I always wonder if there is not some direct correlation between the cranking pressure and the Dynamic CR? It seems that at 150 cranking pressure, you should be at 8.5CR (or some other figure) and then when you get to 170 or 200 you should be at some other known DCR? Am I all wet here?

For example if you buy an engine with and unknown CR you could do a compression test and have a good idea of the DCR. Or you could be like me and have a cranking pressure of 170-174 on my truck and also have lost all the numbers I wrote down and have no way to know what I have!

Just talking out loud!

Thanks

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2 Mopars come with Spark plug tubes. One is a world class, racing machine. The other is a 426 CI. boat anchor!

12.70 @ 104.6

Crank8ng pressure will vary with cam specs too.

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Official Cookie and Mater Tormentor.

That's true! If you had an engine with 12:1 compression for instance, and put in a cam with a 106° separation angle in it, it would have a different cranking pressure than the same engine with a cam of identical specs except with a 116° separation angle. So you can't really compare or equate the cranking pressure to relate to an actual compression ratio.

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There's no such thing as too much cam....only not enough engine!

I have always wondered how the cranking pressure can exceed the actual compression X 15. I can see many reasons why it would be less but none why it would be more. Can somebody straighten me out on this.

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If you didn't drive it there, it's not a street car.

Understood that the cam is coming into play, and that is how we get our dynamic CR right? So why does the actual cranking pressure that we are seeing not correspond to the DCR? When we crank it over to do a compression test the cam is obviously in the loop and doing it's thing.

I guess I am probably wrong but I want to understand why?
I know it will not be a hard an fast rule because of all the variables, but it would seem that you should be able to decifer a rough estimate of the DCR.

I have seen charts where it shows, at a certain cranking pressure you can run on regular gas, at a higher # you would need super, and at another number you are at race fuel. So if you know that, then it would seem to my pea brain that there is a corresponding estimate of a DCR that goes with that number.

Thanks for helping me learn!

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2 Mopars come with Spark plug tubes. One is a world class, racing machine. The other is a 426 CI. boat anchor!

12.70 @ 104.6

What is the X15 number and what is it for?

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2 Mopars come with Spark plug tubes. One is a world class, racing machine. The other is a 426 CI. boat anchor!

12.70 @ 104.6

I think the cranking pressure would come closer to approximating the DCR and not the SCR. In most instances, the SCR can be very deceiving because the actual cam and how its degreed in can affect the DCR drastically. So we have to ask ourselves what we are actually meaning when we say "What's your Compression Ratio"? Are we meaning what everyone traditionally brags about when they say "I've got 10:1, or 12:1 compression(SCR), or the more realistic(DCR) that the engine actually sees itself once all the variables are taken into account. Given the previous example of the 2 different cams in the same SCR scenario, its even very possible for 2 totally different engines to have the same DCR while their SCR is several points different from each other based on their cam selection and how they are also degreed. So I would think that you could compare engines cranking pressures to DCR readings moreso than on their SCR readings and cranking pressure.

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There's no such thing as too much cam....only not enough engine!

Yep, that is what I was asking from the start. See the title of the original post.[/quote]

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2 Mopars come with Spark plug tubes. One is a world class, racing machine. The other is a 426 CI. boat anchor!

12.70 @ 104.6

15 approximates atmospheric pressure so to my way of thinking a mechanical compression ratio of 10:1 couldn't exceed 150#. This must not be true because you will often see more than 150# on a compression gauge with 10:1 even though the valves are open for part of the cycle and the rings don't seal absolutely.

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If you didn't drive it there, it's not a street car.

Heat.

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Joshua

LOL.... PV=nRT as a start of understanding or some other kind of entropy...?

Seymour, Rick, and all,

15 psi is atmospheric pressure at sea level. It is less above sea level. At 2000 ft where I am it is about 14 psi. At the top of Pikes Peak it is 9 psi.

I have noticed what you did, Seymour, but have never really thought about it. I'm sure Josh is right about the heat. Temperature must be higher inside the gauge than in the cylinder you are cranking, assumedly due to friction of the gas on the small tubes and orifice going in the gauge and small gauge volume. It would take some more sophisticated measurements to figure this out. Also, the gauge uses "gauge pressure" (psig), which means at ambient pressure it reads 0 instead of 15 psi. So you can add 15 psi to all your readings to get actual absolute pressure (psia), which is what you would need to calculate a real DCR (= psia cranking divided by psia ambient atmosphere) . Of course, this gives us an offset in the wrong direction, since usually we measure a higher cranking psi than would give us a sensible DCR number.

I thinik the gauge should also give you something that relates to DCR, but only if the gauge has the same temp as the cylinder gas and through the gauge system. That must not be the case.

However, you should get an indication (correlation) between what you read on the gauge and DCR (not SCR). This is the in charts that Rick mentions.

I have found that cranking PSI on my built up engines always correlates well to how much timing I can run and what kind of fuel I need to use. When a motor has 180-200 cranking psi, that is about the limit of what you can run on pump premium.

Interesting topic...

Lou

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Home of Slant6-powered fun machines since 1988

Thanks for the info guys. Maybe we can come up with our own formula!!
True, because I have @170 cranking pressure and add in atmospheric pressure puts me at 185 / 14.4 = 12.84 CR which is obviously higher than I have.

Keep the theories rolling

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2 Mopars come with Spark plug tubes. One is a world class, racing machine. The other is a 426 CI. boat anchor!

12.70 @ 104.6

Heat could explain my quandary. I was going to try to make some calculations but I can't remember what a mole is. We will have to leave this to the scientists.

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If you didn't drive it there, it's not a street car.

Maybe we should start a list and show what people actually have as their Static, Dynamic and Cranking pressure. May be neat to have.

Rick

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2 Mopars come with Spark plug tubes. One is a world class, racing machine. The other is a 426 CI. boat anchor!

12.70 @ 104.6