oil weights

[Thundervalley]

I have a SBC drag race only engine.I currently run 20W50 oil I was thinking of going to a straight 40W in Brad penn.Would this result in the same hot idle oil pressure or about 5-7 psi more over the multi grade oil?I currently have 30 lbs at a hot idle would like to have 40 lbs.

[FastBuick]

That's not the right way to get oil pressure. Adjusting the pump or a high volume pump is a better way to go. The SAE 40w will not flow fast enough for initial startup and you will be getting dry starts which is never good for bearings. You would have to heat the oil before each startup if you were to use a SAE 40w oil. Depending on your clearances and how hot the engine runs, you will most likely need a 10w-30 or 15w-50 synthetic racing oil. In my SBC I use either the Amsoil Dominator Racing Synthetic 10w-30 or Amsoil Dominator Racing Synthetic 15w-50 depending on clearances and time of year I'm running. How much horsepower does the engine make? RPM range? Clearances? And what temperatures do you run in?

[oldjohnno]

Why are you concerned about the pressure? Even if you do increase it via a thicker oil the actual flow won't change at all so I can't see what you'd be gaining. Pressure in itself doesn't really tell you a lot apart from the fact that if it's below the relief valve setting the engine is getting all the available flow. And if the pressure is unusually low then it's an indicator of a problem that needs to be rectified, not just hidden behind a thicker oil.

[BlackoutSteve]

Relabel your oil pressure gauge to "Resistance to Flow".
Perhaps mount your gauge at the far end of the mains, -as far from the pump as possible, and see if you really do see a pressure increase from using such stubborn oils..

There is no way I'd be using a 20W50 in an engine that hardly warms up by the time you stage, let alone a single weight oil that doesn't want to go anywhere when cold..

Take a leaf out of ProStock and head in the direction of the grades they use and are designed to flow well at low temps with minimal warm up.
For example, those engines use something like a 0W5..
https://www.motorstate.com/oilviscosity.htm

[FastBuick]

Just my thoughts. First the dry start deal. Yeah if it sat for a few months or more. And actually I have tore down engines that sat longer and never found a bearing without oil on it, well unless it starved and spun. If high viscosity oil causes dry starts there should be lots of large industrial engines crapping out daily from it. And in the old airplane days running 60 weight they should never have lasted at all from all those dry starts.
Yeah some people are running much less viscosity these days, but still the load carrying ability of the oil increases with viscosity.
What viscosity can depend on clearances and what load at what rpms.

Yes there will be oil on the bearings, however just oil being on them doesn't protect the bearings when the engine starts. You need oil flow and pressure to create the protective oil cushion between the bearings and the crank. The higher viscosity SAE 40w because it has a higher resistance to flow will make that time between startup and having a protective cushion of oil longer. Higher viscosities don't always offer better protection. And yes it was gotten away with before, however engines have developed quickly from those times and have different requirements now. Also I'm sure those engines running those high SAE weight oils had oil heaters to heat the oil before start up. Otherwise in the cold temperatures it would hardly flow at all.

[Olefud]

Consider;

http://www.stle.org/resources/lubelearn ... on/#fluids

[Sparksalot]

Good gawd, I hope that isn't what engineering has been dumbed down to!

Been an engineer and tribology guy for a long time and have never found a way to convey the concepts to lay people, particularly performance engine people. I'm one of those too for over a half century. Opinions are rampant In that community, knowledge is not. I walk away from voodoo.

The only pressures in your race engine lubrication system you should care about are the ones generated by hydrodynamic lift which separate the bearing surfaces from destroying themselves and what delivery is needed to support those many hydrodynamic wedges inside the plain bearings. Those are for most conventional American automotive engines the plain bearings or journals: main, rod and cam.

Clearance, load, area and velocity define a plain bearing, what viscosity of lubricant it needs are defined by those parameters. For a race engine life is a totally secondary matter, it needs to last as long as the race, that's all. There is a big difference between a bearing designed to last for decades and one which will last maybe under a thousand rotations under full throttle. Where are you in that spectrum?

No idea how your engine was built but desiring more oil pressure is absolutely the wrong path to follow.

