New to the site. Ok I’m mostly an outboard mechanic, but I have worked on Chevy engines for years, and raced cars, etc. I picked up a 1996 350 vortec Mercruiser engine for my tritoon. I have had several engines in it over the years, 3.0 4 banger and including a 406 SBC aluminum heads, etc. so I did a compression check on this motor I bought, and I was shocked. 205-210 lbs on every cylinder. This gauge is accurate and have used it for years. I know and understand the vortec quick burn design, but is 93 octane going to be enough for a carb motor with this much compression? Is this normal for these engines? I have had late 80’s 350’s and 165-175 lbs. my 406 I ran 165lbs with aluminum heads.
thanks,
Roy

Ayuh,..... Any chance the motor was mothballed with oil in the cylinders,..??
That would cause higher than normal numbers,....

Fire it up on the floor, run it, then test it again,....

We ran it on a hose when I bought it. Then I ran it about 20 mins when I got it home, and that was the numbers. I figure they must have put flat tops in it. The plugs were black, so it could have some extra carbon on the pistons, but not enough for 20-25lbs of compression. Cam sounds stock Mercruiser idles low and smooth.

I had a 350 with Vortec heads years ago with similar numbers. It had a Comp XE 262 that idled smooth and ran pretty well. I want to say it was right around 200 cranking. I was worried about detonation but it ran okay on 92. May have run okay with 97 but I never ran it that way. This was back in 1999/2000 so my memory may not be the clearest but I do remember it having high cranking compression numbers. Compression ratio for them is 9.3 +/- a tenth and with the truck cam they come with it's probably expected to produce some strong cranking compression.

Also worth mentioning is that cranking compression numbers are not necessarily indicative of compression RATIO which is what is a big driver for octane needs.

If you know someone at a local short track, you may be able to get them to tube and whistle it for you.

I’m going to stick a bore scope in the plug hole see if i can get a look at the pistons. See if they are dished, flat top, or I hope no dome.

The 96-2000 5.7 vortec (rated at 9.4:1) truck engines would typically blow 190psi .

lots of vortec engines (even marine) have a 196*206* at .050” 109lsa hyd roller cam, which by #’s helps create good cylinder psi.