Dolmar 64xx/73xx/79xx series, smurfs and solo 665/675/681 thread

[huskyboy]

What’s the consensus on the 7900 vs the 7910 saws? What cylinder can be made stronger? Or are they pretty much identical?

 

[Wilhelm]

The 7910 piston is preferred over the 7900 one, I am uncertain about the cylinders - but the 7910 has a much larger outlet stock due to the SLR.

The 7900 13.5k coils are preferred over the 7910 12.8k coil - but mostly for tuning reasons.

 

[mdavlee]

Cylinder port timing on 2 of each was within 1 degree on the exhaust and intake best I remember. I prefer the ring location on the 7900 piston and the slab side for the solid side to force the charge into the transfers

 

[mettee]

What’s the consensus on the 7900 vs the 7910 saws? What cylinder can be made stronger? Or are they pretty much identical?

The 7910 with the 13500 coil or 272 coil is the combo I see most(strongest appearing). The 7910 piston is obviously non windowed, but it's also lighter than the 7900 windowed piston(from what I read). So the combo of the new style piston with the cylinder is what I think does it.

In the newer 7910 cylinder the casting is more precise and the transfers seem really uniform from what I have seen comparing the two. Just seems like the 7910 casting is nicer.

 

[huskyboy]

What is transfer timing on the two? I heard the 7910 is different transfer #’s to work with the more restrictive slr muffler to meet epa rules... is this true? Which would explain the lower hp output stock? Seems easy enough to rectify ...

 

[Derf]

Stupid question - what is a slab side piston?

 

[mettee]

Stupid question - what is a slab side piston?

The non windowed 7910 piston is slab side, aka no windows. Side is a slab.

 

[Simondo]

Stupid question - what is a slab side piston?

Pic for you of one........

 

[huskyboy]

In theory has more case compression than the windows one and it is lighter

 

[huskyboy]

But... the windowed one has a centered ring pin

 

[mettee]

How much does that ring gap being centered help compression? Or how much does it reduce friction?

As I understand it the ring does two things in an engine(not oil ring) seals in compression and is a factor of causing friction while it does its primary job.

I'd think that no matter where the ring gap is, it's a round cylinder with a round ring, right? So it's applying the same equal force no matter where the pin is to locate the gap.

The ring is clearanced so when hot it doesn't bind in the cylinder, so it's essentially meant to float in the ring land, and I wouldn't think it was applying any extra force on one side if the cylinder or the other.

What's the concensus from guys like mdavlee, randy, and Carl, and Al? I'm just putting my thoughts out there, Im not saying I'm right.

Oh and I'd tend to think the force on the ring was all up and down, that there is very little side to side. Side to side would be a factor of piston to wall clearance.

 

[Hedgerow]

How much does that ring gap being centered help compression? Or how much does it reduce friction?

As I understand it the ring does two things in an engine(not oil ring) seals in compression and is a factor of causing friction while it does its primary job.

I'd think that no matter where the ring gap is, it's a round cylinder with a round ring, right? So it's applying the same equal force no matter where the pin is to locate the gap.

The ring is clearance so when hot it doesn't bind in the cylinder, so it's essentially meant to float in the ring land, and I wouldn't think it was applying any extra force on one side if the cylinder or the other.

What's the concensus from guys like mdavlee, randy, and Carl, and Al? I'm just putting my thoughts out there, Im not saying I'm right.

A centered ring gap is advantageous at times where port sizes and shapes are concerned.

 

[Hedgerow]

In theory has more case compression than the windows one and it is lighter

More case compression is a good thing..

Until it's not...

 

[mettee]

A centered ring gap is advantageous at times where port sizes and shapes are concerned.

Okay that makes sense, it's strictly a factor of getting every bit of the port work you can to increase flow without having to compromise and make porting smaller.

 

[huskyboy]

More case compression is a good thing..

Until it's not...

They must’ve changed it for a reason... whatever that may be? Whether for epa reasons... or a performance standpoint. I tend to think the latter. At least that is my thinking.

 

[huskyboy]

My red top 7900 seems to be “sportier” feeling with the slab side slug... as in it revs a bit faster and likes to be tuned higher. It also sounds a bit different and maybe a bit smoother vibes. *I have not scientifically proven this*... just my butt dyno. Lol. I really wish I tested before and after I installed the piston. But it would have been meaningless as I tightened squish and matched the lowers to the case and some carb work...

 

[mettee]

She is in good shape. May need new oil lines I can't tell yet, I need to look. Definitely needs new AV springs.

 

[huskyboy]

She is in good shape. May need new oil lines I can't tell yet, I need to look. Definitely needs new AV springs.

It’s junk it’s the small bore you need to get a bb kit! Lol

 

[mettee]

It’s junk it’s the small bore you need to get a bb kit! Lol

You know I toyed with the idea of porting it up. But I'm gonna just square it away and run it as a 7300. It frickin revs right up, it just needs a little lovin. The muffler is already opened up for me. I'll do the AV springs, oil lines, small stuff, and run her.

My makita is gonna be all out

I need another beer. Who else wants one?

 

[mettee]

I see John Reilly running nitrous on one of his saws.

It was .....insane