Mastermind meets the Dolmar 7900

[parrisw]

Its good everyone has fun here but is there not anyone that has enough knowledge of PORTING to tell me what kind of changes in port timing gives you the quicker throttle response? Its obvious quicker flow in and out of cylinder is a big help, but besides flow and widening ports such as timing , duration and crankcase pressure? It looks to me like higher crankcase pressures would be a big factor?

By just widening the ports you'll see a big increase in throttle response.

 

[Mastermind]

Randy she sounds really good and it has snappy throttle response, I think you did an excellent job.

Thanks Andre, I can count on you to tell it like it. I'll admit to being very careful and doing a ton of reading for this saw. The 7900 is a saw that I've heard can go the wrong way.

Poplar and pine is soft to me. Locust is a very dense dry wood,thats why everyone makes posts out of em here because they last in the ground and weather. Dont know, is oak like pine to you too?

Locust is a dense wood, and it lasts forever for fence posts. When I moved here 26 years ago there were locust posts that were in the ground since the late '70s. Those posts are still there holding a fence.

The reason pine was used in that video is because that's what I had at the time. I did some comparison cuts with 372s and needed a clear, consistent wood to make my results meaningful. I downed a whole tree to get the cant I used so for the next couple of weeks all my testing will be done in pine.

If you look through my utube channel you will see videos in all sorts of wood. I just use what I have available at the time. One thing I've noticed in testing these saws is that soft wood will pull the rpm down on a saw because of chip loading, just because a wood is relatively soft doesn't always make for a fast cut.

 

[o8f150]

Locust is a dense wood, and it lasts forever for fence posts. When I moved here 26 years ago there were locust posts that were in the ground since the late '70s. Those posts are still there holding a fence.

The reason pine was used in that video is because that's what I had at the time. I did some comparison cuts with 372s and needed a clear, consistent wood to make my results meaningful. I downed a whole tree to get the cant I used so for the next couple of weeks all my testing will be done in pine.

If you look through my utube channel you will see videos in all sorts of wood. I just use what I have available at the time. One thing I've noticed in testing these saws is that soft wood will pull the rpm down on a saw because of chip loading, just because a wood is relatively soft doesn't always make for a fast cut.

dang,, i just learned something

 

[parrisw]

If you look through my utube channel you will see videos in all sorts of wood. I just use what I have available at the time. One thing I've noticed in testing these saws is that soft wood will pull the rpm down on a saw because of chip loading, just because a wood is relatively soft doesn't always make for a fast cut.

Well said Randy. I've been trying to say that for years, some people never get it though.

 

[srcarr52]

Its good everyone has fun here but is there not anyone that has enough knowledge of PORTING to tell me what kind of changes in port timing gives you the quicker throttle response? Its obvious quicker flow in and out of cylinder is a big help, but besides flow and widening ports such as timing , duration and crankcase pressure? It looks to me like higher crankcase pressures would be a big factor?

Better balance from intake to exhaust flow with help with throttle response but it's more a function of just generally flow through the whole engine. When you have higher peak flow generally the flow will be able to accelerate quicker giving you better throttle response. So when the saw is at idle, the flow through the engine is say 10% of when it's at 10,000 RPM. The faster you can go from idle flow to peak flow the more throttle response you'll have.

Also anything you do to lighten the rotating assembly will help once you have the port flow.

 

[Mastermind]

Better balance from intake to exhaust flow with help with throttle response but it's more a function of just generally flow through the whole engine. When you have higher peak flow generally the flow will be able to accelerate quicker giving you better throttle response. So when the saw is at idle, the flow through the engine is say 10% of when it's at 10,000 RPM. The faster you can go from idle flow to peak flow the more throttle response you'll have.

Also anything you do to lighten the rotating assembly will help once you have the port flow.

Thanks for giving him a good answer. I was tired last night and couldn't really think of a good way to put it.

One reason I think this saw runs as well as it seems to is the new style, light weight piston that was used. I have another 7900 on my bench for port work with a windowed piston. I'll be contacting the owner about changing that.

 

[Metals406]

Another consideration for "softwoods" is where they grew. High elevation pine, fir, spruce, white fir, etc will be harder than pine grown in the south.

Why? Tighter growth rings and tougher wood fibers. It get's cold here, with lots of snow and wind. These trees don't have the growth rate of southern woods where it can stay 50° or better all winter. Northern climates also don't have as long a growing season,giving trees less time between sap up, and sap down.

I've cut Doug Fir with growth rings averaging 1/16" or less apart. . . Some of them 1/32" apart for a 25 year stretch at a time.

There's a reason studs coming out of tree farms aren't as structurally strong as out of State forests, where they had to compete for water, sun, nutrients, battle wind and snow. . . It makes tougher wood.

 

[srcarr52]

Thanks for giving him a good answer. I was tired last night and couldn't really think of a good way to put it.

One reason I think this saw runs as well as it seems to is the new style, light weight piston that was used. I have another 7900 on my bench for port work with a windowed piston. I'll be contacting the owner about changing that.

