Posted by MoparNorm on Saturday, January 20, 2007 at 10:19:42 :
In Reply to: Can unleaded be used in stock 318 poly? posted by Glen Baldwin on Saturday, January 20, 2007 at 00:11:44 :
The compression ratio for a factory spec. varies depending upon type:
318-1,2 Poly, with a manual transmission is 120-160 psi.
318-3 Poly, with a manual trans. is 90-120 psi.
Poly's with automatic trans. the pressure should be 110-140 psi.
Max. variation between cylinders 20 psi.
So, if you have an automatic, you're in good shape, if you have a 318-3, you're in excellent shape and if you have a 318-1,2 with a manual trans, you have a motor that is wearing well but getting near the end of a service cycle and may/will need rings in the forseeable future. If it runs well, don't worry, but look for the tale-tale signs of smoke and oil compsumtion.
Now, back to your first question, valves. Polys, with original equipment, use stelite faced valves with valve rotators, these two items in combination will help the exhaust valves live when using unleaded fuel. Many variables effect valve wear, such as weight, load, altitude, speed etc. Assuming a good condition valve train, you should get at least as much as 50,000 miles without exhaust valve damage.
When I rebuilt my Poly I did not install valve seats, I'm going for the 50,000+ miles theory and here is why. To install valve seats the head must be machined and the seats inserted into the head. The unique nature of the Poly head is the secret to it's power and torque. Looking at a Poly head you will see that the chamber is a near perfect hemisphere, however with a large "squelch" area and valves that take up just about every useable area of that combustion chamber, in short, a Poly is a HEMI with one row of rockers removed. Using only one row of rockers in a HEMI design results in "canted" valves (truth be known, the new generation "Hemi" is really a Poly) I did not want to ruin that combustion chamber efficientcy by cutting it up to insert seats and changing the efficient nature of the stock design. I installed new exhaust valves with stelite surfaces and made sure that the rotators were in good working order (new) I expect to get at least 50,000 more mile out of that motor.
Now about additives, undoubtedly someone will talk about lead additives to cool the valves. NO! Lead is a banned substance, there is no such thing as a legal "lead additive" there are however "lead substitutes" and in my opinion they are a gimic and a huge waste of money. Save what you might spend on additives and put it away in a savings account for a quality motor rebuild in a couple of years.
MN
(the other Poly information source!)
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