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Expected oil pressure for smallblock engines at idle

M

MikeyM

Just got it firing!
Joined
Feb 10, 2024
Messages
5
Location
Belgrade, Belgrade,Serbia
Hello everyone,

I have a question regarding the expected oil pressure at idle.


It happens that when the oil is hot oil pressure switich is activated during idle.

I've tried multiple different oils, 10w40 Eni/Agip, 10w40 Motul, 15w40 Motul, but the result/outcome was the same.

When the oil is cold, there is no issue, oil pressure relief valve reliefs the pressure at 4 bar, I even tested the spring of oil pressure relief valve, and it is within the specs according to the repair manual.

When I tested the oil pressure relief valve I've also checked the sump for debries, still nothing to be found. Before I start further teardown of engine I just want to see what are your experiences about oil pressures in small block engines at different rpms and temperatures in genereral.

My measurements are following :

When engine and oil gets hot, oil pressure drops around 0.1bar @800 rpm.

If I increase the idle about 1000 -1100 rpms value goes up to 1.1-1.5 bar. Warning light is not activated anymore.

I see that the oil pump has the same partnumber for V35 and V75 engines so I assume only difference is the oil pressure relief valve setup in oil system between these two engines, i.e. this should not affect the oil pressure when engine is hot.

Now the question is do we have somewhere the table/data of expected oil pressures at expected rpms?

Also, what are your suggestions about the oil, shall I try different ones then recommended by manufacturer for 750 SB engine?

Many Thanks for support!

BR,
Mike
 
The smallblock should idle at 1100-1250rpm, could be idle too slow will be low oil pressure.
Idle motor like manual says may fix issue. There is a reason for a higher idle on smallblocks and it may be for oil pressure. Just my idea. Have you measured the pump clearance on block?
 
Hi Vagrant and V700Steve,

Many thanks for your quick replies!

Regarding idling speed where did you find this information? To be exact in which manual? I'm asking because I've checked a lot of them, and I couldn't find any information about idling speed for my SB 750 Engine.
I was assuming that idling speed should be around 1000rpm based on charging current curve data from alternator, since the charging starts from 1000 rpm.
And also, is there information about expected oil pressure value @ for example 1100 rpms?

Regarding the clearance between the block and the pump, I didn't check this, since I'm still thinking should I open the engine to do a proper diagnosis.

Is there anything else to be checked just by removing the sump, without doing further engine disassembly?
 
Vagrant said:
If you are basing your assumption on a light and not a real gauge stop thinking bad thoughts now.
Click to expand...
Hi Vagrant,

Maybe I wasn't clear enough. Measurements mentioned in my previous post are done with the pressure gauge.


My assumption regarding proper idling speed(1000rpm) is based on charging curve data from repair manual, since there are no values before 1000rpms.
1707592441900

Since I've got some clear figures from in the post above Vagrant, so I'm just asking about where I could find this data and linking my assumption to the information that he provided in his post.
 
MikeyM said:
Hi Vagrant and V700Steve,

Many thanks for your quick repli

Regarding idling speed where did you find this information? To be exact in which manual? I'm asking because I've checked a lot of them, and I couldn't find any information about idling speed for my SB 750 Engine.
I was assuming that idling speed should be around 1000rpm based on charging current curve data from alternator, since the charging starts from 1000 rpm.
And also, is there information about expected oil pressure value @ for example 1100 rpms?

Regarding the clearance between the block and the pump, I didn't check this, since I'm still thinking should I open the engine to do a proper diagnosis.

Is there anything else to be checked just by removing the sump, without doing further engine disassembly?
Click to expand...
There is idle seed in Engine section at beginning of V7 Classic service manual from 08-09
Older I would have to look but would be under tuning carbs.
OK, page 66 in V35/V50/V65SP book says 1000-1200rpm
Page 2.3 in 750 Breva Service @2003
In 02 750 Nevada Club service it page 9 in inspections & maintenance. Says 12-1300
 
Last edited:
Check the pump tolerances 1st, I have an old car with around 300,000Kms on it, I have changed the oil from its recommend 10/40 when new to 20/60 for classic cars, oil consumption has dropped and oil pressure is fine. Pressure is always low at idle. Your 800rpm idle seems low.
 
Thanks everyone for the suggestions, I'll try also with 20w60 and increased idle to 1200rpm.

I try to post the results before and after.

V700Steve I really appreciate the provided data!

Is there maybe some round figure available, or somebody remembers what would be expected oil pressure when oil is hot @1200rpm, for example?

Thanks!
 
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