I currently have 150 cranking psi in my 340 with a comp xe274. I was wanting to down grade to either a xe262 or the xe268. How much of a bump in cranking psi will I get with these cams and what is the max you can get on a iron headed motor with 93 octane before the dreaded PING!

Changing a cam will not affect cylinder PSI.

just to throw a nut in the mix, on my 2-stroke race quad i can run 216 psi on 93 octane. my 440 octane is determend by timing and jetting. back it off and richin it up i can run 87.etc.

Is there a rule of thumb for figuring this out?

No rule of thumb that I know of but a shorter cam like that will probably bump up the cranking psi by 10 lbs or so.

what is the max psi A person should go on pump gas?

Now lets see; Yes, "generally speaking" changing the cam "will" affect cranking pressure. If the closing event of one cam is say, 13 degrees & the new is 9 degrees, both @ 050 approx,after bottom dead center then you will have more cranking pressure on the cam with the smaller degrees.
Even just advancing a cam on the ajusting slots of 4 degrees can give significant increase in pressure.
I have a programne download from comp cams that has a calculator for cranking pressure; haven't used it for a long time, but when you load all the specs of cylinder & combustion chamber in, plus stroke & bore, it gives the effective cranking pressure to the opening event of your intake valve.
cheers, Mike.

One should solve for dynamic compression vs cranking pressure or static compression. There are still other variables to consider as well such as quench.

By using online calculators such as the one below, you can change intake closing points to get the dynamic compression where you want it, then spec/order a cam.

http://www.wallaceracing.com/dynamic-cr.php

Quench or no quench & this calculator may help you. http://kb-silvolite.com/calc.php?

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Now lets see; Yes, "generally speaking" changing the cam "will" affect cranking pressure. If the closing event of one cam is say, 13 degrees & the new is 9 degrees, both @ 050 approx,after bottom dead center then you will have more cranking pressure on the cam with the smaller degrees.
Even just advancing a cam on the ajusting slots of 4 degrees can give significant increase in pressure.
I have a programne download from comp cams that has a calculator for cranking pressure; haven't used it for a long time, but when you load all the specs of cylinder & combustion chamber in, plus stroke & bore, it gives the effective cranking pressure to the opening event of your intake valve.
cheers, Mike.
Your correct and if your swapping the same camlobe familys pretty easy to get a pattern,the problem is the .050 duration is has little bearing on the seat duration,most calculators are pretty general therefore gigo.

There is quite an interesting article that analyses cam timing vs. compression, at the following url:

http://victorylibrary.com/mopar/cam-tech-c.htm

You can download an excel spreadsheet that estimates cylinder pressure. With my 541ci RB, using that spreadsheet and associated formula's, my measured cranking compression is actually within a few PSI of the predicted value.

For a given engine, the key determining factor to cranking compression is static compression and the intake valve closing point ABDC. You need to use the advertised value (not the value at 50'), as compression cannot start until the valve is fully closed.

Ive taken the liberty of inputing some values into the spreadsheet for your 340.

Based on the calculations, if all 3 cams are installed on the same ICL then:
XE268 calculated to be 154psi
XE262 calculated to be 158psi.

Remember, this is only a theoretical calculation, and your results may vary, as there are numerous real world factors that are not taken into account. I would be interested to see how well the spreadsheet calculations match up to other users known engine parameters vs. predicted values.