Porting a Mac Engine

[mcbob]

Well since this place needs a bit of decent conversation here's my silly question.

Now i apologise for being a tad slow but i need a basic understanding of whats to gain in moving transfer ports ( lifting them ) i visit the odd forum or two and read a lot but i'm sure that some of the more knowledgeable and experianced hands will set me straight as regards our Mac engines ... and what can be done

PS some examples of various port timings and there need to be moved or what advantage is gained in doing so needs to be explained to me .... i'm just a poor old country boy so please have mercy on me.

Mc Bob.

[Gordon Duax]

Bob,

If anyone tries to put this into a 100 word post, they probably will be skipping something.............

I suggest you pick up a copy of
Gordon Jennings 'Two Stroke Tuner's Handbook'.

The blow-down time effects power band width (RPM range).
But here is a lot more involved than just the actual timing.
As someone mentioned in the other thread,
there is what is called time/port area.
A tall narrow port, will act about the same as a lower wide port.

I once ran the numbers on a Komet K78,
to see what could be done to it for more bottom end
(engine was to be used on a direct drive kart).
After figuring rod/stroke ratios,
time/port area numbers (not an easy task),
etc., etc.,
the numbers showed that a little less blow-down time
(taller transfers) was in order.
Cut the ports to what the numbers said,
and it worked great !

But that was on a long rod Komet.
A Mac, with it's ridiculously short rod probably can't use taller transfer ports.
A short rod makes for more dwell at the bottom of the stroke.
This gives it more time/port area than an engine with a longer rod,
and the same physical port dimensions.

From all the fog of odd-ball 2-cycle engines in early karting, One stood out.

It was called McCulloch.
And McCulloch ruled the earth.

Then someone open the door, and let the Italians in............

[Gordon Duax]

Bob,

I still think you should try one of my little superchargers.....

[mcbob]

Thanks Gordon your words of wisdom will help i'm sure but mybe it's time i read the book mentioned above maybe i should leave well enough alone ...................

Mc Bob.

[John Swartout]

Hello Bob,
I rebuilt a Mac 93 a couple of years ago, and while putting it back together, noticed the piston at TDC was about .025 above the top of the cylinder, and the stock head gasket was to thin. (Someone had milled the top of the cylinder down). I inquired about this and was told it may have been a way to 'quick time it', in effect changing the porting. Anyway, I put the thicker head gasket on, and it seems to run real good. I just wanted to share the experience.... I'm sure it's not the first time this has been done.

John

[mcbob]

Well this is the sort of responce i need i'm not judging anything as of yet but it would be great to hear of what has worked and what hasn't ........... i'm sure as far as porting is concerned both in the transfers and intake / exhaust areas 99% of ideas has been tried with better results gained by some now what we need is the results of what the better porting arrangements are ........... P.S Don't forget the days of being the fastest kid on the block are long gone and this is vintage racing maybe the best gains nowadays would be a weight loss of the human kind as all up weight counts.

For those who do not know me i don't ride on or in my Mac but hold it both hands as a hotsaw.

Mc Bob.

[Rob Voska]

Your spot on about the weight. You ask a very tricky question.........The best. fastest Mac's on there day were ported so after a few races the top of the cylinder blew off. Make one like that then leave just a little more meat on the next one. Or use a girdle.

We know so much more now about what makes a engine run than they did in the 60's it's really hard to compare. One thing I always try to remember is a engine of a given size is going to put out XX amount of torque. All I can really do is move that torque around in the RPM range a little by tuning the carb, clutch, pipe, gear, timing, fuel to fit my application better and that's what makes me faster than the next guy. Head design, squish band and port timing are important but rarely the determining factor in who wins.

[mcbob]

Would luv to see one of those that hadn't exceeded the pop-off pressure ....

Gives new meaning to caging the beast within, doesn't it



Mc Bob

[mcbob]

Thanks fellas to those who sent me some reading material on modding Mac's it's appreciated and will be food for thought no doubt about it.

I have a complete 101d on it's merry way deep south maybe it will be a candidate for a little filing and fiddling ................. gee a fella may as well move the old bed into the shed as thats where i spend a lot of my time too many projects not enough life time.

Mc Bob

[Rob Voska]

DON'T HACK UP A GOOD D BLOCK! Get almost anything else but leave it alone!!!!