Wierd Oil Pressure Problem, SBC 383

I have a newly rebuilt SBC 383 that is having some weird oil pressure issues. when its cold it has lots of oil pressure but when its warm at idle it goes down to 20 psi when in gear at the light and 30 if I put it in neutral, but that not the weird part.

The weird part is when I'm driving down the highway. my oil pressure gets worse above 3000 rpm. at around 3000 rpm it will cruise around 38 psi, but go any higher and it starts to drop. at 3700 rpm its down to almost 20 psi. I noticed this the first time I had it on the highway after the rebuild. The oil pump was not changed during the rebuild

Engine bored out 0.010, with new rings, pistons, and all new bearings done professionally after a piston failure. IT has a high capacity oil pan so i don't think its starving for oil at higher RPM. The oil pressure line run right from above the oil filter to the gauge in the truck. IT is the plastic line that came with the gauges and not a metal one.

Not sure where to start looking for the problem. I'm thinking I'm just going to end up putting a new oil pump in it and maybe switching that line to metal. Anyone else have any ideas of what could be causing my issue?

Welcome to CHR!

First, my newly built SBF, everything new idled at about 20 to 25psig, then increased to about 40 or 45 with RPM after it was warm, so I wouldn't be too concerned with that part. The first things that come to my mind when oil pressure falls with RPM are 1) oil level low, starving the pickup, 2) oil filter restriction/very dirty or 3) an oil pump problem. You've said that it's a higher capacity pan with the right amount of oil so #! shouldn't be an issue. It's hard to imagine that your oil filter is the problem, but it might be worth the cost to replace it with a quality filter, like a WIX Gold or ?? so I'm assuming that the most likely is a worn/damaged oil pump. You mentioned a piston failture, so there could have been fine metal passing through the screen and going through the pump, so I'd probably be looking at replacing the pump. Considering your professional build I'd go with a standard pump, not a high capacity/high pressure, but that's just me, others may disagree (how about it Glenn Sexton?). It's likely that you'll see better idle pressure, too.
I would for sure change out that plastic line going to the oil pressure gauge. I use a sending unit style, but a direct piped unit takes away any questions - it is what it reads, but that little plastic pipe getting on a hot surface is a disaster/huge mess just waiting to happen.

My $0.02, and again, glad to have you here!

My initial reaction was to mention the clearance between the oil pump pickup and the oil pan floor. It could be just a wee bit to close and restrict the oil flow reaching the pickup. Folks have reported here that they had a similar situation as you describe, higher rpm's and the psi gauge was going lower. And like Roger says, just replace the pump. I have zero idea why anyone would re-use an oil pump on a fresh motor rebuild. But if you do replace the pump, measure the pickup distance from the block - then measure the distance from the pan flange down to the floor of the pan. Make note also if the gaskets are being included in your measurements. Once you install the new pump with pickup. Repeat those measurements and verify you do NOT get the pickup screen any closer to the pan floor. I'd ideally try to increase that gap - even a little bit - just to remove that as potential problem. Glenn is our resident SBC expert. let's see what he offers as both Mr. Spears and myself bleed Ford Blue! LOL

I agree with Mike, and I meant to mention checking the clearance of the pickup to the pan but forgot and hit the "Post Reply" w/o adding it. Mike's right!!

Welcome to CHR!
You start with saying, “…a newly rebuilt SBC 383.” When I read that I assume that you have a 350 bored .030 over and a 3.750-inch crank from a 400 small-block. A bit later tell us it’s been bored .010 – does this mean that the engine failed and now it has .040 oversized pistons? You also indicate that the oil pump was not changed (during the re-bore?) but you have a high capacity pan. I’m a bit confused as to the sequence of events but there are several indications of a mis-match between the pump/pickup and pan. The other possibility would be if you started with somewhat uncommon 4.005 bore and a 3.800-inch stroke (Chevy high performance engine – very expensive!). In either case, I suspect an improper oil level at speed and you’re experiencing either starvation to the pump or cavitation causing the oil to flow up the sides of the block and away from the pickup. Here’s where I would start:

Replace the following components:
Pump - Miloden 18756 high volume/standard pressure.
Pickup - Miloden 18314
Pan - Milodon 30901 seven quart pan that fits stroker motors – assuming passenger side dipstick.
Gasket – Miloden 41000 one piece
Windage Tray – Milodon 32102

Optionally:
Milodon 23050 oil pump drive shaft
Milodon 81300 is an oil pan stud bolt kit – worth the $20 for ease of assembly (in my opinion).

Let us know what you decide –

Regards,
Glenn

Oil pressure seems to baffle at times. Mike saved me with the oil pump pickup to oil level point, and Roger covered the filter. In the case of the filter, something (too much volume of oil or defective filter media) could be causing it to fo into bypass mode at the higher rpm. As to the pickup level, you may be having an oil return to the pan issue if there is something plugging the openings in the heads/block that allows the oil to return to the pan quickly enough. Another thought might be your oil is getting too hot (for some reason you've not identified yet) and you're losing viscosity as a result.

Edit: Too many people go first to blaming the pump due to lack of understanding. In a simplified description the pump produces flow, not pressure. The pressure reading is a result of resistance to flow, and is better seen as a diagnostic tool.