Idle Oil Pressure

[William G.]

Greetings,
I've searched the archives for oil pressure at idle for Corvette 327 engines. It is common to find concerns about LOW idle pressure, but my issue is the reverse. That is, at reasonable rpms (2000-3000) my 66 300 hp shows normal pressure of 35-40 psi, however at idle it "drops" to about 30-35 psi. Is this a sticky gauge which needs attention, or a partially plugged line, or..........something else? Idle rpm 500-650. I've never seen this before. Oh, when I switch off the engine the gauge immediately drops to zero as it should. Ideas please. And 15W-40 oil... the old CL-4, I believe it is. I'm not GREATLY concerned about this but it would be nice to know what the idle pressure actually is and more importantly what the cause is. Thanks again for all the help.

Bill

[Joe L.]

Re: Idle Oil Pressure

Greetings,
I've searched the archives for oil pressure at idle for Corvette 327 engines. It is common to find concerns about LOW idle pressure, but my issue is the reverse. That is, at reasonable rpms (2000-3000) my 66 300 hp shows normal pressure of 35-40 psi, however at idle it "drops" to about 30-35 psi. Is this a sticky gauge which needs attention, or a partially plugged line, or..........something else? Idle rpm 500-650. I've never seen this before. Oh, when I switch off the engine the gauge immediately drops to zero as it should. Ideas please. And 15W-40 oil... the old CL-4, I believe it is. I'm not GREATLY concerned about this but it would be nice to know what the idle pressure actually is and more importantly what the cause is. Thanks again for all the help.

Bill

Bill------


In the case of ANY indicated oil pressure issue, I always recommend the first step is to install a test gauge and determine what the oil pressure actually is under various operating conditions. Actually, I don't think you have a problem, but checking things out with a test gauge is fairly simple.

[Duke W.]

Re: Idle Oil Pressure

GM never speced idle oil pressure for these engines, but fully warmed up at about 2000 revs they should be 40-45 psi except late '63 to '65 mechanical lifter engines, which are speced at 55-60 at 2000 due to a higher pressure relief spring in the standard volume pump, and they should have 80 psi gages.

Idle oil pressure is dependent on several variables including idle speed, oil temperature, and oil viscosity rating.

Engines barely need any feed pressure at idle because the bearing loads are very low, so anything over 5 psi is probably okay on a mild engine that idles at about 500. SHP/FI engines generate more idle oil pressure than base engines merely because they require higher idle speeds.

As Joe said, checking pressure with a known good test gage is the best course of action. The Bourdon tube gages installed in vintage Corvettes are very simple and reliable, but you never know after over 40 years.

It's possible you have a high volume oil pump installed with a 40-45 psi spring. Many clueless "engine builders" install the high volume pumps, which are delivered with about 60 or greater psi springs, on base engines. Then the gage is pegged at 60 most of the time. The proper fix is to install and standard volume OE replacement pump with a 40-45 pound relief spring, but a "quick fix" is to just replace the 60 psi spring with the 40-45 psi spring. A high volume pump will generate higher oil pressure at idle regardless of the installed relief spring.

Duke

[William G.]

Re: Idle Oil Pressure

Good Afternoon.

I'm not trying to be a s***t a**, Joe and Duke, but I think I already know what you gentlemen are telling me. However, if I remove the copper fitting at the back of the engine and install a "new" gauge in place and then get a, say, ten psi reading I'm still not going to know whether the problem is in the gauge or the copper tube from the engine to the gauge.

And I suspect I'm not alone in this.....but I'm pushing 70 years of age and my back protests mightly when I try to crawl under the dash and if I make it then the rest of my body objects to trying to reach up under the dash to disconnect the copper tubing at the back of the gauge. I was hoping it was easier than that. Can I use another location to attach a known good gauge? How about the pipe plug at the front of the engine above the water pump and adjacent to the numbers pad? Course if I get ten psi there I've still got to deal with the dash issue...but I'd have learned a little anyway.

It seems as though this is an unusual problem and I was hoping that others had seen it. Joe seems to suggest it may not be a problem....but 30 psi at idle seems to me quite unusual.

Thanks again.

Bill

[Joe C.]

Re: Idle Oil Pressure

Good Afternoon.

I'm not trying to be a s***t a**, Joe and Duke, but I think I already know what you gentlemen are telling me. However, if I remove the copper fitting at the back of the engine and install a "new" gauge in place and then get a, say, ten psi reading I'm still not going to know whether the problem is in the gauge or the copper tube from the engine to the gauge.

And I suspect I'm not alone in this.....but I'm pushing 70 years of age and my back protests mightly when I try to crawl under the dash and if I make it then the rest of my body objects to trying to reach up under the dash to disconnect the copper tubing at the back of the gauge. I was hoping it was easier than that. Can I use another location to attach a known good gauge? How about the pipe plug at the front of the engine above the water pump and adjacent to the numbers pad? Course if I get ten psi there I've still got to deal with the dash issue...but I'd have learned a little anyway.

