Dolmar 7900w VS. (vid)

[Jacob J.]

So how would this effect your timed cuts then? Earlier you said for more gains (faster times) with this saw you'd need an unlimited coil...

:sips scotch:

I think Andy pretty much nailed it. There's a timing curve to every limited ignition module. Some have it as short as 500 RPM and some have it as long as 3000 RPM.

Let me take you back to the glory days of the Red-Light 066 Magnum....

Those saws I believe (and the CDIC 064s of the same era) were rev-limited at 12,500. However, when you get into the higher RPM range on those coils, they start retarding the spark at around 11-11,5k. So if you end up modifying the port timing and lighten the reciprocating assembly on those saws in order for them to develop their peak power at a higher RPM, you're going to fail because of the latent timing curve brought on by the rev-limited ignition. So that's one factor. Also add into the mix (this is extraneous but still a factor) that those ignitions would increase their timing curve as they aged. Meaning that, instead of the curve starting at 11-11,500, eventually they go down to 10,5, then 10, then 9,5- etc.

I've seen an older 066 Red light that hit the limiter (with the red LED flashing) at 9,500 according to my fairly new DET-202.

 

[Tree Sling'r]

I looks as if I am not gonna get my point across.
But, when a different coil is on this saw, I will have a faster cut time than before when it was still limited.
The above has been the case on every modded saw I have ever done after switching from limited to non-limted coil.
The switch has made the saw capable of running to it's ported expectations. Which is where the gains are.

 

[Haywire Haywood]

Jacob J is making some sense to me.

In this theory, it's not the actual limiting function of the coil that is the problem, it's what it does to the ignition curve 1500-2000 rpms before it starts interrupting the spark. If the saw is designed to develop peak torque at 11k and the coil starts retarding the timing at 12k, then it's working against a ported saw that develops peak torque at 12K.

Ian

 

[Tree Sling'r]

Jacob J is making some sense to me.

In this theory, it's not the actual limiting function of the coil that is the problem, it's what it does to the ignition curve 1500-2000 rpms before it starts interrupting the spark. If the saw is designed to develop peak torque at 11k and the coil starts retarding the timing at 12k, then it's working against a ported saw that develops peak torque at 12K.

Ian

Granted this is way over my head, but, it does make sense. A different ignition would have a different curve therefore not interupting spark, right?

 

[Haywire Haywood]

It's not the spark interruption that is the problem in this theory. If it interrupts the the spark at 13k, it isn't affecting the saw when it's in the cut at 12k. However, if the saw is designed to run at 11K in the cut, and you have the saw ported to run at 12K in the cut, the ignition timing at 12K might not be what it is at 11K. Spark advance, rather than spark interruption is the issue.

I'm just running with the theory, I'm no saw mech so take it with a wheelbarrow of salt..

Ian

 

[Dapper Dan]

Well...this is way over this dumb old body man's head except for the fact that the modded saw ran 6 seconds faster than the stock one.....

If an unlimited coil made it, say, another second faster would it be worth the trouble and expense?

I say leave it as is and run it side by side with your modded (unlimited) 460 and see how it compares.

Good work Sling'r!

 

[Peacock]

It's not the spark interruption that is the problem in this theory. If it interrupts the the spark at 13k, it isn't affecting the saw when it's in the cut at 12k. However, if the saw is designed to run at 11K in the cut, and you have the saw ported to run at 12K in the cut, the ignition timing at 12K might not be what it is at 11K. Spark advance, rather than spark interruption is the issue.

I'm just running with the theory, I'm no saw mech so take it with a wheelbarrow of salt..

Ian

I'd say you summed it up pretty well. This assuming that the 7900 coil actually manipulates timing.

If not, then any rpm under 13.5k is gonna be unaffected by the coil, assuming the carb is correctly tuned for the jug.

 

[CHEVYTOWN13]

Camshafts and different variables

All this talking about port timing is making me think of camshafts and how different bumps act--valves closing late for top end grunt or closing sooner for torque. Long header tubes vs shorties. Have to change the timing curve.

2 strokes are cool beans man.

 

[Metals406]

But regardless of which coil is installed, you're not going to get the saw to spin to over 13,500 in the cut (as you stated earlier).

I think the point Slinger was trying to make goes as follows (these are purely hypothetical numbers used only for example.)

Let's say the saw is limited to 13,500 out'a the cut... And you tune to 13,500, right at your limiter. Now, let's say, because of a handful of variables--you lose 2,500 RPM in the cut. Let's pretend this 2,500 RPM is a constant.

Now let's say, you remove that limit, and now the saw will spin 14,500 out'a the cut... Now using our rule (imaginary as it may be) of a 2,500 RPM loss... You will now develop 12,000 RPM in the cut.

So to recap:

Limited> 13,500 out--2,500 loss--11,000 in the cut.
Non-limited> 14,500 out--2,500 loss--12,000 in the cut.

As you can see, limited RPM outside the cut, would indeed limit gains in the cut...

 

[Outlaw5.0]

What slinger is trying to say is the peak torque/hp has moved up in the rpm range. Instead of having a 10,000-13,500 power range, it now may be 11,000-14,500 rpms. With the rpm limiter, he is not using all his power.

A good example is take a 500 c.i.d NHRA Pro Stock engine, it has a usable rpm range of 9000-10,500 rpms, what if it is rpm limited to 8000 rpms?, will it be as quick................of course not.

