I understand that compression numbers tend to be higher when testing a cold engine, and lower when that same engine is hot. However, after searching and reading numerous threads, I can't really come to a definitive conclusion why this is? There was some mention of the housings expanding at a higher rate than the rotors, but even that does not seem to make sense. Just curious why this happens, I know it is definitely true, but it still seems counter to logic. Anyone care to help me wrap my brain around it?

 

ive heard opposit. ive heard that a cold motor will not give the corrct compression results. i hope so bexcause my motor was showing around 90 every when it was cold, it runs perfectly, and i take it to 7.5k rpms almost every time i shift.

 

Correct compression test results are obtained when testing an engine at normal running temps. The components don`t fit together as well when cold resulting in incorrect results (lower compression than it actually is)

 

No, see this is the problem, everyone seems to think a cold engine will give you lower numbers...a cold engine gives you higher numbers. A hot engine gives you lower compression numbers. As I said, this seems counter to logic, clearly Im not the only one who thinks so, but it is the case. I am just wondering why that is so.

 

Anyone have a clue?

To reiterate:

Cold engine = higher compression numbers
Hot engine = lower compression numbers (but the proper numbers)

There's really no debating this point, it is stated in the FSM, as well as numerous online resources. So why is this so, thats what I am trying to understand.

 

I remember of a thread someone posted a while back where he actually suceeded to make different graphics of cold/hot and WOT/throttle closed

I dont remember exactly how did he manage all that data to make a graph, but he explains it pretty clearly

the conclusions were that the best number were reached in the : COLD AND WOT

the worst number were obtained with : HOT AND CLOSED THROTLLE

the guy had around 15 PSI of difference between these two.

l-p

EDIT : I dont have a clue about the reason why though... Unless somebody's a mechanical engineer or advanced physic teacher, nobody will be able to point the cause WITH NO REASONABLE DOUBT since there's a ******* shitload of mathematic to understand what's exactly going on with each of the materials under heat and pressure.

 

That is pretty much affirming what I stated, which is fine, but what I am trying to learn here is WHY it is this way. I don't need to know whether it is true or not, we know it is true.

 

that's the reason why I edited my post :P

 

Some proof could be this...

A engine with low compression has hot start problems...(aside from leaky injectors)...hummmmm

James

 

Has to do with intake air temperature, no? Colder air is more dense, and a hot engine will heat the intake air no matter what you do.

 

lol, nice one. Is it really that complex?

And Wankel7, I know it is true, I know about the hot start, I'm just asking why it is true....I guess it really isn't necessary to know, it was more out of curiousity than anything.

 

Boyle's law states the relationship between volume, pressure, and temperature in ideal gases (air in your rotor housing during the compression test). The equation is V x P = kT where v is volume, p is pressure and t is temperature. Assuming a constant temperature, k is the consant that you would always receive by multiplying the volume by the pressure. You can see by this equation what increasing or decreasing t would do to k. Okay boys and girls (my physics teacher used to say that), let's do a few equations solving for k.

P= 5, V=4, T=2, solve for k.

5x4=k2

20=k2

20/2=k

10=k

Now, let's increase the temperature.

V=5, P=4, T=4, solve for k.

5x4=k4

20=k4

20/4=k

5=k

Notice the colder temperature yields the highest result.

I know this is all conceptual and doesn't actually deal with the physics and thermodynamics of a rotary engine, but it gets the mind thinking at least. And it all jives with the results above in your own compression tests. Any more questions... you'll have to go ask your physics teacher or call a Mazda engineer.

 

Why does it matter, we know that this is the case, so just accept it for what it is?

 

even so, its not the compression your looking for. the compression your looking for is the compression your engine uses when its runing. you only test it cold to see if it has compression.

infact i know 100% that if you test it at two different temps you get higher reults on a warmed up engine than on a colder engine.

i was getting around 90 all around ad then i drove it aroud for 12 houre, and the magical number jumped up to 100.

