Port timing numbers

mcbob · Post by **mcbob** » Thu Apr 08, 2010 7:05 pm

Not that i have altered any of my Mac's yet but i'd like to see what experiance you fellas have had and what sort of results was obtained, i know how most of the Vintage Karters feel " Don't butcher good cylinders " but i have bought the odd one or two cylinders that are in need of reboring, most need .060 thou plus and have their ports altered and have the odd bit of cylinder fin damage ........ in hotsaws this is not a problem as the runs are short most times less than 30 seconds with an event taking as little as 8 seconds to complete.

How high can a fella go before the exhaust is too high on a 101 or 91b/1, 92 or 93 ?

Our rpm in a log is between 10,500 and 11,000 rpm under load this is what the saw is actually pulling best whilst cutting not free reving out of the log. The huskies and Stihls (piston ported) pull higher RPM but dont have the pulling power of the old Macs especially at the widest part of the log.

Now if lifting the exhaust port moves the power band up to higher revs then there must be a limit or have i been drinking too much Budwiser again.

Another point is compression with the SP125 chromed bore cylinders the head is fixed so i can't slow the piston by raising the compression.

I have just finished building a SP125c with rear boost ports and now look forward to seeing what sort of lift there is in performance.

McBob.

Rob Voska · Post by **Rob Voska** » Thu Apr 08, 2010 9:44 pm

Don't raise it. Widen it or add more ports. Higher exhause will lower the torque where you are trying to cut.

Post by **Jeff Campbell** » Mon Apr 12, 2010 3:42 pm

mcbob,
I don't have dyno data to answer your question, but have seen the specs for many top notch modified macs. Many are raising the exhaust port 0.060" and some as high as 0.090", but also eye-browing at the same time. Once you widen the port as far as you can go without trashing your rings, then try raising it, going no more than 0.060" to start with. The boost ports when modified, are usually raised to the same height as the transfers, and the top of those are somewhat squared off to be wide at the top. Then you can flow the intakes, without opening them up much, there is already a lot of time-area on the intake ports.
Another thing you might try playing with on a saw, since you are running a fairly narrow rpm range, is making a customer expansion chamber, with steeper cone angles than what has been used for kart racing. This will raise your power peak, but reduce your power band width, which may be good for hot saw competition.
Does the SP125 have a single large exhaust port, or 2 exhaust ports with a central rib?

Jeff

mcbob · Post by **mcbob** » Mon Apr 12, 2010 5:02 pm

The SP125C has a D shaped exhaust port and a chromed bore ... i have just done one by enlarging the exhaust and now it's big enough to stick ya head in plus ground in some boost ports and recut the flywheel slot to get 34 degrees now all i have to do is fit the 101carb and intake and tug her string to see what happens. This saw is for the visually standard or production saw section so no pipe so i hogged out the entrance to the muffler to match the exhaust but i didn't lift the exhaust port and no changes to the other porting. Since this is a suck it and see, saw engine there's a bit more i can do to come.

The other visually standard SP125C i built that won the National's at Dungog is still running strong so hopefully this one will be another step up the chain of evolution.

McBob

Post by **Jeff Campbell** » Mon Apr 12, 2010 6:38 pm

mcbob,
You can simply angle the exhaust port to raise it, without raising the top of the muffler mount outlet.

Can you cut down the height of flywheel fins? or does this violate the visual rules? The stock flywheel fan robs a lot of power at your cutting rpm, and pushes more air than you need.

You could also probably put in a full circle crank and some stuffing and stay stock appearing, this would enhance power at your cutting rpm range.

Jeff

mcbob · Post by **mcbob** » Mon Apr 12, 2010 7:05 pm

As i said i have more i can do ........... visual or production class ... visually standard ... what you do inside the covers is your business ............................

Tom Smith · Post by **Tom Smith** » Tue Apr 13, 2010 1:14 am

DSCN1529.JPG: (812.43 KiB) Not downloaded yet

Bob, here's how to lighten a Mcculloch boost port piston. Don't plunge all the way thru the bearing boss. This one is out a kart racing 101 that broke a bottom ring. I use billet pistons with a single 1mm ring, those thin rings can't take it when you start actually making some power.

mcbob · Post by **mcbob** » Tue Apr 13, 2010 2:53 am

Tom what size are the drillings in the skirts ?

Bob

Tom Smith · Post by **Tom Smith** » Tue Apr 13, 2010 10:41 pm

The 4 holes in the skirt are 1/2" or they can be milled oval and made larger
the center of the hole is .700" measured down from the top of the crown, the hole centers are 1" apart
the 2 small holes in the wrist pin boss are 7/32"

mcbob · Post by **mcbob** » Wed Apr 14, 2010 4:53 am

Thanks Tom

Bob

Port timing numbers

Port timing numbers

Re: Port timing numbers

Re: Port timing numbers

Re: Port timing numbers

Re: Port timing numbers

Re: Port timing numbers

Re: Port timing numbers

Re: Port timing numbers

Re: Port timing numbers

Re: Port timing numbers