 

[CHEVYTOWN13]

Door slammer connection

A good example is take a 500 c.i.d NHRA Pro Stock engine, it has a usable rpm range of 9000-10,500 rpms, what if it is rpm limited to 8000 rpms?, will it be as quick................of course not.

AMEN.

 

[Tree Sling'r]

In a nutshell the last few posters have more or less put some of my thoughts into their own words.
I think where I, Wet and Lakeside are getting crossed up is my ideas of rpm and horsepower and torque. Their terms and basis are for sure more educated than mine.
I have not many clues on the curves, just that it is what is in the way of making the gains I beleive are in the 7900, especially after porting.

Anyway, thanks I have learned a lot here on this thread.

 

[B_Turner]

You'll have to move your flywheel around quite a bit. The 272 coil has a timing circuit and it's not the same as any 3120 coil.
.

Be aware that the 3120 flywheels that go with the green and black coils are keyed maybe (rough guess from memory) 15 degrees differently from each other in relation to the magnets.

 

[redprospector]

In a nutshell the last few posters have more or less put some of my thoughts into their own words.
I think where I, Wet and Lakeside are getting crossed up is my ideas of rpm and horsepower and torque. Their terms and basis are for sure more educated than mine.
I have not many clues on the curves, just that it is what is in the way of making the gains I beleive are in the 7900, especially after porting.

Anyway, thanks I have learned a lot here on this thread.

A friend of mine has a race ported 7900, ported by the same guy that built mine. They are identical, except mine is not limited and his is.
In small wood I can tear his butt up. In bigger wood the only difference in these two saw's is chain & operator.

Andy

 

[Jacob J.]

Be aware that the 3120 flywheels that go with the green and black coils are keyed maybe (rough guess from memory) 15 degrees differently from each other in relation to the magnets.

Yeah- Ed had one with the green coil but he had move that flywheel around close to 180 degrees or so to get it to work with the 272 module. He ground the cast-in key off.

 

[brncreeper]

Make your own custom ignition map.

 

[Haywire Haywood]

I think the point Slinger was trying to make goes as follows (these are purely hypothetical numbers used only for example.)

Let's say the saw is limited to 13,500 out'a the cut... And you tune to 13,500, right at your limiter. Now, let's say, because of a handful of variables--you lose 2,500 RPM in the cut. Let's pretend this 2,500 RPM is a constant.

Now let's say, you remove that limit, and now the saw will spin 14,500 out'a the cut... Now using our rule (imaginary as it may be) of a 2,500 RPM loss... You will now develop 12,000 RPM in the cut.

So to recap:

Limited> 13,500 out--2,500 loss--11,000 in the cut.
Non-limited> 14,500 out--2,500 loss--12,000 in the cut.

As you can see, limited RPM outside the cut, would indeed limit gains in the cut...

Not really. This is why. In this scenario, you are assuming that your ability to adjust the carb stops when you hit the limiter at unloaded WOT. If the correct air/fuel mixture for the ported saw to run at peak power in the cut (12k) is say 15:1, and the ignition limiter starts dropping spark at 13k which is 14:1, there is no reason why you can't continue to lean it out to the correct 15:1 mixture, even though the saw won't wind out to let you see it on a tach. When it's under load at 12k, the mixture will still be correct. If spark advance is taken out of the picture, the unlimited coil simply lets you see the results of leaning on a tach, which is an ease of tuning issue. Not really a cutting issue unless you're limbing at close to what unloaded WOT would be if it weren't limited.

Now this might be an issue and I think it's been touched on before. If you're walking a tree limbing and are constantly in the limiter because it's a mostly unloaded to just slightly loaded scenario, I can see a limiter being a real pain in the azz with the saw never running clear and stuttering the whole time.

Ian

 

[B_Turner]

Yeah- Ed had one with the green coil but he had move that flywheel around close to 180 degrees or so to get it to work with the 272 module. He ground the cast-in key off.

The black and green coiled 3120 flywheels I have all have a separate key, not cast in.

Ed's posted that he's never had a timing light on a chainsaw ever in his life. And when I asked him whether he knew if the top rpm timing was different between the green and black coiled setups (my suspicion is that the EPA version is slightly advanced), he didn't know.

I can't figure out how he knows where to reset the flywheel to...Course I confuse pretty easy.

 

[Jacob J.]

The black and green coiled 3120 flywheels I have all have a separate key, not cast in.

Ed's posted that he's never had a timing light on a chainsaw ever in his life. And when I asked him whether he knew if the top rpm timing was different between the green and black coiled setups (my suspicion is that the EPA version is slightly advanced), he didn't know.

I can't figure out how he knows where to reset the flywheel to...Course I confuse pretty easy.

Well basically I followed his thread, and in order to get the saw to run, he ended up somewhere around 172 degrees from where the flywheel had been in the stock position, according to him. I've never done this conversion so I can't speak from experience. I have however, converted an 064 to the non rev-limited universal ignition using the flywheel for the limited module, and I ended up about 68 degrees ahead of what the flywheel had been at with the limited module.

 

[B_Turner]

I have however, converted an 064 to the non rev-limited universal ignition using the flywheel for the limited module, and I ended up about 68 degrees ahead of what the flywheel had been at with the limited module.

How did you end up with that flywheel position? I mean how did you decide the 68 degrees?