Measuring squish through the spark plug hole

[Guest ratchethack]

MGNX, thank you for shedding even more light on what for me would've otherwise at least partially remained the baffling darkness of the unknown! This is very helpful. The waters would seem to deepen more quickly than anticipated.

 

This further explains why I had such a hard time getting a "clamp" with the solder -- though now I'm mystified by the prospect of working with a squish band of only 1-2 mm!! And now I begin to question the potential return on effort of getting such a tiny squish band down to the recommended .040" (1 mm) without also doing substantial head work, as you've described.

 

Taking .020" or so off the barrels is one thing, but a complete head re-work is another kettle o' fish altogether. . . .

 

[Armed with a few more tools of illumination, thanks to our Forum experts, our intrepid squish-band fisherman again approaches the workshop, embarking upon a second exploration with solder and socket wrench in hand.]

 

But before I do, I wonder if there's such thing as a similar cut-away or drawing somewhere that accurately maps out the V11 combustion chamber as you've described it?? The shop manual has only a crude cross-section drawing through the valves (90 degrees of rotation from what I'd most like to see), and another crude drawing of the piston, neither one of which provide much of a clue about the shape and dimensions of the squish band.

 

I'm wondering now if the "effective squish" isn't above the tiny 1-2 mm "shelf" that you've described? This would make the squish band more conical in shape and easier to access with the solder thru the plug hole. This is what I'd envisioned in the first place.

[mike wilson]

Looks more to me as if there is _no_ effective squish band at all in the V11 head. Even the Breva type head shown is poorly designed that way. All the figures I can find online seem to think somewhere about 30% of diameter is needed.

 

This may explain some of the difficulties with pinking/detonation, etc.....

[dlaing]

Looks more to me as if there is _no_ effective squish band at all in the V11 head. Even the Breva type head shown is poorly designed that way. All the figures I can find online seem to think somewhere about 30% of diameter is needed.

 

This may explain some of the difficulties with pinking/detonation, etc.....

Could it explain why the Mike Rich piston is the best of the three?

And why some get pinging with the FBF piston?

The Mike Rich piston helps create more effective squish, not at, but near the tiny squish band.

[mike wilson]

The squish band is a result of the shapes of the piston _and_ head. It needs both. You can have a whizzo "squish" shape in the top of a piston but, without the corresponding part in the head, it is (at least) useless.

 

There are numerous complications that conspire to make the physical implementation of squish problematic, especially in a four stroke. Not forgetting that it is not compulsory, squish seems to be something that Guzzi engine designers have decided to forgo.

 

One less thing to worry about, Ratchet. Nogbad will be happy.

 

m

[Skeeve]

Could it explain why the Mike Rich piston is the best of the three?

And why some get pinging with the FBF piston?

The Mike Rich piston helps create more effective squish, not at, but near the tiny squish band.

 

Look at the top of the MR piston, vs the other two. See how the former has a nice-flat top, vs. a peaky one on the others? This means in the case of the latter two pistons that the combustion chamber is essentially a "pair of spectacles" shape (essentially, two combustion chambers sharing a distantly offset spark plug), w/ the spark occurring essentially at the "bridge", with flame fronts propating across the top of the "lenses" & then down. With the Mike Rich piston, that flat top means you have one (albeit oddly shaped) combustion chamber w/ a flame front progressing more directly to the extremities thereof. Less pinging is the result.

 

Clear as mud?

 

Ride on!

[Guest ratchethack]

Gents! Coupla things.

 

Though you can get some general sense of a combustion chamber shape by looking at a piston crown, and make the kinds of assumptions that Skeeve makes above (good stuff, Skeeve), there's close to nothing that can be determined about the shape of a squish band by looking at either a piston crown or a combustion chamber alone. As Mike pointed out, it's the proximity of the two in relation to each other at TDC that defines the shape of the squish band. That's why modeling clay (plasticene) is typically used to get an accurate measurement of squish bands -- but for that, of course, you need the heads off, modeling clay put on, heads torqued back down, and off again for analysis.

 

Again, in the interest of maximum up-time, I'm just trying to think ahead, do a little discovery and some preparation. What I'd like to avoid as much as possible is a SURPRISE that I'm not prepared for when the heads come off.

