Weird issue

[Mekanix]

I'd like to be within the spec range if possible, maybe mill the head 0.55mm which will up the compression a bit more but won't cause detonation until I get into the 1.2mm range.

I'll CC the 1.9 head and see what that has.

 

[Sportster-2001-951C-Stock]

Started reading up on how rotary valves work. I guess at high rpm's its inertia that pull's the new charge in and at lower rpm's its the low pressure that draw's it in.

All this time, I thought pressure differential drew the charge in? Port timing/arrangement is very important though, as well as the exhaust tuning, the charge could go straight out the exhaust port or maybe not properly swirl around and cool the plug electrodes. I guess this isn't related to the problem you're experiencing, though.

 

[Sportster-2001-951C-Stock]

More squish reduces octane requirement by reducing annular heating, compression increases octane requirement, right? Seems sorta catch 22 in this case. Squish chamber was invented during WWII/I by the English when they needed to burn crap fuel.

 

[Mekanix]

Learn something new every day

Measured all 3 heads I have and each is 45-46cc Even the one giving high compression.

I think that at 2mm Its almost beyond usefull squish isn't it ?

 

[Sportster-2001-951C-Stock]

2m seems like a lot of squish to me, in fact the 951 has more squish than I thought was normal. I'd like to try less squish but not increase compression. BUT, I'm not an experimenter or a racer, I'm a hit factory spec and be happy guy.

I really don't think your head is the low speed issue here but it's the one piece that isn't the same, right?

I think it's worth while setting the squish to lower limit spec, why not use a thinner base gasket for that?

 

[Mekanix]

Using a 3 hole gasket drops I can drop it to 1.65-1.7mm which is still larger than the spec of 1.2-1.6.

 

[Sportster-2001-951C-Stock]

Maybe that head was meant to be machined and hasn't been (yet)? The compression scares me a little but maybe it's okay, I just don't have that level of experience so can't help. Wonder if it's stamped "Rocket fuel only"?

 

[Sportster-2001-951C-Stock]

Thinking about this a little more and finally realizing you have the same combustion chamber volume between heads, something maybe doesn't add up in comparison to the last build...? That is, why has static compression increased yet squish is larger than spec?

 

[Mekanix]

Changed the raves too. Maybe the compression was higher before but bad raves caused lower numbers ?

 

[Sportster-2001-951C-Stock]

Got any aspirin handy?

http://edj.net/2stroke/jennings/aspirin_takers.pdf

http://edj.net/2stroke/jennings/

 

[Mekanix]

Lol. I was just reading the tuners handbook and racing handbook.

 

[Sportster-2001-951C-Stock]

Are there any obvious differences in the three heads you have (aside from damage)? Like maybe the squish band ID is different?

There aren't so many things that can cause compression to increase yet your squish gap is larger than spec. I wonder if you've got the correct spec... These engines seem to have larger squish gap than most others so say 1mm gap is maybe not enough.

I dunno though, I've never measured squish on a Rotax motor.

RAVES - I doubt removing them will change static compression, let alone replacing worn ones. Deck height and port location along with head volume contribute to compression but are fixed whereas ring seal can vary.

I recall Gordon Jennings said something about a carburetor hump that occurs when compression is too great but it's unclear how much is too much.

 

[Mekanix]

Nothing obvious between the three heads. I wonder if these engines stretch a bit more and need the extra clearance ?


Side note: I have another set of completely stock carbs and new kits and rotary valve cover on the way for testing.

 

[Sportster-2001-951C-Stock]

I guess so, they sure must pump a ton of heat considering the horsepower.