compression ratio

[admin]

rats_ass - 01/21/07 at 11:46am

I have an ace 100 running engine. I am putting it in a super rat frame. The person I bought the bike from put the super rat head on the ace 100 engine. In my manual it shows 10;1 compression for the ace 100 and 5.5;1 atmospheric compression ratio for the rat. What is the difference in ratio types and will I gain with this head swap or should I go back to the stock head? Thanks for the help!
Jack

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Bullfrog - 01/21/07 at 6:59pm

While I can't truly define the terms mentioned in your message ("compression ratio" and "atmospheric compression ratio"), I can offer a couple of guesses and some useful information.
I would GUESS that the 10:1 compression ratio comes from a comparison of the cylinder head combustion chamber volume to the displacement of the engine. I would further guess that the 5.5:1 "atmospheric compression ratio" comes from a comparison of the combustion chamber volume to the cylinder volume from the top of the exhaust port to head gasket surface. Those guesses may, or may not, be accurate. But it really doesn't matter, because those numbers don't really help you in the shop.
What really matters is the compression data you will get by screwing a compression gauge in the spark plug hole and doing a compression check. Anything over about 160psi is going to cause you problems.
The Super Rat head would probably be a bit much on a stock Ace 100 cylinder . . . but we don't really know whether your Ace 100 cylinder has had porting work done to it in the past 30+ years . . . so do that compression check, OK?
Ed

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Forum Admin - 01/22/07 at 9:11am

Jack the message Ed left is totally correct but I would like to try and add to it. I believe the crown height of the Ace 100 pistons ( double ring pistons) was aprox. .065 " taller than that of a Superrat single dykes ring piston. So if you use a Superrat head ( #932501) while using a stock Ace 100 piston I think ( big guess) that your compression will be well over 190 lbs. But as Ed said check it. When you do check your compression your motor should be cold. If the motor has not run in 3 weeks then give a squirt of wd-40 through the sparkplug hole in the head to seat the
ring(s). Also hold your throttle wide open while kicking the bike to check compression. Let us know what your compression is please when you get a chance.
Paul

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rats_ass - 01/24/07 at 12:31am

You guys were right. I checked the compression and it was close to 195. on my gauge . To compare I have an ace 90 that hasn't run in several years and it registered 145. My poor old Bultaco only registered 115. So I will go back to the stock head. How much difference was there between the ace 100 motor and the super rat motor in performance? Should I try to find a super rat top end?
Thanks
Jack

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