McCulloch SP-81 Project (Experiment)

[stinkbait]

Mitch, did you happen to see Scarrs thread on "reworking" the ports on the 394. Maybe something to consider to get rid of the McCulloch groove for compression release?

If you are talking about the McCulloch "Q" Port, this saw doesn't have that. It just has an elliptical exhaust port and an external compression release.

 

[homelitejim]

Another thing to consider is bar length, these saws shine with longer bars especially ones longer than 24".

 

[stinkbait]

Another thing to consider is bar length, these saws shine with longer bars especially ones longer than 24".

Once I finish changing the port timing that may not be the case anymore.

 

[heimannm]

Very interesting Mitchell. I have couple of SP81's that I really like, and they are about 1/2 worn out! I can only imagine what a fresh one with a little massaging could do.

Mark

 

[Jacob J.]

I hope it can handle the carb. I'm thinking that it will probably do fine at WOT, but have trouble idling. I really have no idea if it will work. I know that the Homelite 650 uses the WB carb and its only 18cc bigger than the SP-81. I'm hoping that the porting will help. The only thing that I don't like is that there is no choke.

Another idea is to make your adapter plate somewhat of a "restrictor" plate. This will help get you the velocity you need to
have enough air flow at low RPM for smooth operation. My dad had to do this to a Harley that he put some enormous slide
carbs on. When he was done, it was one of the fastest bikes around. In fact, he could outrun a well-tuned big block in the
short stretches.

 

[stinkbait]

Another idea is to make your adapter plate somewhat of a "restrictor" plate. This will help get you the velocity you need to
have enough air flow at low RPM for smooth operation. My dad had to do this to a Harley that he put some enormous slide
carbs on. When he was done, it was one of the fastest bikes around. In fact, he could outrun a well-tuned big block in the
short stretches.

The adapter does restrict the area some. The area of the top of the adapter is 0.957 in. sq. By the time it transitions to the bottom of the adapter plate it's around 0.844 in. sq. And by the time it reaches the cylinder wall the intake port area will be 0.784 in. sq.

 

[stinkbait]

Got some more work done on the 81. I degreed the ports to see where the engine stands. Don't pay attention to the stuff wrote on the wheel. That's from another saw.

My piston stop tool.

The current numbers on this engine

EX - 100
IN - 64
TRANS - 115
BD - 15

The exhaust is already high enough as is. I'm going to widen it to the max and leave the timing as is. The transfers are higher than I would like, creating a short blowdown. So I'm not going to raise the transfers any. I'm just going to open up the bottom of the transfers and blend the tops towards the intake. I plan on lowering the intake to 100, but that requires removing a lot of material.

The proposed numbers

EX - 100
IN - 80
TRANS - 115
BD - 15

I would like some opinions on my final numbers. Once I remove the material, I can't put it back.

 

[Mastermind]

Got some more work done on the 81. I degreed the ports to see where the engine stands. Don't pay attention to the stuff wrote on the wheel. That's from another saw.

My piston stop tool.

The current numbers on this engine

EX - 100
IN - 64
TRANS - 115
BD - 15

The exhaust is already high enough as is. I'm going to widen it to the max and leave the timing as is. The transfers are higher than I would like, creating a short blowdown. So I'm not going to raise the transfers any. I'm just going to open up the bottom of the transfers and blend the tops towards the intake. I plan on lowering the intake to 100, but that requires removing a lot of material.

The proposed numbers

EX - 100
IN - 80
TRANS - 115
BD - 15

I would like some opinions on my final numbers. Once I remove the material, I can't put it back.

I like the numbers. I wouldn't mind going in a few times creeping up on the intake target though. You may find with that big carb you won't need as much intake duration as you think.

 

[stinkbait]

Another idea is to make your adapter plate somewhat of a "restrictor" plate. This will help get you the velocity you need to
have enough air flow at low RPM for smooth operation. My dad had to do this to a Harley that he put some enormous slide
carbs on. When he was done, it was one of the fastest bikes around. In fact, he could outrun a well-tuned big block in the
short stretches.

Have you known anyone to port a 81 JJ?

I like the numbers. I wouldn't mind going in a few times creeping up on the intake target though. You may find with that big carb you won't need as much intake duration as you think.

How far do you think that I should go the first time? Halfway at 72? To take it all the way to 80 would require that I lower the intake 0.200".

 

[Justsaws]

I would double the height of the adaptor plate, assemble and run the saw before grinding. I would also be putting a 1" long intake tube and running a custom filter on that.
I may have missed it but which muffler are you using?

Excellent project, by the way.

Edit: I just saw the part about using a "stock 81 muffler" with some work done to it.

 

[heimannm]

Keep in mind that a stock SP81 muffler offer very little restriction, a little work on them and they flow very well...

Mark

 

[stinkbait]

Keep in mind that a stock SP81 muffler offer very little restriction, a little work on them and they flow very well...

Mark

I plan to match the flange of the muffler to the exhaust port and make a custom muffler cover.

 

[Mastermind]

How far do you think that I should go the first time? Halfway at 72? To take it all the way to 80 would require that I lower the intake 0.200".

I would probably trim the piston skirt to get the intake timing where I was headed. I've never had a 81 apart so I really don't know if that works on those but thats how I get there on most other saws.

.200 is a big move away from the factory designers numbers. Those guys were pretty sharp.

 

[jnl502]

Sweeeeet project mang..................I really like these kind of builds. Like the old days of Hot Rodding. Buy the biggest engine you could afford and put it in what ever POS car body you could scronge.
count me in!
jnl

 

[stinkbait]

I would probably trim the piston skirt to get the intake timing where I was headed. I've never had a 81 apart so I really don't know if that works on those but thats how I get there on most other saws.

.200 is a big move away from the factory designers numbers. Those guys were pretty sharp.

You think that I would be safe to go .050 off the skirt and another .050 off the bottom of the intake? Lowering the intake is about the only way I can achieve anymore intake area because it's pretty much as wide as it can go from stock. The port is short and wide.

 

[Mastermind]

You think that I would be safe to go .050 off the skirt and another .050 off the bottom of the intake? Lowering the intake is about the only way I can achieve anymore intake area because it's pretty much as wide as it can go from stock. The port is short and wide.

If it's area you are looking for lowering the intake is the way to go. I was just thinking that if by chance you don't like the altered timing you could find another piston.

 

[Macman125]

Whatever became of this saw? I am working on porting a 6-10 and wouldn't mind seeing some pics of some port work that has neen done to the 10 series macs.

 

[homelitejim]

I got stalled on my experiment, seems those fella's at McCullough knew what they were doing. I could never get enough power over stock or durability to make it worth while.