[Rizzle]

I've been doing a lot of oil research over the last few hours and figure I'll add a summary some of things I learned. If something seems off please question it.
There are 4 common base oils used to make the oil. I - IV. There is also type V however it is not commonly used in auto oils since it doesnt have any (or much?) of the seal swelling abilities the type I-IV have.
Most multi-grade oil is made with a thin base oil and an additive pack that maintains viscosity at higher temps. Prob a type II or III base. The problem is over time/stress, this add-pack can break down and you lose some of the high temp rating (cSt @ 100C). Straight grade oils should not suffer from this.
High quality synthetics running a type IV base (typ POA type oils) can usually meet the multi-grade rating without any additives, and so the hot rating should remain the same after hard use. A good many synthetics and semi-'s are made using type III bases and so reading up on what the oil is made from can go a long way. If you look at the oil spec sheets, a high VI number (viscosity index) typically means a higher base/base mix.

Figure out what temps your oil gets to, is also important.
TBH 30 psi hot idle would not have me worried. As long as it remains consistent to the conditions and isn't slowly dropping I'd say focus more on making sure your additive pack matches the type of use the engine sees and your oil change intervals.

[David Redszus]

Figure out what temps your oil gets to, is also important.

Oil temp is very important. Consider two oils both at 250F in the bearing.

20w-50 = 12.56 cSt
40 = 12.93 cSt

The actual differences in viscosity between these two oils is only 0.37 cSt.
Which is nothing at all.

Oil viscosity will vary much more from blend to blend and batch to batch, even
for a given brand.

[Olefud]

Good gawd, I hope that isn't what engineering has been dumbed down to!

Been an engineer and tribology guy for a long time and have never found a way to convey the concepts to lay people, particularly performance engine people. I'm one of those too for over a half century. Opinions are rampant In that community, knowledge is not. I walk away from voodoo.

The only pressures in your race engine lubrication system you should care about are the ones generated by hydrodynamic lift which separate the bearing surfaces from destroying themselves and what delivery is needed to support those many hydrodynamic wedges inside the plain bearings. Those are for most conventional American automotive engines the plain bearings or journals: main, rod and cam.

Clearance, load, area and velocity define a plain bearing, what viscosity of lubricant it needs are defined by those parameters. For a race engine life is a totally secondary matter, it needs to last as long as the race, that's all. There is a big difference between a bearing designed to last for decades and one which will last maybe under a thousand rotations under full throttle. Where are you in that spectrum?

No idea how your engine was built but desiring more oil pressure is absolutely the wrong path to follow.

Agreed. However, the wedge also generates a good bit of localized heat. The pressure/viscosity/clearances have to be such as to provide adequate cooling.

[rally]

I have a SBC drag race only engine.I currently run 20W50 oil I was thinking of going to a straight 40W in Brad penn.Would this result in the same hot idle oil pressure or about 5-7 psi more over the multi grade oil?I currently have 30 lbs at a hot idle would like to have 40 lbs.

Why are you worried about 30 lbs of pressure at hot idle and want to increase the pressure to 40 lbs? Nothing wrong with the 30 lbs you have right now IMO. What kind of oil pressure are you crossing the traps at on the strip? Remember more oil pressure and heavier oil cost horsepower. Bill Grumpy Jenkins always used the low horsepower oil pumps on his Big Block Chevy drag applications. I like your choice using Brad Penn racing oil, its good stuff IMO. If i were you, i would call Brad Penn and talk to the tech, they have two good people there last time i talked to them and helped me choose the right weight oil. I agree with the other poster, at start up you want an oil weight that flows the best when cold starting an engine. Give them a call.

[David Redszus]

My humble anecdotal observations are that a say 40 oil might loose viscosity over time to say 30, 20 etc, but an oil that starts out at a 5 has very little cushion.

A straight 40 weight oil will change viscosity very little over time; perhaps 5 points. But, it could either decrease or increase its viscosity, you don't know which unless you run it through
the lab for a viscosity test.

If the oil is diluted due to fuel wash, it will lose viscosity. If it accumulates sludge from the crankcase or additive
mismatch, it will gain viscosity.

Since we don't know which way viscosity will move, it is best to simply change oil frequently.

If we were to record and plot oil pressure against engine rpm, a rising pressure slope indicates proper flow of oil
at that pressure. If the curve levels off, it indicates the pressure relief valve has opened and we have begun
to aerate the oil. Now the oil viscosity (and flow volume) has little effect on lubrication.

[piston guy]

Richard Petty uses STP in his oil. Lots of ads to prove it!

[wyrmrider]

STP is the Racers Edge
Wynn's is Friction Proffing
now there's Lucas Snake Oil

[Steve Haaf]

Thundervalley, what are your bearing clearances, what coolant temperature do you race at and what fuel do you run? Knowing these are very important.