# 65 GTO 389 cranking compression/leakdown specs and questions



## tallrandyb (Jun 12, 2021)

I got my original car back about two years ago, and haven't torn into the engine. I do know that it sat in two collections for over 10 years, and it was over 10 years before that when it was completely refurbed (thread about the Ron Ham shop in Little Flock, AR), with pix of the engine being built and on a test stand/etc so I assume they knew what they were doing and likely installed valve seats for unleaded gas as it was the mid-90's approx, but I don't know for sure. 

I just did a compression test and a (crappy) leakdown test (crappy, since I wasn't ever really at TDC for each cylinder, I just let the pressure push my finger off the hose then connected the leakdown kit) to try to get some more info on likely compression ratio and potential valve seat issues/lobe issues/etc.

*Engine specs and facts to start:*
1966-dated 389 block with 093 heads (correct replacements in 66 for a 65 I believe; I assume the original engine likely blew in the first year; 10.75:1 approx?)
AC 44S plugs gapped to .035, Pertronix module in stock distributor, 15 degrees base timing + mechanical + vacuum
correct 65 tri-power setup, running 110 leaded for now
Muncie 4-speed, 3.23 safe-t-track rear

*Performance*: 
No pinging, runs great, sometimes just a little smoke when hitting the trips hard, no smoking on startup, no oil consumption, no dipstick pushing problems. Basically I have no complaints, just wanting to figure out what the configuration/condition really is, and will start blending 91 Ethanol free unleaded with the 110 leaded with this current tank.

Warmed it up, changed oil and filter, then hooked up remote starter switch and pulled all the plugs, radiator cap, valve breather.
Plugs look good, they get a little crust on them all over the electrodes, also slightly reddish, I'm assuming from the 110 leaded, no oil on insulators.

*Testing:*
Compression test and leakdown test results (leakdown with 55psi pressure, very likely never right/at TDC):

Leakdown/Compress Cylinder Cylinder Compress/Leakdown
17 200 1 2 190 27
25 190 3 4 190 24
12 200 5 6 195 11
12 200 7 8 200 10

Cranking compression is all within 5% of each other (190-200). 

Leakdown on 3 cylinders 24-27% (maybe, for sure they are just higher than the other 5); I can hear air in the crankcase, and could be convinced maybe some out the exhaust seats, but nothing in the carb and no bubbles in the radiator and with 2&4 side by side I couldn't hear anything out the others plug hole.

So, what do you think? Seems the 093 heads are definitely keeping the compression high, and with the car having sat for long periods of time over the last 20 years, I wouldn't be surprised if the cylinder walls got some rust and the rings have worn a bit from startups. But it still doesn't smoke and runs great, so I'm inclined to just enjoy it and start reducing my octane numbers until I'm maybe down in the 96-98 range (14-15 gallons 91 non-E, 5-6 gallons 110 leaded).

Any thoughts/inputs? Thanks!


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## geeteeohguy (Feb 2, 2008)

I can help. Your cranking compression is right in the ballpark of where it should be. Leakdown over 25% is going to pump some oil at WOT and is due to worn, or possibly sticking, piston rings. Since the engine sat stationary for some time, it is quite likely that the rings are gummed up in the cylinders with the leakdown. I would run about a cup of ATF or Marvel Mystery Oil in each tank of fuel, or possibly do the 'Italian Tune-Up' where ATF or water is drizzled down the middle carb at 3000 rpm or so for a few minutes. 

Just driving it may fix it. 

I bought a '61 Corvette that had a hurt engine and had sat for decades....my compression was 195 in every hole, but leakdown was 30-35% in EVERY hole.....teardown showed worn out rings in all cylinders. Since you only have the issue on 3 cylinders, I'm leaning towards stuck rings. If it were mine, I would not tear it down (yet) I would drive it with some MMO in the fuel. 
My '65 389 has 175 PSI in every cylinder due to the high overlap cam it has. With a stock cam it's more in the 190-200 range.


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## tallrandyb (Jun 12, 2021)

Thanks @geeteeohguy, I'll toss some MMO in the next 2-3 tanks then do a quick leakdown check on #3 (easy, lots easier than #2 with A/C!) sometime after that to see if there is any improvement.

I know the car had only 700 miles put on it in the 10 years before I got it, and I've put over 2000 on it in the last 2 years with no issues, and I'm looking forward to increasing those smiles/year in retirement.


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## geeteeohguy (Feb 2, 2008)

With your low mileage and only a couple-three cylinders involved, the chance of stuck/gummed rings vs worn out rings is very good. If it doesn't 'come around' it will need to be addressed eventually. I have seen broken piston rings in these engines from detonation, but it takes some doing. And when they're broken, leakdown is usually over 70%. I think you'll be okay here.


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## tallrandyb (Jun 12, 2021)

A little confused on this one, as MMO says to add to your crankcase?


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## PontiacJim (Dec 29, 2012)

tallrandyb said:


> A little confused on this one, as MMO says to add to your crankcase?


You can add to the gas tank. They used to have the amount in ounces on the back which was to be added for "X" gallons. It can be added to the oil, but have never used it for that - just in the gas.

Right off the website:

"ADDED TO FUEL: At every fill-up add *4 ounces of Marvel Mystery Oil or Marvel Ultimate Oil to every 10 gallons *of diesel or gas. An average passenger car tank is 15 gallons, so you would add 6 ounces at every fill-up."


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## tallrandyb (Jun 12, 2021)

Yep interesting. They say you can add to every oil change and every tank of gas, so I’ll likely try both eventually then go back and run a leakdown again to see if I see improvement over time.

And congrats on 7,000 posts in 10 years, two a day just like vitamins but this forum helps more!


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