Squish clearance and compression ratio - advice please!

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gangus
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Re: Squish clearance and compression ratio - advice please!

Post by gangus » Wed Feb 25, 2015 1:22 am

You really need both.
You get det damage on the top mating face of the barrel behind the nicasil.
If you look at the heads in the above post, the outer flat section of bronze/brass is there for this reason.
The heads lift and flame travels in this area. I have seen head bolt upgrade reduce this, keeping squish constant instead of opening up, allowing leaner jetting prior to detonation.
This isn't really related to a kr though....

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Re: Squish clearance and compression ratio - advice please!

Post by James P » Mon Mar 16, 2015 9:33 am

After procrastinating for ages on this, I decided to get something done. I made a few measurements and did a few calculations to see how much the port durations would be reduced by lowering the cylinders:

Kawasaki specification:

Exhaust 186deg
Transfer 126deg
Comp ratio 7.4:1


My measurements and/or calculations (standard engine with standard gaskets etc.):

Exhaust main 179.5deg
Exhaust aux 188.5deg
Transfer 127.5deg
Squish clearance 1.25mm
Comp ratio 12.2:1 geometric, 7.4:1 corrected


If I reduce the squish clearance to (for example) 0.85mm by using thinner base gaskets, I should obtain:

Exhaust main 177deg
Exhaust aux 186deg
Transfer 124deg
Squish clearance 0.85mm
Comp ratio 13.3:1 geometric, 8.1:1 corrected


This seems OK to me, although the compression ratio is higher than I'd like - I would prefer to keep the standard (or very slightly lower) compression ratio for reliability with today's questionable fuels. I have therefore had the combustion chambers enlarged by between 1.0 and 1.5ml (each one was milled identically, but my syringe only indicates to the nearest half millilitre!).
If I can obtain exactly 0.85mm squish clearance, the new compression ratio should be about 11.7:1 geometric, 7.2:1 corrected.

The cylinder head in the mill with one combustion chamber roughed-out, ready to start the second one:
Image
I would have liked to have had the squish bands recut to match the piston crowns exactly, but decided that it would be too much faffing about (possibly needing to skim the head as well), for perhaps questionable advantage.

Job done, combustion chambers lightly polished:
Image

Scan of a direct tracing of the combustion chamber profiles, before and after:
Image
For some reason the inner edge of the squish band looks a little different on each tracing. However, no modifications were done in that area.

Now I'm just waiting for the arrival of thinner base gaskets from Dan McBryde. Dan has posted them, so they should be here this week or next.

Regards,
James

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Re: Squish clearance and compression ratio - advice please!

Post by JanBros » Mon Mar 16, 2015 10:45 pm

did you include the head gasket when calculating the cyl head volume ? it's about 0.5ccm³.
I just calculated the head volume from original figures, and it's a massive 19cc :shock:
I always ran 10cc heads because 10cc is always safe on a 125cc cylinder if you have a proper squish.
looking at the cross-section of your head there is hardly a proper squish. the edge of the squish should be sharp without any radius, and for more torgue/less high power band the squish area should be a hole lot more (up to about 50%, which equals a squish width of 8.2mm). a tiny squish band is good for high revving high powered engine, and a road going KR1 isn't that at all.

I don't think you will notice any difference. I would have faffed a hole lot more :mrgreen:
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Re: Squish clearance and compression ratio - advice please!

Post by James P » Tue Mar 17, 2015 7:22 am

JanBros wrote:did you include the head gasket when calculating the cyl head volume ? it's about 0.5ccm³.
I just calculated the head volume from original figures, and it's a massive 19cc :shock:
I always ran 10cc heads because 10cc is always safe on a 125cc cylinder if you have a proper squish.
looking at the cross-section of your head there is hardly a proper squish. the edge of the squish should be sharp without any radius, and for more torgue/less high power band the squish area should be a hole lot more (up to about 50%, which equals a squish width of 8.2mm). a tiny squish band is good for high revving high powered engine, and a road going KR1 isn't that at all.

I don't think you will notice any difference. I would have faffed a hole lot more :mrgreen:
Yes Jan, I allowed for a head gasket of 0.25mm thickness. I measured the volume of the standard combustion chambers at 8.0ml (or 8.0cc if you prefer!) with zero squish clearance (piston taped to head with thin smear of grease to seal, oil injected through plug hole). I then used an approximation of the volume of the desired squish clearance (including head gasket) in calculating the compression ratios (I say approximation because the actual squish clearance volume is curved, whereas I just used the "flat" volume in my calculation - the actual volume will be a little larger).

I agree with your comments about the "proper squish", but that is what the standard Kawasaki head is like :( ! If I had a choice, I'd choose a much wider squish band (which is why I initially wondered about shrink-fit rings).
You are also probably correct in that I may not notice any difference! However, I like to take all reasonable precautions for reliability - time will tell.

Regards,
James

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Re: Squish clearance and compression ratio - advice please!

Post by JanBros » Tue Mar 17, 2015 11:44 pm

I created this : http://users.telenet.be/jannemie/Head%2 ... %2056.xlsx

tnx to MJ on who's head calc it's based.

data in it is from my engine :wink:
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Re: Squish clearance and compression ratio - advice please!

