Stroking a Dolmar 7900

[Fruecrue]

The 'induction stroke' for a two-stroke is from transfer closing to TDC. When you raise the transfer height, you reduce the induction stroke. (you can get the same time/area by widening the transfers without reducing the induction stroke)

It would be interesting to see what a stock 7900 performs like when the crankcase is machined out to give an extra .30" of clearance. The added crankcase volume will reduce the crankcase compression and broadened the powerband.

Gives a guy a lot to think about. My brain wants to trend data. I’m always looking for a relationship of bore to stroke vs. what timing or other characteristics make a certain motor shine. Induction stroke seems relative here, as does blowdown.
I’m still trying to fully understand what makes this saw run best with more blowdown than others.
Not trying to be picky, but that added crankcase volume comes with an added mm of stroke, so saw isn’t exactly stock anymore. I think I’m picking up what you’re putting down though. I’ve been told that tight case helps to make the 7900 what it is.

 

[Fruecrue]

But the crank will be taking up that space that is machined out right?

It will certainly need that space, but will the total volume of the crank be more? Enough to equal the volume of magnesiun ground from the case? I have no idea.

 

[Deets066]

The added crankcase volume will reduce the crankcase compression and broadened the powerband.

You talkin specific to the 7900 or any saw?

 

[Terry Syd]

You talkin specific to the 7900 or any saw?

Any saw, the added crankcase volume lowers the maximum delivery ratio (peak torque) in the powerband. That's why chainsaws have a relatively low crankcase compression ratio as they need a broad powerband.

What I was referring to in my post was by reducing the crankcase compression on a stock 7900, could the width of the powerband be increased. In other words, is a felt increase in torque in such a mod more a result of a bigger crankcase volume and less of an increase in stroke.

I don't know the answer, but I do have a strong aversion to raising transfers on two-strokes. Sometimes it is necessary, but a quad-port chainsaw has plenty of room to widen the transfers and still leave good ring support.

 

[Deets066]

Any saw, the added crankcase volume lowers the maximum delivery ratio (peak torque) in the powerband. That's why chainsaws have a relatively low crankcase compression ratio as they need a broad powerband.

What I was referring to in my post was by reducing the crankcase compression on a stock 7900, could the width of the powerband be increased. In other words, is a felt increase in torque in such a mod more a result of a bigger crankcase volume and less of an increase in stroke.

I don't know the answer, but I do have a strong aversion to raising transfers on two-strokes. Sometimes it is necessary, but a quad-port chainsaw has plenty of room to widen the transfers and still leave good ring support.

Why do you hear about crank stuffers and full circle cranks if in fact it should be the other way around?

 

[Terry Syd]

You can push the 'peak delivery ratio' up higher with a higher crankcase compression. So, if you are racing and not worrying about low-end torque, stuffing the crankcase makes a lot of sense.

Chainsaw racers have mentioned that running a full-circle crank to reduce the crankcase volume pushes the powerband up about 400 RPM.

If you are running a small saw (like for limbing) and you want it to produce more top-end power for the same size and weight, you can do the same trick, but with a reduction in low-end torque.

 

[Terry Syd]

Here's a better trick for the 7900 - find an old Zama C3M 'twin jet' carb and have Poleman mod it for the saw. The increase in low-end torque with a proper fuel ratio has to be felt to understand.

 

[mettee]

Here's a better trick for the 7900 - find an old Zama C3M 'twin jet' carb and have Poleman mod it for the saw. The increase in low-end torque with a proper fuel ratio has to be felt to understand.

How can you tell if you have a twin jet Terry?

 

[Terry Syd]

You have to open up the 'wet side' and see if there is a brass jet on each side of the welch plug (thus 'twin jet').

There have been a few threads that have mentioned the carbs and mods on this forum. Several have pictures of the insides.

Poleman used to be able to get Chinese Twin Jets for less than $20 and then mod them, unfortunately that source has dried up. Now you have to scrounge around and find an old Zama C3M.

 

[Deets066]

You can push the 'peak delivery ratio' up higher with a higher crankcase compression. So, if you are racing and not worrying about low-end torque, stuffing the crankcase makes a lot of sense.

Chainsaw racers have mentioned that running a full-circle crank to reduce the crankcase volume pushes the powerband up about 400 RPM.

If you are running a small saw (like for limbing) and you want it to produce more top-end power for the same size and weight, you can do the same trick, but with a reduction in low-end torque.

Every saw I have added case compression to has been a really good runner throughout the power curve. I have not tested it by itself though. Gunna have to try that one of these days

 

[Fruecrue]

 

[fin460]

View attachment 150333

looks nice.

how are you measuring how much you remove

what are you using to remove material with?

 

[hseII]

looks nice.

how are you measuring how much you remove

what are you using to remove material with?

Sharpie lines are 0.020”.

Well the regular ones.

Those Jumbo are moar.


Sent from my iPhone using Jon1212 Pro

 

[Mastermind]

Are we there yet?

 

[Fruecrue]

looks nice.

how are you measuring how much you remove

what are you using to remove material with?

Measure thickness before and after.
Dremel w/ carbide burr.

Are we there yet?

Yes, yes we are. Wait, where?

 

[Adirondackstihl]

Don’t know and don’t care.....but whatever Moobisicles done to my 7900/7910......
Yeah....that

 

[Redfin]

What clearance between crank lobes and case are you shooting for Dave?

 

[hseII]

What clearance between crank lobes and case are you shooting for Dave?

Sent from my iPhone using Jon1212 Pro

 

[RI Chevy]

I am trying to figure out how he got the belt sander inside that case. LOL. [emoji848]

 

[Redfin]