Formulas for Good Runners Without Porting

Repairs on older models are hard to get done here because IME most of the guys doing the repairs couldn't pour pizz out of a boot if the instructions were on the heel! Seriuosly, it seems that most of the techs working on saws only know about current products and are not interested in anything that they don't know about.


And I agree the 371 single ring has a wider skirt compared to the current 372 piston.

 

I sometimes build one I call 'The Uber Sleeper' if you can find the stuff to do it, or want to pay through the nose for it!

Husqvarna 362XP as pristine as you can find it, decals and tag in tact e.g. legible. This an H-280 small bar mount saw.

OEM 51.4mm 372XPW C/P, delete the base gasket.

16-18 inch bar.........

Looks as stock as possible but goes! A bit of an overkill putting 75cc behind a 16 inch bar, but what the hay, and a lot of fun watching your friends grab on to it and cut. They get this look on their face like 'is this thing on crack, about to blow up or what'?

 

Here is an example from a saw racer buddy of mine, the important thing here is that he started out stock, then did some minor porting then slipped in a Stihl 046 piston with a shoe horn! This is a Dolmar 7900!

 

The 046 piston has a wider skirt and compression height is lower than the 272 piston that is commonly used. Looks to be about .040 or so taller than the stock piston.

Above is why an 046 piston was used in place of the stock or 272XP piston. The saw ran on the stock AM piston and on the 046 but then some minor porting was added to get the last 5-10% and retain the torque. Also note that 'no lowering' was done, in other words no changes to the stock timing. When you raise or lower the ports you change the timing of the engine, the thing to remember here is that if you change one thing (during the porting process) EVERYTHING else changes with it.

 

When I was racing MX for HVA, the factory would sometimes send out some pretty odd-ball stuff. Like pistons with unequal skirt lengths on the intake and exhaust sides of the piston. These were big pistons, a 400cc Husqvarna 400 Cross used an 81mm piston and an 86mm stroke! Anyway what they were doing was changing the timing of the engine without altering the cast iron liners of the race cylinders (e.g. porting)!

 

Timing changes are measured in degrees of rotational position of the engine. A change of approximately 0.018-0.019 results in approximately a 1° change in timing. Example: if your intake opens at 75° and you want it to open at 76° you can lower the floor of the IP by 0.018 or you can remove the same amount from the intake side skirt. Messing with the piston is a very risky business not something to be done on a chainsaw. Our pistons are cast then machined, the race pistons sent out by HVA were machined billet pistons, much much stronger!

 

Correct, he milled some off the top of the piston to get the squish he needed because of the height difference from stock. I am sure a degree wheel was used but he has done so many 7900s that he has most of the numbers in his brain!

 

Master, it is important to note here that the addition of the spacer under the jug was more a matter of returning the engine toward stock timing. Since the 046 piston is 39.5mm OH and the stock piston is 40mm loss of .5mm. And the removal of material from the piston top 0.010 would equal almost 1mm or 0.040 or 2° of timing change.

 

No chit, Red Rider. Change one little thing and everything goes out the window. Add to that different manufacturers measure compression height differently pin center to crown, pin top to crown and pin bottom to crown. Then to further complicate things Dolmar doesn't publish those measurements for their pistons and then there are manufacturing tolerances to consider. If there was universal acceptance of a single 'time/area' calculation specific to two stroke engines we would be much better off, but there is not.

 

Mech, see above the spacer was to regain stock timing also notice that he opted not to mess with the port timing.

 

The 7900 runs best for me with the new OEM lightweight piston. I've tried every combination you can imagine.

 

How is the new reservation system going? Is your calendar out into early 2015 yet?
Cheers
KRL

 

I've not had time to work on it much Brian.......free time in the summer is spent with the family. I'll likely get back on it in the fall.

I'm booked out till mid October right now. I've tried to stop taking anymore for the time being........but people keep asking for a time slot.

I wish the notification system was working here.......I don't get an email.......and I forget to check in.

Great site with great people......but no way I can keep up.

 

A poulan wild thing 505 I built from spare parts. Putting a stihl 066 piston in it. To use the stihl pistons you have to open the pin bosses up to clear the rod.