Working out piston crown volume.

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stevo135+
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Working out piston crown volume.

Post by stevo135+ »

Hi all.

My KMX is in bits again now and I've got 2heads sent away for proper squish band modifications and to get a sensible compression ratio.

However I can't measure the actual total combustion chamber volume with the head on and the engine in the bike, due to a lack of room above the engine and the fact that it's inclined forwards.

I've got a bit more hopefully accurate data now, and I'm trying to work out how to measure one thing to get a cc volume.

The piston is 55.94mm diameter.
The crown height is 3.2mm
The piston edge sits 0.3mm below the cylinder deck height.

What I'd like to work out is the amount of cc that that is leaving in the cylinder to add to my combustion chamber volume.

A simple calculation for a 56mm piston in a 0.3mm stroke cylinder would give about 0.73cc.

The piston crown is way above the cylinder deck height at TDC but the edge of the piston is 0.3mm below deck height.

This is annoying as I really want to know what the space left in the cylinder is as I'm calculating my combustion chamber volume instead of measuring it assembled.

My thoughts are that it's likely to be no more than 0.3-0.4cc left in the cylinder, but it could be alot less due to most of the piston being above the cylinder at TDC.

Does anyone know a formula I could use to try and nail down this volume a bit more accurately.

Finally when I was doing my earlier compression ratio calculations they were way out as I was using the wrong effective stroke length due to getting my exhaust port height wrong.

I'd actually been starting and running the bike with a compression ratio of 9.3:1 from the exhaust port closing, so no wonder it was so hard to start for a 125 (134cc).

I still don't know what compression I need or can get away with for super unleaded fuel, but I've left this up to the experts who are going to drastically reduce it compared to where it is now.

If It runs great after all this I'll be very happy indeed. However if I lose power at low-mid revs compared with standard then I'll be looking to increase the compression again and find a higher octane fuel than pump unleaded. I don't want a bike that screams to the moon but between 4-7000rpm would get left behind by a standard KMX.
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JanBros
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Re: Working out piston crown volume.

Post by JanBros »

use this :
http://users.telenet.be/jannemie/JanBro ... 201.2.xlsm

on the head-page, untick "input from data" and you can use it as a stand-alone.

or here is an excel just for the head :

http://users.telenet.be/jannemie/JanBro ... c%20B.xlsm
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stevo135+
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Re: Working out piston crown volume.

Post by stevo135+ »

Thanks Jan. I did mean to reply on this thread ages ago. I've had two heads sent off again for re-working since I discovered I was trying to run a 9.3:1 CR before.

1, my squish bands have finally been opened to exactly 56mm as you had advised me ages ago.
2, the squish heights have been set to give 1.1-1.2mm piston clearance with my Wossner piston.
3, and the combustion chambers have been re-worked to give 14cc on the high compression head and 15cc on the low compression head.

This should now give me 7.84:1 and 7.2:1 respectively. I've been told that the first head needs better/racing fuel though to not detonate.

I've just tried running the re-built engine on 103 octane non oxygenated fuel and it barely starts and runs at all, and sounds feeble like its struggling to ignite the fuel.

When I started it with the daft 9.3:1 head before it was modified I used 98 unleaded and it started easy (except for the effort to kick it over) and sounded very crisp and picked up cleanly.

Is 103 octane too much for spec of my engine and its compression ratio? And if so should I try a mixture of unleaded with this race fuel? I think it's laced with Toluene and some other chemicals as it smells evil and very strong solvent fumes when you pour it out of the drum.

I'm not bothered about race fuel if I don't need it, but I was told that if I used unleaded I could detonate with the 14cc head?

They asked me for some specs when they modified the heads and said something about maximum squish velocity calculations which is beyond my very limited knowledge of engines I'm afraid.

What kind of fuel does a tuned or race KR1S like to run on by way of very vague comparison? I don't know if I should start with a 50/50 fuel mix of 98&103 octane or more or less? My gut feeling is that it's struggling to burn the race fuel and that's why It won't run well if at all?
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JanBros
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Re: Working out piston crown volume.

Post by JanBros »

my excel's calculate the SQ velocity ...

I always use 10cc heads on my KR , which gives 13.3:1 compression ratio (not corrected) and I just run plain 98 fuel without any problem's. max SQ velocity is about 28.5m/s
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stevo135+
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Re: Working out piston crown volume.

Post by stevo135+ »

Thanks Jan.
My total clearance volume is approximately 10.9cc. when I use the full swept volume (134cc) it gives me 13.3:1 also. I know you prefer to have it quoted that way for accuracy.

The low compression head has a thinner diameter squish band and a clearance volume of 11.9cc. The compression is a very safe 12.2:1 with this head.

I spoke to the pipe builder yesterday and he also thinks that the 103 octane fuel is overkill for 13.3:1 and that it's burning too slowly. He said to try a tank of 98 unleaded and just mix 1litre of the 103 in so that would 8:1 ratio of unleaded to race fuel. He said to get it on the dyno and let them set up my ignition for max power. That way if it does show any signs of detonation I can always increase the dose rate of race fuel, but he'd be surprised if fresh 98RON wasn't sufficient even with the ignition and jetting set up for max power.

I've got a bigger alloy radiator than standard so that might help prevent it getting hot enough to detonate too.
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