Small Block High Ol Pressure
I know this is been hashed over a few times on the discussion board, but I need a little help, please. I have a stock 66 small block L 79 that was rebuilt about 4000 miles ago by the last owner.it runs beautifully in all respects. However, the oil pressure gauge is always pegged at 60 psi except on hot idle when it drops down to 55 PSI. Based on some recommendations on this board I replaced the oil pump last year with a standard Melling M55 since I was under the assumption that the oil pump I was using had the high-pressure spring. After installing the new standard pump I still have the same high oil pressure. This morning I put the car up on the lift and pulled down the oil pan to remove the oil pump. This pump, which is the standard Melling M55 pump, has the yellow spring which I understand is the low pressure spring. Talking with Melling their yellow spring is rated at 58 PSI although I don't know under what conditions whether that is at WOT or idle. I would very much like to end up with a pump that allows me to have oil pressure that is not pegged all time. Since this car may be PV'ed in the future I assume that I will have to have normal oil pressure readings. I have confirmed the accuracy of my oil gauge, so that is not the problem. Is this higher oil pressure to be expected with the M55 pumpin its standard off-the-shelf configuration? And what can I do to bring the pressure down? Obviously monkeying with the spring will do it but it is so much work to change a spring that I'd like to get it right the first time. Can anyone help? thanks in advance, Bill Gould
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Re: Small Block High Ol Pressure
Bill-----
You could try installing the GM standard pressure spring, GM #3814903. However, I don't know if the Melling M55 is enough different than the GM pump (manufactured by Melling, or not) that it will not result in the same pressure when used with the M55.
Actually, I'm not a big fan of the Melling M55. I think I'd rather go with a GM #12555284. It's possible that this pump is now manufactured by Melling. However, that does not mean that it's the same as the M55. Melling makes pumps to different standards and design.
Tip: In the world of automotive parts there are two, basic classes of parts----OEM and aftermarket. The 12555284 is OEM; the M55 is aftermarket. Basically, OEM is a part of the quality supplied to automotive manufacturers for PRODUCTION. Aftermarket is generally a lower quality part that can be sold at a price considerably lower than OEM and will "last the remaining life of the car" much of which has been "used up" at the time the part is installed on the car.In Appreciation of John Hinckley -
Re: Small Block High Ol Pressure
Joe, Is there any way to determine what the output spec is for the GM #12555284? When Ii search on that number it is always described as a high volume pump. I know that high volume is not the same as high pressure but it makes me hesitate.Comment
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Re: Small Block High Ol Pressure
William------
I don't know the actual spec for output. However, I know this is a standard pressure, standard volume pump. It is NOT a high volume pump. This pump is the direct descendant of the GM #3764547 oil pump originally used for your application.In Appreciation of John HinckleyComment
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Re: Small Block High Ol Pressure
Joe, I just checked the Melling website. They say that the M55 is a replacement for the GM1255284. My guess is that it is a functional replacement but has higher pressure that the GM pump. I will call Melling in the morning. Thanks for your help. Bill GouldComment
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Re: Small Block High Ol Pressure
William, you may want to get a copy of the July 1965 issue of Chevrolet Service News (page 6) where the following "Service Tip" appears:
"HIGH ENGINE OIL PRESSURE
Corvette 327 cu in - 365-375 HP
The above 327 cu in high performance Corvette
eninge oil pressures in the area of 80-95 psi (at 2000
RPM) CAN BE EXPECTED AND ARE PERFECTLY NORMAL.
However damage may occur to this engine if the 283
cu in engine oil pump is used which delivers oil pressure
of 65 psi at 2,000 RPM.".Comment
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Re: Small Block High Ol Pressure
William I am having the exact same problem with my 67 L79 with a Melling M55 installed. However, I still need to check my oil pressure gauge to make sure it is accurate. I look forward to reading your post tomorrow.Comment
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Re: Small Block High Ol Pressure
The relief spring determines the highest pressure that the pump will deliver before dumping the excess pressure back into the inlet side. Your engine is developing too much pressure at all times. I would suspect that your oil viscosity is too high. What are you using. Or, your journal clearances are on the tight end of the range. Developing 55 psi at hot idle is not the fault of the pump.Comment
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Re: Small Block High Ol Pressure
Joe, I have tried 10W30 and 15W40 Shell Rotella and there is no appreciable difference in the oil pressure. Are you saying that a softer spring will not bring down the oil pressure?If that were not the case then why the different springs?Comment
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Re: Small Block High Ol Pressure
The higher the engine RPM, the faster the pump spins and the higher the pressure it supplies, given a pump in good condition. The "low pressure" spring relieves (dumps excess pressure) when the pump high side reaches 58 psig., and this limits the pump's output so that as RPM's increase further, the pump pressure is limited to 58 psig. The "high pressure" spring relieves at 70 psig., and this allows the output pressure to continue to rise (above the "low pressure" limit) as RPM's continue to climb. The higher pressures are needed to ensure that the crank journals are lubricated sufficiently in SHP engines which rev higher than lower horsepower engines.
