DIY: Porting & Polishing

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[bholmes]

I was wondering if anyone could tell my the basics of porting.  IE what does raising the port roof do as opposed to lowering the port floor, what effects what RPM range, How manny MM is too much etc.  I know widening is like increasing the duration of the port timing.  Thanks.

[kato520]

Right, i've attached a silly vid.About the only good thing the vid does is show a nice example of a two stroke motor worrking. By changihg the height of the ports, you are changing the timing. Someone with far more knowledge of this can correct me at any-time, but if you lower the port, you would be retarding the timing.By rasing, you would effectively be advancing the timing.As i said, with the IT 175 we modified, we opened up the transfer ports in the bottom of the barrel by 10 mm overall and up into the barrel, in a tapering fashion for about 20 or 30mm. this let the piston stuff a larger volume up the transfer ports to the intake port. now you have a pressure difference between the gasses in the crank-case and the gasses waiting at the intake port, probably not the correct way to word it, but i hope you get the drift. Now, we lengthend the intake port ( duration ) to allow more of this gas into the cylinder ( more fuel=bigger bang= more power, but not that much) at a higher speed than standard ( opening up the bottom of the transfers makes this happen) I then honestly can't remember if we raised and lowered the exhaust port by .5 mm, or if we just raised it by 1mm. Beacause we have just stuffed a bit more fuel in, at a higher than standard velocity, you neeed to start to get rid of the exhaust gasses sooner,and possibly for longer to stop a build up in back pressure. Too much back pressure will slow the intake charge (fuel ) down, costing you performance and power ( the thing will ride like sh_t ), to little back pressure and you loose some of the intake charge (fuel ) down the exhaust, costing you performance, power and fuel ( the thing will ride like sh_t!) As you can see, it's a bit of a balancing act. If you want to make drastic changes you WILL need to re-jet and re-pipe. The changes i'm talking about, resulted in for us, a wider spread of power, plus a very small HP gain.We did not have to change jet or pipe, to this day she is all stock. Another point of note is that we have to chop a bit out of the piston skirt every time we put a new one in, because in changing the duration and timing, you need to uncover the reeds to feed the crankcase. This is also timing, to tie in with what we did with the exhaust port, and to increase  the volume of fuel ( opens earlier, shuts off later, DURATION ) in the crankcase, to balance up what we did with the transfer and intake ports. Chopping the piston requires that it is still in balance when you finish, so what you take off on the intake side, you must take off on the exhaust side. We do this, the balance it on a piece of thick wire ( straightend out coat hanger ) to do the fine balance.WHEW!!!! I will go on google now to see if i can find a copy of that book " THE TWO-STROKE TUNERS HAND BOOK" for you. For such a simple engine, two strokes can be as involved as any four stroke, but you do all this yourself at home with a dremmel or die grinder ( no money!! ) and this book gives you the know how to achieve what you want from your engine. All the equations you need are in it and it explains every thing much better than me!!! Also, if you watch the vid enough ( you don't need to listen to the guy after the first three times ) you can see whats going on , and what you want to change. WHEEEW!! Hope this helps..........

[kato520]

sh_t, i forgot the video!!!! that happened last night as well!!!   Here it is 

<a href="http://www.youtube.com/v/LuCUmQ9FxMU&amp;ap=%2526fmt%3D18&amp;rel=0&amp;fs=1" target="_blank">http://www.youtube.com/v/LuCUmQ9FxMU&amp;ap=%2526fmt%3D18&amp;rel=0&amp;fs=1</a>

[kato520]

 Sorry, i left out the word "Earlier" when i was talking about uncovering the reeds. It should read, "you need to uncover the reeds earlier to feed the crank case "  Sorry......

[kato520]

http://motorbicycling.com/f13/two-stroke-tuners-handbook-link-1264.html      http://toostroke.blogspot.com/2007/12/two-stroke-tuners-handbook.html   Both these books are GOLD!!!!!!

