372 big bore timing....want to port...

[Brownpot Deaton]

ok here's the spill..i recently bought a 365 special in awesome condition and bought a big bore kit for it(52mm)...think its nikasil plated...but anyways...i couldn't find any tried and true numbers for a woods port so i figured id start a new post...keep in mind that all i have is a dremel so unless someone knows a majical trick i can't adjust the transfers...(casting looked pretty good..definantly flow more are than 365 cylinder...)

squish is at 0.031" w/o base gasket....

so after couple times measuring and using a paper clip since i cant do transfers visually and alll i wound up with following numbers...+ or - about 2-3 degrees for paper clip depending on rotation ( paper clip measured 0.041" or 1.07mm for you metric guys; and that was consistently using same side of clip...)


if top dead center occurs at 0 degrees, then

the intake opens at 74° clock wise (cw) of zero,
intake closes at 73 ° counter clock wise (ccw) of zero,

{ this should mean 147° of duration...since piston is up durring intake opening)


the transfer ports opened at 130° (left bank/flywheel side) and 131° (right bank/clutch side) ccw of zero
the transfer ports closed at 128°(left bank) and 130°(right bank) cw of zero

{ this should mean about 101° of duration since piston would be down when transfers are open, or when wheel reads 180° which is bottom dead center)

....the exhuast i widened about 1-2 mm already, but is just under 60% diameter i think...and when i checked today, there was a little bitty hump towards one side that must have come from the grinder, that causes the exhuast to open 3° degrees early and close 3° degrees later than listed if you consider just the hump...will have to go back and bring the exhuast back up to this hieght all over....keep this in mind please....

exhuast opens 109° ccw of zero
exhuast closes at 108° cw of zero

(so about 143° of duration...)

so now i ask that some of you knowledgeable CAD-aholics give me a suggestion and please explain why the numbers have to be at a certain degree and duration for optimum....or just say because it works...

also..how do i modify the ports using degrees...just shave alittle then check and repeat till get it correct????

 

[Four Paws]

Different timing numbers work in different saws based on MANY factors which are all related to geometry. I know of some guys that are consistently in the winners circle and built their saw based on feel and looks. After you do enough of them it becomes kind of like art. Others build their saws based on mathematics and do very well. I am just a firewood cutter, not a pro or competitor, so take what I say at face value. There are ballpark 'cookbook' numbers that tend to do well in most saws. Getting maximum performance takes tuning, testing, and a good bit of knowledge.

My advice is:
Determine what you want out of the saw when finished.
Measure, measure, measure and measure again...mark THEN cut.
Get your compression up.
Widen your ports and adjust timing based on your desired finished product.
Don't touch the transfers on your first several saws until you figure it out.

To mod a port to 'X' degrees, install piston, find TDC, rotate piston to desired open degree, mark bottom of exhaust port location (or top of intake) by scribing a line on the piston. Remove piston and cylinder, mark inside of cylinder with Dykem of sharpie marker. Insert piston and look through ports and line up your scribe line. Scribe a line in your marker on the cylinder wall at the bottom of the skirt on intake side. Line up exhaust markings through port and scribe a line in your marker on the cylinder wall at the top of the piston (go through the plug hole).

Clear as mud?

 

[Crofter]

Have not had my hands on one of the nikasil plated ones (still back ordered) but the early ones had a pretty roomy combustion chamber so not easy to get compression up. Not to knock them but the basic port positions should have been relocated instead of simply increasing the bore. Because the ports are slanted as you go larger diameter the exhaust and transfers drop and shorten duration. As you note from measuring squish you can only drop the jug about 10 thou. or so without running out of squish and still not good compression as 4 paw notes. You will be faced with machining some off the top of the piston and or squish band and taking more off the cylinder base, but the catch is you may upen up the exhaust port to the base at the bottom of the skirt when piston is at top dead centre. Also watch that the material is not too thin at the top of the exhaust port when you grind to get the durations you want. The top of the exhaust port is a bit flatter than I like to see but hard to get the arch in without losing more of the king/ compression. You will be moving a lot of things around and you have to forsee any ooops! that might result. Who said, "measure, measure, measure"? They can be made to really perform but there is a lot of metal work to do it.

