Engine compression
 

I was told that increasing the percentage of oil in my glow fuel will also increase the compression in my engine. Is this true ?

RE: Engine compression
 

True compression ratio is a fixed mechanical value. By adding oil you may temporarily seal some air leaks and in that manner raise a percieved compression level. If your engine has lost compression there has probably been some damage ranging from scored piston/sleeve, broken or siezed piston ring (if equipped) or even a hole/crack in the piston. Might even be other factors too numerous to list here.

I guess the question is "Why do you feel a need to raise your compression?"

A direct answer to your question is: NO

RE: Engine compression
 

Well, sort of, more or less. If you are running castor oil, adding oil will add some more oil droplets which are incompressible and won't burn in your compression chamber. Sort of like adding bb's to your fuel ( bad analogy[&o]); takes up more space in the chamber and raises the compression ratio slightly.

RE: Engine compression
 

I'm going to have to disagree with Jim. The oil is fully mixed/blended in the fuel as it hits the carb spray bar. At that point it is atomized and runs into the combustion chamber with all "vapor droplets" approximately the same size. As the vapor mixture is compressed it detonates before returning to a liquid state.

RE: Engine compression
 

I have to agree with Bruce on this one.
The volume of atomized material entering the combustion chamber does not change with any change of oil content. By far most of what is being compressed is air as alcohol and nitromethane are nearly as uncrompressible as oil. Any change do to simple crompression would be negligable.
The oil content itself however being less combustible may tend to pool somewhat into any gaps between the piston and cylinder wall.
That said any increase in compression would be minor and indeed offset to some degree by introducing a less combustible mixture which in itself would even out any performance increase.
If you are having compression problems it may be time for a rebuild or replacement.

RE: Engine compression
 

I am still in disagreement. If you add oil, you have to richen the needle to have the same air/burnable mixture you had before. Therefore the volume of vapor droplets is greater than without the added oil. Secondly the burn is not a detonation, but rather a flame front originating at the gloplug. Even model diesels do not detonate, but rather have a burn all through the chamber at once.

RE: Engine compression
 

Oil burns, ever seen an oil rig burn? I've seen turbine engines, real ones, blow a seal and keep running until every drop of oil in it was gone and the bearings fried. Couldn't shut it off. Even if the tiny bit of oil required a tiny bit of richening of the mixture, which I doubt< it would be so marginal as to be ignored. He's not, I hope, talking about adding another 20% oil to the mix. Probably about 1-2 % at the most.

I'll stand by my response and you can stand by yours.

RE: Engine compression
 

I think the increase of the oil to the fuel mix reduces the volatility of the fuel hence the need to adjust the needle to balance the air:fuel mix. The point here is that if you increase the oil content you may well increase dynamic compression ratio but at the same time you have reduced the volatility index of the fuel (i.e less burnable content) and reduced or even negated the benefit that the increase in CR would have brought.

RE: Engine compression
 

Essentially this sounds like an urban myth. Old car engines are meant to be improved by using thicker oil (e.g 20/50 instead of 10/30). The reality of that analogy is that the engines are still old... all that the analogy delivers is potentially reduced oil consumption, not improved performance. Not convinced it's true ... thicker oil is meant to be less likely to run down worn valve stems or be drawn up through the rings. At 100C all engine oil is pretty thin and one type over another is not actually going to make that much of a difference to a worn engine...

RE: Engine compression
 

I agree with Jim, oil takes up space in the combustion chamber and gives a slight increase in compression. Yes, oil can vaporize and burn but in order for that to happen, the temperatures have to be over 500 degrees F. In glow engines most of the oil just passes through the engine and comes out of the exhaust, that's why we need the Formula 409 and the paper towels after we are through flying. In extreme cases of flooded engines, connecting rods have been bent because of hydrolock from all of the incompressible liquid in the combustion chamber.
Some worn out old engines may run better with high oil fuel because the oil is helping seal the compression.

RE: Engine compression
 

I would bet any increase would be negated by the reduced air/fuel charge{if there's more oil there's less room for air and fuel}
and as unmeasurable as the compression of liquids without very special equipment and yes liquids are compressible just not for any practical purpose

RE: Engine compression
 

When people tell you that more or thicker oil will increase the compression in your engine, most of them would be talking about helping out an engine that had worn out to the point it was losing compression from the worn fit between piston and cylinder wall.

Guys who grew up building hot rods and lusting after muscle cars, back when they first came out, would understand the idea instantly. A lot of them had engines that were losing compression. Thicker oil was supposed to "give it back". It was a common situation at the time. And a strongly held belief by many. But understood by all. Was it an old guy who said that to you?

Nonetheless, the technical discussion was entertaining.

RE: Engine compression
 

Perhaps someone should ask whether the engine whose compression is in question is ringed or ABC. Ringed engines tend to have less compression than ABC engines, this is by design. Maybe if you fellas knew if it was ringed or ABC, you wouldn't have to speculate about the effects off putting extra oil in the fuel. Just my two cents.

