SBC oil pressure mystery

General engine tech -- Drag Racing to Circle Track

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rfoll
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SBC oil pressure mystery

Post by rfoll » Fri Sep 29, 2017 8:41 pm

My friend has a vehicle he started drag racing. It was originally a circle track truck from American Super Trucks and is based on some kind of GM coil spring chassis. It was converted to run road racing and he ran for a short while with a fresh nearly stock 2 barrel 350. When a weekend of racing started costing $1000, he gave it up and decided to join me at the Sat night Late Night Drags at PIR. We kicked up the compression with a set of 305 heads and put a larger cam in. It has a Performer RPM with a 3310. Rear gears are 3.73, Turbo 350 trans, and still using the road race slicks. The truck weighs 2800 with the driver. It has taken a while to get all of the bugs out, but when it runs down the track, it is rater unique, and he is having a ball. Last Saturday, after the burnout, the oil pressure light came on and the gauge showed 0. He shut it off immediately, and put it on the trailer. After pulling the pan and a fairly thorough inspection, we can find absolutely nothing wrong. The gauge and idiot light are served by the same copper line with a "T" at the gauge. All of the cam journal bearings are in place. None of the rods are discolored. The rear mounted oil pan to pick up clearance is correct. We ran the oil pump with a cordless drill before the engine was removed and the gauge red 60 psi. The plug under the oil pump is still where it belongs. We are down to suspecting a piece of debris hit the end of the tiny pressure gauge line, or the oil pump relief valve hung open. The burnout and launch is very mild due to the limited traction of a road race tire. We are open to ideas about this that we haven't thought of yet.
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Re: SBC oil pressure mystery

Post by Schurkey » Fri Sep 29, 2017 8:51 pm

Plugged/collapsed oil filter?

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Re: SBC oil pressure mystery

Post by pamotorman » Fri Sep 29, 2017 8:58 pm

I have seen a piece of RTV plug the block fitting for the oil pressure gauge on SBC. that is why it should not be used on oil pressure fittings

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Re: SBC oil pressure mystery

Post by MadBill » Fri Sep 29, 2017 9:05 pm

If the engine has pressure when spun with a drill, it must be the drive tang on the distributor not engaging. I seem to recall there is at least one oddball SBC that has a shorter intermediate shaft; perhaps minimal engagement due to the wrong part caused part of the female slot to break off but left enough for the drill mandrel to engage.
Alternatively, the pressure relief valve may have temporarily stuck open.
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Re: SBC oil pressure mystery

Post by Baprace » Fri Sep 29, 2017 9:39 pm

pamotorman wrote:
Fri Sep 29, 2017 8:58 pm
I have seen a piece of RTV plug the block fitting for the oil pressure gauge on SBC. that is why it should not be used on oil pressure fittings
I agree, the line must be plugged or pluging with debris, all the bearings show good correct ??

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Re: SBC oil pressure mystery

Post by GARY C » Fri Sep 29, 2017 9:44 pm

I had one oil pump that would hang up in forward motion but the shaft would still turn, you could spin it backwards with a drill and then forward again and it would work prefect, turned out to be a small sliver if metal in the pump.

MadBill, the BBC pump in a SBC requires a slightly longer rod as I recall but standard one will work but may break a tooth due to lack of engagement.

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Re: SBC oil pressure mystery

Post by exhaustgases » Fri Sep 29, 2017 11:03 pm

Kind of like when I was a kid I had a 327 that would lose oil pressure during my down the street drags. I figured it was pumping the pan dry. And is normally why some use deep pans and oil restrictors. I had it apart many times trying to find a problem, and none found and had great oil pressure if driven normally.

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Re: SBC oil pressure mystery

Post by jdperform » Sat Sep 30, 2017 8:28 am

When I used to build wet sumps the last part I put on was the oil pan. I was always worried about intermediate shaft engagement. On at least 50% of the time I would press the oil pump shaft out of the drive gear just a little. I liked to get no more than .060 up and down play and no less than .030. I would use "common sense" to determine if I still had enough press on the Oil pump drive gear. Of coarse the dist has to be installed and clamped.

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Re: SBC oil pressure mystery

Post by rfoll » Sat Sep 30, 2017 12:31 pm

We haven't cut the filter open yet. We put a fitting on the oil pressure port next to the distributor and pumped the oil through a paint filter and found nothing. The idea of a blinded gauge line sounds reasonable, but why would anything move in a direction where there is no flow? That port only sees flow on startup, or slightly when the pressure increases. We haven't seen any silicone in the oil yet, I'm going to cut the filter open this morning. I'm leaning towards a stuck pump relief valve. We will open the pump when I do the filter. The pump would have been at max pressure during the burnout. Thanks for the replies, Rick.
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4sfed

Post by 4sfed » Sat Sep 30, 2017 4:52 pm

A GoPro camera makes a good data acquisition system to see exactly what is happening.

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Re: SBC oil pressure mystery

Post by CharlieB53 » Sun Oct 01, 2017 9:42 am

Do you know the oil temp and RPM when the pressure loss occured?

I would very closely inspect the oil pump rod engagement as well as the oil pump shaft/gear engagement. Slightly loose press fit of the shaft to gear fitment combined with high RPM and high pressure of cooler oil could possibly cause the shaft/gear to 'slip', loosing pressure.

Then again maybe the shaft/gear is keyed and I'm just stupid.

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Re: SBC oil pressure mystery

Post by Newold1 » Sun Oct 01, 2017 9:58 pm

A question is does the engine have a special side kick out sump section or windage features designed for circle track racing? If it does it may be putting to much oil to the side in that type of pan and at starting line launches and keeping not enough oil at the rear of the pan sump where it feeds the oil pump pickup. Just a thought.

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Re: SBC oil pressure mystery

Post by levisnteeshirt » Sun Oct 01, 2017 11:14 pm

Was the engine warmed up prior to the burnout

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Re: SBC oil pressure mystery

Post by rfoll » Wed Oct 04, 2017 10:23 am

The engine has a stock oil pan, and was up to temp when this occurred. We took the cover off the pump and found a small piece of steel, about the size of a sesame seed in the gears. We also found a significant amount of debris, not engine related, in the filter when we cut it open. I suspect the debris was hiding under the stock splash shield in the factory pan when it was installed. We might not know exactly wht killed the pressure reading but the junk in the oil was significant enough to cause several of the suggested problems. Thanks for the replies.
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