I've recently started logging my C5's oil pressure in HP Tuners, and noticed it dropping with increasing engine load. At WOT, it usually seems to be 5-10 psi lower than it would be with light or no load (e.g., its 70 psi at redline when free-revving, but 60ish under load). I haven't yet gotten an alternate oil pressure gauge up to confirm what the stock sender is reading, but I have no reason to believe the stock unit is inaccurate. You can't really see it in the stock gauge unless you pay close attention, because the C5 gauge reads more of a rolling average (i.e., its damped) than the current oil pressure.

This is on a '99 LS1 with 60k miles, AFR heads and a stock bottom end except for Katech rod bolts, Clevite rod bearings and a LS6 pump. The oil is German Castrol 0w30 (a heavy 30-weight). Oil pressure at hot (190F) 850 RPM idle is 33 psi, cold (85F) is 40 psi, and the car is equipped with a Setrab oil cooler (-10 AN lines, should lower the oil pressure somewhat).

I know, I should have checked the mains when I was doing rod bolts, but I thought mains lasted forever in comparison to the rods, and it always held good oil pressure. The previous owner really screwed up this engine in a number of ways; lets just say when I tore it down there was a hole in the front timing cover.

I'm told this pressure drop is indicative of worn crank main bearings, which allow the crank to be pushed down under load, increasing clearances and decreasing oil pressure. As I see it I have a number of options:

1) Just keep running it, keeping an eye on oil pressure and maybe doing some oil analysis to make sure the bearings aren't chewing themselves up.

2) #1, except run some 40 weight oil.

3) Pull engine and replace rod, main and cam bearings. This is cheap but not easy.

4) Get a new shortblock. This seems kind of drastic for an engine that is otherwise running fine. It makes decent power, doesn't use much if any oil and doesn't have piston slap, so I'm hesitant to want to mess with anything but the bottom end bearings.

5) ...could some drop in pressure be normal in these engines? I'm told the block itself flexes significantly, increasing clearances and thus dropping pressure. Its holding good oil pressure otherwise, so that makes me wonder.

The kicker is that the car is used in track events, and so sees a lot of WOT.

Thanks for any help.

 

i believe a slight oil pressure drops normal because at higher speeds the oil pumps spinning faster, but the oil dosent have the same amount of time to drain back down into the pan as it does when your idleing, or slightly off idle...i would think if the main bearings were worn enough to affect oil pressure, i would think there would be some noise at initial startup, as well as maybe some conrod slap...anyway, sounds like you have all the possible solutions figured out...so goo luck and keep us posted!

 

Unfortunately, the oil pressure drops with load. It raises with RPM normally. For example, cruising at 3000 RPM I might be getting 50 psi. If I left-foot brake and give it some gas while maintaining the same RPM, the oil pressure would drop to 45 psi or so.

 

Bearings. BUT! You can still drive it hard for a long time and not have to worry. I'd go to 10W40 and just drive and not worry about it. If you are still getting 60PSI under full acceleration then all is good. On my original motor, I had the same scenario. It would drop 5 PSI as I stepped on the gas under load and come back as I would let off. I still ran it hard for 2 years before pulling it apart because I wanted to build a stroker at which point the bearing didn't look bad and the crank was mint.

 

I'm not so sure I would worry too much. ANY engine loses a bit of pressure under load. Diesel trucks cars VW air-cooled...any I've ever seen.

I just rebuilt an 01 LS1 Vette motor for my Volvo. The thing had only 20,000 miles on it. I started checking clearances and they were like .004 on the mains and .0035 on the rods. There was NO wear on the journals at all. I ended up using -.001 main and rod bearings. They were still loose according to GM specs.

|If you do rebuild it you might end up with -1 coated bearings.

Even though GM shows tight clearances in the book it doesn't mean they let them go out the door like that.

Also be careful of a huge oil pump. Costs horsepower to run and drains the sump like NOW!

 

I'm now running Redline 5w40 and getting more than 60 psi at redline. The lifters also seem quieter. I'll do some oil analysis before and after my next track event, and report back on the results.

That combined with high-gee cornering is why I was wary of higher-flowing pumps, and opted for the LS6 pump.

Incidentally, everyone I've asked had never heard of a car loosing oil pressure under acceleration. Maybe they just don't notice? Only road racers seem to regard straights as being places to relax and check gauges, and its hard to really spot it unless you're logging oil pressure in your tuning software.

 

Mine doesn't

 

I have minimal fluctuation between free revving oil pressure and oil pressure under a load. If I were you, I wouldn't worry about it. You have plenty of oil pressure. If there were a 10 psi drop from idle, I'd be worried. Temperatures play a big part in the oils viscosity. If you are really worried about it, put an oil cooler on it.

 

^^^^^^^ Ban this troll