Forum » Engine Building Fundamentals » oil weight and pressure

oil weight and pressure

Engine Building Fundamentals

Discussion and questions related to the course Engine Building Fundamentals

= Resolved threads

Page 1

Hi andre, great video. just wanted to clarify your point on using lighter weight engine oils to extract more power on tighter clearances: is there an approximate rule of thumb for how much to change the weight of the oil based on the horsepower and torque increase from the engine? for example if the manufacturer originally used 20-40 weight oil, an addition of 200 or more horsepower would need an increase to 50-60 weight oil?

also, wouldn't it be safer to use thicker oil to err on the side of caution allowing you to have tighter clearances and not worry about the thin film of lubricating oil breaking down between the different surfaces of metal? wouldn't this allow greater overall oil pressure?

While on the face of it, it seems like all you have to do is form a hydro-dynamic wedge to keep the bearing and journal apart - there is a LOT of good info' on-line, King Bearings have a lot of good PDFs - there is a bit more to it than that.

There are several race series' where the engines are very highly stressed for extended periods, such as NASCAR, WEC, F1, where any parasitic losses are minimised and they are using 0-5W oils, or thinner, but they are also using specialised oil additive packages.

The oil film is subject to very high shear velocities and this causes the oil to heat up, so there also needs to be enough oil flow through the bearing to keep it cool and prevent oxidisation - a not-uncommon cause of failure.

The crankshaft, and block to a lesser extent, twists and flexes under the loads from the engine power impulses (and to a lesser extent the other strokes) and centrifugal loadings - these can cause the journals to be very slightly twisted in relation to the bearing, and in extreme cases, there could be contact.

I'm not quite sure what your last paragraph is trying to convey - if you could re-write it?

Hey Gord.. thanks for the answer.

what im basically asking in my last paragraph, is whether it would be safer to increase the thickness of oil as horsepower is increased? the rational is thicker oil with antrifrictional additives (like extreme pressure molys) would mean higher oil pressure and greater protection....right?

You seem to be a little confused and making a common mistake, the oil pressure ISN'T what gives the support film it's thickness/strength.

Oil presure is a measure of the pressure (force, if you like) required at the pump outlet to push the oil through the engine. As you say, a more viscous (thiocker) oil will increase oil pressure, but because it is harder to pump through the engine - in some cases, it may actually reduce the oil supply to the far end of the engine, as the increased resistance to flow may reduce the amount of oil through the oilways and there may be more oil flow from the pump being bled off by the pressure relief valve opening more.

There is usually some question of how much 'pressure' is required, basically enough to overcome the centrifugal force at the main bearings and still provide enough flow for the big ends fed from that bearing. I've seen some people running extremely high pressures, when they aren't needed, and are just adding parasitic power losses to the engine - the high pressures also add to the loading on the oil pump drives and some of the older people here may recall the Ford Windsor hex'shaft drive they used, and the problems that caused with high oil pressures rounding off the drives and blowing engines. Another problem is that at higher rpm, it is harder for the pump to draw the oil into the pump, so there is even an increased chance of pump cavitation/oil starvation at very high rpm!

Back to your questions - and some may disagree, which is fine, the more opinions the greater your understanding and, as you're paying the bill in the end...

In theory, yes, a more viscous oil should aid the bearing support, especially at lower rpm, but it is the wedge that actually supports the load at higher rpm and a 'thicker' oil will often need more clearance to enhance cooling due to the greater frictional heating. As I said, there are some VERY highly stressed engines using very 'light' oils, and many 1000+ hp four cylinder cars running the OEM grade oils, often 5-30, without problems. You may see frequent comments about race engines needing 50 or 60 'weight' oil to maintain pressure - this is often, not not always, due to the builders running excessive clearances and/or the oil too hot - rather than an actual need for the oil they will usually be better off running less loose clearances, and an oil cooler with a 'lighter' oil.

Additives, such as molybdenum or zinc based products, can have friction reducing properties, but not all friction is the same. they are generally surface 'treatments' for highly loaded interfaces, such as camshaft lobes on followers or fingers, and if you reach the point of actual metal to metal contact in a bearing, it has already failed.

Why do you think you need a 'thicker' oil? Have you read the articles that King Bearings provided?

Hey Gord, yes you are right. My original understanding as well was that thickness gives the support film and not the pressure. I have thought about how a thicker oil could reduce oil pressure and flow throughout the entire engine at high RPM.

The engine I am rebuilding for power is a euro S50B30 (stock 282CHP) that I want to push up to 500+/600+ RWHP using a turbocharger.

Do you have any experience with this engine? I am using stock clearances throughout my engine, so would that mean I stick to factory weight oil of 40?

I am thinking I should jump to 50 since theres a turbo going in and the weather this side of the world is unforgiving - hot and very humid.

The kind of power I am trying to squeeze out of this engine from what ive seen others doing isnt that much, as people push this engine up to 1000+HP albeit on flex fuel. I am using RON 95-97 gasoline.

I have not read the articles provided by Kind Bearings but I will. Thank you for that recommendation!

I look forward to your reply.