400 SBC Compression Ratio

I've got a 400 small block I'm getting ready to pull from a donor truck for my '78 K30 which has a very tired 350 in it. The donor 400 sounds healthy, gave it a compression test yesterday and was getting readings of 160-172 psi (average 165psi) which seems high to me. As far as I know, it's bone stock. Don't know the block casting yet but the heads are smog-era 458642s (nothing to write home about).

I have a copy of the old-school How To Rebuild Your Small Block Chevy by David Vizard and it says 140psi would be expected for a 10:1 compression ratio. There's no way a stock '77 400cid SBC should be kicking out that much PSI.

Any thoughts on what could cause this? I used a newish Harbor Freight compression test kit because I'm a cheap bastard doing this out of my old man's shop. I disconnected the battery lead to the distributor and cranked it over for 8-10 compression strokes per cylinder reading.

As an aside, I plan on yanking the smog heads and throwing on a set of 3973487 LT1 heads that I have laying around.

Edit: the engine was essentially cold when I ran the compression test. Roughly 45% humidity, 500ft above sea level, 55° F ambient temperature, brand new fully charged 800 CCA battery.

Well, I guess I’ll say this to start- perhaps double check with another compression tester. Also, the book you’re reading may have good general information, but have you looked up the specs for that particular engine? Good place to start IMHO.

Just food for thought. Or, if it had good oil pressure and was a good strong runner, swap it in and not worry about it. Dismissive response it may seem, but really…I don’t know of too many circumstances when a used engine will actually BUILD compression over time with wear. So what other explanation would there be? I’m all ears.

I don't know the history of the engine, just that the guy I got it from said it was the original engine. Other than an aluminum radiator, electric fan, some cheap headers, and a rattle-canned engine bay, I have no reason to think it's been rebuilt.

My best guess is I was allowing it too many compression strokes before taking my reading.

I seem to recall my old 400 running 160+ when I tested it. It was years ago though.

I actually put LT1 heads on it too. Drilling the steam holes was interesting, lol. Some of them drilled fine, but a couple areas just killed every drill bit I tried. Iirc, I wound up using some type of masonry bit and it drilled right through.

Sterling452 said:

I don't know the history of the engine, just that the guy I got it from said it was the original engine. Other than an aluminum radiator, electric fan, some cheap headers, and a rattle-canned engine bay, I have no reason to think it's been rebuilt.

My best guess is I was allowing it too many compression strokes before taking my reading.

Click to expand...

Good info, and I hear what you’re saying.

I will say this- and if this helps at all- more compression strokes with more cranking will not build an inaccurate number. It’ll simply max out on what is able to attain.

Seem to remember From My youth,, Yeah long time ago,, But anyways Seem to remember someone telling Me CARBON can increase compresson readings

bucket said:

I seem to recall my old 400 running 160+ when I tested it. It was years ago though.

I actually put LT1 heads on it too. Drilling the steam holes was interesting, lol. Some of them drilled fine, but a couple areas just killed every drill bit I tried. Iirc, I wound up using some type of masonry bit and it drilled right through.

Click to expand...

Lucky for me, the LT1 heads came off of another 400 and the steam holes are already drilled

rusted nuts said:

Seem to remember From My youth,, Yeah long time ago,, But anyways Seem to remember someone telling Me CARBON can increase compresson readings

Click to expand...

This too. Forgot about that.

Checking compression with a tester will verify that rings and valves are sealing properly, but there is no direct correlation to compression ratio. Stock engine and heads should be about 8.2 compression ratio.

Shop manual will usually state that your are looking for uniform compression (within about 10 psi) across all cylinders, not a specific number. If you get low compression on some cylinders you squirt in some oil and retest to see if it helps the rings seal.

I have a mildly built 400 in my '57 Chevy and just did a compression test on all cylinders just to check engine health. With the engine cold, all 8 cylinders ranged from 170-180psi. It is best to test with the engine warm, but I was just looking to see what the variation looked like. The actual number is less important (within reason) than the variation across. 5% is pretty darn good, especially since it is a 20yo motor. Not many miles on it, tho.

Disable the ignition, remove all the plugs, wire the throttle wide open.

Crank the engine the same number of revolutions for each cylinder.

The rule of thumb: the reading on the gauge from the first full puff should be at least half the final number. There shouldn’t be more than 10% difference between the highest and lowest number. The actual number isn’t really important.

Cam timing, camshaft installation, and engine build has a huge influence on compression gauge numbers. If the engine is 100% untouched, 100% stock, then the numbers in a service manual will be accurate. If the engine has been rebuilt or modified the actual numbers are meaningless.