 

Under normal circumstances, ROTARY ENGINES LOSE COMPRESSION WHEN WARM. THIS IS WHY MANY BORDERLINE WEAK ENGINES FLOOD AND HAVE HOT START PROBLEMS WHEN WARM, but fire right up when cold.

I refer to normal circumstances as an engine that hasn't sat and stuck for a while, doesnt have oil or atf in the chamber to raise the compression numbers artificially, and isn't breaking in new seals.

OF course, if you have an old engine that sat for 2 years and you test it, sure it'll be weak...but once you get it running, the seals will start to free up a bit, all the crap will be burned out of it, and a fresh coat of oil will be restored, and of course the numbers will go up. Same for a fresh rebuild where the seals haven't seated yet...of course those numbers will rise with run time.

 

Thanks for the input guys.

I have accepted it Kevin, just a little curiousity is all, nothing wrong with learning something new. When I raised this question, I never for a minute doubted the truth behind it.

 

Well its good to hear that. I just baught an 87 SE engine has 158k miles on it car is in great shape, no smoke comming out the back when shifted at high rpms, engine seemed strong, compression was exactly the same all the way around but was at about 70 psi. It sat from early November through Mid August without ever being started. By the time I got it home, about 30 miles it seemed to be running a lot better, had a lot more throttle response and now starts much faster. I guess I need to do another test on it.

As for low and hot compression numbers, there are going to be dozens of factors involved many of which will counter act eachother. On a warm engine the air and fuel are going to expand going into the chambers, since it is less dense compression goes down, but also the heat inside the chambers will cause the air to expand even more and increase pressure which helps to negate the lower pressure from it being less dense. At the same time the seals experience a lot of wear with the engine warm so with it cold they shouldn't match as well which would make you think the numbers would go down when cold, but the air is more dense when cold. There is also the fact that multivisocity oils are designed to be thinner when cold and thicker when hot so a cold engine will be easier for the starter to turn over.

In the end you are worried about how the engine will run so only the specifications of when the engine is warm matters.

 

Working with that concept, the whole issue is a matter of intake air temp then? That would seem to make more sense than the whole rotors expanding at a different rate from the housings suggestion that I've read on here.

How hot are intake air temps typically? I think I'll run a search.

 

Well i sure as hell was wrong. I stand corrected. My earlier post was based on hear-say from many years ago. I`ve learned something. Thanks Guys

 

No, intake air temperature is not the only factor that determines the yield of your compression test... obviously, if a rotor is cold and has not expanded to the wall of the chamber due to normal operating temperature... you'll essentially have a leak between the rotor and the chamber housing. So, the cold rotor will yield a lower compression because of the "leak". Furthermore, the warm rotor will yield a higher compression because of the sealed rotor/chamber housing (no leak where air can escape).

All of these factors go into determining hot/cold compression yields. They have to. Isn't physics fun?

 

My theory is that cold apex seal springs provide more tension than warm ones, which explains the relatively small (yet important if your engine is in that borderline area between okay and weak) 5psi differential between cold and hot. But thats just my theory. There's not much other explanation for it, unless you could say that the rotorhousing (which is aluminum) contracts more than the rest of the engine when cold, which would also provide slightly more spring tension on the apex seals...making for a better seal when cold.

I don't buy into this intake air temp theory affecting compression measureably.

 

So, ultimately we have three factors...

1) Rotor (seal/spring expansion/contraction)
2) Housing (expansion/contraction)
3) And intake air temperature (hot/cold)

To what degree each one of these factors is more important that the other, I don't know... they all make sense and 3 & 2 would seem to counteract 1. So, there is a happy medium somewhere. Good talk people. I know the differences in results of cold/hot compression testing are already known, but a little intellectualism (insert: brain exercise) never hurt anybody. Are there more factors to add to the list? Maybe the viscosity of the oil at different temperatures?

 

Hey, that sounds like a good theory, and I had yet to read or hear anyone mention anything about that aspect. Good stuff, now this is becoming the discussion I had originally hoped for. Thanks all!