j9fd3s

iTrader: (3)

gasoline floats on oil

mike

J-Rat

iTrader: (2)

Dont know anything about that, I always thought that since they were both petroleum based. they would mix.

But I garantee that if you start a flooded motor, that gas and oil are mixed up good.

Rat

Turbonut

iTrader: (1)

Rat-Cold rotaries more compression then warm. Hog wash! We are really getting carried away now.Now you tell me why these cars will not start after sitting for extended periods of time in cold weather....And it's not flooded. The car has never flooded since I've owned it. 13 years. Why is it the dealers in the cold areas are aware of the problems with these cars when they sit, and I've got someone saying more compression. I wish that was true. I guess the seals expand when it's 10 degrees outside. C'mon, let's get realistic.

J-Rat

iTrader: (2)

Woah!

Re-read the post!... I am having a discussion about cold compression vs. hot compression.

I say, cold compression is lower then hot compression. The other guy says its the opposite.

Here is the post...https://www.rx7club.com/forum/showth...hreadid=139706

Rat

*edit* Turbonut, lets continue this via AIM.

J-Rat

iTrader: (2)

So how do you explain pre-mix, or deisel?


rat

Wankel7

Yeah, lets continue it on IM. I am saying compression is higher when cold. Cold parts contract - Warm parts expand.

My main source on this comes from the factory service man.

For a compression test it says to get the engine up to full operating temp. Then let cool for 10 minutes. Then do the test. This is for a reason. The reason is cold engines will not give you an accurate look at your compression.

IM Wankel7

James

Wankel7

Just a side note from aviation.

Read this off a website that tells you how to prep an aircraft engine for compression checks.

http://www.littleflyers.com/engcomp.htm

"...... The engine is ground operated until cylinder and oil temperatures stabilize. This is necessary due to the difference in co-efficient of expansion of aluminum (piston) and steel (cylinder) otherwise the compression test may be unreliable. Also some cylinders are manufactured with the top bore of the cylinder slightly smaller, it is called choke. This is because the top cylinder operates at a higher temperature and thus expands more making the cylinder the same dimension from top to bottom when it is at it’s operating temperature. This warm up is usually accomplished as part of the pre-inspection run-up. Also during this time crankcase pressures are taken. High and increasing case pressure readings will be manifested long before any decay in compression test readings are noticed. Tracking case pressures over a period of time will give a savvy technician additional clues as to cylinder / piston / ring conditions. See TCM SB 89-9 for Continental specifications. We also verify grounding of BOTH magnetos with ignition switch(es) “OFF” for personal safety and to prevent inadvertent engine firing which damages our direct compression test equipment. Compression checks are conducted as soon as possible after shut down to maintain cylinder / piston / ring operating temperature and clearances. "

In that artical it talks about different metals expanding at different rates. Or engines are no different. Steel end and intermidate housings, and alum. rotor housings.

James

dr0x

Do a compression test when its -15C out (or something, just cold) and then warm the car up and do another. I want to see PROOF compression is higher when cold. To say its lower warm is assuming too much. Its assuming the housing expands more than the rotor, shaft, and the seals (and every other internal part).

wind

man alive! what an annoying car sometimes.... stupid mazda! bah! they screw us over with their electrical design....then charge us an arm and a leg to fix it! bah!

luckily....my car hasnt flooded that badly yet...phew! i did the fuse pull a couple times.... but thats about it.....

patman

iTrader: (3)

oil smelling like gas? r u sure its not just the fact that there is now gas all over your engine bay? (remember how it sprayed out the spark plug holes?)

yes, the oil that is injected by the omp is for lubrication, but in lubricating all the parts it also seals them. this is due to the viscosity of the oil. someones example about the glass on the table is a good one. the way oil lubricates metal parts is by forming a smooth layer in between the relatively rough metal. this layer also makes parts stick together, which creates a seal. try it. get two clean peices of metal and rub them together and pull them apart. now put some oil etween them and do it again. they rub easiier but are harder to pull apart.

also apex seals are not dried by gas like an o ring in a pump. they are metal. they are not porous and do not absorb the oil. the gas just lowers the viscosity of the oil that was on them.
as for being distorted by cold, i doubt that one too. did u ever think that after sitting for a long period of time in the cold the oil that coats them may just have run off and be too thick to spread evenly again?

Pat

patman

iTrader: (3)

oh yeah one more thing. the housing probably does expand more than all the rest, but this would make the seal tighter, not looser! think about it, it expands equally in all directions (just like everything else). the open space in the middle, therefore, is smaller.

want another hands on expiriment? get a flat peice of metal and drill a hole in it. now find a round thing of some kind (like a bolt or something.) that fits very tightly in the hole but is still removeable. now torch that ****** for a while. u will not be able to pull the bolt out till it cools.

