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Oil Pressure question

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77_MG_Midget

77_MG_Midget

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Took my 1500 midget for a drive last night. Still 81 degrees at 9pm... nice driving weather!!! Anyway, after about 5 miles, I noticed the oil pressure wasn't quite what I thought it should be.

Cold, it's 70 psi at 3000 rpm, 20 psi at idle. But after the engine warms up good, it drops to 45 psi at 3000 rpm and 12-15 psi at idle.

Engine temp is good at 160-165. The engine is fresh, only 25 miles on it so far after the rebuild. During the rebuild, I put in a new oil pump and pressure relief spring.

The workshop manual says the oil pressure should be 40-70 at "running speed" (which it doesn't define running speed, so is it 40, 50, 60 mph???) and 20 at idle. So, this made me a bit concerned as mine is lower at idle when hot.

Is this normal? I was reading an old thread about making an adjustable oil pressure relief valve. Would this be worthwhile?

Advice is much appreciated!!!!
 
12 is a little low but you sound fine otherwise....
maybe itll change as u break it it and the weather cools more....any leaks?...also tuning makes a difference.......z
 
Assuming everything is square with the world internally on your engine it sounds like you put things together on the loose side.

"""""Cold, it's 70 psi at 3000 rpm,
after the engine warms up good, it drops
to 45 psi at 3000 rpm

Actually thats not too bad.

The oil of choice for me would be 10-30 or 10-40 Chevron Delo or any good other oil with a can of GM EOS one time on break in and then at 500-700 miles d& fill with any good 10-40 in the summer. But thats with factory bearing clearances.

Oil gets hot and thins and pressure goes down. Bearing clearances and rod side clearance dictate oil pressure when cold and hot, as well as the bypass spring in the pump system.

Not being a Triumph motor maven I cant give U specifics on a regulator spring adjuster. If you feel you were on the loose side you might shim the spring and see what the increase is. maybe .060" or so to start.

Also cam bearings are a real source of oil flow...were they new? How about valve train oil loss?? Oil viscosity?
 
No Zimmy, no leaks at all. (should this concern me, it is a LBC after all... LOL).

I know my carb needs tuned, just haven't gotten to it yet. Oh, and forgot to mention, I am running 20W-50 oil.
 
[ QUOTE ]
Also cam bearings are a real source of oil flow...were they new?

[/ QUOTE ]

There are no cam bearings in the Triumph 1500 motor. The cam just rides on the layer of oil between the journals in the block.

Not sure about putting things together "loose". The machine shop cut the crank .010, so in went .010 rod and main bearings. They measured within spec... ???
 
""""""There are no cam bearings in the Triumph 1500 motor.'""""

See I told U I wasn't Stanpart conversant.........but I always thot the Spit motors had cam bearings.....

Anyway.....I looked at the handy dandy Spit motor exploded view of the oil system and it shows an externally removable spring. So it's an easy shim....try that ...all you need to do is pick up 5 psi hot. I wouldnt agonize over the situation as it isnt really critical and you not gonna run the motor a gazillion RPM for 10 hours.

""""""The machine shop cut the crank .010, so in went .010 rod and main bearings. They measured within spec... ??? """""'

I understand what your saying......I do too much work on Lunar Landers and the technology transferrs.

Crank grinder grinds the throws .010"...but does he grind on the low or on the high side of tolerance. And then the rods are resized at the high side (example). So maybe the rod bearing clearance dims are .0012-.0020" and you didnt specify exact dimensions. So even tho you use .010" bearings you might have mebbe .0022" bearing clearance instead of .0011" thus your on the high side. This x 4 and the mains x 4 and excessive rod side clearance and on and on. So there is less thumb over the end of the hose and less pressure.

NOW..most people are not engine builders but engine assemblers and I certainly understand that and Im not talking down to you. This is why some shops charege $2000 for an engine rebuild and some charge $2899 for that same engine....(but done to specific tolerances).

Have fun and shim away......
 
There is such a thing as an adjustable oil pressure valve. Mostly used by the racers.
 
Jerry,

thanks for the good information. I actually never thought of the machine shop grinding the crank to the high side of spec. This would make some sense. Still within spec, but on the high side which could decrease oil pressure.
 
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