So, as some of you know I have a 390 in my truck. I am obviously trying to learn as much as I can about these engines so I have yet another question.

What is the highest compression ratio you should/could run on 87 reg. octane gas? Also, I heard someone through around "dynamic compression ratio"
what does this mean?

 

I was running 89 on a 10.25 to 1 390 with a Crane 272 degree Energizer cam and 12 degrees initial timing. So something less than that, but also depending on cam choice. Bigger overlap cams (narrow LSA) bleed off cylinder pressure allowing high static ratios.

 

For every 10 degrees F temp increase from a 180 degree stat you reduce the compression by 1/4 of a point.
You can be all over the chart with static compression numbers. Dynamic compression due to your cam specs dictates if you end up with a dog, a good running or a detonating useless POS of a motor.

 

Correct. To go further, what Baddad is saying is that any given compression/octane combination may run fine with a big duration cam, but ping with a short duration cam. The reason is short duration cams, like you would typically put in a truck, build more cylinder pressure at lower rpms. This is why they have more low end grunt than a long duration cam. As a result, they need lower compression to prevent ping. There are several factors that determine if ping is going to happen with a particular octane rating, but they always come down to two things, cylinder pressure and temperature.

They are a balancing act. You can raise one and lower the other to compensate. Here is a short list of items that have an effect on one or the other:

Rich=cooler/lean=hotter
aluminum heads=cooler/iron heads=hotter
clear day=more pressure/cloudy day=less pressure
big cam=less pressure/small cam=more pressure
altitude=less pressure/sea level=more pressure

This is why there is NO definite answer as to what compression is right.

My engine, for example, will run fine here near my house on 91 octane with 12.05:1 compression. If I drive to Salt Lake City it will ping itself to death because of the altitude change.

Do a search on the web for "dynamic compression ratio" and get to know it well. It is a great starting point for determining a compression ratio. There are also dynamic compression ratio calculators on the web that will help you make your choice, but you will need to have some precise numbers for your engine to make it work accurately.

If you follow through with your own question you will have a great understanding of basic internal combustion and camshaft design and effect when you are done. It's worth it.

-Scouder