Help with 340 compression + cam change
The 351M in my 75 F250 had 160-165 in all eight about 5 years ago when I put the re-man heads on it. They are rated at 8.4:1, but routinely spec out at around 8:1. The cam in mine I am almost positive is the Melling, Elgin, everybody's grind that's like .486/.512 and 204 and 214 ground on a 112. It has a very slight bump when cold but stock when warm.
Your example is a shining one that all the formulas and calculators in the world sometimes come up short. Did you degree that cam? If not, you can add a good chunk to it if you advance the camshaft timing probably 4*. I have threatened to do that with mine, but I have the rebuilt 400 under the work bench.
My 351C with all the combustion volumes measured and computations made 45 years ago came up with 160 psi with a 10.3:1 SCR. But the cam was 'long tailed' and knocked down the DCR to the low 8's... which goes right to around 160 psi cranking compression. So it works when you put the right info in it. I don't know what to say about your 351M, but it makes a few assumptions on what is in there, and if the valves are moving as thought....
I’ve never understood what dynamic compression ratio means or what it would tell me. And prior to this thread wasn’t sure I had the info to figure it out. Consensus here is that my static should be at least 8.5:1.
So using the Wallace calculator with 8.5 static and this cam (34 ABDC) I get:
Static compression ratio of 8.5:1.
Effective stroke is 3.10 inches.
Your dynamic compression ratio is 8.02:1
So what does this tell us???
You don't use the intake closing at .050" to compute DCR....the DCR number is computed with intake closing at or near the advertised duration numbers. So 8.02 is not correct for an 8.5 SCR..... Buuut DCR does correspond closely to cranking compression, and if you have 150-160 psi cranking compression, then your DCR is close to 8. (Meaning that the SCR has to be higher than 8.5....or the valves are doing something unexpected when cranking.)
The whole point of running the cranking compression after running the engine is to make sure the lifters are pumped up and closing the intake valve when it is supposed to. If you have Rhodes lifters, which lose duration when oil pressure drops, then they will give a higher DCR and cranking compression.
DCR is a good indicator of the motor's low-mid RPM torque performance. You'll feel a 1/4 point change, a 1/2 point change will make the motor 'wake up' some at lower RPM's, and a full point will make it feel like a different motor. If you are spending your engine operating time up in the higher RPM ranges, then it becomes far less of a factor. So it has meaning for street, cruising uses, and less so for drag racing use.
Too low of a DCR will be an engine that bogs at low RPM's..... which is one reason why guys put in higher stall torque converters with a big cam.... the DCR drops and they have to rev the engine past the low RPM bog to get a good launch. This why all the warnings to not over cam a stock mid-late LA 318 which starts with low static CR around 8... over cam it, the DCR drops down into the low 6's and it is bog-city.
