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1959 390 unleaded conversion

Started by Blade, September 25, 2020, 11:42:17 AM

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Blade

I am restoring the 390 in the '59 and decided to do the conversion to unleaded fuel. If anyone has already done this I am looking for what kind of valve seats to use and also if the valves need to be replaced too. Reading some posts from some other later model users they say they use their cars as daily drivers and never even did the conversion and even years later no burns on the valves. Regardless I do would like the conversion done.

Dave Shepherd

Hardened exhaust seats will do it. But it seems use of modern fuels has not been a common problem.

Cadman-iac

 Unless you are working your engine really hard for long periods, you really don't have to worry about burnt valves.
The seats are softer than the valves are, so they take the pounding and the wear. But a set of hardened valve seats for the exhaust valves would be the fix. The intake side doesn't get as hot and therefore doesn't wear like the exhaust side does.
I had an old GMC 305E V6 that I ran hard for a couple of years and the only thing that happened to it was that the two center exhaust valve seats had receded, (worn) from the heat. It had a single exhaust pipe after the two sides joined behind the engine, and so it didn't breathe that well.  It also wasn't a very efficient engine,  it was massive and had the spark plugs down in the inside of the "V" between the intake runners.
But even so,  after putting new seats in just the center cylinders, and installing dual exhaust,  I ran it another 100K without incident.
The only reason I even noticed the problem was because I had to keep adjusting the valves,  but it was backwards. The center exhaust valves had to be backed off each time. At first I thought it had "tuliped" the valves, not thinking about the seats. But once I had the heads off it was obvious what happened.
  So if you have new seats installed it should be fine.

Rick
CLC# 32373
1956 Coupe Deville A/C car "Norma Jean"

INTMD8

I'm sure you could have someone install hardened seats.

That being said, when I rebuilt my 59 390 the machine shop did a valve job, bowl blend, used valves and conical springs from an LS1 engine. 

Has been perfect for 7 years and 12000 miles. 

Not sure if it will become an issue but from the results so far I would do it the same if I could do it over again.

Big Fins

I wouldn't spend the money personally. Cadillac engines are so full of nickel they will never wear to the point where you would get valve leakage.
Current:
1976 Eldorado Convertible in Crystal Blue FireMist with white interior and top. (Misty Blue)
1969 Fleetwood Brougham in Chalice Gold FireMist with matching interior and top. (The Old Man) SOLD!

Past and much missed:
1977 Brougham de Elegance
1976 Eldorado Convertible
1972 Fleetwood Brougham
1971 Sedan de Ville
1970 de Ville Convertible
1969 Sedan de Ville
1959 Sedan deVille

Blade

Thank you everyone for the feed backs!