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Old 11-10-2013
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Default Compression ratios?

O.K.,My 1999 Yamaha 90C manual shows 3 different compression ratios for each cylinder, cyl#1 5.76:1, cyl#2 5.67:1, cyl#3 5.86:1.....my questions are this: why would Yamaha want 3 different compression ratios, and how would they achieve these 3 ratios?....any experts out there who know?.....
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Old 11-10-2013
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What manual are you looking at?

For a 1999 Yamaha C90TLRX, Yam manual LIT-18616-02-05 shows 5.86 to 1 for all three cylinders.

The 1999 Yamaha Product Information Guide shows 5.9 to 1 for all three cylinders. 5.86 is 5.9 when rounded to the nearest decimal.

There are several ways to change the compression ratios for various cylinders. Piston could be thicker, surface of the piston top could be different, cylinder head could be different, etc.

A Yam SX150 for example has 1 ~ 4 at 6.3/1 and 5&6 at 6.0/1. Pistons and connecting rods are the same so it must be done in the cylinder head.

Why they do this is not known to m.

Last edited by boscoe99; 11-10-2013 at 09:18 PM. Reason: change to model year designator
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Old 11-10-2013
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Thanks, but my motor is a 1999 model C90TLRX, build date is Jan. 1999, I.D. Is:6HIL-356808.....just wonder if these differences are related to motor cooling, balance, other?.....since I believe that these ratios are calculated from the moment that the piston closes the exhaust port to TDC, maybe the port could be modified slightly to create the differences?....but why???....Rodbolt might know?
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Old 11-10-2013
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Yes, C90TLRX is a 1999 model year. Don't know how in the hell I typed a K.

The piston and the rods are all the same part number. Unless the cylinder head squish area is different, or maybe the crankshaft main bearing to rod bearing distances are different, I don't see how the compression ratios can be different between cylinders.

But, according to my Yamaha data all three cylinders have the same compression ratio.

You got me wondering now. Why in the heck do some cylinders in some Yamaha models have different compression ratios? What is the purpose?

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Old 11-11-2013
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In my "specifications/Yamaha Outboards" for 1999 it shows only 4 of the motors with varying compression ratios: 200 2.6L HPDI, 175 2.6L V MAX HPDI, 150 2.6L V MAX HPDI and the 90TLRX.....it's just curious why these 4 motors would be like this?....
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Old 11-11-2013
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I recall reading, somewhere, that the compression ratios were different to even out the cylinder temperatures. Going way back in memory I think that the service manual for "old" 200 HP V6's stated different compression ratios for the bottom, middle and top pair of cylinders. I may be wrong but I remember the manual also showed the carburetor jetting to be different as well.
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Old 11-11-2013
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Is that a Yamaha document you are looking at?

Are you in the US or outside of the US?

Yamaha US did not offer the HPDI in 1999. First year for the models you mention are:

Z200TXRY = 2000 Model Year
VZ200TLRZ= 2001 Model Year

Z175TLRZ = 2001 Model Year

Z150TLRY = 2000 Model Year
VZ150TLRY = 2000 Model Year

I see where the USA SX (Ox66) models and VZ and Z (HPDI) models came with different compression ratios is certain cylinders but can't find any reference to a C90 or 90 coming with different compression ratios.

Strange indeed.
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Old 11-11-2013
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The book/literature I'm holding here is titled:Yamaha Reliable And Innovative Outboards"-effective July 1, 2008.....the 200HP 2.6 HPDI is showing cyl #1&4 is 6.3:1 and cyl#5&6 is 6.0:1....and similar on the 175HP, 150HP and my 90C......all the other motors in their line-up from 8HP to the F350V8 show all cylinders the same on compression.....???
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Old 11-11-2013
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sounds like it is not from Yamaha, but is an after market is my guess
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Old 11-11-2013
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It is a sales/technical literature bulletin put out by Yamaha for distribution at dealerships and boat shows...lots of photos/information/features,etc. with 2 page fold out in the back with specs on all Yamaha models from 8HP to 350HP....It's just hard to understand why 4 of their motors are shown to have these different compression ratios and all the rest are the same!....Surely Rodbolt knows something about this....but if he doesn't, guess we'll just keep wondering what the story is?.....
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Old 11-11-2013
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Quote:
Originally Posted by robert graham View Post
The book/literature I'm holding here is titled:Yamaha Reliable And Innovative Outboards"-effective July 1, 2008.....the 200HP 2.6 HPDI is showing cyl #1&4 is 6.3:1 and cyl#5&6 is 6.0:1....and similar on the 175HP, 150HP and my 90C......all the other motors in their line-up from 8HP to the F350V8 show all cylinders the same on compression.....???
Yamaha is known for having crappy data. I don't think they proof read anything. Therefore, I suspect the document is in error with respect to the C90. C90 by the way, not 90C.

It appears that only the OX66 and HPDI models have cylinders with different compression ratios. Bottom two cylinders have the lower compression ratio. But why??? What is so unique about these two models that they benefit from this?
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Old 11-11-2013
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I know exactly what you mean about the Japanese literature/translation/technical information, but the numbers in this bulletin just don't have the look of a traditional typographical error....but who knows, maybe Rodbolt will chime in with his thoughts about this. My C90 continues to run like a brand new one, but I remain curious about these different ratios....
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Old 11-12-2013
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yes Virginia we did get some 99 HPDI V6 motors for the US market

not many but enough.

over the years Yamaha has played with combustion chamber CC"s to allow for something.

a lot of mid 80 through mid 90 3 cyl stuff had 10 psi difference machined in the head, some of the V6 stuff had the same.

I got bit once or twice back in the day.

on 2 stroke V motors ALWAYS compare bank to bank.

4 stroke moters its all the same.

inline 3 you may see 15 PSI top to bottom.
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Old 11-12-2013
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Quote:
Originally Posted by rodbolt17 View Post
yes Virginia we did get some 99 HPDI V6 motors for the US market
not many but enough.

over the years Yamaha has played with combustion chamber CC"s to allow for something.

a lot of mid 80 through mid 90 3 cyl stuff had 10 psi difference machined in the head, some of the V6 stuff had the same.

I got bit once or twice back in the day.

on 2 stroke V motors ALWAYS compare bank to bank.

4 stroke moters its all the same.

inline 3 you may see 15 PSI top to bottom.

Can you give me a model identity please?
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Old 11-12-2013
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Quote:
Originally Posted by rodbolt17 View Post
yes Virginia we did get some 99 HPDI V6 motors for the US market

not many but enough.

over the years Yamaha has played with combustion chamber CC"s to allow for something.

a lot of mid 80 through mid 90 3 cyl stuff had 10 psi difference machined in the head, some of the V6 stuff had the same.

I got bit once or twice back in the day.

on 2 stroke V motors ALWAYS compare bank to bank.

4 stroke moters its all the same.

inline 3 you may see 15 PSI top to bottom.
Any ideas what Yamaha was hoping to achieve with these differences in combustion chamber volumes?...balance, consistent cooling, smooth running?...
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