 

Just got back from Fishing Expedition #2. This trip, I tried 3 times to get a read on the squish by going across the piston as Mike suggested. Unlike the first time, this time I knew I was at the bore because as I slowly raised the piston to TDC (compression stroke), I could feel the solder being pushed up to the cylinder head. First I tried my .084" solder. Nothing. Not only did I again not feel it "clamp" through TDC, but there wasn't as much as a trace of a mark on the solder. Next, I tried the .130" solder. Same thing. Incredulous, I tried it again with the .130". Same results -- not a trace of a mark on the solder. The piston didn't even put as much as a new angle on the nearly straight length of solder, though I did get a fleck of carbon on the top at the end, which indicates nothing of value.

 

CONCLUSION: Hubert and Mike must be right. At least some of us here in the US didn't get any squish band at all with our Guzzi's!! No wonder they ping?!?!

 

I'm amazed that these things can get past the EPA emissions Nazis this way?!?!?!

 

Any further thoughts??

[luhbo]

......

CONCLUSION: Hubert and Mike must be right. At least some of us here in the US didn't get any squish band at all with our Guzzi's!! No wonder they ping?!?!

......

Any further thoughts??

 

 

Paranoia?!?

 

Hubert

[big J]

I've seen engines with one side of the head "offset" ,if you like, to provide a squish band at one side to promote turbulence.

 

Could this be a possibility?

[mike wilson]

Gents! Coupla things.

 

Though you can get some general sense of a combustion chamber shape by looking at a piston crown, and make the kinds of assumptions that Skeeve makes above (good stuff, Skeeve), there's close to nothing that can be determined about the shape of a squish band by looking at either a piston crown or a combustion chamber. As Mike pointed out, it's the proximity of the two in relation to each other at TDC that defines the shape of the squish band. That's why modeling clay (plasticene) is typically used to get an accurate measurement of squish bands -- but for that, of course, you need the heads off, modeling clay put on, heads torqued back down, and off again for analysis.

 

Again, in the interest of maximum up-time, I'm just trying to think ahead, do a little discovery and some preparation. What I'd like to avoid as much as possible is a SURPRISE that I'm not prepared for when the heads come off.

 

Just got back from Fishing Expedition #2. This trip, I tried 3 times to get a read on the squish by going across the piston as Mike suggested. Unlike the first time, this time I knew I was at the bore because as I slowly raised the piston to TDC (compression stroke), I could feel the solder being pushed up to the cylinder head. First I tried my .084" solder. Nothing. Not only did I again not feel it "clamp" through TDC, but there wasn't as much as a trace of a mark on the solder. Next, I tried the .130" solder. Same thing. Incredulous, I tried it again with the .130". Same results -- not a trace of a mark on the solder. The piston didn't even put as much as a new angle on the nearly straight length of solder, though I did get a fleck of carbon on the top at the end, which indicates nothing of value.

 

CONCLUSION: Hubert and Mike must be right. At least some of us here in the US didn't get any squish band at all with our Guzzi's!! No wonder they ping?!?!

 

I'm amazed that these things can get past the EPA emissions Nazis this way?!?!?! <_>

 

Any further thoughts??

 

Is it doing something it shouldn't? If it's running fine, then I don't understand what the problem is. The engineers who spent a long time desiging, building and refining this machine have obviously come to the conclusion that any level of effective squish band is unnecessary.

 

That does not mean that there is not sufficient turbulence (the purpose of squish) already or that there is some overwhelming need to provide it.

 

On such a large, comparatively slow revving cylinder the engineers possibly discovered that creating an excessively compact combustion space _caused_ detonation. With the faster-burning, lower octane fuels used nowadays it is possible that a less fashionable shape is all that is needed.

[luhbo]

Mike, that's a bit of what I meant in one of my previous posts talking about this so generously enlarged S/A wich can be seen on the Mike Rich pistons. I think that what you've written above together with the fact that a however perfectly balanced squish gap is effective only for a very small amount of crank degrees could lead to more problems instead of gain remarkable advantages. At least if you consider what efforts have to be made (milling the cylinders, buy expensive pistons, time and so on).

