Forum

[Phil Ashbrook]

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 ?

[another Allan]

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!

[another Allan]

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

[another Allan]

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?

[another Allan]

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.

[tribonnie]

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

[Alan R]

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....

[Midge]

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