should installing a 488 lift cam and 292 duration lower cranking comp. if so how much. phanks proge

Can you go down from where you are?

if it has alot of duration, it will bled off compression. how much I don`t know.

I remembered reading you other post about this problem. This is a dumb question but did you get the valves adjusted correctly? If they are too tight the valve may be hanging open slightly causing a compression loss.

I read your other post too.... I don't think the cam is the cause of your seeminly low crompression readings.

I say this because I have been using a comp cams XE284 in an 8.5:1 350 (with 76cc heads...). That cam is similar to yours at 284/296 adv. duration, and just over .500 lift. My cranking numbers were between 160-175psi.

Now, this engine was WELL worn, and I'm sure the ring seal wasn't the best. It burned a lot of oil, and had most cylinders with 20-40% leakdown numbers... :eek: After I took it apart I noticed the pistons are very loose in the holes.... I can put a .025" FLAT feeler gage between the piston and the bore. You can move the piston back and forth, or up and down. Now I think that must negatively affect the ring seal. (I built a new engine as my fix...)

So I don't think your low compression numbers are due to the cam size. I also don't think they are due to ring seal, but I could be wrong.... I'm certainly no expert.

As sschevellefan noted, it could be your valve adjustment, or something else in the valve sealing area. Bent valves, valves sticking in guides, knicked or damaged seats or valves... anything that could prevent the vavle from closing or sealing completely. A leakdown test could help you determine if the valves are closed and sealing.

One last thought, you changed heads too, so maybe it's head gasket related? Doubt it, but just a thought. I bet it's in the valve adjustment or sealing though. Good luck, and I hope you find the problem! smile.gif

chris

Edit: It was mentioned in your other post... you are sure the compression readings are accurate, right? The guage is good, and you're opening the throttle, right?

That RPM cam, or generic equivalent, will give you relatively low cranking compression (among other things), compared to some other cams. As far as "how much it would lower it", that would depend on what you're taking out, and on the engine's compression ratio. I'd guess though, on any reasonable engine at all (like 9:1 or more), you should see at least 140 psi; probably more like 150-160. Not being as good a design as the XE cams or any other modern designs, you probably won't see as much as you would with those.

Make sure you take all 8 plugs out, block the throttle wide open, use a good hot battery, and let the engine compress the cyl you're testing about 5 or 6 times at least before you take a reading.

What was the old cam? If the old cam was smaller YES, if the old cam was bigger, NO

what cranking compression do you guys think is optimal for 93 pump gas?
1- with iron heads
2- with alum heads ?

just curious


Mike

180psi iron, 200 psi alluminum would be my guess at 180degrees running temp, there are a lot of variables here

I'm running 195# with 91 octane and iron vortec heads with 9.7 static and 7.8 DCR. As far as I know, I've never run into detonation with the vortec heads.

It appears to me that the optimum pressure would be the absolute highest that you could achieve without detonantion for a given combo.