Does this increase cranking compression or reduce it?

Lee

It supposedly increases psi but it also decreases intake valve to piston clearance.
Perform a test and let us know.

Advancing=increased cranking compression.
Retarding=less cranking compression.

UDHarold

Thanks.

I just built a 632 that is supposed to be 13:1. I did a compression test and all were 180psi. The cam is 284/296 @ .050 112 intake center line and 112 lobe sep. installed straight up. I degreed the cam and it came out fine. Does 180 sound high for the specs?

Lee

low very low,no dome low

A huge part of the actual cranking compression, reguardless of the static compression is the intake valve closing angle of the cam. Think of it this way, if the intake valve never closed(which doesnt happen, but bear with me), if the intake valve never closed, youd never be able to compress anything. So, the later the intake valve closes during the stroke, the lower the cranking compression. And, the sooner it closes, the higher the cranking compression is. Make sense? That may be about right for your motor, but maybe not. I wont try to say til you give a few more specs.

Give me the stroke length of the motor, the connecting rod length, and if you have the spec card give me the advertised intake valve closing angle. The .050 number wont work, so make sure its the advertised IVCA. The specs you provided sound rather large, and even though the motor is huge too(and probably the crank stroke), the cam may still have large enough of an intake valve closing angle to keep those cylinder pressures down.

Out of curiousity, while you are at it why dont you post the rest of your combo. Or, at least the heads, chamber size of the heads, piston(bore size and dome CC), manifold, carb, yadda yadda. It is completely possible for those numbers to be accurate. Also, how did you perform your compression test? Oh, by the way, the reason advancing the camshaft increases cranking compression is based on the Intake valve closing angle. If you were to advance it 4 degrees, your intake valve closes 4 degrees earlier, and like I said, the earlier the intake valve closes, the higher the cranking compression is. Hopefully that helps shed some light on why the numbers change when advancing and retarding the cam, and also why that it is very possible for your cranking compression to be "seemingly" low when your static compression is actually pretty high. Its all in the cam specs, so post em if you can.

Good information.

This motor is in my wife's new tube chassis vette not a chevelle :-(

4.75" stroke
4.60" Bore
.225 (5.72mm) Dome height
6.70" rod length
Brodix CNC M2-BB2-Xtra
119cc Chambers
.039 head gasket
Comp 11-740-9 cam
IVO=30 IVC=74
EVO=80 EVC=36
Holley CMDR950 fuel injection
65# injectors
2000cfm throttle body

The cam card only shows the valve events at .050, the only thing it gave is the duration at .020 which is 322/336.

When I clayed for piston to valve clearance the clay in the quench area was about .050 thick, not scientific but close I guess.

I get the impression from the replies that the compression is actually low, but I felt it's high! Especially with the cam size. Even with 114 octane and cranking timing at 10 degrees it sometimes is hard to start when hot. I have a 3.1HP, 325tq 4.44 geared starter motor plus I swapped it with another one and it still does it. I thought maybe the pistons are giving me more compression then I asked for...

Lee

Originally posted by 70_chevelle:
Thanks.

I just built a 632 that is supposed to be 13:1. I did a compression test and all were 180psi. The cam is 284/296 @ .050 112 intake center line and 112 lobe sep. installed straight up. I degreed the cam and it came out fine. Does 180 sound high for the specs?

Lee At a guess it sounds fine to me, if anything maybe a little high for the 112 centerline I would have guessed that at 108 ICL.
Still just a guestimate though,
This is also assuming you don't live at 5000 feet
:D

Mike, our altitude is 4400 physical, the DA at the time of the test was 6200' (sick that I logged that!)

Maybe the starter cable is too small and wont flow the current..I'll look into that as a possible culprit.

Lee

Way too many varibles to give you a number. I think you are probably fine.
I have 3 different gauges & they all read different by 20lbs or so with everythng the same.
If they are all the same within reason & seem reasonable, I don't lose much sleep over the number.
If I would have had to guess on yours I would have been around 150-160 or so with that much cam on a 112 ICL. I am assuming a pretty aggresive roller lobe