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O.K., my book for my 1999 C90 Yamaha says the compression ratio for cylinder #1 is 5.76:1, cylinder #2 is 5.67:1, cylinder #3 is 5.86:1......why the differences?....Are these differences created by simply different ports?......Surely somebody knows about this!
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Where are you getting your information from? What book? I am not seeing in the service manual what you have posted. -
not uncommon for Yamaha to use ********g compression ratios on both inline and V motors. why worry/
simply run it and enjoy.
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The book is a sales catalog from 2008 showing all models and specifications.....the motors with varying compression ratios are the 200 2.6L HPDI, the 175 2.6L V-Max
HPDI, the 150 2.6LV-Max HPDI and my C90 model.....all other motors in the line up have consistent compression ratios in all cylinders. I've just always been curious about this???
Any ideas?Comment
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your "sales " book is simply wrong.
way more models than that have a varying compression and for a lot of other year models.
it is not like you have the ability to verify it.
that is why when doing a compression test you will note 7-10 psi difference when going from 1 to 3.
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Yamaha is known to be loose with their data. I suspect that the document you have is simply in error. Below is a snippet from the 1999 PIG (Product Information Guide) which indicates a compression ratio of 5.9/1 for all three cylinders. This agrees with the service manual.
If I had to guess how the CR varies from cylinder to cylinder, if and when it does, I would suspect it is the way that the head is cut. Particularly if the pistons are all the same part number.
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This is not a big deal to me.....just curious why compression ratios would vary from cylinder to cylinder...maybe to help balance the motor???Comment
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I have never come across anyone that had a good reason for varying compression ratios amongst cylinders on the same motor. Seems there must be a good reason for doing it but the why is the mystery. Why on some motors and not on other motors?
However Yamaha has a knack for doing some stuff that simply defies all logic.Comment
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Well, if you find out anything about this please let me know so I can just put it to rest....bet someone over at Yamaha can answer this?Comment
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Only in Yamaha in Japan will anyone know why they did what they did. I tried to Google it. Got no where.Originally posted by robert graham View PostComment
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dunno, most all the 3.1L two smokes you will see a 5 psi difference from 1-4 and 5-6. always did.
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Looks like this may remain one of life's little mysteries,...sorta like women-folks....But if anybody figures either of these out, please share the answers....Comment
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