RE: Engine compression
 

G'day Mate,
Be aware, that the more nitro and/or oil we use, the less methanol we're using, so we use a cool(er) plug. But the oil content will not increase you compression ratio. But in some engines, the manufacturers recomend a lower percentage of oil, than normal, one reason for this is because the fuel with a high oil content is slightly thicker & will not pass easily through the fuel system.

RE: Engine compression
 

If we look at the idea that increasing oil content raises compression because more oil takes up more volume in the combustion chamber then for all practical purposes it's myth.

First up, oil out the exhaust equals oil in through the carb (assuming none of it burns so let's call it castor :)). So how much oil could there be? If you look into the venturi while the engine is running it seems as though fuel/oil is pouring through. But the engine has turned hundreds of times while you glance in so we need to know how much oil passes through the spray bar every cycle because this will be the volume available. This is easy to work out. All you need to know is the total volume of oil in a tank, run the engine full throttle and note the rpm, note the time it runs on that tank and you have everything you need. Run time by rpm gives the number of cycles. Divide the number of cycles by the oil volume in the tank and there's your answer.

As an example a 300cc tank with 20% oil has 60cc of oil. An engine runs at 10,000 rpm for 10 minutes on that tank so it turns 100,000 times. So the rate of oil flow is 60cc/100,000revs or .0006cc of oil per cycle. Now considering your average largish engine like a .60 has a roughly 1cc combustion chamber then your compression won't go up by much if the volume is reduced to .9994cc. In fact if the engine in this example was a 10cc (.60) then it's dry compression would be 11:1 and it's wet (with oil) compression would be 11.006:1 which is completely unnoticeable.

Of course if the OP was talking about raising perceived compression with more/thicker oil then I've done all this for nothing :D

RE: Engine compression
 

Being a beginner at this hobby, I ask the question. What is the difference between 2 cycle and 4 cycle fuel. I was told that 4 cycle has more oil in it. And that I could burn it in a 2 cycle engine. I then ask, what would be the benefit of using 4 cycle fuel in a 2 cycle engine. The answer was more compression and a cooler running engine. So this is what created my original question.

RE: Engine compression
 

Ahhhhh, I said. Well, IMO, assuming the nitro content is the same you won't notice any difference in compression but it probably will run the slightest tad bit cooler because you have reduced the HIGH volitility content portion and increased the lubrication level slightly. I don't know that you need this BENEFIT but it won't hurt either.

RE: Engine compression
 

Ok who did you ask ? and who did the answer come from.

Fuel has gotten out of hand with this oil and that oil and this additive
In the ole days you could run any oil you wanted as long as it was Castor ...engines ran good and lasted a long time

RE: Engine compression
 

G'day Mate,
In actual fact 4 stroke fuel can have less oil than 2 stroke, because of lower high speed revs, generally, & blow by lubrication.
In general most 4 strokes will be happy on 15-18% oil and 5% nitro (eg O.S., ASP etc..) - but always check the engine instructions.
In general most 2 strokes will be happy on 18-20% oil and 5% nitro (eg O.S., Irvine, ASP etc..) - but always check the engine instructions.

RE: Engine compression
 

just another thought,when i would test 2cycle/4cycle engines for compression loss,or a bad valve i would do what is called a wet and dry compression test.when you add oil to the compression chamber and don't have a bad burnt or bent valve,the compression always rises?so i would say adding more oil content would raise compression but not to a point to where it would actually matter much,fluids can be compressed such as air,but liquids they are such a minute amount that it would be dangerous to add fluid directly to a compressing piston in excess.i have replaced too many bent piston rods/blown pistons and engine blocks/valves and cylinder heads due to detonation from liquids getting on top of the piston.the oil in either ABC or ringed engines is what aids in compression and reduces friction along with the nitro/methanol.run one of these engines without any oil and see how good they run and how long they last.

RE: Engine compression
 

Here's my approach. I buy a good quality fuel. Each person has their own preference, for me it's Omega. I typically use 15%. Not sure why I don't drop down to a lower percentage, perhaps it's a bad habit from my boating where I run 50%. I don't keep separate 2 cycle and 4 cycle fuels, it's too much trouble. Besides, I've looked up the oil percentages for standard and 4 cycle Omega. Guess what? They're the same! [X(] Same goes for Cool Power.

RE: Engine compression
 

This is a funny thread. I was laughing all the way through it. "I have to agree with Jim, I have to disagree with Bruce," and then along comes Chuck like a speed bump to smooth it over

RE: Engine compression
 

I have to agree with Cyclic. :D

RE: Engine compression
 

Sales pitches on fuel lables can drive a man to drink:eek:[:'(]:D They advertise on different fuels that oil is added to lower heat:( Helis and cars are for an example:eek: All I have seen is a bigger mess and not a change in compression just in temperment:eek:[:@][:@][:@][:@][:@]