Internal Bleeding

Internal Bleeding

Bob Johnson, Pittsburgh, PA, 1968 Dodge Charger 440

Rick, thanks for providing all of the useful tech articles on –

I’m sure you are busy, but I was hoping you could point me in the right direction on an oil pressure issue I’ve got in my ’68 440. My motor does not tick, smoke or display any signs of wear and has been driven very little since it’s rebuild. The block is a ’71 440 block and I’m using’68 906 heads with a valve job, hardened seats and a comp-cams valve train with pro magnum roller rockers. The machine shop cut the crank ten thousandths, installed new main and cam bearings, decked the block and bored the cylinders .020.

Since day-one the motor has had low oil pressure, I initially thought it was just a bad gauge, but when I had them re-calibrated it showed the same thing- 20 PSI hot. A mechanical gauge confirmed this,showing actually closer to 10 or 15 PSI at hot idle. The motor builds great pressure as RPMs increase, but returns to very low pressure at idle.

I’m even running 20w-50 in a brand new motor to offset the low oil pressure! My brother later had a 440 machined at the same shop and his displays the same behavior. I have checked the pressure relief valve on the oil pump and even replaced the oil pump I first used on the motor (both Melling units). How can I have such low oil pressure at a hot idle only?

If you’ve ever seen a situation like that or have any advice on how to go about diagnosing the problem, I would greatly appreciate it. Thanks a lot, Bob



Simply put, there’s too much clearance somewhere. Most likely places are:main bearings, rod bearings, cam bearings, and / or valve lifters.

The numbers you quote are about typical when a shop builds an engine “on the loose side”. This would be about normal for an old-school “race” engine -about 0.003″ on the bearings will produce just about what you’ve got. (The theory used to be that this reduced friction and made more power. Now we know that this is more than offset by the increased windage losses from all that oil flying around, getting whipped up by the counterweights).

If the pressure builds rapidly above idle, this proves that the oil pump just can’t keep up with the demand at idle. At higher revs, say increasing from 750 to 1500, the pump’s output almost doubles, but the oil demands of the engine increase by only a small amount, so the pressure jumps up to the setting of the regulator valve.

What you have can be improved in any of three ways:

> > Disassemble the engine, measure all clearances, correct those that are too loose. I, personally, shoot for 0.002″ to 0.0025″ on hi-po street engines.

> > Install a high-volume oil pump.

> > Use a motor oil that thins out less when hot. (Read: 100% synthetic).

While I don’t especially like the idea of a newly rebuilt engine acting more like it has 200K on it, I think the engine(s) will live a long and normal life, however, if you do nothing at all. I would only temper this judgement if the engine(s) are frequently subjected to heavy loads at low RPM(lugging), which is only really possible with a manual transmission and an idiot behind the wheel.


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