Head Volumes - does this sound right???

[bally123]

Measured my chamber volume today and got the following results: Bike is an EC250 2005

Trapped volume 17.8ml (1st try) and 18ml (2nd try). Using a proper graduated calibrated glass burrette. Measured with piston at TDC using a little grease to seal rings. Hoisted front of bike up to get it level and filled plug hole to a level to meet the plug face.

On a 250 i make this a comp ratio of just below 15:1 - this sounds high to me??

Squish measures at 2.00mm so that needs skimming for sure.

Does this volume sound about right??

I have no pinging issues on pump fuel (98ron here in UK)

Cheers Andy

[Jakobi]

Hey Andy. The 250s do seem to run a lot of compression compared to the 300's.

I reported all the volumes and ratios from when I did my 250 conversion and had the head modified. We ended up taking some material from the chamber to drop the CR to around 14:1.

Lotsa reading here

[bally123]

Got a lot further today than I planned - which makes a refreshing change!

Head reshaped and skimmed - took some time tweaking the design to get the chamber shape to give the right volume and match the piston dome. End result is an on the bench head volume of 20.7ml and squish at 1.18mm. (Jakobi: Thanks for the link, your posts provided me with the reassurance I needed - )

See below for a photo with head being cut on lathe (forget mounting on mandrel via plug, 3 jaw chuck with soft jaws on OD gave the perfect mounting condition) ... the dark ring is a witness to the old squish band, as you can see the angle was very different to the piston dome.

Rechecked all the basics and gave carb a serious clean. As a conservative starting point jetted with JD blue#3 Pilot 42 slide #7 main 185 air screw at 2 turns.
Have only started the bike up so far and it idled really well on the choke and stalled when choke was knocked off - good signs as its never done that before!!

Attached another photo of my static idle setting on the slide, does this look like an adequate gap for idle? (surprised at how far in the idle screw goes to achieve this gap)

Serious test ride planned for Sunday, hope it finally puts some stability into the jetting.

Andy

[Jakobi]

All looks good. Did you measure your trapped volume again after the work or just going with the figures?

With the carb slide thats about where I run mine. Only 1-2mm visible under the slide. All good

[bally123]

I didn't check the trapped vol, I was happy with the bench top math in that the increase in vol (18ml to 20.7ml) offsets well against the theoretical change in vol due to amount skimmed. In addition I was keen to get it back together. I will check it at some point. Just hope I don't hole a piston in the mean time!!

Whilst on the subject I am not completely happy checking trapped vol as it can be difficult to see and to know where to fill it to, or how to allow for the plug volume. Bench top head measures are naturally much more consistent even if thy don't provide the absolute comp ratio.

Thanks for confirming slide position, I will keep you posted.

Andy

[Jakobi]

Works for me mate Enjoy!

[Diggs345]

Hi andy

Out of interest what cutting tool did you use in the lathe , did you grind your own up ?
Nice work by the way
Regards
Chris

[GMP]

Just settle on a base gasket thickness, start with a good top end, and measure squish and actual compression with the stock head. Recut for correct squish, do the volume measurement and calculate that neede to be removed for the correct PSI for fuel used. RBD does it this way, about 200 PSI max for a 250 on USA pump gas. This way your basing it on a real number for your motor, not a theoretical CR. Ultimately your squish band width will be more narrow but the MSV will actually be greater due to consistancy or the tighter clearance across the width. It it what it is unless you add aluminum, which is not neccessary. Everyone has had excellent results here.

[bally123]

Hi Chris

The machine was a cnc lathe so I could follow the whole profile from squish band to plug hole in one smooth move with a little boring bar (we have some tiny little tools that love aluminium, only about 8mm dia shank, no chatter and a great finish) I personally would not go the route of a form tool. Too much hassle with chatter and poor finish. Having a CNC lathe to use is of course a luxury not everyone gets!

The chamber and squish band was modeled in 3D CAD including the piston, this enables me to establish all the parameters of the head to piston relationship. I can then produce the profile data to put into the CNC. I have done this many times with road race motors and the real world results can be very close to the CAD design.

The real trick is knowing what volumes and squish areas are needed, for me the motocross / enduro engine tune is a new thing. More compression and larger squish bands compared to my road race builds. (mind you they were running about 12.5 K rpm)

Glenn, I manged to get the profile without sacrificing much squish band, from memory I think the chamber diameter increased by about a 1mm. If I can get hold of a compression tester I will double check against that 200 bench mark - thanks for the info...

Now if I could just get the carb's damn fuel valve to shut off I could have some fun!!

Andy...

[GMP]

From what Ron has said you can get a very nice balance by running a slightly more narrow band (not much), keeping it tight, and being reasonable on the compression as stated. Over here our results seem to back that up 100%. My two bikes will lug down to just about zero and still rev out very far.