Dolmar 7900w VS. (vid)

[Gambler]

I am going to post a FACT. Rev-limiting coils limit RPM by: cutting spark, which floods the cylinder and takes several RPM to clear it back out, which kills torque by not having preasure against the piston on the down stroke.

Also Treeslinger is right about moving the torque curve up on the RPM which is more HP.

If you argue with this at all, then I will excuse you as ignorant and expect you to hit the books and then apologize to the class room for your outburst when you realize you don't know your bung hole from a hole in the ground.

Slinger, nice work.

 

[Cheese]

I appreciate the videos.

The sidetrack of what they are actually showing amuses me. In the standard set, in order to show something, a video must have two saws with the same bar and chain in the same wood looking and sounding the same to reflect actual gains.

I think that I learned something about the RPM's reflected by sound but wonder how that is the biggest gripe.

Does the video not show one saw outperforming the other?

Do we need a dummy to push saws at the same speed through the same wood, etc?

I look forward to the next round, if it weren't a Dolmar that I was not familiar with, I would be tickled to own it!

 

[MCW]

Love your work mate and basically anything to do with 7900's
Where's pancake gone and who the hell offended the poor fella?
Anybody???
Well???
Oh, I see, nobody cares

 

[JT78]

I reckon sometimes you just have to ignore the ignorant they dont know any better. Slinger the saw ran good did sound like it had more left so I would imagine that after a retune and an 8 pin it will be one quick SOB.

 

[Wet1]

Slinger, how about running those same saws, chains, and wood with the 8 pin and compare them to the 7 pin w/o any other adjustments.

Have to agree the video doesn't mean much since the chains, chain tension, and pressure on the bar in the cuts could very well be different (and likely is), but I'm sure the modded saw is quicker in the cut regardless. Since the saw's aren't hitting the limiter in the cut, I also question what removing the limiter will gain you. It will be running at a slightly higher rpm as it hits the wood, but there isn't a lot of inertia there so it might only be worth a few tenths of a second at best.

Either way, nice work.

 

[Tree Sling'r]

UPDATE:
Removed the H limiter, same everything 18.79 compared to a previous 19.82


So pretty much a hair over a 1 second gain, making the mod a 6 second gain.

Got the saw to hit the limiter WOT, backed it off (13,530) - It wants to run, but can't.

 

[Wet1]

Is it hitting the limiter in the wood???

 

[Tree Sling'r]

Is it hitting the limiter in the wood???

13,500 in the wood on a 7900?????

 

[sprung22]

Did you get a vid with the H out?

 

[Wet1]

13,500 in the wood on a 7900?????

Probably not very likely with anything over a 2" bar. So why does it matter if the coil is limiting the saw since it's not going to hit the limiter in the cut anyway???????????????????????????

The only advantages I see to a unlimited coil is it makes it easier to tune and you might be able to start the cut a hair faster... but the second the saw gets into the wood, I see no advantage.

What am I missing here???

 

[Tree Sling'r]

Did you get a vid with the H out?

Yeah, but don't really feel the need for the whole production process for 1 second.

 

[Tree Sling'r]

Probably not very likely with anything over a 2" bar. So why does it matter if the coil is limiting the saw since it's not going to hit the limiter in the cut anyway???????????????????????????

The only advantages I see to a unlimited coil is it makes it easier to tune and you might be able to start the cut a hair faster... but the second the saw gets into the wood, I see no advantage.

What am I missing here???

RPMs, the more RPM a saw has the more power it has, can't really explain it any better than that.

A saw that will safely rev 14,300 out of the cut will cut much faster in the wood than a saw that is at 13,500, chain speed.

The whole max RPM in the wood versus RPM out of the wood must be oblivious to many.

It would be impossible to get a 7900 to cut 13,500 in the wood. Maybe if it reved in the low 14s you could bury a bar and maybe get 12k outta it, but I'd doubt.

I will post this as an example:

http://www.youtube.com/watch?v=pvqZPy0LwHM&feature=channel_page

 

[sprung22]

Right on.

 

[Wet1]

RPMs, the more RPM a saw has the more power it has, can't really explain it any better than that.

A saw that will safely rev 14,300 out of the cut will cut much faster in the wood than a saw that is at 13,500, chain speed.

The whole max RPM in the wood versus RPM out of the wood must be oblivious to many.

It would be impossible to get a 7900 to cut 13,500 in the wood. Maybe if it reved in the low 14s you could bury a bar and maybe get 12k outta it, but I'd doubt.

I will post this as an example:
http://www.youtube.com/watch?v=pvqZPy0LwHM&feature=channel_page

I must be a canidate to ride the short bus, because I still am failing to see what difference it makes in the wood.

Work with me here... I understand a modded saw might be able to spin faster out of the wood, and that it can certainly make more power in the cut since it's modded. Even if you can get it to spin 15k out of the cut, yet it will only spin 11k in the cut, what difference does it make if the coil is limited to 13.5k? The limited coil isn't effecting anything, right?

I still can't see how it makes any difference in the cut if it's not hitting the limiter anyway. What am I missing???

