S3 Race 250 Port Matched Cylinder and Head

[Jakobi]

Piston and head look ok I guess. I've run a bit lean at some point as there is oil build up on the underside of the piston. Not black or carbon, but definite heating of the oil to leave residue like you see on the side ports of the piston. There is a small carbon mark in the middle. I've run so many different combinations of needles, clip positions, and mains it could have happened at any time however I think the 172 main I've run for the last 2 rides could be the cause. I had a small black mark in the middle only of my last piston too which I knew I ran really lean on the needle on one instance. It didn't have this same glazed effect though. Just a neat black dot. Could also be the change from GRO to Amsoil. Never the less, it'll be back to the drawing board looking for new base lines when putting it back together as a 250.

A bit of carbon build up on the head exhaust port side. And some carbon build up on the dome of the piston. Bringing the squish band in should help to cool the piston as it will assist in flowing the fuel across its dome towards the center. At the moment its like it just burns whereever it is.

[Jakobi]

And the last couple. I didn't take any pics of the S3 cylinder of head installed as they were only snugged down to take some squish measurements and cc the trapped volume. I was getting a bit tired by that point and looking towards just getting it done so I can pack it all up to be shipped off tomorrow.

[GMP]

Jake,

Those comp ratios are not correct, you are not allowing for the exhaust port(s). The calculated theoretical comp ratio should use the swept volume of the cylinder from the edge of the PV flap to the top of the stroke, not the full displacement of 249 or 293 cc. This is not the right way to do it anyway as compression changes (lowers) with PV opening and there are some leakage losses at kickover speed. Thats why it should all be based on cylinder pressure using a good guage. The guy doing the head work should adjust the volume of the dome(compression) as the final step based on your fuel and original head volume (that he should do accurately with a burette/pipette and glass plate).

I never had a piston come out of a bike without some golden brown glaze under the dome, its normal.

[Jakobi]

Thanks for the info Glenn,

I realise the numbers I crunched are only for an uncorrected compression ratio. Its a quick and easy calculation only. To work out the corrected compression ratio which will be a much lower number there are many more variables to take into account.

Dave has had his hands on a few gassers now and is building up data which he uses to do simulations. I will be sending the entire cylinder and head to him with the info I have provided here. He will then take further measurements and then let the software do all the work as to determine the best setup. With any luck we'll get him chiming in here and posting some more graphs to demonstrate the effects of raising compression, altering port timing, etc.

I really appreciate the input and am enjoying learning more about how these things work. Glad to hear the glazing under the piston is not a huge concern. This is only my 2nd time in a 2 stroke and both on the Gasser. Every time I pick up a tool its a new lesson waiting to be learnt.

[(F5)]

Duck me fays! Those CRs seem high! My 300 measures at 12.6 I think standard setup. 16.5 is higher than I run in my race 50 with Av gas!!! Is it a methanol head, or how is it measured?

One has to be careful to get the engine upright without air pockets & I go to 2 threads up the spark hole to account for the plug. I use a light oil like 5W fork oil.

I'd argue that a gauge is only useful for gauging wear, not effective compression. I've measurably increased compression and ended up reading the same on a newish German comp gauge. OK it was a high compression, but it illustrates the issue.

Besides when you start the bike dynamic compression is quite different due to pipe action, that's why I don't buy the whole ex port 'corrected' thing.

Now I'm not saying that you can't have a situation that off pipe the compression is effectively higher with the PV down, but I still don't buy that 2 strokes are measured one way & 4 strokes another despite the cams overlapping into compression stroke. When you have an effective pipe on a competition 2 stroke engine, compression ratio will guide you to maximum you can safely run with a given fuel.

[Jakobi]

I thought the same thing with the CR for the 250 aswell. Very high but then they do label the black insert as "high high".

To measure I used metho and a 12cc syringe. A slight smear of grease to help the rings seal and then top dead center (locked in place at the flywheel). I did both the S3 300 and S3 250 both with a gasket stack of 1.3mm and both had the ports timed the same with this setup. Once I hit the bottom of the threads (which was hard to see - had a little torch to shine some light in to help) I then continued to fill to the top of the plug hole as a reference. Both took 5ml to fill the plug hole to the very top threads (28ml and 21ml respectively).

Today I measured the piston dome heights to also pass onto Dave to assist in calculating the trapped volume for comparison against my measurements. The 300 came in at 3.3mm where the 250 runs a 3.8mm.

[(F5)]

it still seems about 2-3 points too high even for crazy octane gas. I had to lift the front wheel up 1/2M to get the engine level. Then I measure as many times as it takes to get a consistent reading with cleaning in between. I'm used to doing small cc cylinders where tiny changes mean large ratio differences. Even so the 300 took bleeding ages as it was sat in the bike & makes it hard to see.

[Jakobi]

You make some very valid points F5 and in all honesty I wouldn't be surprised if there is some error on my part (especially on the 250) as I was getting a bit mentally fatigued by late in the afternoon. It'll be very interesting to see what figures dmcca comes up with when he calculates it. He advised he should be able to get pretty close by cc'ing the combustion chamber and using its volume in conjuction with the piston dome height and the squish readings I sent him. I'd be more inclined to trust his figures than my measurements. As they say, experience comes with practice and thats what it was for me.

In hindsight I should have done as you suggested and taken a few different readings. Ie triple checking my work.

[Jakobi]

Disregard - used wrong calculation for Dome volume. Should read 6.6cc

I just did many runs ccing the red head insert using a piece of glass some grase and some metho filing in the plug hole. Came out to 17.5cc and I believe all the head inserts are the same volume.

Deck height was calculated from the squish measurement 2.05mm, and I used a dome calculator (http://www.monolithic.com/stories/dome-calculator) to work out the volume based on a dome height of 3.8mm. All cubic mm were then converted to ml.

Combustion chamber vol = 17.5cc
Deck height volume = 7.27cc
Dome volume = 8.77cc

Trapped volume = (Combustion chamber vol + Deck height volume) - Dome volume

I was wowed to see the figure come out to 16cc. Maybe my measurements weren't so sketchy after all. In any case that gives us a really high CR. Imagine what it would be like if I dropped 1mm of base gaskets.

If anything I now think the measurements I did on the 300 were wrong as they were my first attempt and the maths shows that it should have been closer to 24.12cc instead of 23cc. This gives an uncorrected CR of 13.11:1. What a learning curve these last 2 days have been!

[MrBlah]

I would say that's been more than just a little lean