The New And Improved Unofficial R65 Forum V2
Technical Discussion => BMW Technical Q&A, Primarily R65 => Topic started by: lauripalokangas on April 30, 2023, 05:39:05 AM
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I'm trying to calculate the compression ratio of my modified engine.
- R45 engine, year 1979
- R80 cylinders and pistons with reduced piston skirts and increased valve recesses on the piston head
- R65 head with improved exhaust valves with 8 mm stems
I know bore (84.8mm) and stroke (61.5mm), but I haven't been able to find out the volume of the head (or the combustion chamber). Would someone have ever measured it and had the values handy?
I'm trying to determine if I should consider compression lowering rings in case the compression ratio is uncomfortably high, or whether it's in the reasonable (9.5:1 or less) range.
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Hello,
I do not have the values you ask for, but I wonder if the piston will travel the full length of the R80 cylinder because the stroke is different compared to the R45. The latter has a shorter stroke so the cylinder from the R80 should be longer, no ?
Or did I miss something ?
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Hello,
I wonder if the piston will travel the full length of the R80 cylinder because the stroke is different compared to the R45.
The R80 stroke is 70, 6 mm, so the final bore and stroke is the combination of R80 bore and R45 stroke. R80 cylinder is indeed longer, so without machining the skirts, it won't be able to crank in the case.
If I understood the focus of your question.
Edit: I think you question is dawning on me, and I'm not sure what the answer is. If I understand your question right, it would not be enough to know the volume of the head, but also the length of the R80 cylinder that the piston doesn't travel (because that volume would be part of the combustion chamber in the short stroke engine, but not in the long stroke). I don't think it's really that much, because the valve recesses in the piston had to be machined to be deeper, to prevent valves from the R65 head from hitting the R80 piston.
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Hello,
There is more to it. The pus rod tubes have a different angle on the two engines, the conrods are also different, etc...
But a French fine mechanic build 750 big bore engines for racing use.
See here : https://bienvenueauflatistan.motards.net/t51124-moteur-r65-transforme-en-750
He shows a lot about his work but never speaks about the price, workmanship, or time....
The link is in French, but plenty of pictures and I bet Google or Deepl will translate it.