I do like the design of that piston... it's strong, light, and most likely allows more flow to the transfers then a ported piston. The only downside is without the flow through the piston the underside of the dome is not being cooled as much. So they must be banking on the pin boss wings to act as cooling fins for the piston dome. If you start seeing carbon build up (or massive discoloration) on the bottom of the piston dome then you know you got problems.

 

[nmurph]

Another consideration for "softwoods" is where they grew. High elevation pine, fir, spruce, white fir, etc will be harder than pine grown in the south.

Maybe, but a 30" pine in this part of the state will have heartwood for 2/3 of the diameter. That heart wood is packed full of hard sap and is like cutting through bubble gum. If you pick up a piece of what we call "fat" wood and a piece of oak the same size, the fat wood is noticeably heavier. That also depends greatly on the species. Loblolly, and Slash to a lesser extent, are softer until they get to the 25-30"+ size. Now, anything up to about 20" is pretty soft.

 

[blsnelling]

Have you compared it to the 460 you did for the build off? How'd it fair against the 372s?

 

[booger1286]

Was not criticizing for using pine just thought it would be interesting to see a comparison between that and something like locust, because i just cut some the other day and it was just so much slower than red oak cutting.

 

[steve316]

wrist pin

Randy the only 7900 cylinder I have looked at is the new style. My question is dose the old style also have the tapered pin; as it is lighter than the 6400 witch is a standard pin.I read a post that the new piston was 12 grams lighter and was thinking ( this causes most of my problems ) the pin might be part of the weigh savings.Steve

 

[Hedgerow]

Better balance from intake to exhaust flow with help with throttle response but it's more a function of just generally flow through the whole engine. When you have higher peak flow generally the flow will be able to accelerate quicker giving you better throttle response. So when the saw is at idle, the flow through the engine is say 10% of when it's at 10,000 RPM. The faster you can go from idle flow to peak flow the more throttle response you'll have.

Also anything you do to lighten the rotating assembly will help once you have the port flow.

What effect would advancing the timing a couple degrees have on throttle response?

 

[HEAVY FUEL]

Another consideration for "softwoods" is where they grew. High elevation pine, fir, spruce, white fir, etc will be harder than pine grown in the south.

Why? Tighter growth rings and tougher wood fibers. It get's cold here, with lots of snow and wind. These trees don't have the growth rate of southern woods where it can stay 50° or better all winter. Northern climates also don't have as long a growing season,giving trees less time between sap up, and sap down.

I've cut Doug Fir with growth rings averaging 1/16" or less apart. . . Some of them 1/32" apart for a 25 year stretch at a time.

There's a reason studs coming out of tree farms aren't as structurally strong as out of State forests, where they had to compete for water, sun, nutrients, battle wind and snow. . . It makes tougher wood.

We've used some fresh cut cottonwood for gtg racing 24-26 inches, your lucky to get thru it without pinching or plugging up with chips.

 

[blsnelling]

What effect would advancing the timing a couple degrees have on throttle response?

On some saws, it has a huge affect on throttle response. It's a big part of the fix on the new 201T.

 

[Hedgerow]

We've used some fresh cut cottonwood for gtg racing 24-26 inches, your lucky to get thru it without pinching or plugging up with chips.

That's why I like the harder stuff for comparison cutting... You take the chain's ability to clear things out, "out" of the equation... I wonder if this is why race chain shines in competition cutting?? :msp_confused:
Slice and clear as fast as the saw can move it???

 

[Hedgerow]

On some saws, it has a huge affect on throttle response. It's a big part of the fix on the new 201T.

Just wonderin'... Seemed to help the old 028 a little, but I've convinced myself of things that aren't necessarily so before...
:confident:

 

[Mastermind]

What effect would advancing the timing a couple degrees have on throttle response?

I won't advance ignition timing on a worksaw. I worry about the added heat, especially when I'm adding compression.

The beveled wrist pin is part of the weight savings and is different than the windowed piston.

 

[srcarr52]

What effect would advancing the timing a couple degrees have on throttle response?

It can help. But only if the saw needs more then a few degrees of timing for max power. Most saws are fixed timing so at lower RPM they are wasting some of the cylinder pressure while the piston is still coming up in the bore from too much ignition timing. Since the time in seconds till peak pressure (dwell) in the cylinder is roughly the same for all RPM of the engine a fixed timing will only produce the maximum output from the combustion in a short RPM band, the rest of the RPM band is a compromise.

Advancing the timing usually increase the throttle response because we are using better (higher octane) fuels then what the engine was originally designed for. The higher octane causes slower combustion dwell time and the more ignition timing needed to absorb the most of the combustion process. Think of cylinder pressure as a function of time after ignition so you have to place that spark at the appropriate time such that the most of the pressure is caught while the piston rod has good leverage on the crank pin. The leverage of the piston rod assembly is a function of angle of the crank. So our cylinder pressure is time dependent, our leverage is angle dependent, if our ignition timing is fixed the two equations can only be maximize for one point in the RPM band. Which is fine because we uses saws at a small RPM range when in the cut anyway.

 

[blsnelling]

In using pump gas and find significant improvements in both throttle response and power under load. Not all saws respond the same though. It's just another part of tuning a saw, similar to tuning a carb. It's just more difficult to adjust.