Detonation, lean seizure, or....?

[a.lo.v2]

what would have caused the hole at the 3 o,clock position in the cylinder head photo?

[holty]

hi ryan, above post made me look at the photos again, and i noticed that your head is not the same as mine, yours has the squish band at 2 heights, mine is only one, what was your head off, heres a pic of mine
IMG_1722_zpsu0s1igd4 by stephen holt, on Flickr
i was thinking that maybe the raised part of the squish was too close, you can see a ring all the way around the outside of the piston, wonder if this might of added to the problem, what did you measure the squish at ?

[RManson]

what would have caused the hole at the 3 o,clock position in the cylinder head photo?

That's for the compression release for the electric starter.

[RManson]

hi ryan, above post made me look at the photos again, and i noticed that your head is not the same as mine, yours has the squish band at 2 heights, mine is only one, what was your head off, heres a pic of mine
IMG_1722_zpsu0s1igd4 by stephen holt, on Flickr
i was thinking that maybe the raised part of the squish was too close, you can see a ring all the way around the outside of the piston, wonder if this might of added to the problem, what did you measure the squish at ?

Our heads are the same, but I machined that recess due to the tight tolerances I noticed upon initial assembly. I turned down that section to increase the squish, but obviously not enough (2mm). I think this time around I will use a thicker base gasket and measure the cylinder head cc as well as that of the piston crown at TDC to be absolutely sure it's within spec.

[holty]

i thought it looked different to mine, that explains the step, if it was 2mm that would be well safe, i think mines about 1.5 mm, i think rotax recommend 1.8 to 2, i was thinking if you used solder and it wasnt right to the edge you might have got a false reading on the lower part, rather than the higher section on the outside, you might do better to machine the squish all the way accross so you dont have to alter your port timings, im pretty sure a large chunk of performance is lost if the squish band, and angle is not done correctly, i am not an expert on this matter by any means, i know darrell is, he did an article in scootering about that very subject a while ago.

[Stampede]

I personally think it was a compression problem. Did you measure your compression ratio? Did you also use a high quality fuel with an higher octane?

And 2mm squish is way too much in my opinion. If you´re increasing your squish too much, you´ll lose the squish effect. The fuel will detonate in an area then, were you doesnt want to have it.

All my engines (Lambretta) are running on a 1.2mm squish at max, with a matching compression ration. My RD350 engine is also running with a smaller squish at about 1mm, I could go down to 0.8 as a max.

Please see also the three articles on:
http://www.rd350.info/articles/squishing/

In your case, I would get the squish to a useable measure again and set the compression ratio as well by machining the head, perhaps to the original level of the donor engine.

[Darrell Taylor]

Forgot to mention the exhaust port side failure of the piston eroding away around the ringland ive only seen in situations of small stinger diameters ,although other contributing factors like high compression or over advanced ignition timing will make matters worse
im not too familiar with the original use for the cylinder /engine design and its suitability ,is it a jet ski cylinder , cvt transmission,2 into 1 pipe ,,whats its original rpm range for peak power , whats the original ignition curve look like and compare with your own ?
the head design/band width /msv etc. will all be to suit the original design so worth looking at for its new use.

[RManson]

Good points all around! I will definitely be looking closer at the combustion chamber as the new top end goes back together, including cc'ing the unit. Stock Ski Doo squish spec is 1.2 to 1.7mm and if I remember correctly, it was smaller than that, hence the cut. I already have a new head, piston, and cylinder and will triple check things this time around. Will probably make for a couple good, new how-to threads...

Darrell,
I agree with you in the stinger diameter as I've been crunching the numbers on what pipe dimensions the engine would be happy with and the stinger diameters all come in around 30mm plus another 2-3mm to make up for the silencer. That's 6-7mm over the current stinger. On the dyno, the power started tipping over around 4,500 rpm as if it had a soft rev limiter set, though it did not. In hindsight, I'm starting to suspect that's where the pipe was at its max in 3rd gear and WOT before it started melting down the piston. The bike revved past 4,500 on the street, no problem. With a larger stinger size, it should have been able to bleed off the extra volume before the temps rose as they did. Of course, an EGT gauge would have added to the story. Needless to say, I have one on the way for the next chapter!

The original application of the top end was a Ski Doo snowmobile, making peak power around 8,500 rpm, set up as a twin with a 2-in-1 pipe. I have an original pipe, it's BIG.

The timing curve I had programmed at the time had 16 degrees initial ramping up at 1500 to 25 degrees total at 2500 then back down from there at 6500. Seemed nice and happy with that curve as I tried a half dozen and this was by far the best. Looks like Ski Doo specs initial at 17 degrees so with the MSD box, that sounds about right. Hard to find a good curve source online as the Ski Doo CDI has no adjustability so it's basically a plug and play deal for those guys.

On another note, we were talking about BMEP on another thread, which reminded me of something I was playing with the other night. We did manage to get a number from the meager pulls we accomplished, 24hp at 4000. Using a BMEP calculator, that's equivalent to 134.48 psi or 9.27 bar, which seems pretty good to me. If the motor can retain that through 8,500 rpm or so, it should be good for 40hp, plus or minus. We shall see!

[Darrell Taylor]

can you calc out the rpm back from the gearing , i dont think 24 at 4000 is correct id expect 13 to 18 bhp at 4k so possibly an rpm error that would also throw the ign curve if based on same rpm measurement
most dyno runs start at 3k so be at 4 almoststrsight away

no extra increase in stinger size would be required due to a silencer if its bigger than the vent if end of cone is 30mm connecting tube 30mm + and perforated baffle tube 30mm+ then be ok , if the silencer is smaller then its a problem as it effectively becomes the new outlet/vent size

[RManson]

Tore the motor down this afternoon and upon dry reassembly found a couple interesting things that I either overlooked or thought I'd compensated for.

First up, piston raised 1.5mm above top of barrel...

IMG_6231 by PotvinV8, on Flickr

Squish test reveals it comes in at 0.700mm with a 0.500mm base gasket. Spec squish is 1.5mm so a tad tight, eh? The piston doesn't contact the head in dry, cold assembly but as you could see in the previous pics, it obviously did at some point once it's hot. I've ordered a 1.2mm base gasket, which should get the squish around 1.4mm, which is more like it.

I think Darrell asked about the ring coverage of the exhaust port at BDC. Turns out it was a touch over 1mm, so the thicker gasket should help this a bit as well and get the port timings very close to stock spec.

IMG_6236 by PotvinV8, on Flickr

I think the high compression along with the small stinger size contributed to the seizure. To be honest, I'm surprised it lasted as long as it did!