You might be referring to a statement I have made several times.
To clarify, I stated "On a slant six, there is no down side to increasing the compression ratio". Of course this means staying within reasonable limits. Most slants have a actual CR of less then 8.0-1 If you limit the CR increase to between 8.5 and 9.0-1 you should have no problems. Of course the higher the CR the more critical the "tune up". The cam will also make a difference in how much of an increases can be tolerated. I know people running slants on the street with over 10 to one, but that is not to be taken lightly. I have a 170 that is a true 9.5 -1 but it was a "bear" to tune. The cam is too mild for that CR and it wanted to ping on anything but premium.

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 225 nitrous
64 Valiant Signet
64 Valiant 4dr 170

Are you talking static or dynamic compression ratio? Dynamic compression ratio is a much more useful number than static.

Either or both. Makes no difference. There is no downside to increasing the CR on a slant six, within reason.
My original statement was based, on questions of "what can I do to get more power out of my street slant"? I stated the best bang for the buck is to increase the CR, as there is no downside. It improves the engine efficency, and will help torque/HP, and fuel mileage.

_________________
Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 225 nitrous
64 Valiant Signet
64 Valiant 4dr 170

My eyes kinda glaze over whenever anyone mentions dynamic compression ratio. It implies they are talking about something they have a handle on.

Who knows the dynamic compression ratio of an engine? It's fair to say Lexus, BMW, Dodge, Ford, etc. could tell you the dynamic compression ratio of just about any engine they mass produce. They have the resources to do that. They can probably actually measure it, and they can probably also calculate it within a very tight tolerance. Of course, they are not about to share that information with you, but they probably have it.

And, of course, there is no 'one' dynamic compression ratio for any engine. So what you'd need is a graph that defines CR under any one of about 5,000 combinations.

Anyone else is just guessing within a loose tolerance. That's why static compression ratio is as good a tool as any.

What we call dynamic compression can be calculated with pretty much the same accuracy as calculating static compression.
It's simply based on the piston position when the intake valve closes rather than basing it on the full stroke of the piston.

Nobody is referring to trying to measure a running motor.

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Nowhere near good enough. That relies on a lot of assumptions which might or might not be true. You can assign a DCR value to any moment in an engine's rotation but if it's not real, it's not very useful and might even mislead you.

RPM, runner lengths, cam timing, ambient temp, intake temp, exhaust backpressure, exhaust pulsations, intake charges being 'stolen' by other cylinders, timing chain harmonics etc. all play a role. It goes on forever.


Then you have to also consider a bunch of external factors take a role when it comes to just how much CR you can tolerate, anyway. Lots of engines will handle 15:1 CR until you connect them to something.

Trial and error, imitation, and luck are what the average Joe really relies on.

No. I will say it again. DCR is a simple calculation based on how much stroke is remaining at the point where the intake valve closes. RPM, runner length, etc does not enter into it. DCR is calculated with STATIC measurements. It's just a calculation that is very useful for making engine build decisions.

When the intake valve closes the trapped air is compressed a certain amount. That ratio is the DCR. We CALL it dynamic but only to differentiate it from that other kind of compression ratio.

We like to talk about static compression numbers but in reality you never get full compression of the entire cylinder volume because the intake valve is still open at BDC.

_________________
--> Check out my FI Turbo build <--
--> And the race truck build project <--
--> The Diesel Corvette <--

Yes, there is no way for the home hobbyist to calculate true absolute dynamic compression ration factoring in things like ram effect on the intake charge, scavenging effect on the exhaust charge, etc... However, for purposes of ballparking the preferredcompression ratio of an engine, it is better to calculate the dynamic compression ratio like ProCycle discusses than to go with static. If you shot for a dynamic compression like 8.1:1 to run on regular pump gas you will have more than enough wiggle room to not worry about things like intake turbulence etc... We all know that none of us has the resources of a big manufacturer, but we can do pretty good with a little math and leg work.

Charlie is right. Increasing the compresison ratio of an engine, especially of a engine like the slant that has such an amazingly low compression ratio from the factory, will make significant improvements to power and economy. If done within reason, incresed compression ratio will not put enough added stress on the factory parts to cause premature failure. Further, when doing calculations for what machining must be done to achieve the desired compression ratio, one is better off is one calculates the dynamic compression ratio rather than the static. This is because an engine that looks like it has a high static compression ratio will actually almost certainly have a lower dynamic compression ratio since the exhaust valve doesn't slam shut instantly at TDC and the intak valve doesn't slam shut instantly at BDC.

OK, then we are using a looser definition of dynamic CR then.

I want to make it clear...I'm as Average Joe as anyone. The following are topics I spend little time worrying about because I know I know little about them and have little access to knowledge. These are car related topics...I will leave 'women' out for the sake of brevity:

Exhaust systems
Cams
Dynamic compression ratio
Motor Oil
Spark plugs

The good news is I can go just about anywhere and find any number of geniuses that can tell me all about them!

I don't know anything, but I think what I'm hearing is: dynamic compression ratio as measured from cam specs, stroke, etc, doesn't tell the full story but it tells a lot more of the story than static compression ratio, while still being relatively easy to calculate from readily available numbers.

Thats about the sum of it. When you are talking about real-world actual dynamic compression on a running engine, you would need to be able to take things like exhaust scavenging, intake charge pulsing, backpressure, etc... into consideration and there is no way that the home hobbyist could measure any of these things. But you can do the measurements and math to know when the intake valve closes and when the engine actually begins to build compression on the compression stroke. This lets you get closer to being the ballpark of the "true" dynamic compression ratio your engine will see when running.

Tire wear. DL points, neighbors complaining about burn outs, wife wanting you to act your age, for starters.

Chrysler kept CR low for the same reason that GM and Ford did. There would have been a lot more warranty work for tweaking of the timing. Variances in fuel and summer weather would send them back with complaints. If you're comfortable with tuning you can raise the CR, maybe run 92 octane. Before engine management systems it just wasn't acceptable for cars that the owners already had enough trouble changing the oil on schedule. That's probably why the combustion chambers weren't too advanced, just wouldn't have made much of a difference with the carburetor systems of the day. Build them as low cost as practical, tell them to buy a V8 if they want more power.

It's also a bit of a myth that one must pursue the highest possible compression. If you're at 8:1 and go to 9:1, you'll make 'more power' but less 'more power' than, say, a supercharger or a turbo or adding another 200 cubic inches.

But raising CR by milling the slant six head is still the most HP/torque per Dollar, then any other mod, if you can do the head R&R your self.

_________________
Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 225 nitrous
64 Valiant Signet
64 Valiant 4dr 170