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post #1 of 3 (permalink) Old 05-08-04, 06:47 AM Thread Starter
tony xa's Avatar
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Matching piston size to comp ratio

How do you work out pistons to use in relation to comp ratio.
I want to use 3.850 stroke crank 6 to 6.125 inch rods and run my engine at 11-1
comp using 98 octane fuel . Heads will be 64cc
Would like to know in there is a formula that is used .
Any help would be great . Thanks
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post #2 of 3 (permalink) Old 05-08-04, 07:33 AM
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Re: Matching piston size to comp ratio

This is how i would work it out tony.Firstly we need to assume your swept cylinder volume and this is worked out using the following formula. lets assume you have zero deck height for this example and that * is a multiplication sign. bore * bore * stroke * .7854. lets assume the following; 4.03*4.03*3.85*.7854=49.109. This figure represented here is the cubic inch capacity of one cylinder. We need to convert this to cubic centimeters somultiply the following. 49.109*16.387064=804.75348. we now know the swept volume in cubic centimeters. 804.75348 cc is now divided by our known combustion chamber size of 64cc and we now know that our c/r is 12.57 to 1. This is obviously way too high and it is clear now that we are not going to be able to achieve our intended goal with a flat top piston design .we have 2 options,those being that we can open up our chambers to suit or use a dished piston which is the preferred option here. again we can use simple math to tell us how much dish in cc,s we actually require to come up with 11 to 1. Using our known swept volume of 804 cc we can divide this by 11 and this figure will tell us where we need to be. 804 divided 11=73.09cc.this figure represents our combustion chamber volume inclusive of cylinder head chamber size of 64 cc.The dish in the top of the piston now becomes part of the combustion chamber.It is clear that our piston will need to have a 9cc dish machined in the top. hope this helps.cheers, russ
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post #3 of 3 (permalink) Old 05-08-04, 07:40 AM
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Re: Matching piston size to comp ratio

This will make it easy for you...

I needed 20 cc dish to get 8.5 to 1..

Don't be afraid to try something new. Remember, amateurs built the Ark...Professionals built the Titanic!
Tuned & maintained by "Peppertree Perfmormance". Dart block, Scat 4340 steel crank, Custom C.P pistons.Oliver rods.. Mal Wood twin plate clutch.. Twin SC61/2's,482 rwkw/ 645 rwhp..
Passengers, myocardial infarction material..
R.I.P Possum...
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