I built a 632 with 13:1 compression, total seal gapless 2nd rings and a comp cams 11-740-9 cam (284/296 @ .050, 112 ICL and 112 Lobe Sep) installed straight up. When I did a compression test cold I got 180 cranking PSI and I thought that was high for 13:1. Well since then I advanced the cam 4 degrees since everyone said the engine should like it and in fact in the drag simulator it did favor it. So now that's it's advanced the cranking PSI is no over 200! In my mind something's wrong. I degreed the cam and it was correct so I'm wondering what my compression really is. Could the compression be higher then 13:1?

I'm going to the dyno Saturday so I'll see how well it runs then. I'm just concerned the C12 wont be enough octane and it kicks back sometimes when starting it hot.

Lee

Advancing the cam will do that. Close the intake valve sooner (by 4 degrees in this case) and you build more dynamic compression.

Hi,
Cranking pressure is not a good gauge of compression at all. The cam shaft specs change what you see on the gauge. For example, a compression check on a stock 3.8 v6 should show around 175#. If your engine had a small cam in it, your gauge would probly peg out.

180 psi will work with 93 octane. 200 psi will not.

Thanks for the replies.

I'm running C12 not 93 octane!

Anyone running that big a cam have any idea what their cranking compression is?

Lee

Lee,

Cranking compression is not a gauge of static compression. It's really nothing more than an indicator of where the intake valve is closing.

That being said, there are some general rules of thumb.

180 psi cranking compression will usually run fine on pump gas. 200 psi is pushing it for pump gas. As you mentioned you are running race gas so that is not an issue. The generally accepted target for a race gas motor is 230 psi.

13:1 compression is really in no mans land for race gas in my opinion. Too high for pump, but leaves power on the table for race gas.

For your compression the optimal cam would likely be a little smaller than what you have. Then again, you've got a BIG motor so giving up a little bottom end shouldn't be a problem graemlins/thumbsup.gif

Let it rip on the dyno and enjoy yourself. Let us know how it goes graemlins/beers.gif

Dragracer - Thanks for the reply. I guess I've been worried because the chevelle and camaro with much smaller cams only have 160 psi.

This motor is going into a 2200# tube chassis vette and all three cam manufacturers recommended a cam close to this size. I too would have gone smaller if it was my choice so we'll see how it works out.

Lee

As the others pointed out, static compression is only one component of cranking compression. FWIW, I did the calculations and came up with 181 psi cranking for the 112 icl and 194 psi for the 108 icl. So your results are pretty much on target.