V85TT compression ratio

[noamlu]

Hii,
I was wondering about the compression ratio stated on the manual for the v85tt it's stated as : "10.5+-0.5:1"
what is meant by "+- 0.5"? is do they refer to some kind of "EFFECTIVE" compression ratio which varies with ignition timing?
I'm familiar with the formal definition of compression ratio, isn't that a fixed property?

[Huzo]

The compression ratio does not vary with ignition timing.
It is the ratio of the swept volume of the cylinder which is the area of the bore x the stroke + the volume of the combustion chamber @ TDC divided by the volume of the combustion chamber @ TDC.
As an example…
If the swept volume of the cylinder is 250 cc and the combustion chamber volume @ TDC is 25 cc
Then the ratio would be 250 + 25 = 225/25 = 9:1

A simpler way is to consider the volume of the chamber @ BDC divided by the volume @ TDC

[DesertPilot]

Hii,
I was wondering about the compression ratio stated on the manual for the v85tt it's stated as : "10.5+-0.5:1"
what is meant by +- 0.5?1 is do they refer to some kind of "EFFECTIVE" compression ratio which varies with ignition timing?
I'm familiar with the formal definition of compression ratio, isn't that a fixed property?

Imagine they mean the compression ratio on different engines is going to be between 10:1 and 11.5:1, depending on manufacturing tolerances, carbon buildup, and the like.

[Speciality]

Shouldn’t that be between 10.0 and 11.0:0?

[guzzisteve]

I think it would mean differences in the base & head gasket thicknesses from their supplies for mfg.
OR what ever they feel like making that day at the factory
Could be their engineers don't know, just somewhere around that number.

[DesertPilot]

Shouldn’t that be between 10.0 and 11.0:0?

That's what I would have thought as well.  It's exactly what I'd expect if they'd said +/-0.5.  But for some mysterious reason, they said -0.5/+1.0, which would work out to 10.5-0.5 = 10 and 10.5+1 = 11.5.  I have no idea why the plus and minus variation might be different. 

For what it's worth, if my compression tester is entirely accurate, I was getting 11.98:1 on the left cylinder and 11.91:1 on the right when I checked a couple of months ago, but I assume my gauge was reading a bit high.  Either that or I was at the bottom of a 1000' deep mine shaft and didn't notice...

[Vagrant]

We might be barking up the wrong tree! When I worked for Stihl they had to do different compression ratios for some countries that had poor quality gas. Usually accomplished by different head gaskets.
Same was true with mix oils. Even though we've had good 50-1 oils since the seventies some countries still use straight 30# no detergent at 24-1.

[noamlu]

The "1" was just a typo I made... ratio is stated as 10.5+-0.5:1.
If your guess is correct, at least you can say they are candid about their mfg tolerances. But what does 5% in any direction mean in terms of thermal efficiency? Will there be a significant difference in torque between an 11:1 v85  to a 10:1? Or perhaps an 11:1 variation be more prone to detonation ?

[Speciality]

Impossible to determine, I’d say. Who’s to say that both cylinders are the same? Other factors affect performance too; none of the engines is blueprinted AFAIK. Interesting question…

[DesertPilot]

Impossible to determine, I’d say. Who’s to say that both cylinders are the same? Other factors affect performance too; none of the engines is blueprinted AFAIK. Interesting question…

Good points, but I would expect both cylinders to be close.  Mine only differed by 1 PSI.