Mastermind meets the Dolmar 7900

[deye223]

Here's the vid I mentioned.

gday brad was that a ported 460

 

[Mastermind]

Numbers.....for the junkies.

82 intake
98 exhaust
126 transfers

.030 popup
removed .010 from the squish band....

I'm putting it together now...

 

[watsonr]

Randy,
Did you do anything to the upper transfers or the exhaust? Would like to see a picture if you did, I may have missed it?

 

[Mastermind]

Randy,
Did you do anything to the upper transfers or the exhaust? Would like to see a picture if you did, I may have missed it?

A lot has been done to both. I'm in the shop on my laptop now though and can't post pics.

 

[srcarr52]

Numbers.....for the junkies.

82 intake
98 exhaust
126 transfers

.030 popup
removed .010 from the squish band....

I'm putting it together now...

Or in durations...

Intake 164
Exhaust 164
Transfers 108
Putting blowdown at 28 <- IMO thats a lot of blowdown. :msp_biggrin: Going for fuel economy? Or with the crankcase compression of the 7900 you just don't need that much transfer timing?

I also like keeping the intake:exhaust around 1:1, what goes in must come out.

 

[blsnelling]

Sorry about going of topic here Randy. I'm anxious to see this baby run!

 

[Mastermind]

Or in durations...

Intake 164
Exhaust 164
Transfers 108
Putting blowdown at 28 <- IMO thats a lot of blowdown. :msp_biggrin: Going for fuel economy? Or with the crankcase compression of the 7900 you just don't need that much transfer timing?

I also like keeping the intake:exhaust around 1:1, what goes in must come out.

Everyone says that the 7900 needs mucho blowdown. We shall see.

Sorry about going of topic here Randy. I'm anxious to see this baby run!

Oh heck, this thread has had some great info shared in it. Anytime I post a thread I only do it to share what I've learned........derailing is where the best stuff comes from sometimes.

 

[blsnelling]

It surely does like lots of BD. You can make a great runner of other models with maybe half that amount.

 

[srcarr52]

Everyone says that the 7900 needs mucho blowdown. We shall see.

I've also heard that they have a very small crankcase that gives them a lot of crankcase compression. You think this has something to do with not needing much transfer timing? :msp_confused: I see for once you've done extensive lower transfer work, thinking it already has enough transfer port velocity? oke:

 

[Mastermind]

I've also heard that they have a very small crankcase that gives them a lot of crankcase compression. You think this has something to do with not needing much transfer timing? :msp_confused: I see for once you've done extensive lower transfer work, thinking it already has enough transfer port velocity? oke:

Exactly. The 7900 has a small crankcase, and the ring end makes it hard to widen the uppers enough. Plus if they aren't being raised too much there's not a lot of time/area there. I widened the entire transfer tract so I could open the exhaust side of them a bit without losing the angle of entry.

This saw has more time in the port work than two normally do.

 

[wendell]

Pics....

Stop hijacking this thread! :msp_mad:

 

[booger1286]

Im excited to see. When is the video comin.

 

[Mastermind]

Stop hijacking this thread! :msp_mad:

If we ever get to meet Ima kick you in the pants. :hmm3grin2orange:

 

[srcarr52]

Exactly. The 7900 has a small crankcase, and the ring end makes it hard to widen the uppers enough. Plus if they aren't being raised too much there's not a lot of time/area there. I widened the entire transfer tract so I could open the exhaust side of them a bit without losing the angle of entry.

This saw has more time in the port work than two normally do.

I do the same on 394's because of the locator pin location is already close to the intake side of the transfer ports and there is plenty of room left on the exhaust side to open it up without loosing the inlet angle.

Did you cut the squish band or use the sand paper mandrel trick?

I spent all last week making the fixtures to mount a cylinder in that lathe to cut squish bands but my first saw in that I was going to do it to (Stihl MS660) has a large ring of unplated cylinder. So I don't think I'll drop the cylinder much on this one, just a quick touch up with sand paper and 0.005" off the base. I’d rather it be a little lower on compression than I like then dropping the top ring into the ring of death. :msp_mad: I was really interested in see what they would do with less then stock exhaust timing since they are already at 167 stock.

Wait... :arg: Sorry Wendell.

 

[Mastermind]

I do the same on 394's because of the locator pin location is already close to the intake side of the transfer ports and there is plenty of room left on the exhaust side to open it up without loosing the inlet angle.

Did you cut the squish band or use the sand paper mandrel trick?

I spent all last week making the fixtures to mount a cylinder in that lathe to cut squish bands but my first saw in that I was going to do it to (Stihl MS660) has a large ring of unplated cylinder. So I don't think I'll drop the cylinder much on this one, just a quick touch up with sand paper and 0.005" off the base. I’d rather it be a little lower on compression than I like then dropping the top ring into the ring of death. :msp_mad: I was really interested in see what they would do with less then stock exhaust timing since they are already at 167 stock.

Wait... :arg: Sorry Wendell.

I just finished a 394. I used a 395 piston since the locator pins are both above the intake port. That way you can go wild on the uppers.

The BB 064 I just built has much lower intake timing than stock. It turned out to have #### load of torque, but not so much rpm.....13,400 unloaded.

 

[Metals406]

Correct me if I'm wrong, but as a rule of thumb, is it?
↓
↓

The less crankcase volume, the more velocity in the trans, and they can benefit from a little area work on lowers and directing uppers.

Larger crankcase volume, lower trans velocity, benefit from less trans area work on the lowers to retain that needed velocity, but benefits from directing and smoothing/blending.

 

[parrisw]

Correct me if I'm wrong, but as a rule of thumb, is it?
↓
↓

The less crankcase volume, the more velocity in the trans, and they can benefit from a little area work on lowers and directing uppers.

Larger crankcase volume, lower trans velocity, benefit from less trans area work on the lowers to retain that needed velocity, but benefits from directing and smoothing/blending.

That's what I always thought Nate.

 

[Mastermind]

Correct me if I'm wrong, but as a rule of thumb, is it?
↓
↓

The less crankcase volume, the more velocity in the trans, and they can benefit from a little area work on lowers and directing uppers.

Larger crankcase volume, lower trans velocity, benefit from less trans area work on the lowers to retain that needed velocity, but benefits from directing and smoothing/blending.

Yep, that's what I think. The 7900 is known to have a very tight crankcase with a 79cc topend.

That's why I did so much transfer work on this saw.

 

[Jon1212]

Correct me if I'm wrong, but as a rule of thumb, is it?
↓
↓

The less crankcase volume, the more velocity in the trans, and they can benefit from a little area work on lowers and directing uppers.

Larger crankcase volume, lower trans velocity, benefit from less trans area work on the lowers to retain that needed velocity, but benefits from directing and smoothing/blending.

Uh Oh, now this is gonna get personal. Please don't make "thumb" jokes in the presence of Randy.


He is quite sensitive about his 3/4 digit, especially since the last time he hitchhiked the guy thought he only wanted a ride part way there.............LOL!!!!

 

[Mastermind]

Uh Oh, now this is gonna get personal. Please don't make "thumb" jokes in the presence of Randy.


He is quite sensitive about his 3/4 digit, especially since the last time he hitchhiked the guy thought he only wanted a ride part way there.............LOL!!!!

Your 261 is in a box.......not sure it'll ever get mailed now......:msp_confused: