Squish clearance and compression ratio - advice please!

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James P
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Squish clearance and compression ratio - advice please!

Post by James P » Sun Jan 18, 2015 7:19 am

Although I have been working on two-stroke engines for a while, I am relatively new to the KR-1 and am in the final stages of finishing my first engine rebuild.

I was hoping to optimise the squish clearance which (for mass production reasons) the factory sets at a rather large figure. I was originally aiming for 0.8 to 0.9mm.

A quick bolt-together of the top end with standard base gaskets (but no head gasket) revealed a squish clearance of 1.0mm. Add the thickness of a standard head gasket and the squish clearance will be 1.25mm.
I measured the volume of the combustion chambers in the cylinder head (at zero squish clearance - piston taped to head and oil injected through plug hole) and they came in at 8.0ml each.
Using a quick calculation for the volume of a 1.25mm squish clearance, I got a total head volume of 11.08ml (it will actually be slightly greater, due to the curvature of the squish volume - my quick calculation was for a flat squish volume).
Using the head volume of 11.08ml with a cylinder volume of 124.6ml (or 71.4ml above the exhaust port), I got a compression ratio of 12.2:1 geometric (or 7.44:1 corrected). This aligns fairly well with Kawasaki's figure of 7.4:1.

If I reduce the squish clearance to (for example) 0.8mm, I end up with a compression ratio of 13.5:1 geometric (or 8.16:1 corrected). This seems a bit high to me, especially with today's lower quality pump fuel.
I was hoping to avoid having to grind or mill out the combustion chambers to get the compression ratio back to something sensible (i.e. about standard). I'm not interested in squeezing every last drop of performance out of the engine - it will be fast enough for me in standard form. However, I was also hoping to maximise reliability by reducing the squish clearance to lessen the chance of detonation occurring.

One other factor to consider is that, using standard base gaskets, the edges of the piston crowns are exactly level with the floors of the transfer windows at bottom-dead-centre. Reducing the squish clearance by fitting thinner base gaskets (effectively lowering the cylinders) will reduce port durations and the port windows will never be fully open (...although they would only be "blocked" by 0.4mm). I'd prefer not to remove any material from the top gasket faces of the cylinders because they have been replated and I don't want to risk chipping the Nikasil. I could instead remove material from the gasket face of the cylinder head, but this could make the head more prone to warping!

For those who have past experience in building and running these engines, what would you recommend, taking in to account all of the factors above?

Thanks & regards,
James

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Re: Squish clearance and compression ratio - advice please!

Post by Luders » Sun Jan 18, 2015 9:12 am

Fitting thinner base gaskets will effectively also lower your transfers to something like 39.45mm off the top of my head, unless you have already raised them, by way of porting?

I know you say you don't really want to go down this route, but if you haven't tuned it, or intend on running it on equivalent 5 star gasoline, I would suggest getting the head re profiled to achieve the correct squish/compression ratio.

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Re: Squish clearance and compression ratio - advice please!

Post by JanBros » Sun Jan 18, 2015 11:50 am

you could just ditch the head gasket and have made grooves in the head to fit O-rings. The rest of the head can be sealed with a silicon sealant, I have ridden KR's like that for almost years.

You'd end up with a just slightly higher compression ratios and a quish of 1.0mm which would still be ok. Don't know exactly who, but someone who knows explained each surface has a boundary layer of 0.5mm . you can "discard" those as not much happening in the layer, so that's 0.5 for the piston and 0.5 for the head = your 1mm.

making grooves in the head shouldn't cost much, anyone with a lath big enough could do it in an hour or so. make them as far away as possible from the combustion chamber, and make sure the area-surface of the cross-section of the groove is just a bit more than 100% of the cross-section of the ring. I use 2mm thick rings and make the groove 2*1.6 deep.
groove = 3.2mm²
ring = Pi*d² = Pi = 3.14²
it needs to be this way as one doesn't squeeze the ring together, one simply deforms it.
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Re: Squish clearance and compression ratio - advice please!

Post by James P » Sun Jan 18, 2015 2:05 pm

Thanks for the replies so far.

Ben: The cylinder porting is standard and will remain so. I usually use 95-octane pump fuel and intend to do so in this engine too. I'm trying to avoid having to modify the head but, if I did go to that trouble, I would want to machine the squish bands to match the piston dome profile exactly, as well as removing material from the combustion chambers to obtain the desired compression ratio. At this stage, I'm not sure how much material may safely be removed from the head (whether from the gasket face, the squish band or the combustion chamber) before it becomes susceptible to warping.

Jan: I did consider converting to O-rings instead of using a head gasket. There are several advantages to this method, but it does involve some expense. It would be much cheaper to buy some of Dan McBride's thinner base gaskets and obtain a similar result, BUT the exhaust and transfer windows would end up 0.25mm lower with the thinner gaskets than with the O-ring method.

Can anyone suggest how much material may be removed from the gasket face on the cylinder head before it becomes susceptible to warping?

Regards,
James

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Re: Squish clearance and compression ratio - advice please!

