Oil Viscosity and Horsepower

Keep in mind though that many smaller air-cooled engines do not have an oil pump and rely on splash lubrication. The bottom of the conncting rod, or a small 'dipper' that is cast into the rod literally scoops up oil from the sump and flings it around inside the engine. Run the oil level too low and you run the risk of reducing or stopping lubrication on these engines.

Also keep in mind the choice of viscosity (whether single-weight or multi-weight) is very dependent on ambient temperature. Run too high a viscosity when it's cold outside, or too low a viscosity when its hot will shorten the life of the engine and will put your warranty in jeopardy. You do have some leeway, but stay within the bounds of the mfgr. viscosity/temperature charts.

Believe me, if the engines could live with a way lower viscosity oil and make more advertised HP the mfgrs. would do it in a heartbeat! But running at the low end of the approved chart will make the engine a bit more efficient.

SO if I put a 5w-30 in my Kohler 25 I should see a horsepower increase corect?

Will this shorten the engine life in hot desert climates?

I always thought it was all about starting your TM.. 5w-30 in the winter and 10w in the summer(I've also ran straight 30). My TM runs hot and I don't know if I would run 5w in the summer.. Your example is a 350 gaining 10 horse what would my 25 horse gain? I haven't really noticed a difference between the two.

I'll stick with the lube over the power that might be gained.

I just rebuilt the Honda GX340 (11hp) in my generator. It uses splash lube. There are others too.

Stock Chevy 350's over the years ranged from about 160hp to 300hp (not including a few HO motors). If we use a 240hp 350 then a 10hp gain is about 4%, so a similar gain on a 25hp motor would be 1hp. That's if you run it at 5500 RPM, and would be less if you run at a lower RPM, as truckmounts do. Friction losses are exponential - double the RPM and the friction increases by four times. Halve the RPM and the losses are only a quarter of what they were at the higher RPM.

You would only see a hp gain if you impose a higher load on the motor by running at a higher RPM or perhaps increasing the vacuum setting on your relief valve (if you have one...). If you run the lower viscosity oil and don't change the RPM or load then rather than a HP increase you would see a decrease in fuel used per hour, in other words you would get the same RPM with a lower throttle setting.

At 2500 RPM you could realistically expect to realize 1/2% - 3/4% fuel savings.

If your Kohler is a CH-750 25hp then the mfgr recommends 5-30wt oil for 32degF and below (winter climates) and 10-30 for 0degF and above (warmer climates) so yes, using 5-30wt in a hot desert climate will probably shorten the life of the motor. How much, and whether the fuel savings will offset the additional cost of more frequent engine replacement is something you should consider.

Sure, let me explain. The main difficulty here lies in the way dyno runs are done versus real-world loads.

To do any work, an engine has to exert force on some load, be it the wheels of a car or the lobes of a blower. We measure the force in foot-pounds (in the US). Force exerted over time is horsepower. One horsepower is defined as 550 foot-pounds for one second. So if you had an engine that turned 1 RPM and it took 550 ft-lb to turn its load, it would be delivering one hp to the load. 2 rpm and 275 ft-lb is the same thing, as is 10 RPM and 55 ft-lb, 100 rpm and 5.5 ft-lb, etc.

Disconnect the load and the hp output drops to zero, as no torque (ft-lb) is being developed. So, the hp is dependent both on the load placed on the engine AND its RPM.

power = work/time

To increase power according to the formula, you can either increase work (more ft-lb, change the load) OR decrease the time (higher RPM).

In a dyno run, the load is artificial, and can be varied at will. Dyno tests are done at WOT (wide open throttle) and the load or torque is varied to achieve a certain RPM. In the case of your 350 engine, it was loaded down to 5500 RPM, and the hp measured. Then they changed the oil, and loaded it down to 5500 RPM again. With the lower friction it produced more torque. So in this case, time (RPM) stayed the same, but work (ft-lb) increased, so there was a hp gain.

If they had instead loaded the motor down to the same torque reading (the second run) instead of the same RPM, the motor would have been running at a slightly higher RPM because of the lower friction. So, in this case, there would have still been a hp increase. Work (ft-lb) stayed the same, but time decreased (higher RPM). Same difference.

So - take a 25hp engine and hook it to a set of sheaves and belt so that it turns a blower. Turn the engine at a certain RPM, say, 2000. Measure the torque, calculate hp. Lets say you find that 20hp is being developed.

Now keep everything the same, but remove the 25hp engine and install a 500hp engine. Start it up and adjust it to 2000 RPM. Now how much hp is being developed? Still 20. Why? Time (RPM) hasn't changed, and torque (ft-lb) hasn't changed either, because the load (the blower) is still the same.