Eye up the possibilities real close and you may decide to keep it fairly simple and stay away from the more exotic stuff. They perform well as stock but not super.

 

[J_Ashley]

Ok, here's a real newbie question- what is this "squish" that you all keep referring to? I'm assuming this is the distance between the piston and the clyinder jug's "cieling" at Top-Dead-Center?

If this is the case, then I guess removing the cylinder jug and machining back it's base will 'tighten' this distance and ultimately raise the compression? (I'm assuming this is where the critical numbers of before and after TDC & BDC cylinder volumes must be calculated to insure you're removing the propper amount of material to reach the desired compression ratio...)

And to make matters more complicated, if one chooses to machine his cylinder base, then he has lowered all the intake/exhaust/transfer port heights in relation to piston travel, and has therefore changed all of the port's timings, correct? Now, any effort to restore the original heights of the intake/exhaust ports in one direction would simply lengthen the duration of that port(s), correct? And for a closed-port saw, again, you could only compensate one way, therfore lengthening the duration, again. I guess for an open port-saw, you could raise the port height the distance that had been machined from the cylinder base to return to the stock transfer port timing...

And having said all of that, what effects are created when increasing the duration of each of the ports?

I assume you'd want the longest possible Intake duration, in attempt to draw in as much charge-air as possible. So, opening the intake port on the bottom (?) sounds like the way to go??

The exhaust port seems tricky to me, because you want to release the spent gasses as quickly as posible, but without wasting, or 'over-scavenging' the charge air. Um, I'm clueless as to what one should do to the exhaust port here...

And then transfer ports... What's the goal here? I've read you don't want to open them too much in order to keep velocity up, so what effects do the transfer ports make?

Ok, that's a lot of questions, but they kind of all came out at once...

 

[timberwolf]

I think you may want to do some reading, these are questions that are well answered here.

http://www.chuckbunnell.com/kart/jennings/jennings.html

 

[Brownpot Deaton]

and adjust timing based on your desired finished product

.

i know what i want(work saw, nothing peaky), but ive never heard how to adjust timing for given desire, just seen people give ideas or numbers (which is reason for posting)....

so here's the milliion dollar question:

how do i find what timing i want?

 

[stihl 440]

Bb

I'm putting a BBkit in a blown 371xp for a guy I know, and the saw will have a proted muffler as well. Will it lose many rpms with the base gasket removed and squish set to .020? The saw will primarliy run a 20"-24" bar......:greenchainsaw:

 

[Brownpot Deaton]

not at all...i dont think it has an effect ..unless its something riduculous, then who knows...

if it does, id imagine effect rpm by single digits.



finished widening the exhuast port to 33mm raised just past the point that top edge opened with previous "bump" i mentioned, matched the muffler, smoothed the entrance to the transfers( made the begining of the "wall" inbetween ports skinnier tapering to origonal size about 3/4 of way up transfer and shaped the entrance where it meets the sylinder wall, but didn't raise it..can try later if want to...), matched the intake to the boot, lowered barely enough to notice, smoothed every thing with a old sanding stone .... then couldn't find my fine grit sandpaper ...sigh...well it was pretty decent, im sure there some texture to it, but cant really feel it....hmmm.oke: oke:

chamfered all the ports, decided to atleast try it ...bolt everything on, open low jet 1/4 turn or so, and turn high out about 1/3 turn out....srew in idle 1/2 turn...whamo...starts right up, idles perfect, snaps to attention faster than a communist soldier durring inspection....(im sure ill have to fine tune the carb, but wont get to till this weekend...)

response is incredible!!! its like it knows what im thinking. can feel it torque itself outa position in my hands with just barely a blip of the throttle. whats wierd, is didn't change the muffler at all, but seems to sound a bit higher pitched...

 

[J_Ashley]

I think you may want to do some reading, these are questions that are well answered here.

http://www.chuckbunnell.com/kart/jennings/jennings.html

Thank you...