Blowdown and Raising Transfers

[Chris-PA]

I've been playing at porting my cheap clamshell Pouns, and I'm running into a couple of limitations. I have one with decently aggressive timing, but now the transfers are too low.

Intake duration = 159
Exhaust Duration = 164
Blowdown round 24

Ports are widened to 60% of bore diameter, the spark timing is advanced about 6[SUP]o[/SUP], it has a larger carb from a 46cc Poulan and the muffler is opened up. It runs quite well, however, it just does not feel as strong as I think it should based on the mildly ported 46cc Poulan I have (different engine series). I'm wondering if the transfer height is a major problem?

I have not figured out any way to get in there and raise the transfers. None of the tips I've tried will allow me to grind with any kind of accuracy. They don't really cut much at all, especially through the chrome on the shot old cylinder I'm practicing on.

Can anyone give any tips on how to cut them, or is it just a limitation of trying to mod small bore clamshells? Or is that amount of blowdown not worth bothering with, as I'm not trying to make a race saw.

The other limitation with this engine is probably compression, as the squish is around 0.040" - I suspect this is the bigger limitation.

 

[cobey]

does someone on here build up the tops of pistons?

 

[Chris-PA]

does someone on here build up the tops of pistons?

Yes, and I may eventually see if one of my buddies can add some material to the top of the piston, but I don't have that capability. I can turn the piston at work once it's added. I'm thinking about that project but wanted to figure out what I should do with the transfers first.

 

[duramax99]

You can trim the crown of the piston. Its the same effect as raising the transfers in the cyl.

 

[Chris-PA]

You can trim the crown of the piston. Its the same effect as raising the transfers in the cyl.

I thought about that, but it is not really equivalent. The main drawback is that it reduces the compression. Another is that I need to raise them about 0.045" to get to 20[SUP]o[/SUP], which is too close to the ring. And if I just beveled the edge of the piston at the transfers I suspect the flow patterns in the cylinder would be poor, plus the ring would still be too close there.

 

[tree monkey]

your intake and ex ports are to high for a small bore woods port saw

 

[Chris-PA]

your intake and ex ports are to high for a small bore woods port saw

Well, that could well be! This one was just a hacking around engine I'm playing with, not one I need. It actually runs pretty strong, but I tend to think you're right in that my more conservatively ported 46cc Poulan is silly strong and hold the revs much better. But that is a different engine design.

My thoughts were that the intake and exhaust as I have them are not coordinated with the transfers, and I can't put them back, so I figured I'd try to deal with the transfers. Or I could leave it alone since it works pretty well. It's probably really the compression, but I need to pick up a gauge to tell.