playing with a pop-up, 066 BB

[Brmorgan]

Just a thought on the cylinder differences - the one I have that you think has better porting numbers has a big "A" stamped on the cylinder base on the intake side. I wonder if that means anything?

 

[mtngun]

Just a thought on the cylinder differences - the one I have that you think has better porting numbers has a big "A" stamped on the cylinder base on the intake side. I wonder if that means anything?

Maybe one of our 066 gurus will chime in with the answer.....

In the old days, didn't they mark cylinders and pistons with A's and B's to designate minor variations in diameter ?

 

[mtngun]

Rechecked compression today -- 128 psi cold. That's a whopping 5 psi more it blew before the pop-up. I'll try to do timed cuts later this week.

 

[russ61]

missing something???

Am I missing something?The top of the piston was machined(metal removed)And chamber volume decreased??I understand how a pop up can change flame travel ,but not decrease volume in this case.
Russ

 

[4492011]

Am I missing something?The top of the piston was machined(metal removed)And chamber volume decreased??I understand how a pop up can change flame travel ,but not decrease volume in this case.
Russ

He also milled the cylinder base.

 

[russ61]

oops

didn't see that part

 

[parrisw]

rechecked compression today -- 128 psi cold. That's a whopping 5 psi more it blew before the pop-up. I'll try to do timed cuts later this week.

whooo hoooo

 

[Meadow Beaver]

Am I missing something?The top of the piston was machined(metal removed)And chamber volume decreased??I understand how a pop up can change flame travel ,but not decrease volume in this case.
Russ

When the piston is for a saw with a centered pop-up it does help evenly distribute the charge across the top of the piston. No it won't decrease case volume either.

 

[willysmn]

Pop up piston timing

You get more compression with the pop up piston mods, but it also changes the timing for intake and exhaust. How does all this come out? I liked the diagram shown for the 066, but I need to know the effect of the timing changes created by machining the piston. What would be the ideal port timing for a saw. I also don't know what SPI is? Something to do with compression?

 

[parrisw]

You get more compression with the pop up piston mods, but it also changes the timing for intake and exhaust. How does all this come out? I liked the diagram shown for the 066, but I need to know the effect of the timing changes created by machining the piston. What would be the ideal port timing for a saw. I also don't know what SPI is? Something to do with compression?

It usually doesn't change timing enough to worry about. If you machine .030 off the piston, then say .035 of the base of the cylinder, the change is only
.005, that probably wouldn't be noticeable on a degree wheel.

 

[mtngun]

You get more compression with the pop up piston mods, but it also changes the timing for intake and exhaust. How does all this come out?

Dropping the cylinder increases intake duration. This is true whether or not there is a pop-up.

Dropping the cylinder without a pop-up (i.e., removing the base gasket) decreases exhaust duration.

Dropping the cylinder with a pop-up, while maintaining the same squish, the exhaust timing is unchanged.

I thought I had shown the before and after timing numbers, but it's been a long thread, so I don't remember. It picked up a few degrees duration on the intake, no change on the exhaust.

What would be the ideal port timing for a saw ?

Depends on who you ask and what the saw is used for, but something like 150 degrees duration on intake, 160 degrees duration on exhaust, seems to be pretty common for work saws.

I also don't know what SPI is?

I couldn't find where that term was used in this thread ?

 

[Meadow Beaver]

Square pounds/inch? lol

 

[willysmn]

Port timing

Thanks for the explanations. Cutting off the piston and the cyl and not changing the port timing is obvious, but I did not see it.

That SPI term was on the diagram for the 066 cylinder. It looks like the time the piston is stationary at TDC, but that's a guess.

 

[mtngun]

That SPI term was on the diagram for the 066 cylinder.

OK, I went back and found it.

SUB-PISTON-INDUCTION. It's what most people on AS call "freeporting", when the piston skirt rises above the exhaust port, allowing exhaust to be sucked back into the crankcase. It sucks, literally.

 

[mtngun]

Finally made time to do a speed test.

As part of my milling chain comparison tests, I've been doing timed milling cuts on a 12.5" pine cant.

The saw had been previously tuned by ear to 13,500, where it was still blubbering. It didn't clean up until 14,000+.

1st pass, 3/8 lo-pro milling chain. 0.76 inch/sec. Seemed happy running 10,400 RPM. About 6 feet into the cut, the engine bogged. After idling for a couple of seconds, it was willing to resume cutting. There were two big knots at the location where it bogged.

I was paranoid that the saw might have overheated, so I retuned to 12,000 RPM before the next cut.

2nd pass, 3/8 lo-pro milling chain. 0.76 inch/sec. This time it only pulled 9500 rpm -- yet the cutting speed was exactly the same ! ! ! Once again, it bogged about 6 feet into the cut. Once again, I let it idle for a couple of seconds and then it was willing to resume cutting.

I had planned to make a couple of passes with 3/8 x 0.063" milling chain, but canceled that due to the mysterious bogging problem.

Here is how the same lo-pro chain did with other top ends:

popped & ported BB -- 0.76 inch/sec

popped & ported OEM -- 0.73 inch/sec

stock BB -- 0.69 inch/sec

Despite the stalling problem, and the rings failing to seat well, this top end still holds my current speed record with lo-pro chain.

I'll let it cool down, take another compression measurement, then this top end will come off and the jug sawed in half so I can examine the funny lines near the top of the cylinder.

 

[parrisw]

Finally made time to do a speed test.

As part of my milling chain comparison tests, I've been doing timed milling cuts on a 12.5" pine cant.

The saw had been previously tuned by ear to 13,500, where it was still blubbering. It didn't clean up until 14,000+.

1st pass, 3/8 lo-pro milling chain. 0.76 inch/sec. Seemed happy running 10,400 RPM. About 6 feet into the cut, the engine bogged. After idling for a couple of seconds, it was willing to resume cutting. There were two big knots at the location where it bogged.

I was paranoid that the saw might have overheated, so I retuned to 12,000 RPM before the next cut.

2nd pass, 3/8 lo-pro milling chain. 0.76 inch/sec. This time it only pulled 9500 rpm -- yet the cutting speed was exactly the same ! ! ! Once again, it bogged about 6 feet into the cut. Once again, I let it idle for a couple of seconds and then it was willing to resume cutting.

I had planned to make a couple of passes with 3/8 x 0.063" milling chain, but canceled that due to the mysterious bogging problem.

Here is how the same lo-pro chain did with other top ends:

popped & ported BB -- 0.76 inch/sec

popped & ported OEM -- 0.73 inch/sec

stock BB -- 0.69 inch/sec

Despite the stalling problem, and the rings failing to seat well, this top end still holds my current speed record with lo-pro chain.

I'll let it cool down, take another compression measurement, then this top end will come off and the jug sawed in half so I can examine the funny lines near the top of the cylinder.

What the heck is all over your truck? Running down the sides.

 

[mtngun]

What the heck is all over your truck? Running down the sides.

I have no idea. The truck has never been washed for as long as I have owned it.

 

[parrisw]

I have no idea. The truck has never been washed for as long as I have owned it.

ha ha, nice. I used to have a truck just like it.

 

[Hddnis]

I have no idea. The truck has never been washed for as long as I have owned it.

Don't wash it now or it will fall apart!




Mr. HE