Squish clearance

[Boot]

Anyone know what the KDX200 squish clearance should be, approximately? I'm sure I read it here a while ago and recorded it, but now can't find the clearnce figures. I've got 0.3mm in my head, but am not sure.

My 240-kitted bike is pinging a little at lower revs under certain conditions, and I'm pretty sure it's because the cylinder head has not been machined properly to match the new piston and bore.

Boot
Melbourne, Oz
91 KDX200 (240)
85 Virago 1000
http://www.copperleife.com/craig/bikes/

 

[KelvinKDX]

This would be a good question to post in Eric Gorr's forum.

 

[Robcolo]

More like 1.0mm would be a good minimum clearance [0.3mm [.012"] is way too tight. [I'll bet your squish is greater than 1mm though ] The squish clearance isn't what causes or corrects pinging though --It's your final head volume. You can compute your [new] compression by measuring the head volume and dividing into the new cylinder volume. going up to 240cc, the comp ratio is likely a bit too high, but to lower it you want to enlarge the hemi part of the head -- not the squish area. Yea, you could just add a head gasket but that also increases the squish clearance which in turn decreases performance and increases cylinder & head temps.

 

[David Trustrum]

Yes & no, too tight a squish clearance will increase MSV & if other factors present (ie: large squish area ratio etc) will lead to detonation.

Put some solder down the plug hole & wind the engine over. Measure. Then repeat in 90 deg spacing & take smallest measurement. 0.8 would be the tightest you would want to run without the piston hitting the head at peak revs. 1mm probably better. This may as stated above raise the comp too high & especially if big bore kit fitter then remove more from head.

You could try adding some leaded av as an experiment to increase octane to see if it goes away. On the left field just check your powervalve is opening properly.

 

[Boot]

Thanks for replies so far. I had in fact tried the solder trick before pulling the head off (I accidentally snipped off a piece of solder in a transfer port !!!). Minimum squish clearance at the edge seems to be more like 1.5 mm, so probably a bit much?

I'm sure that the overall head volume is too low as well. I hadn't noticed any pinging until after having it bored to the first oversize, which increases the compression ratio. Looks like I'm going to have to try to track down somebody who has appropriate machining experience. The avgas idea is a good one, though, David. It pings on premium pump unleaded.

Comments from Eric Gorr's site are below for anyone interested:

After overboring the cylinder, the head's dimensions must be changed to suit the larger piston. First, the head's bore must be enlarged to the finished bore size. Then, the squish-band deck height must be set to the proper installed squish clearance. The larger bore size will increase the squish turbulence, so the head's squish band may have to be narrowed. The volume of the head must be increased to suit the change in cylinder displacement. Otherwise, the engine will run flat at high rpm or ping in the midrange from detonation.

 

[Boot]

Forgot to ask: David what would the powervalve have to do with this? It pings at low revs when the valves are supposed to be closed. So if the valves were stuck open this would reduce the compression ratio surely?

 

[David Trustrum]

No, I was thinking that the valves may have stuck closed & it was pinking at higher revs (read the post to fast & missed the low revs bit)

1.5mm is ok, sort of. If you machined it to 1mm you may get a little gain all things being equal as there is less gas squished that doesn’t burn. However all things won’t be equal as this will also increase the compression ratio which sounds a touch to high as it is. It will also increase the MSV, but I doubt is this would be a problem.

I assume the head was increased in diameter when the barrel was bored. If not then the compression would likely be way too high & the pattern in the solder would have a sharp corner edge reducing down to bugger all a few mm in from the edge.

It would appear that not enough was machined out & also the squish area to chamber ratio is higher now so machine some of the squish area away. If you felt like getting all scientific you could slap a piece of Perspex on the upside down head & glue it on, then through a small hole pour a measured amount of oil until full (fill spk hole with an old plug). A horse syringe would be the go, or a normal 5cc one filled a few times.

This way you can measure before & after to see how much material you have machined & if you did decide to give it a 0.5mm skim before you machine the chamber you can see if it is still bigger after. Suspect you will have to add a good CC to change the comp ratio ~ ½ a point, more would be safer if you can hear it in normal riding.

