[Lear35A]

...about compression ratio. Everything written by Porsche says our cars are 11:1. I don't believe it.

I bought a cheap burette, and today cc'd a stock piston dish and a couple of combustion chambers in my head. The piston dish was 28.5 cc and the combustion chambers were both 44.5 cc. And my head has been milled just a bit to make sure it's flat. This gives a CR of about 10.4.



A message from another member said his shop usually comes up with CRs in this range too.



Does it matter? I don't know. But I am curious as why Porsche advertised 11:1 when the motors are actually 10.5:1. Our marque should be above such hyperbole. Heck, guys maybe we can run 87 octane gas and save big money.



Now that I've got a little experience doing this, I'm going to repeat everything to be sure. I'll bet though I'm pretty close.

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[Cloud9...68]

Yep, that's what I was told, and that was my experience as well. I know this is blasphemy, and I've never personally done it (and this could really open a heated debate), but I bet you could run 87 octane in these engines with no perceptible loss of performance or gas mileage. Not saying you wouldn't lose a few horsepower, but I bet most people would never feel it.

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[rhudeboye]

That's it! I'm sending mine back!



Now where did i put that receipt?

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[flash]

yup - they use a different method of calculating compression ratio. 10.4 is about right. that being said, 87 will not work. it needs 90 minimum. i've tried 88. trust me. it's not pretty. the car ran, but i couldn't get on it. it went past the limits of the knock sensors and ECU.

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[Cloud9...68]

[quote name='flash' timestamp='1355011812' post='135986']

they use a different method of calculating compression ratio.

[/quote]

Isn't that kind of like saying, "In Porsche's world, pi is equal to 8.325." How could there be a different method of calculating the ratio of two fixed volumes?

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[flash]

sort of.



in the simplest terms, it's a lot like the difference between static and dynamic compression. i think they were factoring in the dual resonant intake manifold boost.



what i do know is that the engines are NOT 11:1. they are also NOT 3 liters. they are only 2.99. you have to bore it out a half millimeter to make it 3 liters.

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[MB968]

[quote name='Cloud9...68' timestamp='1355021463' post='135991']



How could there be a different method of calculating the ratio of two fixed volumes?

[/quote]



It's not that simple if you sit down a group of engineers and ask them to calcualte the same thing. There may not be a set of international standards for how you compute the volume. For instance, there is always a tolerance on the size of the bore. So when calulating, do you use the largest it could be and still be in spec, or the smallest, or some statistical basis. The same would be true for the actual stroke. Same is true for the piston dish, and combustion chamber (especially if the chamber is a cast, not machined surface. Then you have the valve, it's tolerances, and how much it protrudes into the combustion chamber. And finally, what about the area below the top of the piston just above the first ring. Do you include this area or not. And, this doesn't include any extra volume of air ingested by a tuned resonanace intake system.



This is exactly why the ISO (International Standards Organization) was formed. Without it, each person/group/organization/country could establish their own norm for how to do things. Don't know if there is an ISO standard way to calc CR.

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[tamathumper]

It's not that simple if you sit down a group of engineers and ask them to calcualte the same thing.

Boy is that ever the truth - across all disciplines. Electrical, mechanical, civil, sanitary,... <img src="/forum/images/smilies/968/wink.png" class="smilie" alt="" />

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[Cloud9...68]

MB,



I understand what you're saying, and I would agree that this would explain the discrepancy if the measured CR of my engine, Lear35A's, and the others I've heard of, was on the order of a tenth of a CR unit or so. Also, if it were just a matter of allowing for tolerances, I'd expect to see an equal number of engines read above 11:1 as below. But what we're seeing is a consistent offset of about a half a CR unit - much too large to be explained by tolerance variations. There's something far more sinister at play <img src="/forum/images/smilies/968/ninja.gif" class="smilie" alt="" /> .



OK, I'm kidding about the sinister part...

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[flash]

you would be amazed at how little it takes to change the CR a lot. taking .005" off of the surface of either the head or the block can result in between .02 and .03 increase in compression ratio.

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[Lear35A]

[quote name='flash' timestamp='1355068455' post='136008']you would be amazed at how little it takes to change the CR a lot. taking .005" off of the surface of either the head or the block can result in between .02 and .03 increase in compression ratio.[/quote]

That reinforces Cloud's point. Decking the block or milling the head by .005" is a lot. Changing the CR by .02-.03 is very little.



Our piston dish is machined. I'd be surprised to see a variation of .1 cc piston to piston. The piston to bore clearance is only .0008" so our bores better be quite precise. I think the fact is that by ANYONE's method we only have 10.5:1CRs. (And 2990cc displacements). I vote for the sinister theory.

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[Fox944]

3.0L is correct. All engines are advertised as a rounded number. 2,990CC rounds up to 3.0L. If the engine were 2,945CC, then it would be advertised as a 2.9L motor. This is true for just about every engine in a production car with very few exceptions. For example, the fox body ford mustang 5.0 is actually a 4.9L engine (4,945CC)

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[flash]

re: ratio - this was "explained" to me once before, but frankly i was not really paying attention. i'll contact them again and see what they say.



re: displacement - yeah - knew that one. just not quite as cool as really being 3.0

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[Cloud9...68]

But again, if we're taking about variances in dimensions and their impact on CR, we should see an equal number of engines measure above 11:1 as we see below. A large shop that builds and races front engined water cooled Porsches as their primary source of business told me they see a bimodal distribution of CRs in these engines - one mode clustered around 10.5:1, and another clustered around 10.3:1, using their particular instruments and CR calculator. Never anything above 11:1, or even much above 10.5:1, for that matter.

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[flash]

that's not what i was saying about the explanation, which was basically a "why" porsche compression numbers are inflated (the irony of a "compression" number being "inflated" cracks me up)

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[lbpesq]

Porsche calculated their specs in Europe. You guys just aren't taking into account the metric system, Daylight Savings Time, and the variable gravitational force of the moon!

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[Lear35A]

Makes you wonder why they bother to make the thicker 1.4mm head gasket. It changes the CR by 0.32. So if you milled the heck out of the head and stuck with the standard head gasket thickness, you might start getting close to 11:1.



And lbpesq, don't forget the (at the time) DM to US$ conversion rate.

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[Scott Collins]

93 never knocks for me. I would use 91 but it's hard to find around here. Most of the stations have 87, 89, then jump right to 93.

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[flash]

there is nothing whatsoever that says 11:1 is safe either. our engine requires a minimum of 90 octane (RON+MON/2) at the compression of stock, regardless of whatever number that is. i can tell you for sure that 88 knocks on a full height head. i had it happen with only 23000 miles on my unmolested engine. i had to get 88 octane in texas, as it was all they had. i really had to keep my foot out of it until i could get other fuel.



but, it is very possible that the 11:1 number is indeed at minimum thickness, and that they advertised accordingly.



they made the 1.4 because that allowed for .012" of milling. it is very common to have to take .005" off the head each time it is serviced. by making a 1.4mm gasket, they provided for a couple of passes before having to replace the head. blocks often need skimming too.



as for fuel requirement, you would have to do a bit of math to figure out what the dynamic compression is, at the valve settings you set at the cams, in order to know what octane you really need. cam overlap changes affect dynamic compression, and alters the knock threshold.

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