Pat

Turbonut

iTrader: (1)

Just jumped on for a second and we seem to have very different opionions on the subject. What I can't understand is what Pat is saying that the oil coats the seals, no absorbtion...The reasion I need an explanation of this, is that I find it difficult to believe that oil can stay on the surface of the seals after combustion. Meaning that during combustion any oil in the chamber would be ignited and blown out the exhaust if indeed oil were on the surface of the seals. And the oil injected by the pump is such a miniscule amount. Believe me, I'm not trying to argue, just get a reasonable explanation of what takes place. Why after flooding, or sitting in cold, something happens inside a rotary to cause no compression, and I don't know what it is. Every dealer here in Central Jersey, that has a mechanic that knows rotaries say it's the seals, but can't give an explanation of what happens inside the engine.
Rat, I misinterpreted you post, I apologize. I thought you were agreeing with the cold vs. hot compression.

Wankel7

I dunno......about the oil staying....but carbon and other junk stays behind. And when that car floods out....gas just washes it clean and it losses compression.

James

dr0x

Yes I know. However, for compression to drop when everything is warm the internals would have to shrink, and the housing would have to expand outward only. And things dont work like that, which is why compression rises when it gets warm, not drops. My front rotor showed 90PSI when it was about -18C out a while ago. Some day when its warmer (or I get an exhaust on the car so I can warm it up w/o pissing neighbors off) Ill compression test it. Im willing to bet that 90 almost jumps to 100.

And for your example, you are heating the ball or bolt and not the plate you put a hole in. Honestly, I have done that but never left the ball cool and heated the plate.

Turbonut

iTrader: (1)

James, that might be possible, but I would think if just gas could wash the carbon & junk out, combustion pressures would do a better job. If given that point, what happens to these engines in cold weather to cause no compression? As stated earlier, I leave the car in the garage for the winter. If I start it every 2 weeks no problem. If it goes 3 weeks or longer I've got no compression.
I'm going to show my ignorance now...Are the side seals the same consistency as the apex seals? Are they made out of the same material?

J-Rat

iTrader: (2)

I think the facts speak for themselves. Keep in mind, that during cold weather, the rotor housings AS WELL as the rotors contract. Now lets assume for a minite that the rotors have a greater expansion coefficient then the housings...

What does this mean? If the rotors contract to a greater degree then the housings, then compression is BOUND to fall on a cold motor. Simple chemistry tells us that all metals react differently to heat and cold.

Assuming the housings are aluminum, and the rotors are steel (I dont know exactly what they are made of), that means they are dissimilar metals. One of them expands and contracts to a greater degree then the other.

Hence, low compression in colder weather.

Rat (NEXT THEORY!!!)

And TURBONUT...COME OFF YOUR AIM HANDLE SO WE CAN CONTINUE THIS WITH JAMES OFF THIS THREAD!! WE ARE HIJACKING YET ANOTHER THREAD!!!!

Jarrett

opelbits

iTrader: (5)

A few thoughts to confuse things:

1- In the 'bolt through the plate' post, as someone else said, you would heat the bolt not the plate to make it jam. Heating the plate around a frozen/stuck bolt (but not the bolt itself) is an excellent way to unstick the bolt. It seems logical that if you heat the plate it would expand in all directions, but that is not quite right. The plate will expand outward, not inward. The hole in the plate does not get smaller in diameter, it gets larger. Also, things do not really expand or contract equally. They do so 'proportionally'. (It's a nit-picky clarification, but I had to say it.) Not to mention the effect of a 'heat sink' or a large dense piece of metal being heated by the same source as a smaller piece of metal with different thermal expansion properties.
J-Rat- aluminum expands and contracts more than steel. If the rotors are steel and the rotor housings are aluminum, then the aluminum would contract farther, theoretically creating a tighter seal when cold.

2- Centrifugal force plays a big role in making the apex seals work correctly. The FSM specifies 250 rpm for the compression test with the factory tester. I presume this is for two reasons. Centrifugal force to 'fling' the seals further out. And also, so the rotors are spinning fast enough to suck in enough air for a reliable reading. How many people have tried a compression test with a weak battery or starter?

3- I am wondering if, over time in a cold environment, those little pieces that the apex seals are seated in are not contracting and pulling the seal away from the housing surface. Or if the seals that are compressed a little through contact with the housing while sitting, are not 'sticking' in a compressed state. Thereby reducing the amount of time that you have a seal and allowing time for combustion gases to escape into the next chamber. Do not unerestimate the strength of carbon buildup.

4- On the topic of 'cold compression/hot compression' (I've not read the other thread yet), what about the temperature of the incoming air? Cold air is denser and hot air is thinner. All drag racers and turbo nutters know this. Why? Colder air makes more power. Aside from the A/F ratio, could the denser air create 'more compression'? The more you air you put in, the more pressure it will be put under during compression?

If I am off base on any of this, please give some comments.

J-Rat

iTrader: (2)

Okay, keep in mind that I wasnt sure of the metallic composition of the rotors in an FC. It was merely an example of thermal expansion. If that is the case, and the rotors have a lower coefficient then the housings, my theory is shot. Not a big deal.

