughts??
I'm usually about 18-20 in most quad ports. Even on dual ports I rarely go less, except on the 440, 460, 461, and hybrid.
I don't understand this in concept. I don't see how the case could remain pressurized, or rather how it could accumulate pressure. After the case (primary) compression angle, then the transfers open connecting the case to the combustion chamber (with the exhaust port open). Then the case is closed off again and the piston draws on the case for the same number of degrees it pressurized it. Than the intake opens and vents the case to the atmosphere. If there were any positive case pressure remaining when the intake opened it couldn't draw any fresh mix into the case and it would stop.I was told to listen for hissing coming from the intake right after you shut the saw down. Too much heat/pressure vaporizing fuel in the case I assume.
A well known builder told me what I posted, and I trust what he says more than anyone that's been on AS in a good long while.I don't understand this in concept. I don't see how the case could remain pressurized, or rather how it could accumulate pressure. After the case (primary) compression angle, then the transfers open connecting the case to the combustion chamber (with the exhaust port open). Then the case is closed off again and the piston draws on the case for the same number of degrees it pressurized it. Than the intake opens and vents the case to the atmosphere. If there were any positive case pressure remaining when the intake opened it couldn't draw any fresh mix into the case and it would stop.
I can maybe see how an excessive case compression could waste power and reduces delivery ratio (volumetric efficiency).
I couldn't be more pleased with how this saw is running. I modded it very similarly to the 7910 I did and got the same fantastic results. I'm currently at 32° past stock, running the 272XP coil. Compression is 215 PSI cold with the exhaust at 100°. Transfers are at 125 on the mains and 130 on the secondaries. Intake is at 82°. The intake and exhaust were both widened to skirt limits. The main transfers were widened .100" towards the exhaust. I then case matched the cylinder base to the case, and carried that widening all the way up through the duct, matching up with the widened transfer. I also moderately opened up the secondary transfer ducts. I know it goes against traditional thinking, but these saws seem to need more transfer area and this is how I accomplish that.
There's more left in this saw. The baffle in the muffler is still fully intact. It may also do well with 2°-3° more timing advance. I'm still running the stock carb and plan to put a RWJ4 on it.
The saw is tuned to 14,300. It's hard to hear the 4-stroking with the baffle still in the muffler, but it's plenty rich. It's holding 13,000 in the first cut of the video. The next cuts are 12,000 and 11,000 respectively. IIRC, the dogged in cut was around 9,000. The saw has a super user friendly broad powerband, incredible throttle response and is just an all around fantastic runner. Now I just need to get it in some bigger wood and see how it compares to something like my 390XP.
As much as I love a MS461, it annihilates one I just finished. It runs good too. This 681, just turned out fantastic.Sounds healthy!!!
I'll have to listen for it.A well known builder told me what I posted, and I trust what he says more than anyone that's been on AS in a good long while.
That's because of the additional muffler work I had done. Thanks.The saw has a throatier tone with the larger carb in the 2nd video, but doesn't appear to cut much faster, if at all. It sure does scream like a banshee in both videos. Great job!!
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