| readthegrain | -- 10-25-2015 @ 12:44 PM |
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Hello Gentlemen I have recently received my Grandfather's '36 Cabriolet. Overall the car is in pretty good shape and runs and drives well. The oil pressure, however, worries me a bit. The gauge reads about 20 lbs when it is first running but as it comes up to temperature, the pressure drops to 5 or even 3 pounds. It fluctuates a little when the engine is revved up but does not come back up to even 8 pounds when warm. I have clean 30 weight oil in it now. My question is two part. 1) What pressures do you guys see on your cars? And 2) Are these stock 6 volt gauges accurate or are they finicky after all this time. I haven been able to find a new, aftermarket 6volt gauge to test on the car with and Grandpa is not around to ask questions of. Thanks Arvid Lindstrum
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| MG | -- 10-25-2015 @ 1:11 PM |
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"The Flathead V8 ran between 10-15 pounds of pressure, but is not considered a pressurized system by today's standards." - from Ron Bishop's 'Rebuilding The Famous Ford Flathead'. The sending unit in on your engine might be suspect. A more accurate reading will be had with a mechanical pressure gauge. Hook one up temporarily where your sending unit is now located on the rear of the block.
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| joe b | -- 10-25-2015 @ 2:37 PM |
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The stock gauge on my '41 read quite a bit like yours. Do what MG suggests and check the pressure with a mechanical gauge. I installed a Stewart Warner oil pressure and temperature gauge under dash. My car is not Comcourse so it didn't matter to me. It was stunning how much better these gauges worked. When the dash gauge shows 20 psi the SW shows 60, 10 psi is 0 on the stock gauge'
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| Drbrown | -- 10-25-2015 @ 9:57 PM |
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Pending testing results with a mechanical gauge as described, although straight 30w oil should be okay, could consider going to 20w-50 oil.
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| TomO | -- 10-26-2015 @ 9:13 AM |
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Joe B, It sound like you have an 80# sending unit with a 50# pump. You may also have a poor ground at the sending unit. I have checked several sending units and gauges using air pressure with a good regulator and have found the King Seely gauges are as accurate as a mechanical gauge when they are in good condition and installed properly. Readthegrain, Your readings are a little on the low side, but nothing to worry about. A check with a mechanical gauge will tell you how accurate your dash gauges are. A good 10W 30 oil will work well in your engine, the 20W 50 will give better pressure, but not necessarily improve lubrication. A good rule of thumb is that the gauge should read close to the middle of its range when you are driving 30-40 MPH. The pressure should not drop on turns or acceleration. If your dash gauges do not read within 10% of the mechanical gauge, you can remove the sending unit, make sure that it is a 50# sending unit (they usually are marked on the square portion, just under the electrical unit with an 80 if they are the 80# unit). You can connect an air pressure source to the bottom of the sending unit and connect the gauge wire and a ground wire to the sending unit. Adjust the air pressure to 30# and the dash unit should read just above the center line. If does not read correctly, you can pry off the cover of the electric part of the sending unit and clean the contacts. Clean the threads on the sending unit and re-install it with no sealant or Teflon tape. Tom
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| readthegrain | -- 11-02-2015 @ 8:34 PM |
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Thanks for the advice guys. I hooked up the mechanical gauge and got similar results. It showed almost 25 lbs when cold but after 15 minutes of high idle and coming up to temperature, the mechanical gauge showed only 1 or 3 lbs. The pressure came up slightly on revving the engine but only a pound. This was very close to the readings I get on the electronic gauge. I also checked the sending unit (which is an 80 lb unit by the way) with compressed air and it read 1/2 to 1/3 of what the compressor gauge showed. I also checked the mechanical gauge on the compressed air and it showed about 15% lower than the compressor gauge but otherwise working. After all that, I am thinking I have a real problem with low oil pressure once the engine warms up. Any suggestions on checking the oil pump? Who rebuilds them?
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| ken ct. | -- 11-02-2015 @ 10:19 PM |
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Last post you just answered your own problem. You say you have a 80# sending unit. 36 to around 47-8 have 50 # oil pumps. You must use a 50# sending unit for gauge to read right. ken ct. Owner of a driver 36 coupe for 25 yrs.
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| TomO | -- 11-03-2015 @ 8:02 AM |
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Before you go to the effort of pulling the engine to replace the oil pump, I would replace the oil pressure relief valve plunger and spring, at the front of the engine under the intake manifold. The spring may be weak or the plunger may have sludge buildup. Tom
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| keith oh | -- 11-05-2015 @ 2:03 PM |
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You did not state if your engine is a 21 head bolt ( requiring a 50# sender) or a 24 head bolt (requiring a 80# sender). Also if you have a oil filter it may not have the restrictor fitting needed.
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| flatheadfan | -- 11-20-2015 @ 2:08 PM |
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This is not scientific but the old rule I have used is 10 pounds per 1000 rpm. Which means a typical idle of 500-600 rpm should be safe with 5 pounds. Tom
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