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post #1 of 67 (permalink) Old 10-21-2012, 11:59 AM Thread Starter
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compression ratio

reading in my service manual for the 2012 bonnie's it SAYS compression ratio in models with electronic speedometer is 10.2, is this correct? i believe my 2012 bonneville has electronic speedometer from prior readings. with triumph lower fuel requirements other aspects such as timing advance or cam shaft etc is prolly altered to control final cylinder pressure which is the bottom line pretty much to control knock. input appreciated since its my first bonnie since 1970!!! i am in the USA, do models differ over the pond, excepting emmissions of course
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post #2 of 67 (permalink) Old 10-21-2012, 12:37 PM
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Compression ration of 10.2:1 sounded too high to me... so I looked it up online. A ratio of 9.2:1 is what I found on several sights. This sounds about right. I have to assume you manual is incorrect.

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post #3 of 67 (permalink) Old 10-21-2012, 12:48 PM
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My Haynes manual also says 9.2:1.
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post #4 of 67 (permalink) Old 10-21-2012, 02:31 PM
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Originally Posted by rodhotter View Post
reading in my service manual for the 2012 bonnie's it SAYS compression ratio in models with electronic speedometer is 10.2, is this correct? i believe my 2012 bonneville has electronic speedometer from prior readings. with triumph lower fuel requirements other aspects such as timing advance or cam shaft etc is prolly altered to control final cylinder pressure which is the bottom line pretty much to control knock. input appreciated since its my first bonnie since 1970!!! i am in the USA, do models differ over the pond, excepting emmissions of course
Models with cable driven speedometer 9.2:1, model with electronic speedometer 10.2:1.

This from a genuine Triumph manual.

Regards

Jim
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post #5 of 67 (permalink) Old 10-21-2012, 02:41 PM
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Models with cable driven speedometer 9.2:1, model with electronic speedometer 10.2:1.

This from a genuine Triumph manual.
Yes, I've read that. A consequence of the raised CR is an increase in power and torque and the service manual does state that power output has increased from 66.1 PS at 7500 rpm to 68.1 PS at 7400 rpm. That's some 3% which is consistent with raising the CR by 1 whole unit.

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post #6 of 67 (permalink) Old 10-21-2012, 02:53 PM
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Yes, I've read that. A consequence of the raised CR is an increase in power and torque and the service manual does state that power output has increased from 66.1 PS at 7500 rpm to 68.1 PS at 7400 rpm. That's some 3% which is consistent with raising the CR by 1 whole unit.
So how did they accomplish the increase? What's the difference in the two motors? Please enlighten us... I'm confused.

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post #7 of 67 (permalink) Old 10-21-2012, 03:02 PM
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So how did they accomplish the increase? What's the difference in the two motors? Please enlighten us... I'm confused.
I've looked at parts sites and compared part numbers of the components that could affect the CR. The cylinder head, barrels, gaskets, etc have remained unchanged. The pistons, however, have changed at the same time as the changeover to electronic instruments. This was from VIN number 436004 (for the alloy-wheeled models).
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post #8 of 67 (permalink) Old 10-21-2012, 03:13 PM
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The oldlder 865 motors the pistons are dished on top.So if they rased it they would have to of changed that .Iwould like to see the new piston to get 10.2 I would think they would have to have a small dome on top.The aftermaket hc pistons are around 10.8to 1 and they have a big dome.

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post #9 of 67 (permalink) Old 10-21-2012, 03:27 PM
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I'm surprised they made a change of pistons... not saying I don't believe it, just shocked that Triumph actually made such a change. I know very little about the specifics of engine internal workings, but wouldn't a 10.9% increase in compression make considerably more increase in horsepower output? (I do know a little about math. )

Did they also change cams, or something else at the same time? And what was the reasoning behind the change(s)? Still confused.

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post #10 of 67 (permalink) Old 10-21-2012, 04:02 PM
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I'm surprised they made a change of pistons... not saying I don't believe it, just shocked that Triumph actually made such a change. I know very little about the specifics of engine internal workings, but wouldn't a 10.9% increase in compression make considerably more increase in horsepower output? (I do know a little about math. )

Did they also change cams, or something else at the same time? And what was the reasoning behind the change(s)? Still confused.
The camshafts part numbers have remained the same.

Apparently there are other factors that influence how much power you get from a given CR increase. I've read somewhere that 3% increase in power per unit of CR increase is a rule of thumb, consistent, of course, with the right fuel grade being available and possibly the right ignition timing.

This site has a chart halfway down the page giving examples:

http://www.popularhotrodding.com/tec...h/viewall.html

Last edited by Forchetto; 10-21-2012 at 04:11 PM.
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