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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Compression ratio choice
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<blockquote data-quote="Chris-T" data-source="post: 5286" data-attributes="member: 1167"><p>Hi folks,</p><p> </p><p>Having seen the success of others posting questions, I thought I'd have a try.</p><p> </p><p>I have a series C Rapide engine which is fitted with new 9:1 specialloid pistons. I have not run this engine. I am wondering if I am going to run into problems with the increased compression ratio. Increasing the CR is clearly going to change the characteristic of the engine but I don't know how significant this would be. For instance as the CR changes at what ratio would I notice a loss of bottom end torque.</p><p> </p><p>My quandry is should I reduce the CR or not. Stevens in KTB states that there is nothing to be gained by exceeding 8:1, indeed he says its advisable not to exceed 7.3:1. I also remember reading somewhere that higher CRs can eventually cause failure of the kickstart mechanism. Will I need to rebalance the crank? Paul Richardson says no, it seems although the pistons vary in weight the engine doesn't rev enough in normal use for it to be noticable. Will the engine be more difficult to start?</p><p> </p><p>So I can leave it at 9:1 or I can try to reduce it. If I reduce the CR I have either to resort to skimming pistons which I don't particularly want to do or I can use compression plates. I've calculated crudely from Tuning for Speed that I would need approx 0.112" of compression plates to go from 9:1 to 7.3:1. PEI states that a maximum of 0.062" can be used else problems will be encountered with rocker adjustment. 0.062" packing should give approx 8:1 CR.</p><p> </p><p>So is this all resonable? Has anybody out there got any practical experience of all this. Its over to you.</p><p> </p><p>Thanks</p><p> </p><p>Chris</p></blockquote><p></p>
[QUOTE="Chris-T, post: 5286, member: 1167"] Hi folks, Having seen the success of others posting questions, I thought I'd have a try. I have a series C Rapide engine which is fitted with new 9:1 specialloid pistons. I have not run this engine. I am wondering if I am going to run into problems with the increased compression ratio. Increasing the CR is clearly going to change the characteristic of the engine but I don't know how significant this would be. For instance as the CR changes at what ratio would I notice a loss of bottom end torque. My quandry is should I reduce the CR or not. Stevens in KTB states that there is nothing to be gained by exceeding 8:1, indeed he says its advisable not to exceed 7.3:1. I also remember reading somewhere that higher CRs can eventually cause failure of the kickstart mechanism. Will I need to rebalance the crank? Paul Richardson says no, it seems although the pistons vary in weight the engine doesn't rev enough in normal use for it to be noticable. Will the engine be more difficult to start? So I can leave it at 9:1 or I can try to reduce it. If I reduce the CR I have either to resort to skimming pistons which I don't particularly want to do or I can use compression plates. I've calculated crudely from Tuning for Speed that I would need approx 0.112" of compression plates to go from 9:1 to 7.3:1. PEI states that a maximum of 0.062" can be used else problems will be encountered with rocker adjustment. 0.062" packing should give approx 8:1 CR. So is this all resonable? Has anybody out there got any practical experience of all this. Its over to you. Thanks Chris [/QUOTE]
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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Compression ratio choice
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