Post by JanBros » Sun Jan 18, 2015 2:37 pm

James P wrote: Jan: I did consider converting to O-rings instead of using a head gasket. There are several advantages to this method, but it does involve some expense.

Can anyone suggest how much material may be removed from the gasket face on the cylinder head before it becomes susceptible to warping?
the cost is minimal, and the result would be big : you'd still have a standard engine but it will run signifacantly better.

I don't know exactly, but I have used heads of which almost the entire outer egde of the head was milled off.
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Re: Squish clearance and compression ratio - advice please!

Post by maccas » Sun Jan 18, 2015 3:00 pm

James,

I think you'd be fine with 13.5:1 compression ratio on 95 Ron fuel.

I've ran my 3xv on 95 Ron fuel with 15.3:1 compression ratio with no Det. Some head shapes you can get away with a higher compression ratio. I'm not saying this will work in all cases. I had to back the ignition advance off a touch but that was all.

Anyway, if your bike is a road going machine, lowering the cylinders a touch and increasing the cr will maybe lose 3-500 rpm at the top but it will make it far more torquey low down and much more useable.

Maybe give the thinner base gaskets a go first as at least that is reversible? Then if you find you miss the revs at the top then revert to standard thickness base gaskets and carry out the o-ring mod as Jan suggests?

Dan

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Re: Squish clearance and compression ratio - advice please!

Post by JanBros » Sun Jan 18, 2015 7:34 pm

maccas wrote: Maybe give the thinner base gaskets a go first as at least that is reversible? Then if you find you miss the revs at the top
but as he will lose important TA (time area), he will probably miss revs anyway :?:
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Re: Squish clearance and compression ratio - advice please!

Post by maccas » Sun Jan 18, 2015 8:03 pm

Yes, dropping the cylinders and increasing the cr will stop the motor from revving as hard.

But a standard kr shuts up shop about 11 grand anyway due to the stock ignition. Lifting the entire torque curve for the sake of missing out on 500 or so rpm at the top is surely worth it on a road motor? And if James doesn't like it he can raise the cylinders again and cut o ring grooves in the head like you suggested. I know the exhaust is pretty low on a kr1 anyway, but so are the transfers in stock trim so blowdown TA should be ok for under 11k road work.

Just my two pence anyway. I'm no expert and i'm just speculating.

Dan

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Re: Squish clearance and compression ratio - advice please!

Post by Luders » Sun Jan 18, 2015 8:46 pm

The 1S makes peak power around 10500 rpm, the KR-1 is less. Using thinner base gaskets will lower the transfers and indeed stop it revving as high.

Dan does make a good point. This is a road bike, so losing a bit of the revs at the top end and gaining some mid range wouldn't be a disaster at all and like he say's it's easily reverseable.

With these things in mind, I retract my initial suggestion and say go with the thinner base gaskets and see how you get on.

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Re: Squish clearance and compression ratio - advice please!

Post by JanBros » Sun Jan 18, 2015 11:04 pm

I agree with all, my point was : Maccas said try lowering the clinders which will raise compression and because of that he'd loose some rev's (that's what I made out of it).
so I replied that doing that will also lower TA's and because of that he'd lose rev's to. So one can not tell for sure what (if there is) made the lost in rev's.

I would find it ok just for testing, but would always raise the cylinder again so that the bottom of the transfers is equal with the top of the piston in BDP. fresh charge flowing over the piston top is probably the most important thing for cooling the piston.
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Re: Squish clearance and compression ratio - advice please!

Post by James P » Thu Jan 22, 2015 10:53 am

Hmm...what to do :? :? ?

Thanks for your advice Dan (and Ben & Jan for continued input). I'd also appreciate some input from someone who has built (preferably more than) one of these engines and just left everything completely standard (i.e. standard gaskets with large squish clearance). The bikes obviously came from the factory like this and, although I would rather optimise several of the features, I would consider running the engine as it came from the factory if nobody has experienced any problems with that set-up...

Thanks again & regards,
James

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Re: Squish clearance and compression ratio - advice please!

Post by James P » Thu Jan 22, 2015 11:17 am

I've just remembered reading about rings being shrunk into cylinder heads on older model parallel-twin TZs (maybe later V-twins too - not sure :? ). I understand that these rings were made of some type of copper alloy and are/were basically just new squish bands.
In theory, this may help in my case, as the squish clearance could be closed up with only a very small increase in compression ratio and no reduction in the port durations. Could someone who knows more about these shrink-fit rings please expand on the matter?

Thanks & regards,
James

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Re: Squish clearance and compression ratio - advice please!

Post by jarno » Thu Jan 22, 2015 11:42 am

those rings your talking about James are detonationrings the where put in top of the cilinder to eliminate detonation.
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Re: Squish clearance and compression ratio - advice please!

Post by gangus » Tue Feb 24, 2015 5:08 am

They don't eliminate detonation, rather are more durable whilst operating with detonation.

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Re: Squish clearance and compression ratio - advice please!

Post by KR-1R » Wed Feb 25, 2015 12:06 am

in head or top cylinder - dont know advantage of one over the other...
easier to install in cylinder, however heat better removed in head

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