French flathead oil pressure

T

Tony Burdett

Member
I’ve just rebuilt my French flathead, at starter cranking speed the oil pressure was 50 psi when it started it went up to 80 psi do reckon this is too high ?
 
My 1953 "Motor's Repair Manual" specifies that normal oil pressure for an American 8BA is 57 psi. I don't know if that applies to the French engines, but it seems plenty to me. I can't think of any period production engine that runs at 80 psi.
 
No worries, I have a french motor in my 34 and have used an electric gauge and now a SW mechanic gauge for oil pressure - both showed up to 100 psi when cold and still 80 psi hot but I have closed the camshaft rear bearing as I don´t use the mechanic fuel pump anymore. Before that it had around 10 psi less so there was pressure lost through that pushrod bush. When cruising with around 2000revs at 65 mph I had 85psi yesterday while it was quite cold outside but even on very hot summer days I never have less than 75 psi at 2000. At the moment running 20W50 "classic oil" because I have it in my shop but I guess a straight 30 will not lower the pressure much. Have done at least 7000 miles since I put this engine in the car - purrs like a kitten.
 
I've been doing some work with various engines and I believe the French oil pump has greater volume capacity than 8BA or 59A pumps.
I will be covering my theories in future videos.
But my position is that a French pump is not the best pump for any good condition flathead.
All that high psi is robbing horsepower.
Look out for videos on the subject, but they will be a while.
Mart.
 
With pressure like that you could easily run a lighter oil. Straight 30 will give you more pressure than the 20.
 
I agree with Marko.
I'd try 15w40, 10w30 or even 5w20 purely as an experiment.
High psi wastes horsepower.
Mart.
 
Well, Tony asked for typical pressures on a french flathead and I gave him the answer that his engine seems to be showing the same pressures as mine.
I don´t think the french engineers were wrong when they decided to run the engines on a higher oil pressure but look forward to see serious testing of different pumps to find out how many horsepower you gain by running the engine on a lower pressure.
 
My theory is that the French engineers upped the flow capacity to compensate for the extra oil feed to the air compressor, the governor and hours counter. Take these things off and the extra flow capacity means more pressure than is ideally required.
My French motor is similar.
The French pumps are great for combating worn bearings in an old 59A for example.
 
The SUMB engines were designed for fairly large military trucks. I like to see around 75 psi on an average engine and this was the pressure I'd adjust aircraft engines to. The pressure will generally fall off when warmed up but I've never run a SUMB engine before. The regular 8BA type set up generally runs less pressure than that with good bearings. It must be the way that SUMB designed their oil system. As was mentioned, a lot depends on how many leak down areas the engine was originally designed to have. The 8BA cars and pickups only have the bearings that are fed and the bypass oil filter with the .060" orifice to control leak down on it. Add more oil fed components and it may have needed to be bumped up some.
 
During my early racing days we ran 75/85 lbs of pressure, using 40 wt racing oil, Checking the oil after every race showed some silver flaked of bearing material in the oil (we didn't Have a filter then), At the mit point of the season I put in the spare engine and checked the bearings of the race engine and found the bearings to be full of pock marks in the rod bearings ( floaties) The next year we broke a crank, wrecked a block, Took the pieces into the inspection lab at the Bullard foundry and their inspection said the crank was broken from clavation erosion due to excessive oil pressure. We then switched to 10/30 oil and never had any more bearing or crank failures/ Also switched to a full flow oil filter. I have the picture someplace of the crank failure. I've posted it several times.
Gramps
 
Years ago I used a French block for some of my testing on flathead life expectancy under all out land speed racing. Here's one possibility why the French engines have high oil pressure.
One area that's different between Ford and French blocks has to do with oiling the engine its the main oil tube that's located in the valley of the blocks.

This tube as we all know supplies all the oil to center and front of both blocks. I discovered the French tube has a much smaller inside diameter than the Ford tube this reduced flow capacity is one reason the French block would show a higher pressure reading since the oil is not capable of flowing as well as the Ford blocks.

Since the planed use of the French block was all out competition one of the improvements I did was increase the internal diameter of the oil feed tube on the French block. With this improvement the oil system worked exactly the same as the Ford blocks which I ran before the French block and all of the Ford blocks I have run after as well.
My results testing with the French block was mixed initially we felt it would be better overall compared to a Ford block but in the end it really did not show any additional strength or longevity.

All of our flatheads are 292 cubic inches burning gasoline with a supercharger the French block got me up to 182 MPH and as our Ford block development continues we are now over 219 MPH. We are still working on more power and time will tell when the peak has been reached at the size we build those old Ford blocks.
Ronnieroadster
 
In the chat on my recent video I had an interesting comment from Harold Degand. He said the French 255 motor has a bigger volume pump than a French 239. I asked about the air compressor and governor and he says only the French 255 has those.

This seems to support my theory that the higher volume pump is better suited to a full military spec 255 motor with oil being supplied to the air compressor and governor. The much more civilianesque 239 motor has a pump much like the stock 8BA.

Just a bit more info from a chance conversation.

Mart.
 
just a curiosity question what oil specs are set down by the French military it may be that they spec an oil such as 15w-40 detergent [ie common diesel spec ] hence the changes internally to suit . after all its their engine they just bought the rights to copy henrys basic design
 
found the reference you spoke of my French isnt that good but i think nominal would mean normal and would seem about right still looking thru for oil recommendations but cant see any military using multiple eng oils they would have one maybe two due to battle logistics
 
Flatheads can continue to operate with fairly low oil pressure and long as they aren't pulling heavy loads everywhere they go. A lot depends on the era the engine was built. The early engines could run all day with 20/25 psi. The gauges topped out at 50 psi on the early engines. The later post war engines came with 80psi senders and gauges in the 1945/46 time frame and later.
 
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found the oil spec as suspected it is OMD110 which was a 30wt mineral oil that now has been outdated and a genrally accepted replacement is now 15w/ 40 diesel oil [used in most heavy duty diesels ie CAT or CUMMINS ]
 
My French engine differs slightly from the one Mart shows on YouTube, although it’s been cast with the ability to be machined for governor it’s not been machined for one , it has the 8BA manifold instead of the swept back ones and a cast aluminium sump pan that is the right way round to fit in a pickup . The engine has a date cast into it which appears to indicate it’s construction of 1984. I failed to measure the stroke but the bore is 72.4 mm and pistons have 38 stamped on top which I assumed was an oversize but have subsequently been told that isn’t the case. I took the pan off with the intention of fitting the smaller 8BA pump i have but out of interest i stripped the French pump and found someone had fitted a steel billet about the size of an aspirin behind the relief spring I’ve removed this and refitted the French pump and pressure is now 60 psi.
 
38 on the piston is 0.38 mm and equates to 15 thou over. This is the first rebore size for the French rebuilds.

From your description it sounds like yours might be a 3.75" stroke motor but without looking for specific details on the crank or measuring it it is impossible to say for certain.
 
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