Post by James P » Sat Mar 21, 2015 12:44 pm

Thanks for that Jan. I've converted it back to "old" Excel, so I can use it on my ancient computer :lol: !

Regards,
James

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Re: Squish clearance and compression ratio - advice please!

Post by JanBros » Sat Mar 21, 2015 2:48 pm

James P wrote: If I reduce the squish clearance to (for example) 0.85mm by using thinner base gaskets, I should obtain:

Exhaust main 177deg
Exhaust aux 186deg
Transfer 124deg
Squish clearance 0.85mm
Comp ratio 13.3:1 geometric, 8.1:1 corrected
if you'll notice a difference, it'll probably be because of your changed timings, which do not realy look good imho.
only 177° main exhaust is only good for about 8000-8500 rpm's, and it limit's your blowdown too much. your engine may be strog in midrange but I wouldn't ecpect much above 10.000rpm.
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Re: Squish clearance and compression ratio - advice please!

Post by James P » Thu Apr 02, 2015 11:25 am

JanBros wrote:...your engine may be strog in midrange but I wouldn't ecpect much above 10.000rpm.
That will suit me fine Jan! I take the engine speed up to the maximum once in a while, but I spend most of my time in the 3500-8500rpm range.

Further on the engine assembly; I have fitted 0.15mm base gaskets from Dan McBryde and obtained 0.8mm squish clearance. A final check of the combustion chamber volume with the engine assembled (piston at TDC) revealed a volume of 11.5ml. The final compression ratio works out at 11.8:1 geometric, 7.2:1 corrected.
Dan also sent some 0.25mm base gaskets - in theory these would give me about 0.9mm squish clearance, with compression reduced slightly further.
All I've got to do now is find my resonator chamber gaskets (temporarily misplaced...), reinstall the engine in the frame etc. etc. and see how it goes.

Regards,
James

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Re: Squish clearance and compression ratio - advice please!

Post by scooble » Tue Jul 03, 2018 11:46 am

I have a copy of the latest Head Calcs spreadsheet. I'd like to set it up using standard pistons.
Please could someone tell me what the standard piston dome radius is, as well as the standard conrod length, and all the other input data needed for a standard engine.

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Re: Squish clearance and compression ratio - advice please!

Post by Gerrit » Tue Jul 03, 2018 6:16 pm

Standard rod length is 106 mm centre to centre.
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Re: Squish clearance and compression ratio - advice please!

Post by JanBros » Tue Jul 03, 2018 10:49 pm

scooble wrote:
Tue Jul 03, 2018 11:46 am
I have a copy of the latest Head Calcs spreadsheet.
don't know if you have this one, I made a new one at the beginning of the year : http://users.telenet.be/jannemie/JanBro ... c%20B.xlsm

working on a porting file to, here is a workable test-example :
http://users.telenet.be/jannemie/JanBro ... 0Test.xlsm

KR1 data is in the head-file
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Re: Squish clearance and compression ratio - advice please!

Post by James P » Wed Jul 18, 2018 1:38 pm

scooble wrote:
Tue Jul 03, 2018 11:46 am
Please could someone tell me what the standard piston dome radius is...
I'm not sure if you still need this info, but the dome radius of a standard piston is about 120mm. I found it difficult to measure with accuracy, as I could only use a profile gauge to get a trace of the dome profile, then compare it to curves of various radii which I drew using a pair of compasses.

If anyone has obtained a markedly different measurement of the dome radius, please let me/us know!

Regards,
James

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Re: Squish clearance and compression ratio - advice please!

Post by scooble » Mon Jul 23, 2018 2:24 pm

According to the default figures in 'JanBros 2stroke Head Calc B', the Piston Dome Radius is 197mm and the piston dome height is 2mm.
I assume the the spreadsheet then calculates the tangent, and then subsquently the corresponding correct squish angle.
The default sqish angle according to this spreadsheet tells me its 9 Degrees.
However, according to recent discussions, it appears that the actual piston dome radius is actually 120mm.
Plugging 120mm into the spreadsheet still gives me an angle of 9 Degrees.
In fact, I can put in whatever figure I like, and its still 9 Degrees.
Do I have to enter some figure somewhere else in order to get the spreadsheet to calculate the squish angle with a dome radius of 120mm and a height of 2?

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Re: Squish clearance and compression ratio - advice please!

Post by JanBros » Tue Jul 24, 2018 11:13 pm

sorry Scooble, maybe my figures are wrong .

Have to check why, maybe they are from my YZ125 piston's.

place the piston you are going to use on it's head, and measure the distance from the floor to the piston crown. In my new calculator, select height and enter the measured height, and it will give you the radius.
On Mark's sparrow-site is mentionned that the radius is 120mm for stock piston's.
normaly, when data is given about a piston at all manufacturers I have seen, the height is a simple number like 2 or 2,5, I have never seen numbers like 3.31 which would be the height if the radius is actualy 120mm. I then presume the height should be 3 or 3.5mm giving 132.17 or 113.75mm
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Re: Squish clearance and compression ratio - advice please!

Post by scooble » Thu Jul 26, 2018 11:53 am

I've inputted all the standard data and this is what I get for a Conical head.
and this is what I get;
Calcs
I am only interested in the squish angle.

When Bob Farnham did my head, he said he set the squish to 17Degrees (although I haven't measured it myself), however, the spreadsheet recommends 13.7degrees

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