It seems as though this is an unusual problem and I was hoping that others had seen it. Joe seems to suggest it may not be a problem....but 30 psi at idle seems to me quite unusual.

Thanks again.
Bill

Tee in a second gauge at the existing location for test purposes. Yes, you can also use the 1/8" NPT tap into the front of the block.

If, when you shut it off, the indicated pressure drops immediately to zero, that's an indication that the tubing is not obstructed.

One more reason for low oil pressure (which is not, I understand, your situation), which was not mentioned, is wide bearing clearances.

[Jim T.]

Re: Idle Oil Pressure

Greetings,
I've searched the archives for oil pressure at idle for Corvette 327 engines. It is common to find concerns about LOW idle pressure, but my issue is the reverse. That is, at reasonable rpms (2000-3000) my 66 300 hp shows normal pressure of 35-40 psi, however at idle it "drops" to about 30-35 psi. Is this a sticky gauge which needs attention, or a partially plugged line, or..........something else? Idle rpm 500-650. I've never seen this before. Oh, when I switch off the engine the gauge immediately drops to zero as it should. Ideas please. And 15W-40 oil... the old CL-4, I believe it is. I'm not GREATLY concerned about this but it would be nice to know what the idle pressure actually is and more importantly what the cause is. Thanks again for all the help.

Bill

Bill my 68 327/350 L79 is only a couple years newer than your 66. After reading your post I later drove my 68 10 miles coming home. The air temp outside was about 65 and most of the trip was on highway or interstate so the operating temperature was normal. Idle oil pressure was not centered on the 35 mark, but just a little to the left of center 35 mark and was a steady reading at the left margin of the mark. Have 10W30 oil in it, Rotella. Driving the interstate at 70 mph the oil pressure was above 35 but not at the next mark on the oil pressure guage. Original factory engine, 96,550 miles.

[Timothy B.]

Re: Idle Oil Pressure

William,

Read my post from a few days ago about the Melling M-55 oil pump on my 67 300hp engine. I think the biggest factor when talking about hot idle and 2000 + rpm pressure is the bearing clearances and relief spring.

My engine carries approx 30-35 at hot idle and pegs the 60 lb. gauge at 3000 rpm. It takes horsepower to drive a pump like this so I am not happy with the excess pressure. After talking to the Melling teck rep I am informed the relief spring in there standard pump is 55-60 and probably closer to 60 he told me.

If the main and rod clearances are in the .002 range, I suspect the engine will see higher pressure than the .002-.0035 some factory engines are built with.

I don't understand what you are worried about, I think your engine is fine. My 63 300hp engine sees about 23 lbs at hot idle and acts as yours at higher rpm, I like this better than the other..

[Stuart F.]

Re: Idle Oil Pressure

Bill;

I'm 71 and still going pretty strong regardless of my arthritis. But what has helped me for nearly the past 20 years was to leave my oil line disconnected and plugged at the connection to the block due to a gauge leak (dripped a drop or so on my E-brake handle). I have since (about 1 year ago) re-connected it by t******* the line by the gauge and using a new fitting - still leaks. Mine idles at 35 to 40 lbs, and gets up to about 80+ on the highway. It doesn't bother me anymore cause I seldom look at it due to my 20 years of training myself that nothing is wrong. I have another gauge to put in, but my back says to leave that to my heirs. In the mean time, I'm going to drive it to my chicken point (redline)and forget it.

Stu Fox
63 L-76

[William G.]

Re: Idle Oil Pressure

Thanks for the replies.....and the last few are enlightening....or encouraging, not sure which. But to add to this and further explain my concern.......I don't believe this engine has ever been apart(don't get me wrong, I can't PROVE this) and hence any idea that it has a high output Melling pump or tight bearing clearances is out the window.

Now I know how unusual it is for a forty plus year old engine to never have been apart.........but quite a few things do point in that direction. To wit: -no exhaust manifold gaskets
-factory style intake gaskets
-french locks still on the exhaust manifolds
-factory style self locking nuts on the exhaust manifold flanges
-all original head markings on the engine fasteners
-no apparent "buggering up" of fastener heads as usually
occurs when attempting to remove things
-clearly visible "HE" hand lettered in yellow on the front of the
right head
Now none of that taken individually proves anything, but all taken together............well.

To me an engine of that vintage carrying 30 psi oil pressure at idle would be a little unusual!

Note to Jim Trekell: Small world. Up until about four years ago I owned a '68 350 hp L79. (I can't exactly recall the idle pressure on that rebuilt engine but I think it was over ten psi but certainly not 30. I'm not questioning your observation.....just noting what I think I recall mine as having been.

The oil is my present 300 hp 327 is 15W-40 and a CI-4 plus classification.....for what that is worth. I bought this base engine '66 car this summer and the oil in the crankcase at that time (to the extent that oil is a factor....not much on a hot engine, I would guess) I have no knowledge of but the engine idled then also with a gauge reading of 30 plus psi.

So am I still concerned? Well, yeah, somewhat. I need to install a known good gauge and check things, but winter is fast settling in on us so that may not happen. It would be good to be able to report what another gauge shows but our roads are starting to get a little slick.

