S3 Race 250 Port Matched Cylinder and Head

[Jakobi]

Originally Posted by MrBlah

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

Come on MrBlah How about you put some effort into it. Lets keep this thread informative. I'm not disagreeing with you, but I wouldn't mind hearing your reasoning behind your comment. Maybe that way everyone can learn something from it.

[MrBlah]

you should post a good picture of the inside of that piston

[Jakobi]

Definite hot spot in the middle of the piston dome which is undebatable. Caused by a lean run at some point in the 85hrs. I am in no way denying this. It has seen so many different needles in so many different positions.

The rest is a very light caramel colour. No heavy carbon deposits internally or white flaking/chipping. No damage to the piston edges around the ring lands. No sign of detonation on the piston dome.

[(F5)]

Originally Posted by Jakobi

. . . 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!

so 13.1 is close to the 12.6 or 12.7 I measured in my 300, easily within tolerances of whatever gaskets they throw in at random. Pretty high for a such a big slug that could be run on any old pump gas.

But 16:1 is mental. Why would they make it so small? Looking up a chart that really is methanol numbers.

[(F5)]

Gee I wouldn't lose any sleep over a dark spot on the bottom of the piston. - If it was grey - then I'd get worried.

[Jakobi]

Originally Posted by (F5)

so 13.1 is close to the 12.6 or 12.7 I measured in my 300, easily within tolerances of whatever gaskets they throw in at random. Pretty high for a such a big slug that could be run on any old pump gas.

But 16:1 is mental. Why would they make it so small? Looking up a chart that really is methanol numbers.

My thoughts too. Out of curiosity I grabbed the silver insert and cc'd it aswell. Same same figures.

With the 300 I can confirm both the stock chamber volume and s3 chamber volumes are the same. I obviously don't have a stock 250 head to cc but following the trend you'd imagine it would be the same as well. I wonder if this means that all the 250's run high CR's straight from the factory? Using the same volumes and a 3mm squish height (epicly huge) the CR would still come to 14:1 with a 19.12cc trapped volume.

It feels like somethings not quite right with these numbers. I wonder if the people at S3 have made a mistake on the 250 chamber volumes?

[(F5)]

yeah seems like a lack of understanding to run squish clearances you could stand up in. But heck how can I criticise people who make their own engine parts? Must be a 'back story'.

PS dmcca pops up in another forum from time to time, so I have heard of him.

[Jakobi]

A 'back story'??

Not being critical of anyone here, just putting my thoughts down. I can't really think of any pure bolt on head that would work for all situations without offering multiple inserts (different volumes) and a way to adjust for squish dependant on the base gasket stack. Any way you look at it there's still work involved to come to the right answer, so its never as simple as bolt it on and go.

I'm certain that Dave will have a record of the stock 250 chamber volume so will be able to fill that void for us.

While probing your knowledge F5, what CR would you be looking at to achieve my goals? Premium pump gas (95RON) minimum, although I always run 98. Looking at a soft yet torquey and tractable bottom which builds into a strong mid - top with good over rev. Not a real MX kind of hit, but enough to let me know its still a 2T.

[dmcca]

Originally Posted by Jakobi

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!

Hi jake, check the dome vol... Should be 6.5-7cc for a 250 domed piston from memory. Pretty sure I got 17.6cc for the last GG250 head I measured. I'll get back to you tomorrow to confirm. If your dome vol was closer to 6.5cc then you'd end up with a CCR of around 15:1 which is about what I got last time... They seem to set up their 250s and 300s very differently.

[Jakobi]

Dave, I just used a calculator (listed above) to calculate it based on a 66.4mm diameter and a height of 3.8mm

Double checked my work and I used 'Oblate Dome Calc' instead of 'Sphere' ...Oops - The later gives me a dome value of 6.6ml. This would bring trapped volume to 18.17cc and CR 14.7:1.

The strange thing is that using the 'Oblate dome calc' not only gets the calc close to what I measured for the 250, but for the 300 too! I think I must have measured them both incorrectly (or not spotted the line at the bottom of the plugs correctly). On my notes I had jotted down values for what I thought was the bottom of the spark plug gap and the top. There appeared to be a 5ml difference. Today I just measured and there is only 2.5cc of thread so working backwards from the full I get..

Measured 300: 28 - 2.5 = 25.5 Mathematically 26.4
Measured 250: 21 - 2.5 = 18.5 Mathematically 18.17

On the 300 I made the noob mistake of drawing to a line and filling right in instead of dropping to another measured mark so may have added an extra cc or two through the tip of the syringe which would bring it back into spec with the maths. I think the angle of the engine must have been trapping some air when I thought the fluid was touching the spark plug. Ie touching 1 side but not the other.

If I could have my time again I'd spend more time and effort to get it 100% right. Hindsight is wonderful.

Thanks Dave.