Rule of thumb for bearing clearance HTHS or Visc?

Status
Not open for further replies.
Joined
Nov 4, 2008
Messages
54
Location
GA
Is there a rule of thumb for this? Something that relates tight bearings like .0012" to an ideal range HTHS #'s, or Visc in 100deg cST? Then .0018", and .0025" to the same?

In my case the application is an '87 BMW race car. Engine is unmodified.
 
First HTHS is viscosity.

Yes there is a rule of thumb for bearing clearances.
A loose engine is one with clearances larger than 0.0027".
Under 0.0027" to 0.0025" a 30wt oil with HTHS vis' from 3.6cP to 3.3cP should work.
Under 0.0025" to 0.0020" a light 30wt with HTHS vis from 3.2cP to 2.9cP.
Under 0.0020" is a tight engine and a 20wt oil can be used with HTHS vis under 2.9cP to 2.6cP.
This is all based on an engine reaching normal operating oil temps of close to 100C.

Of course the bottom line is oil pressure which is oil temperature dependant. Don't know what engine you're running but maintaining minimum oil pressure of 10-11 psi/1,000 rpm is what you want.

Hope this helps.
 
I don't consider .002" really tight . Esp on smaller engines.

An oil pressure and temperature gauge should be installed on the race car. You need hard information.
I would not start with a 20.
 
Originally Posted By: CATERHAM
First HTHS is viscosity.

Yes there is a rule of thumb for bearing clearances.
A loose engine is one with clearances larger than 0.0027".
Under 0.0027" to 0.0025" a 30wt oil with HTHS vis' from 3.6cP to 3.3cP should work.
Under 0.0025" to 0.0020" a light 30wt with HTHS vis from 3.2cP to 2.9cP.
Under 0.0020" is a tight engine and a 20wt oil can be used with HTHS vis under 2.9cP to 2.6cP.
This is all based on an engine reaching normal operating oil temps of close to 100C.

Of course the bottom line is oil pressure which is oil temperature dependant. Don't know what engine you're running but maintaining minimum oil pressure of 10-11 psi/1,000 rpm is what you want.
Hope this helps.

Interesting. What you've described jibes with info that used to be on the Joe Gibbs Racing Oil website. For us it seems too thin tho. Our motor is called an M20B25. BMW initially spec'd a 20W50, but in the late '90's also recommended 10W40 for summer operation.

My Dodge Ram oil pressure is probably 80psi or better at 2krpms with hot 10W30. But the OP release valve on the BMW is at about 70, so even when the oil is stone cold, that's as much as you'll get.

While racing a visc oil with a 100deg Visc of >20cST will get a person 55psi or so in a new tight engine at 6000rpm, but the averate xW40 will result in around 45psi. And the average xW30 will result in around 35psi. I'm talking 220deg oil at 6000rpm.

I first read about the 10psi/1k rpm rule in AE Haas's writings. But I'm thinking that it's just a very general rule and has to be applied with care. If I put xW20 into my engine with it's .0018 mains and .0020 rod bearings, I'd have prob 20psi at red line. For a couple seconds and then Boom.

So what do you figure is going on here? What was the design concept that resulted in me having to use an oil so much thicker then the bearing clearances would seem to suggest?
 
That's the 4 cyl engine right?
I am surprised you require such a thick oil to get proper oil pressure with thoughs tight clearances.
I know the minimum OP on my M52 BMW engine is 59 psi at "elevated rev's", so if you have to run a 50wt oil to get 60 psi hot at 6,000 rpm then that's what you do. I'd also run a high VI oil as well to minimize the amount of time the oil pump is in by-pass when the oil is cold.
 
Bearing design doesn't use or refer to absolute bearing clearance in determination of the correct operating viscosity.

2 thou on a 2 inch journal is way looser than 2 thou on a 4 inch journal.
 
Originally Posted By: RangerGress
Originally Posted By: CATERHAM
First HTHS is viscosity.

Yes there is a rule of thumb for bearing clearances.
A loose engine is one with clearances larger than 0.0027".
Under 0.0027" to 0.0025" a 30wt oil with HTHS vis' from 3.6cP to 3.3cP should work.
Under 0.0025" to 0.0020" a light 30wt with HTHS vis from 3.2cP to 2.9cP.
Under 0.0020" is a tight engine and a 20wt oil can be used with HTHS vis under 2.9cP to 2.6cP.
This is all based on an engine reaching normal operating oil temps of close to 100C.

