I guess I'm kinda up **** creek on this one. The shortblock is less than 30k miles old. All cylinders read real close to 94 psi during a cranking compression test.

I thought my old 193 swirl ports were 76cc. I guess they were actually 64cc. So now I have ~8.3:1 static compression with that cam that's in my signiture.

blow...

Guess I'll just have to get some 10:1 pistons...

 

Are you holding the throttle open when you do the compression test?

 

I think you mean to say that you thought they were 64cc but they are actually 76cc? Yes, a miscalculation on compression can be very annoying.

Before you tear it down, go get the COMPLETE casting number off the head and X-reference it here:

http://www.mortec.com/castnum.htm

 

No I meant what I said. I thought the old heads were 76cc. So when I got my new 64cc heads I thought I was raising the compression to a decent amount and bought a cam to match. Well the old heads turned out to be 64cc also so there wasn't any increase in compression. I've been to that site before...the old heads were '193's.

Yeah I had the throttle jammed wide open.

 

Well, were the pistons in the motor dished? If so, then your compression is pretty much low; not quite 8.3 low, but probably 9¼ low. The only real sensible way to fix it is with flat-tops.... not completely practical at this time.

That's not a whole lot of cam.... I wouldn't worry too much about it being too much or anything.

The psi number sounds really, really low though. Even for that low of static compression. Are you sure the cam isn't retarded a tooth on the timing gear?

What does it run like?

 

The cam was installed at -2°. Yeah the pistons were dished. I guess I could break down and degree it. I had such a difficult time getting all the parts in one place at one time with the money I had that degreeing the cam just kinda got left behind.

It runs like a bandit. It FEELS like it lacks a lil bit of low end, which is what I expected going from the old combo to this one. Nothing absurd tho. It'll rip to 5500 rpm no prob and pulls hard all the way. It really starts singing around 3000 rpm...

 

Sorry, my mistake. I had it backwards. Pretty much everything cast iron built for 87-up small blocks is 64cc (or 58 for 305s). Your 193s come up as an 87-up head.

Anyway, I come up with about 9:1 mechanical compression with typical 12cc dished stock 350 pistons. Little low, but like RB said, not 90 PSI low.

 

Weird huh?

 

I've measured 14cc on stock-style dished pistons, including ring depth, valve reliefs, and deck height.

In addition to valve timing (intake LCA), duration and overlap can create very low apparent cranking pressures while the dynamic compression is really more than adequate. Your cam data doesn't indicate a lot of overlap, however, and that's a pretty low result.

KB or Badger cast hypereutectic jugs might be an answer. Might be a good time to go with 6" rods if you've been thinking about it, but the balance will be affected.

 

Shouldn't be that low... my 350 has 8.5:1 compression and still cranks 140psi.

I had a 305 that read about 85psi and it turned out the piston rings were shot. Hope you dont have the same problem...

 

that does seem kind of low. You should be able to get at least over a 100 psi gauge pressure. Dumb question but was the motor warm during the test? I guess some of it does depend onthe overlap of the cam (didnt see the 110 LSA) so i guess it varies form engine to engine.

 

btw, how does the SD behave with the 110 lsa? Ive gotta pull my engine anyhow so i was thinking of getting a comp cams XE256 flat tappet for the hell of it to replace my present cam.

 

Have you tried using a different compression gauge? I've seen more than one gauge go bad before. I would think you should have at least 120 lb.

 

dimented24x7, if it wasn't for extensive PROM tuning the engine wouldn't even run on the stock programming. It's been about 3 months since I got it running and I'm still tuning it.

DartByU, I dunno if it's the gauge or what. I don't think it is tho. Somehow I think the combination of cam degree and specs, as well as too low a compression ratio make for the low cranking pressure. That's just what I think...