[kato520]

UUUMMMM...... You probably don't want the one on motorized pushbikes    Here is the right one....http://www.vintagesleds.com/library/manuals/misc/Two-stroke%20Tuner's%20Handbook.pdf   I'm on fire today!!

[kato520]

That last Book is the exact one we used for the  175, i'm getting a copy for myself right now!  Time to tune my 490!!! Best of luck, Hope i helped

[4V5]

my DT175 got a 'lazy sunday afternoon with a file' port job. I raised the exhaust about 1mm and wider by about the same on either side, the inlet port's bridge was 'bullnosed' with a smooth radiusand the fith port above it had the casting imperfections removed (was half closed over) the transfer port's bridges was bull nosed too and all of the rough casting finish on all of the ports were cleaned smooth(er).
I also cut a new base gasket out of thicker material to raise every port and then lapped the head again.

That made a fair improvement, I'd already lapped the head down for a 0.9mm squish, boyesen reeds and a naughty muffler. it was a work in progress this bike and on the 20km commute on a 110km/h road everyday the mods were noted and more done. originally the 175 would do only 80km/h with my 110kg on it, the boyesens gave me 5km/h the squish another 5km/h the muffler gave me 10km/h to do a solid 100km/h. the port job upped it considerably to 115km/h then when the airbox was ditched for a pod and the muffler from a KR250 fitted it reved right out to 130km/h (even up hill into a headwind with two fingers on the clutch)

it sounded naughty, a high pitched wail that could be heard 10km away from the farm and it survived 2 years of this highway abuse doing 40,000km in the process.

I kept the autolube on the bike and towards the final incarnation I added a dash of oil to the tank every fill to lean it out a fraction. (cause I'm lazy and don't know how to tune carbs properly)
I also used caster when I was due for a rebuild anyway, cranks lasted 20,000km and rings about 10,000. spark plugs lasted 1000km

what I did was not rocket science but fun as all hell. the look on people's faces in their cars as I overtook them on this stock looking (but not sounding) DT175 was always priceless...



Cheers!

Leslie

[dar373]

quality

[fatterhatter]

this guy gives alot of info

(he does mainly quads)

my Dt250 now pulls 90 with ease !

http://www...om/atc.htm

[CBR600]

Widening the port does not change duration. Changing the port heights, changes duration.
Have a read of this .

http://www...m-bell.pdf

4v5 did you measure the deck clearence after fitting the base gasket?. Was thinking if you used a thinner base gasket to made the piston flush with the cylinder then recheck squish, ditch the h/g machine the head out a few cc, running 105oct.

[Griffo]

But it does change time/area. Which still gives you improved flow.

[CBR600]

Griffo, im not under standing that, as i put the port heights into my porting program then i get a answer of the durations. The duration numbers only change depending on  port height. Pull the flywheel off and put a degree wheel on.
Widening the ports will allow more volume of air to flow thru, but you still have to make sure if your putting more air in, it still needds to flow it out.
Btw, the  link i posted has some it / yz tuning extras on the last pages page 180 on....

[Griffo]

What software are you using? Is it the Dos based software that Eric Gorr is/was using? I can't remember who wrote it now...?

[4V5]

yes software would be nice, where can I get a copy?


... my 175 engine, I'd checked the squish after changing the base gaskets.. but didn't check the deck clearance there's plenty on room in the DT head I'm sure the original squish was 1.8mm and I'd dropped it to 1.0mm..

current dt #3 is a stocker 3000km in 18 years   I can't bring myself to modify it... but... I do have the rebuilt engine from DT #1.. while it was apart I raised the exhaust 3mm and widened it about 5mm the transfers are still bullnosed but the transfer bridge has bevelled edges this time... I haven't done the head yet though...

I fear the barrel has been done too far... Lucky I have a good suply of DT175 barrels... good to play with things you can get parts easily for sometimes 

Cheers!
Leslie