Straw, also remember that "what you see" is a function of all the parts currently in the engine. So if the current cam is retarded, or advanced, or the timing set worn a little, or the rings/valve job has some wear or are too new to have fully seated... the reading will be affected one way or the other. One needs to always consider the measured pressure is a result. Not a prediction. As you note, the only thing that SHOULD affect the cylinder pressure indicated on a gage is the closing of the intake valve on a given engine. However there are always many other factors (including starter circuit and battery strength) that DO affect it.
I agree 100%. And that is inherent in my point
What he has is what he is working with--unless he wants to do a rebuild. As far as I think I read, he has a freshened up engine with a very mild cam in it at the moment. Obviously, I don't know if the current cam was degreed in, or just stabbed in as so often is the case.
If I understand corectly, he wants to change cams and the rest of the factors that you mention are basically constants in this case unless he is going to spend more money on other things.
Someone else mentioned the supposed factory compression of the engine....My guess is that the actual compression ratio was probably at least half a point less than advertised because all the heads I have seen had a lot more volume than the specs listed. That is pretty typical among all brands, imo. Perhaps his machineshop decked the heads and block and got things close to spec..I don't know.
I do know that it is really not about compression ratio, but, it is all about low speed cylinder pressure, and that if he currently has relatively low cranking pressure now, it will get a lot worse if he sticks that Mopar cam in it.
I learn slowly
, In January of 1965, I bought a new Belvedere II 383 and in February of 1965, I stuck a 310 Isky magnum in it....sure sounded good. I think it was about 1990 when I finally began to resist the urge to overcam engines-particulary street cars.
My latest project is a Chally with a 440 that some prior owner stuck that particular Mopar cam in that the OP likes. Sure sounds good...but a stock piston'd '70 block with 452 heads does not make much cranking compression even tho the the heads are decked pretty good.
I figure that the cam you suggested and a lot more compression will make it run a lot better than it currently does. I might be wrong, but, I have finally come to the conclusion that math does work and the cam companies know what they are talking about.
I am pretty much like you, I am not sure there is much magic between cam companies as long as they grind the lobes as they are designed to be ground. I don't think they are perfect, or we would not have to degree them in, however so I always check 'em.
I have a comp roller in one of my Buicks and it works well. Not so high on their roller lifters, but, maybe they will get them where they all are good in one box one of these days.
So, while I agree with you in essence, I still think he is going to need a lot more cranking pressure than he probably has....as he has no info, a compression test is about as good as we can do unless he has it taken apart and everything measured. It is not that hard to extrapolate if the cam is the only change. We don't need engine analyzer software to do that like we would if he was changing rod lengths, etc.
But, hey, it would not be fun if there was one answer, and only one answer for every problem...that would be boring as heck.
