Static/Dynamic Compression and Advancing Camshaft Question

[LR1970]

Hi, so in my research about compression ratios I have discovered all about Dynamic compression and cranking PSI. I am reading that for pump gas your dynamic compression should be around 8 to 1 and 165 psi cranking pressure for an iron headed big block. Does this sound right?

The camshaft I want to use in the build has a 74 ABDC. To get the 8 to 1 dynamic ratio I will need a 10.9 static ratio.
Is 10.9 static and 8 dynamic still ok for pump gas?
I plan to use Edelbrock 84cc RPM heads or 915 heads to get close to the 10.9 ratio.
Will advancing the cam change these ratios?
If I plan to install the camshaft 4 degrees advanced, should I be recalculating the static compression at 70 ABDC so then bringing the static compression down to 10.5 to 1.
Cam has same LSA and centerline.
Or am I just splitting hairs here?
I would like some real word opinions before I go to the machine shop for head cc milling.

 

[496 polara]

Go with the aluminum heads regardless. Advancing the cam generally raises cranking compression. Someone smarter than me will be along with the math.

 

[Geoff 2]

Forget about DCR. It is meaningless for determining the use of pump gas because the DCR doers not take into account things such as port flow, which among other things, controls how much air is drawn into the cyl.
Adv/ret the cam makes a difference. I would advance the cam 4*. It will make a small difference; whether that is enough for you depends.....
It is easy to do.

 

[Kern Dog]

If you are trying to do a workaround for high compression, I've been there.
For years I dealt with detonation. I backed of the timing, ran race gas, tried the bigger cam routine and even slipped in thick head gaskets.
I was hesitant to pull the engine and change the pistons.
The thicker head gaskets changed my compression ratio from 10.9 to 10.1 and the knocking stopped. There was no quench but at least the knocking had stopped. I had a cam go bad so I took that as an excuse to pull the engine and do it right. I changed to quench style dished pistons....

I'm at 9.8 to 1 with quench and have driven in 110 degree heat with no detonation.

 

[lewtot184]

I think trying to get 165psi cranking compression and 8:1 dynamic won't happen. I think it's like trying to mate ducks and chickens together. It's my understanding that 8:1 dynamic is maximum for pump gas , and 165psi isn't. Don't chase the maximum in either direction if you want a pump gas engine. Too many variables.

 

[451Mopar]

If it is a true street pump gas setup, I would error on the lower compression side because pump fuel changes all the time.
If you don't mind mixing in some high octane once in awhile, you might be good?
Alot depends on how the engine is loaded, and how cool the intake charge and chamber are.
If it is a big engine in a light car with high stall converter, then engine is not likely to get lugged around at low RPM with wide open throttle so easier on octane than a small engine in a heavy vehicle with a low stall or manual transmission pulling a load. Lugging the engine at WOT at low rpm is going to make alot of heat and more likely to ping/detonate.
Same for higher air and engine temperatures.

 

[BSB67]

Pretty much what everyone said.

Furthermore, there are at least 3 different calcs available out there to use and they will give different results. Which to use? Seat timing? 0.050” timing, some modification of those. Does cylinder pressures really not start until the intake valve touches the seat? Why is 0.075” inch valve lift a magical number?

I hate to leave cylinder pressure on the table. I have used these calculators and adjusted for variables and have become comfortable with blending it all together and making a choice for my stuff, understanding that I’m okay if I get it wrong.

My suggestion is to get someone with experience to do this for you, understanding that they will be conservative.

 

[dan juhasz]

If you are trying to do a workaround for high compression, I've been there.
For years I dealt with detonation. I backed of the timing, ran race gas, tried the bigger cam routine and even slipped in thick head gaskets.
I was hesitant to pull the engine and change the pistons.
The thicker head gaskets changed my compression ratio from 10.9 to 10.1 and the knocking stopped. There was no quench but at least the knocking had stopped. I had a cam go bad so I took that as an excuse to pull the engine and do it right. I changed to quench style dished pistons....

View attachment 1734086

I'm at 9.8 to 1 with quench and have driven in 110 degree heat with no detonation.

how did lowering your cr affect the engines power? was it very noticeable ?

 

[Kern Dog]

Right now, the engine at 9.8 to 1 feels just as powerful as it ever has, Maybe that is because I'm able to run more spark, more timing without knocking.

 

[steve340]

Be careful advancing the camshaft. Most cam companies grind in 4 degrees of advance when they manufacture the cam.

 

[dvw]

I don't know what cam you are running. I do know that Eddy RPM heads with a Mopar .557 solid running 10.0-1 will run on 93 without issue. In a stock street 65 Belvedere running a 4.10 decent converter and S/S springs the car will run 11.40's anywhere with open exhaust.
Doug

 

[451Mopar]

I calculate and look at DCR, and then calculate cranking compression ratio, but only to see it the compression ratio is way off. I select the cam I want to use, and use the DCR to figure the compression ratio I want, not the other way around.

 

[LR1970]

I don't know what cam you are running. I do know that Eddy RPM heads with a Mopar .557 solid running 10.0-1 will run on 93 without issue. In a stock street 65 Belvedere running a 4.10 decent converter and S/S springs the car will run 11.40's anywhere with open exhaust.
Doug

Thanks, I have pretty much the same set up. With XE285HL cam and 3.9 gears, 65 Coronet. I was going to use the new version of the 509 cam but decided on the XE285 instead.