Background on engine is it's the numbers matching engine that appears to be all original. It has as far I know around 150k miles on it and it doesn't look to be rebuilt. It has stock air cleaner, Thermoquad, intake, exhaust etc. Here are the results...



1. 140 2. 115/(130)
3. 139 4. 126/(142)
5. 134 6. 125/(140)
7. 139 8. 144

(xxx) = added tablespoon of oil to cylinder

Things were looking good till I got to the passenger side. From what I've read the readings indicate worn cylinders on 2,4 and 6. My plan was to pull the motor to clean and re-gasket it as well as put on an aluminum intake and Holley carb. Clean and paint the engine compartment while the motor is out. Later install a TTI exhaust. My question is should I expect the motor to last for awhile or should I bag it up, put it aside and go through the expense of replacing it? Replacing it would be putting in a 408 with aluminum heads etc. which is obviously a lot more than cleaning up what I have. It does run real good so the low cylinder pressure was a surprise.

If it's going to be apart already why not just dingleball hone and re-ring at the same time as the re-gasket?

See what the bores look like and go from there.

Leave it alone you may get those numbers up using sea foam and a hard run

If you pull the heads be sure to check for burned valves/seats.

keep it running (downtime is a frequent trap) plus all OE not rebuilt is a huge plus. keep driving it, it runs fine & build a 408 in your spare time & drop it in & have fun & if you ever decide to sell it you can drop the OE eng back in & assuming it still runs fine (your numbers ain't that bad)/all OE/not rebuilt which are all good selling points ($$$).

Thank you guys for all your replies.

@RapidRobert, I really think that's what I'm leaning towards. I definitely don't want it down too long, I've been that road before and sold things I wish I'd kept. If the compression really isn't that bad then I think I'll try to clean it up and just go through the maintenance stuff to keep it happy for now. I'll just slowly build a stroker on the side till it's done and then change everything over when I'm ready.

Even though it goes against all of our Puritan beliefs, the first thing to do is to run it hard for 100 miles or so. If you want to add Seafoam or Techron that'd be okay too. It is possible that carbon deposits are keeping the valves from seating. Running it hard with plenty of full-throttle charges should clean out most anything.

Then with engine fully warmed up, run it at say 2000rpm and trickle water down the carburetor. Be prepared for clouds of dirty steam. I try to do it in out of the way places where neighbors won't complain.

R.

Some of these posts seem to be ignoring the fact that adding oil to the low cylinders for a retest did in fact result in significantly better numbers and therefore indicating a ring sealing issue, not heads.

I am wondering about smoke and oil consumption. If acceptable then I would be REALLY tempted to 'just run it'.

I checked this car out for the owner earlier this spring. It didn't smoke at all when it was started and idled great for the few minutes I listened to it. Nothing that would indicate anything was wrong with it. If I recall the inside of the exhaust tips were dry too.

This thing had zero speed parts on it, other than some aluminum wheels, so I suspect it has just been used as transportation, rather than a hot rod.

I vote for good old "motor honey" to help increase the compression on 2/4/6. SeaFoam does not contain any ingredient to "build up" on your rings/cylinders, so that won't help anything.

I had a similar situation MANY years ago with a 383. I "dingle-ball" honed & re-ringed 3 cylinders and put it back together. I'm not sure if the angle is right, but can you get a look at the cylinder "ridge" with a boroscope through the spark plug hole on a small block? I just bought a boroscope for $25.

my first question is what do you want from this engine? are you going to race this thing for max et? or you just want a good running engine that sounds cool? if you are happy with the way it runs and its performance use it as is. if its not strong enough for you rebuild it. Keep it simple

Keep in mind adding oil to cylinders wil raise compression on just about any used engine.

Bad ring seal can come from worn rings, but it can also occur when the rings are not free conform to the cylinder walls due to carbon build up in the grooves.

Since you got improved results on the three low cylinders by adding oil to improve the ring seal, and considering the age of the engine, you might have rings in those cylinders that are not functioning right due to carbon build up.

It might be worth trying to decarbonize one of those 3 cylinders
by removing the sparkplug,
draining the oil pan,
and continuously pumping a solvent like
"Yamaha Ringfree" or
"Chevron Techron Concentrate Plus"
down through the sparkplug hole
and out through the oil pan drain
to a catch pan where an electric fuel pump would re pump it up to the sparkplug hole again.
8 to 24 hours would be typical times.

Before restarting the engine, add some high quality synthetic oil into the decarbonized cylinder, and turn the engine over by hand to recoat the cylinder walls and rings.

If you are worried about using a decarbonizing fluid in an old and valuable engine,
you can also just pump a very light engine oil like fresh Mobil One synthetic 0W-20,
But do it longer, 24 to 72 hours,
and turn the engine over by hand often.

Marvel Mystery Oil works pretty good freeing up stuck rings as well.

Pacnorthcuda, that's what I thought as well. I haven't seen any oil consumption and every plug that came out was gorgeous without carbon build up or oil fouling.

Neil, you are correct. Tips are dry and no clouds of smoke when engine is fired.

yella71, I'm not looking to race the car at all. I just want exactly that, a good running engine that sounds cool.

360view, one thing I did notice is a lot of dry carbon build up on the inside of the exhaust tips. The decarbonizing idea you gave sounds good, but I would be nervous of all that fluid getting into bearings etc as well. I think I like the Marvel Mystery Oil idea as I've heard good things about that oil and I feel it would be less invasive to other parts and bearings it will come in contact with.

Supercuda, I think I like that idea. Maybe a few hard runs first to see if that changes anything. If not, maybe try the Mystery Oil.

I can't thank you guys enough for taking the time out to reply. 👍🏼