Magnum blocks only have about 8.2:1 compression
I don't think that's right; the pistons are only down a short ways, and the heads are closed-chambers. to be down at 8.2 the total chamber volume would have to be; about;
(360/8 x 16.387)/(8.2 less1)=
102.42cc, lol. Naw that'll never fly.
To be at 9.1, the total chamber volume maths out to (360/8 x 16.387)/(9.1 less1)=
91.04, still pretty high.
IIRC the closed chamber Magnum heads are about 60cc; so with an 8.8cc gasket, the pistons would have to be ;
91.04 less (60 +8.8)=22.24; sounds close for a dish. If the piston was down .012( 2.5cc), that leaves 19.74cc for the dish.
So by the specs, 9.1 seems more plausible. I have never taken one apart, tho.....
But no matter, you can/ may have already corrected all that at the rebuild. Just keep in mind that
the street engine only cares about Dcr, which includes the cams Ica or Intake closing point. Scr is just the tool we use to get to the required Dcr.
There are a few ways you can calculate your Scr:
If the heads are not yet on, you just measure all the pertinent parts and a lil math spits it out.
If the heads are on and the engine is not yet installed, here is what I have done;
I roll the engine over on the stand and put either #1 or #6, up as the high point, then adjusted the sparkplug hole to be the highest point. But you can use whatever plug is conveniently the highest.
Then crank the piston for that chosen cylinder to TDC (TDC-compression if the cam is in); try to be very accurate. Next
I get my trusty 100cc pig-sticker medical syringe, and fill it full of oil, to exactly 100cc. Next
I inject as much of that oil into the chosen cylinder as will fit, to the bottom of the plug hole. Be careful, as it approaches full, the level will rise very rapidly. Finally
I measure how much oil is left in the syringe and do the math, according to; 100 less what is still in the syringe. then
the formula is
Scr= (CV+cv)/cv, where
CV is the swept volume and cv is the amount you injected. The swept volume is that part of the cylinder that the piston sweeps thru as it travels up and down. Which is
(Bore/2)squared x pi x stroke x 16.387( the conversion factor to get cc)
pi is 3.1416
example at stock bore and stroke;
(4.00/2) squared x 3.1416 x 3.58 x 16.387 =737.214cc
If you measured 91cc, then
Scr = (737.2 + 91)/ 91 = 9.10
Here are some examples of various cams installed in a 9.1 Scr engine.
If the cam you install has an Ica of 60* then your Dcr will be
Static compression ratio of 9.1:1.
sealevel/ Ica of 60*
Effective stroke is 2.88 inches.
Your dynamic compression ratio is ...................
7.52:1
Your dynamic cranking pressure is 150.79 PSI.
V/P (Volume to Pressure Index) is ...................
131
if the Ica is 56* then
Static compression ratio of 9.1:1.
sealevel/ Ica of 56*
Effective stroke is 2.97 inches.
Your dynamic compression ratio is ....................
7.72:1 .
Your dynamic cranking pressure is 156.09 PSI.
V/P (Volume to Pressure Index) is .....................
140
If your Ica is 52* then
Static compression ratio of 9.1:1.
sealevel/ Ica of 52*
Effective stroke is 3.06 inches.
Your dynamic compression ratio is ..................
7.92:1 .
Your dynamic cranking pressure is 161.41 PSI.
V/P (Volume to Pressure Index) is
..................... 149
If you can get a tight Q down to .040 or less,
AND your elevation is 800ft, then
Static compression ratio of 9.1:1.
800ft/ Ica of 48*
Effective stroke is 3.13 inches.
Your dynamic compression ratio is .................... 8.08:1 .
Your dynamic cranking pressure is ....................161.41
Your effective boost compression ratio, reflecting static c.r., cam timing,
altitude, and boost of PSI is ...............................7.92 :1.
V/P (Volume to Pressure Index) is ....................... 152
VP of 152 makes a firecracker street engine.
Pressure of 161 is pushing the limit for pumpgas, but with a tight-Q should not be a problem
Ica of 48* is pretty small, ending up at something like a 200@.050 or smaller.
Speaking of VP:
Anything over 140 with 3.55s and a manual trans is already fun. Same for an auto with 3.23s and a 2800TC.
By 152 you have a serious tire-fryer.
131 is ho-hum, could be fun with the right TC/Gear combo. It will likely cost you more for the TC and gears than hi-compression pistons installed. And the Hi-C pistons keep giving back forever.
Read about VP here
V/P Index Calculation
But if the engine is already installed you are kindof screwed.
You can get a rough idea by back-calculating from the Cranking cylinder pressure BUT
you will need to also know your Ica pretty accurately and your elevation. However, if your pressure is over 150/155 you're in pretty good shape