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Discussion Starter #1
I have asked a couple of people about closing the squish, and the corresponding jump in compression.
I'm still kind of confused about this subject though??

Now on the Yamaha 490 motor I'm working on, I believe the squish to be around .090 (I'm just going off the most common take on this).
I was told that the squish should be around .065?

Now getting the squish to those numbers isn't a big deal.
But I have to wonder with taking .025 of material off, what will that do to my compression ratio?? I plan on running pump gas in this motor, and was told that doubling up, or even making a thicker base gasket will help out with that..

But why even do all the work to close the squish if your just gonna add it all back (plus some) with a thicker gasket?

The concept behind this is confusing the shit outta me, and I need some help to understand it.
 

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well that is some what of a band aid fix and really there is alot more ot it than squish clearance alone . the head by design is prone to detonate .id say .090 is a bit of a high estimate too. also there is much to consider like the squish angle the squish width the bowl shape and volume the radius that blends the squish band ot the bowl the port timing the target rpms of coarse the fuel being used the port area the pipes scavenging effect .allowing for plenty of squish clearance can reduce heat and allow fuel time to effectivaly burn off at a slower rate also it increases voume and decrease static compression ,large bore air cooled motors tend to run hot and have alot of rod stretch as they heat up which reduces squish clearance ,basically just becuase you have alot wh n kicking it iand testing in your garage doesnt mean it will have same amount under pressure while running ,this is where the new 2 cycle designs are doing amazing things like variable timing and programmable variable squish clearances and compression to favor different parts of the powerband at certain times .hope that helps some



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Discussion Starter #3
well that is some what of a band aid fix and really there is alot more ot it than squish clearance alone . the head by design is prone to detonate .id say .090 is a bit of a high estimate too. also there is much to consider like the squish angle the squish width the bowl shape and volume the radius that blends the squish band ot the bowl the port timing the target rpms of coarse the fuel being used the port area the pipes scavenging effect .allowing for plenty of squish clearance can reduce heat and allow fuel time to effectivaly burn off at a slower rate also it increases voume and decrease static compression ,large bore air cooled motors tend to run hot and have alot of rod stretch as they heat up which reduces squish clearance ,basically just becuase you have alot wh n kicking it iand testing in your garage doesnt mean it will have same amount under pressure while running ,this is where the new 2 cycle designs are doing amazing things like variable timing and programmable variable squish clearances and compression to favor different parts of the powerband at certain times .hope that helps some
To bad they don't have a smiley face that's drooling, with it's eye twitching.. lmao

So let me see if I have this strait?
As the motor warms up, the compression ratio drops??

So doing the 30 degree 8mm head mod will blend the bowl more into the squish area, but lower compression... Right??

How much compression would I gain every .010 I take out of the equation?

And how much compression is to much to run 91 octane pump gas????

I think I'm still confused.. :Dope:
 

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compression ratio never changes, whether the motors running or not. cylinder pressure does change ( many people get compression ratio and kicking compression aka cylinder pressure mixed up). i think thats what flotek was saying
 

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Discussion Starter #5
compression ratio never changes, whether the motors running or not. cylinder pressure does change ( many people get compression ratio and kicking compression aka cylinder pressure mixed up). i think thats what flotek was saying
Well think I might have this figured out? It's about as clear as mud, but makes more sense all the time.

I think I'm just gonna do the head mod, then knock .010 out of the jug just to true things up.. I'll see how it runs like that? I'm also starting to contemplate some port work?

I also have a 82' YZ 465 jug, and head. That option is starting to creep into my mind as well.
 

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well i guess the point is there are many variables and even under load and when heat is added into the equation there are even more changes most people dont think about ,yo u are not going to get a complete understanding of a the 2 cycle concept with all the theories and variables involved in one thread or overnight lol



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Discussion Starter #7
well i guess the point is there are many variables and even under load and when heat is added into the equation there are even more changes most people dont think about ,yo u are not going to get a complete understanding of a the 2 cycle concept with all the theories and variables involved in one thread or overnight lol
Yah that's very true, I am gaining a lot more knowledge than I thought was necessary.. But It's all good, I'll figure it all out eventually.
 