Thanks again.

Bill

[Duke W.]

Re: Idle Oil Pressure

There's really no need to be concerned about your oil pressure.

And there's an easy way, combined with your other evidence, to pretty much prove that your engine has never been torn down.

Measure the head thickness using feeler gages at the corners of the engine where the head mates to the block.

If it's .018" that pretty much proves that it's never been torn down since it left Flint. The GMPD replacement gasket was .026", and virtually all rebuilders use a thick composition gasket.

In the current issue of The Corvette Restorer I mentioned something like "all indications were that the engine had never been opened up" when discussing John McRae's '67 300 HP engine that was rebuilt to Special 300 HP specification. One of those indicators, and really the key indicator, was that it had the correct part number .018" head gasket that Flint used in that era. It also had the correct OE pistons and it had never been bored.

Let us know what you find.

You're using the best oil money can buy for a vintage engine, so the engine will probably last a few more decades.

Duke

[William G.]

Re: Idle Oil Pressure

Duke,

Did your feeler gauge check a few minutes ago. I found the best spot to insert a feeler gauge to be about 1/2 to 3/4" outboard of the edge of the number pad. I could insert an .011" or .012" feeler in about 3/16" max. before contacting the head gasket (other places I tried on both heads I really couldn't insert that thin a feeler much at all). I then tried a .020" and a .022" feeler at the same spot (and again tried the left head also) adjacent to the pad with no luck--couldn't slide the feeler in at all. I guess that pretty much nails it along with the other evidence mentioned earlier. Thanks for the advice.

This is a Bowtie car and it is great to confirm that it's never been opened up. It has a 1970 350 hp distributor in it and a service replacement List 3367 Holley--other than that the engine appears completely as it left the factory--down to the original water pump, fuel pump, alternator, etc. Too bad on the distributor and I'm going to try and get the correct "fire plug" for it. Think I'll leave the carburetor as is. The dist. number is 11112021 with what appears to be a January 1970 date--and this dist. has the small hole drilled opposite the tach gear whereas as I understand it the correct '66 distributor would not.

Regards
Bill

Thanks again to all who responded.

[Stuart F.]

Re: Idle Oil Pressure

I believe I went through the same or similar excercise on my 63 L-76 about a year or so ago to verify whether my engine used double head gaskets, which Duke advised was done on L-76 production engines. I would have to check my notes as to the exact gauge thicknesses in question, but mine did not indicate it had double gaskets. This would have meant that the actual compression ratio was about a point or 3/4 less than advertised/published. My engine too is mostly original and I know for a fact the heads have never been off as I have owned it since new. So, the jury is still out on that for me. Certainly, I would like to believe I have 11.25 to 1 compression ratio, but I would be more concerned if I thought someone had torn my engine down for what ever reason without my knowledge. The car was in storage with a close friend while I was in SEA, but I can't believe it was touched as there's no other evidence of that happening.

Stu Fox

[Dan D.]

Re: Idle Oil Pressure

I would be more concerned about rod failure on these original engines. I guess it is crap shoot as to leave it alone and original and take a chance on loosing the block if a rod or rod bolt breaks, vs tearing it down, loosing the originality but insuring it is now bullet proof. GM did specify dual head gaskets for the 11:25 piston engines back then, but they were the thin metal gaskets. -Dan-

[Stuart F.]

Re: Idle Oil Pressure

Bill and Dan;

I checked my notes when I did the feeler gauge check on my L-76. I too could not get a .015" in the area by the pad. I show in my notes that the original gaskets were supposed to be .018", so I assume a total of .036" when 2 were installed (before crush). I think I have more of the story, but have not located it yet.

Anyone else have some input on this?

Stu Fox

[Duke W.]

Re: Idle Oil Pressure

Gasket thickness specifications are nearly always compressed. The .018" shim gasket used by Flint in the sixties is the compressed thickness. And figure a tolerance of about +/- .001".

If you couldn't insert a .015" feeler gage between the block and head, you may have made the measurement incorrectly.

Also, the OE gaskets have and identifier you can see at the corners - a round hole.

Chevrolet offered a .017" stainless steel gasket (listed in the Chevrolet Power Manuals of the seventies) and it has a square hole in the corner than you can see.

There was a thread on this a few months ago.

The double gasketing began as a 1962 model year running change on 340/360 HP engines due to customer detonation complaints according to a TSB, and I believe it ran through the 1963 model year.

It may not apply to 1964. The additional 2 cc chamber volume from the 2.02" valve unshrouding cut and later closing inlet valve of the 30-30 cam reduced the propensity to detonate. The additional chamber volume is why the '64 SHP/FI engines are advertised with 11.0:1 CR rather than 11.25:1 for the '63 SHP/FI engines (with a single head gasket)

In any event, typical deck heights higher than blueprint means most engines left Flint with about 1/2 point lower compressen than advertised, with a single gasket, and a second head gasket knocks off about another half point.

Duke