C2 High Oil Pressure

[Donald T.]

Hey everyone. My oil gauge had been reading too high, so I finaly got around to hooking up an oil pressure test gauge on my 65 L79 today. Unfortunately it turns out that the gauge is not the problem. At start up my oil pressure is 75 psi at idle. It comes down to 65 psi at idle once up to normal operating temperature. I am currently running 15W-40, and the readings were not much different when I used 10W-40. I had already replaced the high volume oil pump with a standard unit, so that's not the culprit. The motor was rebuilt prior to my ownership, so I don't know what all was done. Any advice would be greatly appreciated.

Thanks!

[Duke W.]

Re: C2 High Oil Pressure

Pump volume is not the issue. It's the calibration of the pressure relief spring. You should check with the manufacturer to determine what the normal hot pressure should be at about 2000 or above.

I believe your engine engine originally had a 40-45 psi relief spring. The contemporaneous mechanical lifter SBs were 55-60. Up to late '63 they were 40-45 and raised to 55-60 through '65.

What is the maximum hot oil pressue you see above 2000 revs.

Corvette engines DO NOT NEED HIGH VOLUME OIL PUMPS!!! A high volume pump will provide more oil pressure at idle, but you only need 10-20 psi. The standard volume pump will provide full oil pressure on a fully warmed up engine by about 2000 RPM. At the point where the pressure relief valve opens the excess volume is shorted back to the pump inlet. High volume pumps just create more parasitic drag and heat up the oil.

The standard volume pump was used for both 40-45 and 55-60 psi engines and the only difference was the pressure relief valve, and the 55-60 psi engines may have needed a few hundred more revs to achieve maximum pressure.

The best advice to those who replace the oil pump is to just get a standard replacement pump from any reputable manufacturer with a pressure relief valve matched to the OE spec. You don't need a "NASCAR racing team approved" high volume (and high cost) oil pump from some hot rod parts vendor.

Duke

[Donald T.]

Re: C2 High Oil Pressure

Thanks Duke. The car had the high volume pump when I bought it. I replaced it with a melling standard volume unit. I got the pump from corvette central as correct for my application, so it should meet OE specs. The change dropped pressure by about 15 PSI, however pressure is still high as outlined in the original post. Reving the engine over 2k increased pressure to over 75 PSI.

After changing the pump and double checking the reading with a test gauge, I don't know what to do next.

Thanks!

[Tracy C.]

Re: C2 High Oil Pressure

Don,

I think you are lathered up about a non-issue here. Strong oil pressure isn't a bad thing. Low pressure is something to be concerned with but your situation is something that wouldn't bother me in the least.

tc

[Mike McKown]

Re: C2 High Oil Pressure

I believe Melling makes a standard pump, a high pressure pump which is the standard pump with the high pressure spring and a high volume pump. Three different part numbers.

This be the case, you may have the high pressure pump which can be easily converted by changing the relief spring in the pump.

The biggest problem I see is if you have the four quart oil pan, you stand a chance of sucking it dry at high rpm's with either the high pressure or the high volume oil pump.

[Donald T.]

Re: C2 High Oil Pressure

Thanks Mike and Tracy. When the motor was rebuilt, they put on the larger 6 qt. pan with the a melling high volume pump. I replaced the pan with the correct 5 qt. pan, and the standard volume pump. Is there something that the engine builder would have done to create such high pressure? The only thing I can think of would be incorrect bearing clearances, however I'm not an engine builder. When I've had the pan and valve covers off, everything looked okay - very clean, no sludge, no shavings, etc. Changing oil and filter several times has not had any affect. So I don't think it is clogged filter or oil galleys. Anything else I might be missing here? If the bearing clearances are too tight, is there potential for engine damage? I haven't seen any sudden drop in oil pressure to indicate running out of oil in the sump. I'm just want to make sure I don't do any engine damage to a very nice original motor, and it will also be an issue if I were to ever go for PV.

Don't mean to get paranoid on this; I do tend to get over protective of baby!

[Duke W.]

Re: C2 High Oil Pressure

As mentioned the current pump probably has a high pressure spring in the relief valve assembly. You could change to a standard spring or just buy a generic small block oil pump from NAPA.

Check with the manufacturer of the installed pump to verify the expected oil pressure for the part number you installed.

Duke

[Mike M.]

If its a melling pump

GM still has a spring part number 3814903. This is the standard presure spring. Its about $5.00. This spring will work in a melling pump. Mike

[Donald T.]

Re: If its a melling pump

Thanks Mike and Duke. Appreciate the assistance as always. I just sent an email to the melling technical support website, to inquire about their specs and recommendations.

[Mike M.]

Re: If its a melling pump

I tried email with them and they never got back to me so I called tech support.
Just letting you know. Mike

[Joe L.]

Re: If its a melling pump

Don-----

The problem is that you'll need to tell them the exact model number of the Melling pump that you have. And, if you know that, you don't need to contact them, at all----I can tell you. What you want is a Melling M-55. What you DON'T want is a Melling M-55A (the standard volume, high pressure pump) or a Melling M-55HV (the high volume, high pressure pump).

You can easily convert the M-55A, if that's what you have, to the standard pressure, M-55 configuration by installing the GM #3814903 pressure relief spring previously mentioned. If you're not sure which pump you have, install this spring and then you'll know which pump you have.

[Donald T.]

Re: If its a melling pump

Thanks Joe. The pump I installed is supposed to be the M55. The unit I removed had a tag with M55HV. I contacted tech support at melling and they were not much help. If he would have actually read my inquiry, he would have seen that I already checked the reading with a test gauge. Here's the response:

I would check the gage, because a M-55 pump is not capable of 65 psi at
idle.

Technical Manager
Cal Rydjord
2620 Saradan Drive
Jackson, MI 49202
Phone: 517-787-8172
E-mail: technical@melling.com

[Clem Z.]

melling pumps have a tag on one of the

cover bolts stamped with their part number. if it were me i would switch to some 5W-30 oil and see what you have