318 heads on a 360
I'm already running 3:55 gears with a revearse manual. I am trying to find a smaller cam kit, but I also want something big enough to give me a good lopey or choppy idle and I was told the duration @50 has to be fairly high to get that.
Partly/ mostly true; it is a combination of the late closing intake and the overlap that makes the sound.
But the late-closing intake also softens the bottom end, usually requiring a hi-stall to overcome.
And the overlap is a result of the long durations. Overlap can be exploited with headers to power up the midrange and over the top rpm. But these same headers can pull fuel charge straight across the pistons and into the headers at low rpm. And out the tailpipe the unburned gas goes. resulting in, you guessed it, terrible fuel economy at, city driving.
And the large amount of exhaust duration has to come from somewhere, so it gets stolen from the power-stroke, resulting in, you guessed it terrible fuel economy ALL the time.
Lemmee show you something;
Here is the Wallace predicting your cylinder pressure and V/P, at 500ft elevation, with that 274 cam;I upped the Scr to 8.2 to reflect the slightly smaller 318 chambers.
Here she is with the Comp
Static compression ratio of 8.2:1.
Ica of 64*, 500 ft elevation
Effective stroke is 2.79 inches.
Your dynamic compression ratio is 6.61:1 .
Your dynamic cranking pressure is 124.49 PSI.
V/P (Volume to Pressure Index) is 106
read about V/P here; V/P Index Calculation
Notice the pressure is 124.
Notice the very weak VP, about what a strong slanty makes. This engine needs a hi-stall and gears to be fun
So here is the 360 2bbl cam
Static compression ratio of 8.2:1.
Ica of56*,still 500ft
Effective stroke is 2.97 inches.
Your dynamic compression ratio is 6.97:1 .
Your dynamic cranking pressure is .................. 133.78 PSI.
V/P (Volume to Pressure Index) is .................. 121VP
Here we have typical MAGNUM 5.2VP
Next, lets put a 318 cam in it, with no other changes.
Static compression ratio of 8.2:1.
Effective stroke is 3.10 inches.
Your dynamic compression ratio is 7.23:1 .
Your dynamic cranking pressure is ..........140.55 PSI.
V/P (Volume to Pressure Index) is ........... 133
Notice the pressure has risen to 140 or plus 12.9%
Notice the VP has risen to 133,which is about where the fun starts.
I would not run less than ~124VP, for me, that is already pretty sucked out.
Next lets compare the cams as to all their durations, and I'll use the advertised numbers. First the Comp274, then the Mopar 360 2bbl, and then the 318 2bbl
>276int/118comp/101pwr/286exh/110LSA/54*E-overlap/Ica of 62*
>252int/124comp/116pwr/260exh/112LSA/32*E-overlap/Ica of 56
>240int/130comp/122pwr/248exh/112LSA/20*E-overlap/Ica of 50*
Compare the power strokes; the more degrees you have in a cruiser, the higher the potential is to make good fuel economy.
Compare the Effective overlaps; the more you have, the greater the idle lope.
Compare the Ica's; The bigger the number, the later the intake closes and the lower the idle vacuum
.
I like to add the compression degrees to the power degrees to come up with a number that helps tell me how suited to an application, a certain cam is. Among these from top to bottom the numbers are 219/240/252.
for your application, I know 227 is a good number, for both economy and cruiser-fun. but 227 is not on the chart.
No big deal cuz you have a 2800TC, so your bottom end can be a lil soft, cuz when you floor it, you will only be in the soft part for a second. So while I don't like 219 for your application; If you got to have idle lope, then of these three your Comp is the best choice.