The above explanation in no way affects pump operation at pressures lower than their relief spring limiting pressures.
Here's a list of the springs available from Melling:
Comment
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Re: Small Block High Ol Pressure
Joe, I spoke with George at the Melling technical hotline this morning. He said that he has this question come up for five times a year mainly from people like us who are concerned about having their oil pressure register on a stock gauge for PV requirements. He said that the old GM oil pumps were not very efficient so they did not put out much oil pressure. George said that the current Melling pumps are much more efficient and therefore do a better job of providing oil pressure to the engine. He is going to send me a green spring which is a 49 PSI spring and is their weakest spring. But as you also said, he is not sure if this is going to help me at the low-end idle if my engine is tight. theoretically it should limit my oil pressure to 49 PSI. Joe and others, thank you for your help. Bill GouldComment
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Re: Small Block High Ol Pressure
The spring won't help you on a PV, if they are looking for oil pressure at idle/low RPM. Normal hot, idle oil pressure range should be around 15-40 psig. I would NOT install the lower pressure relief spring unless you don't plan on revving your engine past about 5000 RPM. A good rule of thumb is to have about 10 psig of oil pressure for each 1000 RPM engine revs.
The "line" that the tech gave you about pump efficiency is possible, although not plausible. If I were you, I'd verify AGAIN, with a brand new gauge plumbed DIRECTLY into the tap on the rear of the block. Make sure the test gauge puts your target oil pressure in the middle of its range. There is no danger of engine damage with the higher than normal oil pressure, although the oil pump is working harder than it should be, increasing parasitic losses. You might try a light oil, like 5W-20 during the PV to temporarily drop the oil pressure. For normal use, continue with the Rotella 10W-30. The best way to verify what you have, is to try to find the journal clearances that were used when the engine was rebuilt. You can try using Plastigage while the pan is still off. Best bet is to talk to the engine builder and/or machine shop. A good engine builder will keep a journal of dimensions for all of his jobs.Comment
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Re: Small Block High Ol Pressure
Bill,
I also have encountered the higher oil pressure with the Melling M-55 pump. You could try to locate a cover from a original used GM pump and plain the surface on a flat table with 220 wet paper than install a new GM 45 PSI relief spring. You could also try to locate a new GM pump standard pressure/standard volume pump on ebay. The L-79 is supposed to have a 45psi relief like the 300hp engine.
If the motor makes 55psi pressure at hot idle than it's going to peg the GM pump relief spring and will show 45 psi all the time. With the M-55 in my 67 300hp engine the hot idle oil pressure is aprox 30-35 psi and at aprox 1500rpm it's pushing 55-60 psi, I don't like the high pressure but I think it's a very nice oil pump. At 55psi hot idle oil pressure, that is a really tight motor, I would be curious what the rod/main bearing clearances are. You could always remove a main cap and check the bearing size to see if oversize bearings are present, .0005 to .002 can be used to tighten the clearances and this could also cause such high idle pressure.Comment
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Re: Small Block High Ol Pressure
What a load of bulls...!
Aftermarket pumps are the same design as the OE pump with only minor geometry differences of the pump housing, which actually weakened it, structurally.
The problem is that Melling DOES NOT supply a proper OE replacement relief spring for most OE engines. Shame on them! The vast majority of small blocks including your L79 are speced to have hot oil pressure of 40-45 psi at 2000 RPM, and millions of these engines lived long lives with no oil related problems in passenger cars, trucks, and even racing. Thottle position has no effect on oil pressure. It's a matter of the pump's basic flow rate, engine RPM, relief spring rating, and oil temperature, and there is usually no difference in hot oil pressure between 10W-30 and 20W-50 or anything in between.
GM does not specifiy idle oil pressure, but on a fully warmed up L79 it's typically about 30 psi at 750.
Melling's 49 psi spring is still too high. If you talk to them again ask them why they can't supply a proper OE replacement 40-45 psi spring. In the mean time if the proper spring is still available from GM, either buy it from GM or see if you can cross reference it on the NAPA or other online catalog.
This issue has been discussed numerous times, and I think others have posted spring numbers and their corresponding hot oil pressure as observed on the in-car gage.
DukeComment
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Re: Small Block High Ol Pressure
Duke, Thanks for your assessment. I will see if I can find a proper spring.Comment
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