 

My concern is that the MR-pistons may give a perfectly compact combustion chamber, but only exactly at TDC. The crankshaft moves on and "suddenly" all around this perfect chamber deep and relatively cold "slots" or pockets are being opened and they possibly spoil a lot of what has begun so promissingly.

 

 

Hubert

[mike wilson]

Mike, that's a bit of what I meant in one of my previous posts talking about this so generously enlarged S/A wich can be seen on the Mike Rich pistons. I think that what you've written above together with the fact that a however perfectly balanced squish gap is effective only for a very small amount of crank degrees could lead to more problems instead of gain remarkable advantages. At least if you consider what efforts have to be made (milling the cylinders, buy expensive pistons, time and so on).

 

My concern is that the MR-pistons may give a perfectly compact combustion chamber, but only exactly at TDC. The crankshaft moves on and "suddenly" all around this perfect chamber deep and relatively cold "slots" or pockets are being opened and they possibly spoil a lot of what has begun so promissingly.

 

Well, that is entirely similar to what happens with a "proper" squish band so I'm not at all sure that it would be a problem if _all other factors_ were properly dealt with.

 

For example, on a twostroke, the combustion chamber is shaped like a beaker on top of a tea plate (both upside down) with the plug in the centre of the base of the beaker. A very regular shape. There are other parameters that affect the flow of gas in there, such as the direction of the transfer ports but, because the chamber can be formed to such an ideal form, squish flow generally overcomes them.

 

For fourstrokes, there are so many other parameters to take into account in forming the chamber, not the least of which is the presence of at least two dirty great valves and associated ports. Because you can't make the chamber anywhere near perfectly shaped, you have to allow some of the other parameters to take a hand in assisting the combustion process, for example the inlet ports are angled to promote swirl. Piston crowns are shaped to help promote a best-form chamber, as well as defining compression ratio. It's all rather subtle and intertwined.

 

Not to say that creating a definite squish band would not be beneficial - it's just that the work involved may not be propotional to the reward. It just seems to me that simple lumpy pistons would have to strike a magnificent vein of luck to come up with a shape that added a fully functional squish band to increased compression ratio.

 

m

[motoguzznix]

Ratchet

 

I took some measurement from the head design when they were off, I will post this soon.

 

Mike, Hubert, Skeeve

The combustion chamber shape resulting from that head design is not favorable, as shown in the following sketch:

 

 

 

These big pockets let me doubt about the serious work in the factory on the combustion chamber shape. Previous Guzzi heads were not of that design and used a large squish area.

[Guest ratchethack]

 

 

These big pockets let me doubt about the serious work in the factory on the combustion chamber shape. Previous Guzzi heads were not of that design and used a large squish area.

Thanks again, MGNX. I, for one, will be MOST interested in your measurements. If your drawing is a valid indication of the combustion chamber shape, It'll change my entire angle of approach on what I do when the heads are off. How do you suppose that no one (to my knowledge - maybe someone already has?) has as much as mentioned this here (or anywhere else to my knowledge) in recent years, let alone publicly exposed this, fully documented it and/or come up with a clear, proven procedure for a "generic" kind of head rework (as opposed to a common power-peak-chasing head rework) with dyno charts before and after, etc.?? The only thing I can think of is that somehow the consequences of what would appear to be a frightfully poor design didn't result in poor enough real-world efficiencies to cause worse emissions, worse mileage, and worse detonation problems.

 

This warrants further investigation, and you've got my full attention!

[dlaing]

Ratchet

 

I took some measurement from the head design when they were off, I will post this soon.

 

Mike, Hubert, Skeeve

The combustion chamber shape resulting from that head design is not favorable, as shown in the following sketch:

 

 

 

These big pockets let me doubt about the serious work in the factory on the combustion chamber shape. Previous Guzzi heads were not of that design and used a large squish area.

Notice that the Mike Rich pistons help fill in that pocket of un-squished mixture.

So, Mike appears to have been on the case for years!

[mike wilson]

That interpretation of the head design [drawn on a napkin 8-)))] does not seem to be favourable. I'm not convinced, though, that any of the pistons shown previously will deal with the "problem".

 

Looking forward to proper measurements.