Compression ratio's

Paul Williams

Active Forum User
Non-VOC Member
Hi All
Been thinking about giving the Rapide a rebore and new valve guides as she is a bit oily and hrd on plugs. I have been offerd a set of 8:1 pistons but the factory paper work says she is 7.2:1. Will the higher compression ratio make any difference.
 

davidd

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Paul,

I don't know what is available. The two easiest ways to get more performance are increasing the compression ratio and increasing the displacement. Increasing the CR a little probably would not do too much for the performance. 8:1 is about as hight as I would go for a street machine. I feel that the higher you go the harsher the engine becomes. Some folks like it and I sure do on a racer, but it is a pleasure to cruise around on a sweet engine that will not start "pinking" when the throttle is twisted.

I think it is more about personality than mechanics, although it would be easiest to stick to a piston that was around the same weight so you can avoid any balance issues with the flywheel. A minor weight difference is OK.

David
 

Paul Williams

Active Forum User
Non-VOC Member
David,

I was not looking for extra performance from the rebore. I think its the valve guides are the oiling problem but thought as i was going to strip the engine a rebore might be worth it. The pistons were offered to me by a mate and are 30thou oversize
 

Bill Thomas

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You don't tell us what make of pistons, Some are not good, Also the old ones had a lot of clearance and the latter ones not so much. Cheers Bill.
 

Bazlerker

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7.3:1 will give you all the performance your brakes can handle. All manner of swill masquerading as petrol will work and the rest of the machine will thank you. Disassemble the beast first, then make a decision about which pieces need to be attended to. Spend the money on the best - that way you will do the work less frequently - which means you will ride more often.
 

Robert Watson

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I run 9.4 on the street and it is a very civil machine. I put Spares Co 8:1 in my son's Rapide and it is a very nice strong runner. Both those on MK2 cams.

I have a Rap on 7.3, small carbs and MK1's and it can run out of steam a bit.

I'd wouldn't hesitate to run 8's if they are good pistons, and even better rings!
 

Bill Thomas

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Hello Paul, I like those, Some don't. I should use the rings you have because I don't think you can get new ones, I would love to be proved wrong !!. I used leyland piston rings and had to get the ring Grooves made deeper and wider. The clearance is as in the old books. Good Luck, Bill.
 

Albervin

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My B Rap has 7.3:1 Omega pistons, slightly modified Mk1 cams and 276 carbs. It will pull 70mph like a 14 year old boy in 3rd with plenty in hand :)
 
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