Where do they get their GO?

[Crofter]

The 681 / 7900 cousins are much in the news recently as pretty hot contenders right off the sales counter. The rather meager improvements that modification yeilds would lead you to believe that a lot of that technology was put in before they left the factory but the size of transfers and ports or the timing durations do not fall in line with what has to be done to make the other ones go. What effect are they capitalizing on to make them such good performers? They seem to be following a different set of rules. How come a good 372 can be made to skin them at higher rpm? (or can they?)

 

[RiverRat2]

The 681 / 7900 cousins are much in the news recently as pretty hot contenders right off the sales counter. The rather meager improvements that modification yeilds would lead you to believe that a lot of that technology was put in before they left the factory but the size of transfers and ports or the timing durations do not fall in line with what has to be done to make the other ones go. What effect are they capitalizing on to make them such good performers? They seem to be following a different set of rules. How come a good 372 can be made to skin them at higher rpm? (or can they?)

all I know is The EHP 372-5 of Haywire Haywood that was At the PNW GTG Smoked the 681's and 7900's hands down!!!!!!!!!! actually it was _ _ _gging ugly!!!

 

[Crofter]

The 372 has a lot more room in the base percentage wise. I thought making the base tighter always made more power?

 

[drmiller100]

all I know is The EHP 372-5 of Haywire Haywood that was At the PNW GTG Smoked the 681's and 7900's hands down!!!!!!!!!! actually it was _ _ _gging ugly!!!

second day the 372 was about the same as the 681's / dolmars.

very impressive running saw.

i really am not that impressed with any of the chainsaws. 80 cc's, and we're all happy with 8 or 9 horsepower??????

i'm convinced the 7900 is limited by transfers. still debating what to do about it.

 

[Crofter]

Transfers are small but some fellows have hogged them away out and made a good percentage loss of power. Maybe they are matched with the tight cranckcase and high velocity. Loose the transfer velocity and maybe they dont scavenge as well? Thoughts?

 

[drmiller100]

my red top east coast saw has huge timing numbers, super lightweight piston, gutted muffler, and makes good go at big rpm. more revvy then adam's, but somewhere in the ballpark on power.

my blacktop homebrew has HUGE hogged out transfers, almost stock piston, hole cut in the side of the muffler, and more moderate timing numbers. It makes for sure more power then the redtop, and a much wider powerband.
even a monkey like me can cut reasonably well with a dull chain.


again, we can discuss all we want, but when all done the following is my opinion.

- a 5 degree advance in timing helps them a lot.
- a larger exhaust outlet for the muffler helps
- bigger transfers pointed in the right direction helps
- increase in port timing numbers helps
- lose the base gasket and enjoy larger compression

i'm working on two other things. I'd like to run a full circle crank. and something else.

port timing is always a tradeoff. crank the numbers up, and you lose power band width and fuel economy.

 

[4492011]

7900

Hi I would like to know on a stock 7900 what would be an acceptable squish measurement? I would like to remove the base gasket and mod the muffler I do not want a full mod just a little more power. I would be running premium pump gas.

 

[drmiller100]

create a new thread for the 7900, and several of us will give you ideas......

 

[husq2100]

Frank,

do you think it has something to do with Time-Area, as described by Gordon Jennings?

im reading bits and pieces from his book, i noted he also talks about stufing crank cases... may be not so desirable???

serg

 

[drmiller100]

i'm a very serious minority, but i believe strongly in keeping the crankcases as small and tight as possible.

 

[husq2100]

im very seriously a novice so i dont know at all : )

serg

 

[romeo]

I have noticed that reed motors like a tighter crank case, where piston port motors like a little room. The 681/7900 have a very large crankcase compared to the other saws. I think the transfer arrangement is set up as nice as any saw can be from the factory. But look at the transfers in a YZ80, no saw is set up with nostrils like that.

None of those Solos or Dolmars at that GTG were turned up full blown R@CE$@W$, they were work mods.

Redprospectors race 7900 hasn't been beat by a 372 yet, get past the coil and run the port numbers high and they will hold torque at extremely high rpm's.

 

[Crofter]

I think there is another factor to the time area equation and that would be velocity. I think the resonant tuning (ram effect) is important to the 7900 681. Gas velocity in intake and transfers has a big effect. If you push the rpm up as Romeo suggests and open up transfers etc, that is probably taking you back to the same gas velocity to make the resonance work again, however at lower rpms the enlarged transfers will lower transfer velocity and scavanging will not be as good (hurt low end torque) With the inherent limits to exhaust port widening I think they start to eventually run out of wind before the modded 372/375, no matter what you do to the transfers. (unless of course you go to a different piston and put a block on the exhaust and add additional transfers!)

Romeo, did you measure crank case volume on the 681 and 7900?

 

[Jacob J.]

Maybe the answer in the 7900/681 then is not to open up the existing transfers but to add two more small external transfers which would enter near just over the intake port.

im reading bits and pieces from his book, i noted he also talks about stufing crank cases... may be not so desirable???

Thing to remember there Serg is that Jennings' book mainly focuses on motorcycle theory, which has a lot of different parameters than saw motors.

 

[drmiller100]

motorcycles need to make a lot more power at lower rpms to get the bike moving off the line.

motorcycles have a lot more competition between amongst themselves.

 

[rbtree]

all I know is The EHP 372-5 of Haywire Haywood that was At the PNW GTG Smoked the 681's and 7900's hands down!!!!!!!!!! actually it was _ _ _gging ugly!!!

Interesting. Ed did mine about the same time. I've talked with Haywire about his saw. Mine underwhelms...AND has 145# compression. Methinx Ed farked up the port timing....

 

[Crofter]

Interesting. Ed did mine about the same time. I've talked with Haywire about his saw. Mine underwhelms...AND has 145# compression. Methinx Ed farked up the port timing....

That seems really strange. Without measuring it up and degreeing it, I wouldn't like to speculate but usually Ed seems to go for high compression and fairly reasonable exhaust duration.