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Old 12-11-2005, 06:38 PM
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Oil Pressure Drop at Idle

My friend, a very experienced hotrodder who has built several very high hp engines over the years going back to 1960, recently built a Merlin 540" engine.

He first fired it up about 8 weeks ago and all seemed good. Oil pressure was at about 50lbs. He first drove the car about 2 weeks ago and pressure during the 5 mile ride was at about 50 lbs but when he slowed down to turn the corner the red light flickered and pressure had dropped to about 20 lbs.

He increased rpms to about 2k and pressure went back up to normal.

He's running a Crane solid roller, Brodix heads and a high volume oil pump. The short block was put together with extreme attention to detail. Crane says not to do anything to restrict oil flow and consequently increase pressure. He can't for the life of him figure out why this drop in pressure is happening or what he can do about it.

He's figuring on tearing the car down for more mods this winter and of course wants to fix this oil pressure problem. He asked me to check with you folks in hopes that you might have some ideas of what he should be looking for as he tears the engine down.

Any ideas as to why this would occur? What clues should he be looking for? Any ideas I can pass on to him?

Thanks, Jim

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Old 12-11-2005, 06:43 PM
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Sometimes a high volume oil pump will empty the pan before the oil can drain back into it. They usually recomend a extra capacity oil pan with the high volume oil pump. Did he put on a bigger oil pan? If not, then maybe that is what he needs.
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Old 12-11-2005, 09:06 PM
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I agree, please post more details about the engine and the mods done to it.
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Old 12-11-2005, 09:46 PM
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How far is the pickup off the bottom of the pan?
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Old 12-11-2005, 10:08 PM
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Has he cut an oil filter open to check for bearing material thats always a good start to see if there is a deeper problem and whether he should not even fire it up anymore
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Old 12-11-2005, 10:08 PM
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I dont see a problem....If it is going down to 20 at idle, that is pretty normal. When you speed it up, it goes up. The rule of thumb is 10#/1000rpm, so you should be good.

It's not draining the pan because that would cause the pressure to drop when you rev it.
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Old 12-12-2005, 12:13 AM
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I'd go with the aforementioned filter inspection. Is he running a vacuum pump? The Vaccum pump can act to suspend oil in the top end if a few precautions aren't taken to let it vent back down. Does he have the fine mess screens epoxied into the oil returns on the heads? What type of oil is he using? Merlins have priority main oiling so he is probably ok. What type of idiot light is he using? What pressure will it "set" the light?
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Old 12-12-2005, 07:13 AM
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Quote:
Originally Posted by poncho62
I dont see a problem....If it is going down to 20 at idle, that is pretty normal. When you speed it up, it goes up. The rule of thumb is 10#/1000rpm, so you should be good.

It's not draining the pan because that would cause the pressure to drop when you rev it.
That is EXACTLY what we wanted to hear!

Still will be checking it out to make sure. Thanks, Jim
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Old 12-12-2005, 07:26 AM
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Quote:
Originally Posted by 56Maynard
I'd go with the aforementioned filter inspection. Is he running a vacuum pump? The Vaccum pump can act to suspend oil in the top end if a few precautions aren't taken to let it vent back down. Does he have the fine mess screens epoxied into the oil returns on the heads? What type of oil is he using? Merlins have priority main oiling so he is probably ok. What type of idiot light is he using? What pressure will it "set" the light?
No vacumn pump. Has the mesh screens epoxied in place. He's using 20-50 oil. Auto meter idiot light set to scare the hell out of ya at 20 lbs.

The corner he was taking was very slow, lots of oil, Hamburger extra capacity pan, lots of good baffles. The pickup is set very closely to the bottom... Hamburger set up.

Maybe it's not a problem... like poncho 62 says (my favorite car from my past was a 62 Grand Prix with the 8 lug finned wheels!). Scary deal to have that light flickering ... but that's what it's there for I guess.

He hasn't cut open the filter yet but plans to do that.
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Old 12-12-2005, 07:33 AM
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One thing I have noticed is that if the dizzy is retarded too much, the oil presure will suffer. I would just double check that as a precautionary meassure, but 20psi at idle is not bad. I personally don't like lights, I prefer the gage.
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Old 12-12-2005, 07:50 AM
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That is a classic sign of large bearing clearance.

What are the mains and rods clearance???

FYI, a BBC needs to be tighter than a sbc.

Pull the engine and use plastigage on all the bearings. Use +0.001" bearings if needed.

I like to use 0.002" to 0.0025" on the mains and rods, 0.003" to 0.0035" on the rear main. Call World Products tech line and get their recommended bearing clearance for a 540.
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Old 12-12-2005, 09:22 AM
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454c10-You are correct. I kind of forgot all about that.
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Old 12-12-2005, 10:02 PM
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Dont only use plastigage also measure journal dia. and bearing thickness also the block and rods and see if there is excessive ovality
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Old 12-12-2005, 10:08 PM
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As the engine breaks in, oil pressure is probably going to drop slightly. I wouldn't be worried as long as the pressure that is there holds.

Like was said, a general rule of 10lbs for 1000RPM is okay. Id start to worry if you see pressure dropping at higher RPMS.

If you dont find any particles in the oil filter, Id be hesitant to pull the engine and tear it apart for inspection. I know better safe than sorry, but at this point, I still think its early to freak .

When stopping the oil is thrown towards the front of the oil pan (away from the pickup). Its possible he stopped a bit harder than he thought and the oil pan was empty enough that even the baffle didnt contain enough for that very brief second.
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Old 12-17-2005, 07:09 PM
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