I'm not trying to be a smart azz, I'm just having a very hard time wrapping my tiny brain around how it can make much of a difference...

 

[Lakeside53]

My tiny brain is in the same slot... But I trust Tree Sling'r's observations also, so I'm "confused" at best.

The only thing I can think of is a different advance curve, and that should be obvious with a timing light. For example, Stihl later limited designs (mainly weedeaters) have a 4 stage advance, but at "operating rpm, they are advanced as stihl wants to go, and often slightly more than an older unlimited coil.


Yes, by moving the torque curve up, you get more HP, but that peaks way less then any spark limit.

 

[Tree Sling'r]

I must be a canidate to ride the short bus, because I still am failing to see what difference it makes in the wood.

Work with me here... I understand a modded saw might be able to spin faster out of the wood, and that it can certainly make more power in the cut since it's modded. Even if you can get it to spin 15k out of the cut, yet it will only spin 11k in the cut, what difference does it make if the coil is limited to 13.5k? The limited coil isn't effecting anything, right?

I still can't see how it makes any difference in the cut if it's not hitting the limiter anyway. What am I missing???

I'm not trying to be a smart azz, I'm just having a very hard time wrapping my tiny brain around how it can make much of a difference...

I am not good at explanations, but I will try.

The limited coil on this saw is not letting the saw reach it's capabilities after the mod. So the modded saw is only hitting 13,530 max RPMs out of the wood. The lower RPM out of the wood, makes for a lower RPM in the wood making a longer cut time. Guessing this saw was maybe 9500-10,000 in the cut.

If I had an unlimited coil, I could get the saw to run out of the wood to it's ported potential at an estimated 14,300. With my other saws in the 7900s class the MS460 and the 372xpw this is the case, the the extra RPMs when put in the cut get me to around 11,500 to 12K in the cut.

So more RPM outta the wood means more RPM in the wood, with correct porting of course.

Don't know if this is clear or not.
Maybe some can explain my thinking better. Remember I am a timberfaller...

 

[Wet1]

My tiny brain is in the same slot... But I trust Tree Sling'r's observations also, so I'm "confused" at best.

The only thing I can think of is a different advance curve, and that should be obvious with a timing light. For example, Stihl later limited designs (mainly weedeaters) have a 4 stage advance, but at "operting rpm, they are advanced as stihl wants to go, and often slightly more than an older unlimited coil.

I guess I was hypothosizing all things being equal since timing hasn't been brought up until now.

I'm going to be swapping the limited coil on my 3120 soon. I don't know if the timing is any different with the 272 coil I'm going to stick in it, but I'm going to check it at the time. Regardless, I expect to see an improvement in the cut because I often run a small bar on this saw and the stock limited coil is limited to a low RPM and I occasionally hit it in the cut. But with the 7900, I never come close to hitting the limiter in the cut so I fail to see how swapping the coil would make a difference in itself.

 

[Lakeside53]

Usually when we get to these impasses, someone PMs Timberwolf for his input...


My 2c -a spark that is sparking every compression stroke at the correct timing is doing exactly what it's supposed to do - spark...

Spark-skipping limiters are "top hat" - they slice the top rpm off. Yes, there is a recovery time when they are flooding, but not before.

"Intelligent" limiters - first retard the ignition at high rpm, then start skipping spark. Spark advance is good around peak torque (oem designed peak torque) but may be detrimental to piston life at higher rpm. MAYBE, this is the effect being seen...

I have no idea what flavor or possible combination of the two Dolmar uses.

 

[Wet1]

I am not good at explanations, but I will try.

The limited coil on this saw is not letting the saw reach it's capabilities after the mod. So the modded saw is only hitting 13,530 max RPMs out of the wood. The lower RPM out of the wood, makes for a lower RPM in the wood making a longer cut time. Guessing this saw was maybe 9500-10,000 in the cut.

If I had an unlimited coil, I could get the saw to run out of the wood to it's ported potential at an estimated 14,300. With my other saws in the 7900s class the MS460 and the 372xpw this is the case, the the extra RPMs when put in the cut get me to around 11,500 to 12K in the cut.

So more RPM outta the wood means more RPM in the wood, with correct porting of course.

Don't know if this is clear or not.
Maybe some can explain my thinking better. Remember I am a timberfaller...

I guess I'm equally as bad when it comes to comprehending then. There's no doubt in my mind you know far more than I do about porting and modding 2 strokes, so I'll have to take your word for it, but I still can't understand how the coil is holding the saw back in the cut if it isn't hitting the rev limiter.

Perhaps I should just wear my dunce cap and keep my mouth shut on this one since I'm clearly not getting it. Thanks for trying to explain it.

 

[Tree Sling'r]

But with the 7900, I never come close to hitting the limiter in the cut so I fail to see how swapping the coil would make a difference in itself.

On a stock saw it would never matter, on a modified saw it makes all the difference in the world.
You kinda get where I am coming from?
I am on a mission now, Romeo did 7900 coil surgery a while back, I may have to see what I can and make a final video.
A modded saw with stock ignition timing is still a stock saw.