Educate Me on Cams and Compression Ratio

I bought a 455 short block off of a member and I want to understand some basics that I don't understand.

It is a 462/0.030 block, forged TA pistons (23cc dish), measures about 0.040 down the hole, 0.040 head gasket, Edelbrock (68cc) heads, TA 212 Cam, 1.6 ratio rockers. By my calculations I believe I am about 9.5 static.

I've found calculators for dynamic compression ratio but don't know how to calc the necessary timing value to generate it from the cam specs.

I definitely want to run on pump gas and my current stock '71 non-stage head, stock '70 piston and Comp Cam 268 cam runs just fine on 87 with a little timing pulled. I didn't know any of this stuff 25 years ago when I built the motor, just enough to get the clearances and have good oil pressure. Trying to figure a comparison here with realistic differences.

Should I run a thinner head gasket to get a better quench? I figure a 0.020 head gasket will get me almost 10.0 but is that safe for pump gas? I figure aluminum heads and forged pistons buys me a little more head room than stock cast heads and pistons.

 

Greg

The value you need is based on when the intake valve closes on the seat.
The closest way to figure that event is to use the advertised intake duration which is generally excepted as the .006" lobe lift value.
In reality the actual .006 lobe lift duration of the cam you receive may vary a bit from the specified value as it is not held solid like the .050" lobe lift duration,
but it is all you have to work with.

One of the first things you need to know is the installed timing specified from the cam card, which indicates the actual position of the intake lobe center.
Both the TA 212 and the Comp 268H are installed 4 degrees advanced.

The TA 212 has a lobe separation angle of 112 degrees.
Installing the cam with a 4 degree advance puts the intake lobe center at 108 degrees ATDC.
The advertised intake duration is 265 degrees.
The intake valve will close 1/2 of the 265 duration from the intake lobe center position or 108 + 132.5 = 240.5 degrees ATDC or 60.5 degrees ABDC.

The Comp 268H has a 110 degree lobe separation with a 268 degree advertised duration.
With a 4 degree advance, that puts the lobe center at 106.
Add 1/2 of the 268 duration to 106 = 106 + 134 = 240 degrees ATDC or 60 degrees ABDC

Both cams have the intake valves closing around 60 degrees ABDC.

Using 60 degrees in the DCR calculator should show a DCR around 7.9

IMO going from .080" to .060" piston to head clearence (with the standard round dish)
is not worth the gain in quench at the risk of not being able to run low octane gas.

Paul

 

Good to know. Leave well enough alone then.

 

Careful. That only applies if the lobe is symmetrical about the centerline. If the profile is non-symmetrical, you will have to have the timing card value.

 

Scott

You are correct.
That's why I specifically talked about the TA 212 and the Comp 268H which have symmetrical lobes.
We installed a Crane cam yesterday where the halfway point between the .050 lobe lift opening and closing is at 110 degrees ATDC
but max lift or the nose is at 109 degrees ATDC.

But like I said, the .006 lobe lift points can vary a bit from the actual advertised cam spec as they are not held as tight as the .050 spec.
and until you get the actual cam the DCR calculations are just ballpark figures.

Paul