Athena 170 porting
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@theportingmaster nice info thank you
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@theportingmaster Reading your post is almost like reading my final calculations,little different numbers by +0.1mm,I did not skimmed the cylinder.
I did work on crankcases,transfers,exhaust,exhaust port,pv,cylinder head(compression,squish,volume) and custom gaskets. -
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@calum No destruction to the plating,everything was done very slow and carefully,the edges are very smooth.
Only sandpaper (dremel and hand) was used,it took A LOT of hours.If I used dremel carbide bits or sandstone it was faster but risky. -
How much of a diffrence did the 5mm widening of the port make?
I've just completely rebuild my engine. (Gearbox, bearing, new piston) Already have 320km's on it and it's pretty solid. The 170 kit is a huge improvement over the 125. Is it worth messing with the ports?
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I received my 170 kit recently and had a look at port timings etc, so here are my thoughts:
As mentioned above, the ports seem basically the same as on the 125 Athena. However, on the Athena 125 that came on the bike when I bought it, I had a thin metal head gasket, but the 170 kit came with a 1mm+ thick head gasket. Not sure if the 125 kits come with such a head gasket too? Using a head gasket that thick would make the port timings even worse on my new 170kit when squish is set correctly, and I don't know of any thin metal head gaskets that fit the 170 cylinder diameter? Could there be some DT175 thin gaskets that could be used?
I decided it was not even worth mounting it in the stock form, with transfer durations around 110deg (or was it less, can't remember now), and also a lot of port area is covered by the piston. IMHO they should be 120-130deg. Fully open they would be over 130 though. However, exhaust duration seems ok out of the box, and gets a bit high with the cylinder raised. I would prefer staying under 190. Based on the high duration I don't want to widen it at this point, thinking blow down area should be sufficient as is, and I'm not aiming for a really high revving engine.
I put the head insert in the lathe and removed 1,4mm, and made some spacers from 1,4mm sheet aluminum to put between the insert and the head. This way it's easily reversible by replacing the insert and removing spacers.
For now I'm waiting for a piston pin needle bearing, since the one delivered with my kit was only 17.4mm wide (from Götz Germany), and I ordered a 22mm wide bearing separately. Inside with of the piston is abt 23mm.I also made some small rectification to the ports, removing sharp edges, and also matching of the exhaust port to the power valve. There was a couple mm step/shape mismatch with the power valve in the open position, so I removed some material in the upper part of the exhaust port towards the header-flange.
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The spacer I used was just a 1,5mm thick alu sheet that I cut to the shape of a base gasket. Remember the head has to be 'moved down' too, to keep the squish nice and tight. Can be done by machining the top of the cylinder down, or machining the head.
IMHO the head volume is too low from factory, and combustion chamber needs to be enlarged to have a safe CR if squish is below 1mm as I want it (0,8mm now). The 125 and 170 inserts seem to be the same from Athena..No porting done on mine. I think widening the upper part of the exhaust could add some power at higher revs and keep the low band pretty much the same. Pretty easy to do a portmap on paper, but I did not do it before I installed the cylinder. Just measured the port height and calculated the port timings to get a rough idea what height I should try. Raising it the full 3mm to get the 'full opening' of the transfers gives pretty wild port timings, but it would probably work with custom ignition curve and pipe to make it a screamer (power at high revs). Not what I'm after though. Pretty happy with it as it is now.