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New here, looking at old threads, and always willing to spout off . . .

Static compression is what you calculate from cylinder volume and stroke. In a 2-stroke there are two ways to calculate it, two figures you can get, and there are many places you can find this stuff on the internet. I think that ultimately it doesn't mean too much, because dynamic compression is what your engine sees as a practical matter. Have you put on a bigger, better flowing carb or air filter or reedblock? Then more air/fuel is (probably) getting sucked in the intake side of your engine, therefore more dynamic compression. Have you fitted a different pipe that sucks the intake charge through the motor (better than the previous pipe) and then rams a bunch of it back into the cylinder with the return wave? Again, more dynamic compression. With changes like those, you raise the effective compression of your engine without ever having done any machining on the cylinder or cylinder head. And with those changes, you might not need or want to change the volume of the cylinder head, because that might FURTHER increase the effective (dynamic) compression so much that the engine can't take it without detonating, sticking pistons, etc..

HOWEVER, that said, getting the squish (Brit. "quench") height optimized is always a winner. It doesn't increase compression very much, and it's anti-detonation effect will more than override the small increase in compression. In fact, suppose that you have optimised squish height for your 490 (this is a big dirtbike single, right?)(I'm an outboard guy) at something like .040". But then lets say you have made other changes such as described above. Maybe you find to your dismay that the engine is now showing signs of detonation. Lets say you try different amounts of ignition timing (less) but don't solve the problem unless you retard the timing so much you lose power. At this point you might figure that if you install a thicker head gasket, to put the squish height back where it was and therfore lower the compression back to where it was, you can reduce your detonation . . . .

That won't work! In fact, even though you reduce compression (a little) with the thicker head gasket, the engine will still detonate, in fact will very likely detonate WORSE. That is because the thick head gasket has increased your squish-height away from the optimum. So the final answer on this is that you must FIRST set your squish height at the ideal. Roughly stated, this would be to reduce it it to the point at which the piston starts smacking the head at peak rpm when you come of the throttle and the bearings/rod/piston is all stretched out, and then add back maybe .005". Of course, since you don't really want to go that far, find out from the better racers using that engine what they recommend for squish. After that, if you have to re-set (lower) your compression you must do so by remachining the pocket in the head as needed. It's more trouble than changing head gasket thickness, but it's what works.

My guess is that the desirable squish-height for your engine is going to be in the .038"-.042" range; I'm guessing that that range will be considered a little conservative by the top racers, but they're the ones who'll know. HOW they know is by having tightened it up too much, and had the piston crown "print" against the head! Generally, smaller engines, better-made engines, engines with less "lash," can have tighter squish-heights than bigger and/or looser (more lash) engines. Again, as with compression, it's the dynamic, engine-running, effect that counts here; you build extra squish-height into a bigger, looser engine only so that when it's running hard, the piston comes very close to the head without hitting it.

You can find this in a lot of places (probably including old threads here), but this is a quick way to measure squish in your assembled engine: take a short length of solder, bend it to a right angle (about 2" in, for your big-bore engine); insert the solder into the spark plug hole and feel for the end of the solder to touch the clylinder wall at some point; have a helper use the kick-start to roll the engine to and past TDC. This will squish the solder between the piston and the squishband of the head (if it didn't, your solder is too small or you aren't holding it right). With a micrometer or precision caliper, measure the squished end of the solder. That is your current squish height.

IIRC, the site to read about squish is www.speedomotive.com. As you'll see if you go there, a proper squish height is desirable on 4-strokes, too, for racing or for plonking, including street cars (if they have closed chamber heads rather than smog heads, which don't have squish areas). I'm proud to say (though I had nothing to do with originating the practice, and only followed the smart guys' advice) that alky outboarders in Seattle were, so far as I can tell, among the earliest adopters of the practice of machining for optimum squish-height, back in the mid-Sixties. We tended to use .035" on any of our engines in those early days.
 
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