 

[Boot]

Nope, believe it or not the head does not seem to have been machined at all! The wonder is that it doesn't ping a lot more with what is roughly a 20% increase in swept volume and who knows what sort of squish dynamics. The piston crown, however, is about 0.5mm shy of the top cylinder deck height at the edge, and added to the thickness of the head gasket (~1mm) means that my minimum clearance of about 1.5mm is measured at that corner you mentioned (where the curved piston crown comes closest to what was once the edge of the combustion chamber). I've now filed and sanded this edge to blend with the existing head squish angle, but of course the squish band is now doubtless too wide besides being not tight enough. The hemispherical part is too small. It's all wrong.

Left powervalve broken again too... Sigh.

 

[David Trustrum]

Urrgh! Yeah, not ideal! Oh well, - all room for improvement & you know the direction.

Sorry to mention the PV, hope I didn’t jinx it. Make sure new one is nice fit ie: all carbon cleaned out of barrel pivot points so takes longer to jam up.

 

[Boot]

Thanks David. Yes, I now know what needs to be done with the head and I can try to work out how to go about it.

I'm off to pick up my powervalve this afternoon. I'll make sure everything's clean, but this time I think I'll have a quiet word to it when I assemble it and ask it to please, please be good.

 

[shr]

IMO, the head should be cut to the 72 mm bore (240 kit). The squish band (head to piston would be ideally at .040" (1 mm) not less than .030" and not more than .050").

The width of the squish band would be around 10.5 mm in @ 13 to 14 deg's then cutting sharply into the combustion chamber.

With the deck height you talked about. the total head volume should be about 20cc to 22cc.

 

[canyncarvr]

I'm not a machinist with extensive kdx experience (shr is), but it seems reasonable that any º change to the squish band that involves removing metal will increase the width of the band.
Thus: The process is likely STARTED with a milling procedure, THEN machined to fit the spec you want.

Sounds like you're on the way toward, 'I cut it twice and it's still too short!!

 

[David Trustrum]

At 10.5mm you are aiming at 50% squish area to chamber area. If you are going to run 1mm squish, making some assumptions that may put you MSV kind of high, at 8mm that is 40% & might be a bit smoother.

Having said that I haven’t tried a 240 kit but something to think about if detonation is a problem.

 

[Boot]

Wow, some actual numbers to use as a starting point. At least I can now take the head to someone who isn't necessarily familiar with KDX240s and hope to end up somewhere close to where it needs to be. Given that it already runs pretty well except for somewhat temperamental idle (typical for 240s) and the one narrow zone of pinging at low revs when under load, surely even a small step closer to optimal head dimensions would be a big help.

It seems to have about 13mm of squish band at the moment, though I'm not sure of the angle or head volume -- presumably they are stock.

Thanks heaps fellas.

 

[David Trustrum]

But presumably the bore size has gone up ~6mm so squish has grown 3mm per side? Area = pi x D sqd /4 so you could work out the relative squish area before & after.

My only point is that if you close up the squish clearance - the squish velocity shoots up,(msv affects turbulence in the combustion chamber, too little or too much is to be avoided) so lowering the area of the squish band might be a wise idea.

 

[Boot]

Yes, absolutely David. You're right, it's an extra 6mm in diameter giving a 3mm ring around the circumference. And yes, I could calculate the relative squish area before and after and shoot for something similar. I understand about the MSV and its role in flame propagation. I'm thinking that I'll have to shave the deck height down very slightly to get the edge squish I need because I've got almost a milimetre in the gasket anyway. It could be that the way it is set up now it has too much squish area sort-of accidentally compensating for too loose a squish clearance, so if I adjust only the one I might find that the bike runs worse. Will have to do it as a total job.

I'm off for four days, but please do add any further comments and I'll respond later.

 

[David Trustrum]

Yes I expect that manufacturers set the engines up to allow for tolerance of bigger pistons, longer rods & shorter barrels so that there can be no way the piston hits the head.

They then consider the multiple types of fuel that will be used & lower the compression to allow for most of these, but then they increase the squish area (hence MSV) so that the majority of engines will run smoothly at lower revs where they will spend most of their lives.

Well it’s a theory. . .

 

[Boot]

Yes, I reckon you're probably right David. They would have to err on the side of caution with a clearance like this that is affected by so many factors. Blueprinting the engine when its actually assembled is the go, and then an adventurous tuner might even tighten the clearance up slightly more.

I've always wondered about carbon buildup too. I assume that it won't accumulate where it doesn't have permanent space to do so, but in some engines I've seen with excess squish clearance I'm sure that carbon effectively tightens things up at the edge (not an ideal way to achieve this).