Now here's the key. Blowers don't have a variable load dial like a dyno. Turn one faster and it takes more torque to turn it, turn it slower and it takes less. So, if you don't change anything (pulley ratio, vacuum setting, etc.) that increases the load, the only way to make an engine develop more hp is to increase the speed. In the case of our blower, increasing the RPM both decreases time AND increases torque (because the blower is harder to turn at the higher RPM) IF the engine has the hp to spare.

Now it's true that every engine has a torque and horsepower peak, and if you exceed the peak RPM the horsepower will go down. I was assuming that your hypothetical engine has the necessary power reserve. If we are past the peak, The gain would be reduced or even worse, and the hp gain would be less or none. I was giving you the benefit of the doubt, and assuming the engine had enough oomph.

I am having a hard time with this one. As Ara was eluding to, every engine has it's own HP/torque curve, so how could we just assume that a 25 hp and a 500 hp engine are going to produce the same HP at 2000 rpm with identical loads? There are just too many variables in each engine, ie, compression ratio, valve clearance, cam lift, stroke, etc.

I agree with you that the blower will have the same HP requirement, but that has nothing to do with what the engine(s) are putting out. It's not like electrical current, amperage, in particualr, where the powered componenent will only draw from the power source what amperage it actually needs. Basically there is no amperage present in a circuit until a load is applied.

I am not a performance engine builder and have never ran an engine on a dyno, but this is just my understanding of things. I am not saying you are wrong, I am just having a tough time wrapping my mind around the concepts you introduced.

So what do you guys make of the 0W multi grade oils? They sell it here, but it would seem to me that it would have to get pretty **** cold before I would feel comfortable putting that into my engine(s).

Good discussion by the way guys!

Great post Keith! I couldn't agree more. Although, starting the TM in relation to oil is only part of the equation.

For the cleaner out in the field all of this mumbo-jumbo doesn't mean much. It sounds like you did your homework on your oil and have a solid game plan for keeping your TM efficient and protected.

You will never notice a HP gain on your engine by the type of oil you use; even with synthetics, you won't notice a HP gain, especially on the 20-25 Hp engines that are so common in our industry. Having said that, I am a firm believer in synthetic oils.

Actually, it is exactly like electrical current. Technically engines don't produce any horsepower unless there is torque. There is no torque unless there is a load. So, the horsepower produced is directly proportional to the RPM (volts) and the torque delivered to the load (amps). You can even convert horsepower (mechanical power) to watts (electrical power). One horsepower is equal to approx 746 watts.

When you say "the blower will have the same HP requirement" then by definition you are saying that the two engines are going to deliver the same horsepower under the conditions we set forth. First, the RPM is the same. Second, the torque (which is determined solely by the load, in this case, the blower) has not changed. If you spin it at 2000 RPM it requires the same torque no matter what prime mover makes it go.

Suppose I put the engine in a soundproof box and bring the output to you via a long driveshaft. You can't see or hear it run. You have no idea whether it is a little kohler, a funny car drag motor, a steam engine, or a guy pedaling a bicycle. But you can measure the RPM of the shaft, and you can measure the torque. Do you have enough information to calculate horsepower delivered to the load? Yes! You have time (RPM) and force (torque). If I make the driveshaft turn your blower at 2000 RPM, you will calculate the horsepower delivered to be 20. Every time. I can swap motors and you would never know. You would still calculate 20hp delivered to the blower.

If I go inside the box and disconnect the driveshaft but leave the engine running (or make the poor guy keep pedaling) at 2000 RPM, then ask you to measure horsepower, you will calculate zero hp. Why? because the blower is not turning, therefore torque is zero. No torque = no horsepower. Yet the engine is still cranking away at 2000 RPM! It's exactly like unplugging a lamp from the wall. The voltage is still there, it's still 120 volts (in the US), yet because the LOAD is no longer drawing any amps, power (watts) is zero.

If you have ever taken any mechanical or electrical engineering classes, you would notice that the analogy is drawn between electrical and mechanical systems as behaving the same, just different units.

Argue/discuss all you want, the laws of nature are pretty set in stone, at least in the known universe. Here's a more scholarly treatment of horsepower, if you're interested:

http://en.wikipedia.org/wiki/Horsepower


Ciao!

When I have a few spare hours I will research this a little; it has been a few years since my last physics/mechanics course!

Again, great topic!

I use 5w20 in the winter (it gets well below freezing around here) and 10w30 in the summer as my operations manual from Prochem recommends.

The first number represents a "winter"thickness rating, and I believe the second number is the thickness or "viscosity" for summer and higher ambient temperatures and ultimately much higher temps inside the engine.

Agreed! Good thread. Always beneficial to think about stuff we don't use everyday.