picked up a 340 long block w/out specs

yellow rose said

You don't need to pull the heads to check the compression ratio and you don't need to screw with cranking compression.

Get the engine on the stand and roll the engine on the stand until the number one spark plug hole is straight up.

Get yourself a real graduated burette, not a turkey baster or one of those hardware store bull semen things. You have to move fast.

Get the piston to TDC and fill the burrette with with water and a touch of dish soap.

Fill the cylinder until the fluid reaches the spark plug hole and read the burrette. Then you can determine your CR.

You have to move quickly. Once you know the volume at TDC you can calculate the CR. If you are concerned about the fluid getting into the pan, you can use solvent, or denatured alcohol and it will dry up.

Again, you have to move quickly because you can't deal the piston with grease. I've done it many times, when some joker would come in and claim 11:1 CR and in truth he was 9.5:1 or worse.

Click to expand...

That will get you the compression ratio, but it still doesn't tell you much about the engine components used. A borescope would help some more.

If it were just a stock rebuild I'd say fire it up, you know roughly what you've got. But the more modifications that have been done the more specific the tuning has to be and the wider the range of possibilities. For my time and money, pulling the heads is worth while. All it costs you is a set of head gaskets, and you can inspect the pistons, bores, get a look at the top ring, the valves, head chambers, etc. That's a lot more than just knowing the compression ratio and taking a look with a borescope. Same for pulling the pan, you get a look at the whole bottom end and all it costs is a gasket.

I wouldn't do all that if I knew the shop that built the engine, or the guy I bought it from. But on a completely unknown engine? Especially on a forced induction build. A few gaskets now will be a lot cheaper than replacing engine components if something goes wrong later because something was assumed.