Hitchcocks Forum

I've just had my twin spark conversion on my cylinder head done and on it's first test ride the compression ratio felt slightly softer , is this my imagination or could the space around the extra electrode make such a difference ?
I'm not sure what my ratio is now because in the begining I had the head and barrel skimmed by 60 thou then at a later date I fitted the 8.5:1 535cc piston with a 2mm compression plate and 1 extra barrel gasket and a 1mm solid copper head gasket . I'm not that good at maths but I think the current comp ratio might be 8:1 , how far off the truth am I ?

With all those changes, I think I'd be looking to calculate the compression ratio from first principles, i.e. measuring the volume above the piston at tdc and bdc and working it out from that. I would use a calibrated vessel such as a burette to measure the amount of oil you can pour in. There may be an easier way, of course, so I wait with baited breathe to hear the wisdom of others!

Of course, that should read 'BATED' breath.............

Thinking further, you already know the swept volume (535cc) so you only need to know the volume above the piston at tdc (Vtdc) which you can measure using a burette. The compression ratio is (535 + Vtdc) divided by Vtdc. I think that's right.........any more offers?

Of course, this will only give you the static compression ratio, not the dynamic compression ratio, but I would think that all the info we see about compression ratios in our hosts' catalogue are static figures.

However to answer you question ......... no, such a small change will make no noticeable difference. The most likely cause for you different "feel" is the ignition timing. Suggest you retard you fully advance ignition by about 6 degrees

Hi guys--------------- first, here's all the definitions and maths to go with our "Compression Ratio" subject matter..............Please take a few minutes to read it, especially the "Dynamic" version >>>>>>>>>>>>>>>>>>>>>>> http://en.wikipedia.org/wiki/Compression_ratio --------------- As you have seen this is about RATIOS ie a comparison of mathematical values and as such these can't be felt or touched.......What you may experience is a DIFFERENCE in the MEP ( Mean Effective Pressure) and thus performance as a result of a CHANGE of a CR.......PHIL, I think the guys are correct and you must measure your current set-up to obtain the static CR.}------ SWEPT VOL. + CLEARANCE VOL. divided by CLEARANCE VOL........... With 2 sparks ie 2"flame fronts" you can now RETARD your ignition a tad as the time for full combustion has been reduced.....BUT, if you have now also increased the CR then a touch of ADVANCE might be called for OR--- the two might just have balanced each other out !! --------------------------- You really ought to do ONE change at a time, and log each result thus found.....PS}--- You may also need to change the spark plug value depending on what's going on inside....

And of course take into account cam timing, the valves may not necessarily be fully closed before piston BDC.