I am going to take some readings on my N/A FC. Cold and hot. Seems like a prime candidate, since I seem to have a hot-start problem.

Jarrett

opelbits

iTrader: (5)

When working with piston engines, some of the more particular builders will machine the cylinder bore to different sizes for pistons that are the same size but have different amounts of expansion. This means that the "cold" engine would have a little leakage and maybe even some piston slap. But when 'hot' all the dimensions are perfect. Too much for me think about.

dr0x

A lot of people do that. That is the exact reason you are to let everything get to operating temp before you start driving.

racermike

A couple of notes: 1. Reason for checking compression (and other things) with a warm engine, is to define a particular standard, ie. "operating temprature". Regaudles of ambient temp. (-20 to +100) or whatever, operating temp. is the same ~180~. and so the manufacturer can define the standard at THAT temp..
2. If you were to be trying to determine the internal condition of a piston engine with a problem, you would first check compression of all cylinders and note it. This is called "dry". Next you would add some oil to each and check again. This is called "wet". You then compare the two. If the compression stays virtually the same (and is low) you probably have bad vaulves. If however the compression goes up and becomes "good", your problem is worn rings. The reason is that the oil has helped to "seal" the rings by filling up the small imperfections. The amount of oil metered or injected in a rotory IS small, and is increased with RPM. It IS there to lubricate the apex seals, but it also, by default, helps them to "seal" small imperfections. This sealing effect is almost ZERO at normal operating speeds because the engine doesn't have "time" for the pressure to escape. At cranking speeds however, there is much more time for the pressure to escape, so extra oil can help. The "sub zero" starting fluid is 90% antifreeze and 10% water, not what you would think could be used to help start a motor (like either). But it is not used to "ignite", but rather to "seal". Mazda puts it in the car therefore, to help RAISE the compression when very cold conditions are being experienced, and the normal "sealing" is not enough to provde (sufficient) compression.
3. One reason for waiting 10 minutes before removing plugs is that it is not good to try to untorque them while they and the aluminum housings are hot. It can tear the threads.(I use antisieze)
4. The reason that letting a car set for an extended period of time leads to hard starts is that the small film of oil left on the surface, does in fact drain off and pool at the bottom of whatever space there is. This has the effect of "lowering the compression" simply because the very small imperfections can again be leaky. If you were to attempt to start it normally, it may not have enough time for the oil to be redistributed and reseal before enough gas is added to the mix to bring on a flooded condition. This is a good reason to disable fuel flow (or do not step on accelerater with a carb) until you crank it for a bit. Crank it a little while to move the oil around, THEN step on the gas, set the choke, or enable fuel flow.
5. Not all the oil gets burned off or out of the motor because it resides on the surface, and gets replentished somewhat with each revolution. If it were "misted" into the mixture, most of it WOULD burn off and/or be ejected with the exhaust flow. Even with pre-mix or 2 cycle motors where the oil IS misted with the mixture, some of it doesn't burn and is DEPOSITED on the surface where it can lubricate.
6. Oil and gas DO mix, but to varying degrees, and will reseparate over time. 2 cycle oil is formulated to mix VERY well and stay suspended for a very long time. The small amount of gas that could work its way into the main oil supply from a single "flooding", should evaporate and be burned off thru the crankcase ventilation system. Repeated "floodings" in a short period of time and without a good "run time" in between, may be cause for more concern.

Finally, when I am preparing a "junkyard" motor for installation, or any used motor that has set for awhile, or trying to bring back one listed as bad; I use ALL KINDS of stuff. Mystery oil, ATF, carbarator cleaner, WD-40, etc., all in the attempt to loosen stuck seals, remove varnish and carbon and rust etc.. I do it however with the exhaust pulled off so none gets downstream into cats or anything, and with the oil plug or pan off so it doesn't stay in the motor. I spend SEVERAL days working the various things thru, untill I can turn the motor and get only clean fluids out. Once it is in the car and hooked up however,,,,ONLY motor oil or 2 cycle oil. I have never tried water, but would not be afraid to. ATF, I don't think I would.

patman

iTrader: (3)

well Turbonut i think that ur question about what i was saying has already been answered, but here's my answer anyway.

Yes, most of the oil on the seals IS burned/blown away. that is why rotaries consume oil regularly. however, this is not important for compression because there is a period in the rotation of the rotor where compression is not important, and this is where the omp recoats them with oil. it only takes a very small amount to do this.

as for my bolt example, i was not talking about the way to free a sticky nut like some of you seem to think, i was just using it as an example for equal expansion. it seems that everyone is now in agreement on that issue now tho so who cares? btw it is not true that the hole gets bigger. the reason that a bolt is freed when u do that is that when it gets tighter in there it "squishes" so to speak the rust or whatever other **** is in the threads keeping the bolt from turning.

hope all my bullcrap is actually helping someone fix their car and not just idle chatter!!

Pat