Of course the bottom line is oil pressure which is oil temperature dependant. Don't know what engine you're running but maintaining minimum oil pressure of 10-11 psi/1,000 rpm is what you want.
Hope this helps.

Interesting. What you've described jibes with info that used to be on the Joe Gibbs Racing Oil website. For us it seems too thin tho. Our motor is called an M20B25. BMW initially spec'd a 20W50, but in the late '90's also recommended 10W40 for summer operation.

My Dodge Ram oil pressure is probably 80psi or better at 2krpms with hot 10W30. But the OP release valve on the BMW is at about 70, so even when the oil is stone cold, that's as much as you'll get.

While racing a visc oil with a 100deg Visc of >20cST will get a person 55psi or so in a new tight engine at 6000rpm, but the averate xW40 will result in around 45psi. And the average xW30 will result in around 35psi. I'm talking 220deg oil at 6000rpm.

I first read about the 10psi/1k rpm rule in AE Haas's writings. But I'm thinking that it's just a very general rule and has to be applied with care. If I put xW20 into my engine with it's .0018 mains and .0020 rod bearings, I'd have prob 20psi at red line. For a couple seconds and then Boom.

So what do you figure is going on here? What was the design concept that resulted in me having to use an oil so much thicker then the bearing clearances would seem to suggest?



I put either 10W-40, 20W-50 and even 5W-50 in my M20B27, thats the ETA. It seemed to like the 20W-50 the best, that engine will rev with anything.

Its a "High Torque Low RPM" motor, and im surprised your 2.5L has an Oil Pressure gauge!

Sweet car. If it is happy with 20W-50, keep using it.

Im serious. if it likes it, keep using it...
 
Originally Posted By: mechtech2
What oil for 'X' clearance also greatly depends on the oil pump.
And the operating temps of the oil ,then there is the side clearance on the crank and rod bearings.
 
Originally Posted By: mechtech2
What oil for 'X' clearance also greatly depends on the oil pump.


Long as there's enough volume to feed the bearing(s), the oil pump is simply there to shift the oil to it.

Bearings can (and do often), create a suction at the oil supply point.
 
If the bearing and side clearances are too much the oil will leak out of the bearings too fast then there will not be enough oil to protect. Ther is a balance between volume and visc.
 
That 10psi per 1000rpm rule dates back to at least the 1920's when Harry Miller was developing engines to race at the Indy 500. It has more to do with overcoming centrifugal force at the OD of the crank to assure that oil can be forced to the center of the shaft, and on to the rod journals.

Once the oil feed issue is overcome, there is almost no relationship between oil supply pressure and acceptable lubrication of conventional bearings. As long as there is enough pressure to replace the oil in the bearing squeeze zone, thereby keeping it in an acceptable temperature range, that's enough. This is partly why the HTHS test is run at 150C.

The actual film pressures within rod bearings is measured in the thousands of psi, and there is no way that an oil pump can supply that. When loaded, a bearing journal is eccentric in the bearing, and most of the oil flow goes out through the unloaded zone. When the load is removed from the bearing, the journal changes position in the bearing, and the oil within the loaded zone is replaced.

In general, I advocate "low" oil pressure. "High" oil pressure forces more oil out through every bearing in the engine, requiring more power to drive a bigger oil pump. Once more oil has been forced out through the bearings, there is more oil flying around the crankcase, being whipped up by the rotating assembly. This costs more power and increases the heat load on the oil cooler. "High" oil pressure also means pumping more oil to the overhead in the engine, where it can overwhelm the oil drains, making the oil stay in the top of the engine, instead of in the sump, where it should be to ensure a steady supply to the oil pump pickup.

Most people try to counter bearing problems by increasing oil pressure. Most of the time, they end up making the problem worse.
 
It is an engineering balancing act. Too much ,too little, just right. Then as things change, wear, extreme heat , load, so does the too much, too little and just right.
 
A Harmon, your explanation of low-er oil pressure is in keeping with my non-professional understanding of engine lubrication, which is why the "more pressure is better" idea was perplexing to me.

Thanks for posting that description.
 
Status
Not open for further replies.
Back
Top