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Discussion Starter #1
Hi, Can any one help me with a spec for my k5 450. Im about to bore the cylinders Ive got a choice of some nos .25 o/s honda pistons or a set of jap reproduction items 12Th o/s (about 3mm)and lighter with teflon type pin retaining plugs THese are 2mm higher than the standard pistons which will obviously raise the comp from the honda spec of 9:1 What would be the resulting increase in compression or how do i work it out, and will it run on 98oct fuel without detination problems (av gas getting expensive here in New Zealand.)Thanks.
 

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the engine will be harder to start, a bit louder; somewhat more powerful (do us a dyno run before and after...); require a higher idle speed; require a more retarded spark (some say more advanced so again go to the dyno and let us know); and there will be more strain on your crank, rod and rod bearings (as in bigend/smallend). make sure you aren't bashing valves when you put it together.
-parks
 

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Discussion Starter #3
thanks for the prompt reply Parks61. Im just working on some figures to try and get a ratio now,(swept area and combustion chamber volumes etc). the combustion chamber measures out at 38cm3 per cyl and cyl volume is 222.37cm3 on std bore plus say 2mm for gasket gives aprox 267cm3 total, the piston is domed shaped and my geometry is crap!Need help with a fomula for volume of a partial sphere, to work out the space thats left in combustion chamber at t.d.c. for the ratio figure. steve.
 

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there is a mechanical way of "doing the math". if you can do a test build (only one side req.d), measure the volume of light oil accurately in cc.s at top dead center (using the spark plug hole as a fill hole, and then bottom dead center. assuming you can actually fully fill (no air pockets)the combustion chamber (orient the cylinder with the spark plug hole plumb) you'll come pretty close. did this while developing the crazy head on my taco roadracer. this will get you in the ball park with careful measuring. in the end though it doesn't really matter...if it runs better you're happy, if not you're bummin, and a ton of measuring and math before hand won't likely predict the final result. all it will do is quantify a change (from stock if you bother to determine where you are now) and give you any idea of what to do if the result (your new piston set up) isn't good. bottom line; i suspect todd henning did a lot of trial and error to come up with his magical motors.
-parks
 

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I don't know - I'd say that's true to a point, but the math will go a long way toward predicting the behavior of your engine (i.e. what octane level is needed to avoid pinging at higher compression). The trouble is that, as far as I can tell, there isn't enough information there to get to what we need mathematically. The dome isn't a proper half-sphere and we don't know the curvature; then there's the problem that the top of the piston has an irregular shape, not to mention the air displaced by the protruding valves.

If I were trying to get at it, I'd drop a big blob of play-doh on the top of my piston, affix the head with both valves closed and without attaching the the timing chain and with no spark plug installed, and crank it over past TDC. Then I'd pull it apart, cut off the excess that was forced into the plug hole, and drop the rest into a graduated cylinder with water in it. Then you know the precise volume at TDC. Then the math should be easy.
 

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Xander,
With the convoluted shape of the piston crown and the combustion chamber, how do you get the volume at BDC, so you can calculate the CR. I like the idea of getting the physical measurement at TDC.

Ken

AHRMA 412
Vintage racing - old guys on old bikes
 

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stock cr calculates out to 8.7 something on my bike.

Stock hg has a compressed thickness of .028 inches.

I had some custom pistons made that gave me a final CR of 10.5:1 and a final bore of 73mm.

I wouldn't call it too hard to start, but it takes a good heavy kick. The stock electric starter's been ditched but I doubt it could keep up anyway.
 

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Ken,

If I understand your question correctly, the volume at BDC can be calculated once you measure the physical volume at TDC... VolTDC+(pi*(bore/2)^2*stroke), because even if the piston crown is a convoluted shape, it remains the same convoluted shape from BDC to TDC, so it can be calculated as though it were a flat cylinder. Compression ratio would be (VolTDC+(pi*(bore/2)^2*stroke))/VolTDC, with VolTDC measured as above.
 

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Oh Yeah
What was I thinking? We already know the bore and stroke. Duh???

Ken
Maybe I just wanted to use the term convoluted.

AHRMA 412
Vintage racing - old guys on old bikes
 

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Discussion Starter #10
Thanks everyone for some great info!very helpful.Lying in bed thinking about it,( as has been known to occur) Ive come up with a similar mecanical means as mentioned here, simply involves pressing the piston crown into some puty type stuff then cc the indent left be hind with oil in a syringe, as per the way i did the combustion chamber, deduct for the diff in volume and devide into over all swept area for ratio . should get me pretty close. only pobblem now is the pistons are at the shop 3 hrs drive away! BUGGER! guess i can take a day of work.
 
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