I think Steve should continue his testing with bigger clearances until it does make a difference. Inquiring minds want to know.
I also agree pumps take the blame for worn out / poor clearances in the motor. And just as the pump clearances grow, so do the bearing clearances and locally they could be much hotter. The other thing to understand is the oil passages are effectively much small with thick cold oil and much larger with hot thin oil. This is because the oil at the walls doesn't really flow and the oil in the very center moves the fastest. As the oil heats up this only changes in the fact the oil becomes thinner and the "boundary layer" at the walls is greatly reduced but "effective cross section of flow" is much greater and requires much less pressure to accomplish and reduces the back pressure required.
I wonder if the issue could be resolved if the driven pump gear was made with aluminum that expands at the same rate as the cover? What do you guys think?
Steve, Please remind us of the purpose of this investigation? The pumps are relatively efficient and sufficient, on a well built engine? Is this a 350 thing vs. a 455 thing? Honestly, I haven't re-read the entire thread but I love the discussion. Is this "What is the cause of Hot Engine oil pressure loss", "The impact of pump dimensional changes on engine oil pressure" or "What's important in a better pump system" ? Cheers, Mike
I wish we could use the same oil pump the 3800 uses. My Riviera SC3800 engine has nearly 200K miles on it. It uses 10W/30 Mobil 1 oil. Fully hot off the highway, the lowest I have ever seen it at idle in gear is about 37 psi. It is 60+ psi at anything over 1000 RPM.
Ok. Back to viscosity or aeration or better yet poor design. I really don't understand this move from straight grades Do the race cars run less than fully warmed up
I have an extra timing cover with gears. I think I will bake it in the oven and take measurements for proof of concept. Lots of engines have Aluminum oil pump housings and make perfectly good oil pressure. My background is in AMC engines. The early sixes used an iron oil pump housing and the later engines used an Aluminum oil pump housing. The rest of the engine was unchanged. They two pumps make the same pressure.
I've got 40 years experience in industrial pump applications in pulp and paper and powerplants. I've seen wore out pumps with excessive pump clearances have reduced or no pumping at all. I remember an old millwright that would adjust an impeller or wear plate on a running pump until it made a slight noise to restore pumping with very little clearance. Basically a slight touching.
The original intent of this post was to inform people of what I was observing during bench testing of rebuilt cover/oil pumps. What I was finding raised more questions. Since I've built a fixture lets see what else we can learn. Now that being said I baked a cover and got before and after measurements. Temps were 60 degree cold and 230 hot. There were no gears installed, I measured the width across the driven gear pocket using a bore gauge and micrometer. I used a digital caliper to measure depth. The depth of the pocket was .871 both cold and hot. Across the pocket was 1.675 cold and 1.6775 hot. A difference of .0025". Now keep in mind there was no cover bolted to the end to help hold the cavity stable if it even would. To answer the question, is air being drawn in from the pump shaft. No. Oil is actually pushing up thru the shaft and bore. Just a trickle. I firmly believe the oil pumps get a bad rap from other problems within the engine. Mainly too much bearing clearance. Yes pump cavities can wear but from what I've seen with end clearance changes the wear would need to be ridiculously large. I'm also calling myth busted on the cover expanding enough to make any measurable difference. This leaves us with the cause of hot oil pressure drop at idle being caused by hot oil, oil chemistry change since 1970, or aeration. If an oil temp sensor would be installed someone could determine if it's due to heat. Then maybe an oil cooler would correct it. I just don't see the temp. changing more than 20 degrees from Hwy. to city. Could that be enough to make a difference? Aeration could be checked if anyone was running a dry sump system on the street. 2 things here worth noting. All the oils I tried would make some foam when I installed a dry oil filter. It took time to work the air out of the media. When I reused a filter with different oil this did not happen. I will from now on pour some oil into new filters when doing oil changes to saturate the media. Don't have to fill it, just wet the media. Also, the synthetic oil always made foam. It was thinner (0w20) so maybe it was cavitating. But it never stopped. Let it sit over night and try it again and foam. As for oil chemistry change, I'm no chemist but they are out there. Larry, I will add some gaskets to see at what point pressure starts to drop. I may still try to heat the oil if I can come up with a hot plate maybe.
I'd like to hear about the chemistry change in oil, not just about the ZDDP. No flat tappet cams anymore, much less ZDDP, ok fine, we understand that. What about engine rpm? Newer vehicles turn much less rpm than earlier vehicles due to overdrive. Do newer formulated oils have less anti-foaming agents in them? Crank driven oil pumps turn twice as fast as our cam driven pumps, plus they're gerotor design verses our spur gear type, are they more efficient? Im going with the aeriation theory
Per Larry's request today I increased the end clearance to see what would happen. Tests were done back to back within 1 hour. 65 degree oil temp. With .006 clearance the reading was 52.5 PSI With .018 clearance the reading was 48 PSI With .0025 clearance the reading was 54 PSI Going from .0025 to .006 only dropped 1.5 PSI. Amazingly jumping up to .018 it only lost an additional 4.5 PSI however it would not prime the pump until the drill speed topped out (1500RPM max) for about 12-14 seconds. Running at lower speed it would not draw the oil to the pump. I also put the bare cover back into the oven, heated to about 200 and measured the width of the cavity across the wide opening. Used digital calipers. The OD was 3.807 cold, 3.811 hot, an increase of .004". The ID was 3.0125 cold and 3.014 hot, an increase of .0015. Measuring the outside was more difficult since there were no parallel surfaces. I made marks and attempted to repeat the same points but am less confident of the result. The ID result was repeatable.
Steve, Seems like with the largest clearances, you get a bleed out condition causing the pressure to drop some, same as larger than stock rod & main bearing spec's. With most all 350's making it to around 100k + miles in stock form, the oil pump system designed by GM has been ample for the most part. Bearing clearances spec'd by GM probably need to be adhered to.
Right Mart. What you call bleed out I would call bypass. Like the oil is just squirting under or over the gear rather than out of the pump. The pumps really do seem more robust than we have given them credit for.
