Why do engines loose compression slowly?
 

Hey Everyone,

I've been wondering for a while now why our engines sometimes loose compression slowly instead of going all at once.

I've done some searching, but haven't been able to dig up all the reasons.

Here is one reason, but it's not what I'm looking for:
Stuck seal - I understand this, but its fixable and doesnt ruin an engine unless the seal becomes unstuck in the wrong position and breaks.

If you don't have a stuck seal, why would an engine slowly loose compression? Do the the apex seals just wear down over time?

And aside from that, if you have a pretty new motor (meaning no wear on the seals), and it was built properly (good clearances), what would explain that loosing compression?
- Could this come from a chipped apex seal due to detonation? And the person just got lucky that the broken piece exited the motor without breaking anything on its way out?



This may lead you to think I'm worried about my engine (it feels down on power), and I am haha. I know the first thing to do is to do an actual compression test, which I plan to do. But I want to know a little more about the potential causes as well.

Colin, I have a rotary-specific compression tester I'm bringing with me to the Carlisi meet.

Off the top of my head, possible reasons include improper exhaust porting, lack of premix/failing OMP, and tuning issues.

Even if you do everything right, nothing lasts forever—the same goes for piston motors, over time the seals wear, you get oil blow-by, and lowered compression.

One question I have is, N/A rotaries maintain power and compression for a long time. Witness Formula Mazda and other motors going for several seasons without rebuild.

I understand that with a turbo motor there is more heat, and greater "strain" on components due to extracting greater power from the same moving parts, but does that necessarily equate to increased wear on seals, i.e: compression lowering more quickly? If so, how?

I totally get that there is far greater chance of catastrophic failure, and so most turbo rotaries burn out rather than fade away, but wondering if there is any reason they should "fade" faster than N/A if they avoid catastrophic failure.

I think they tend not to last as long due to all the heat generated, as well as the higher power output for the same size engine. This along with possible slight detonation events that can weaken the apex seals due to poor gas quality and suspect tuning. Don't forget the 'unbreakable' seals which tend to be unkind to the rotor housing surfaces. For modified FDs I've come to believe in premix more and more over the years.

the faster those rotors are running up in RPMs the faster they are wearing every part attached and come in contact with

i believe that it all has to do with heat and simple metal wear, the seals are made of either carbon or metal, our side seals corner seals and seal springs are also all made of metal. when heat gets to them they expand and contract with the heat and cooling done in the motor, thus making the springs weaker putting less tension on the seals pushing on the housings and causing a slow loss in compression. thats what i believe anyways

sure the springs can fail and loose strength but more importantly it's wear on the the actual apex seal (height shortens over time) and this combined with wear on the housing and irons makes for a deminished seal between the various seals and housings. so with that said as the rotor is asked to spin faster in a shorter time zone it creates more and more heat causing more and more wear. fix the heat shorten the wear, which is why we like premix and water inject........etc......

I was just hoping for input from some of you experienced builders in another thread regarding what happens during hard de-acceleration.....

Since the S6 and later OMPs are rpm and load dependent, there would be (brief) periods where little or no lubrication is being provided to the hard seals during times of hard de-acceleration. Also true (possibly more so) if someone is pre-mixing, since injector duty goes to zero too. I don't track my car, but guessing these periods would be longer (from higher rpm) and more frequent if you did. Now I understand that there is no combustion event taking place during hard decel that would burn it up. But RPM would still be relatively high...possibly sweeping away whatever residual lubrication was present. ???
I'm not a guru by a long shot, but wondering if over time, this could be a contributor to hard seal/housing wear...which would apply the OP's original question about compression loss.

Thanks Rich, I think it might get some use :)

I definitely get the general wear over time theory (well i guess its more than just a theory haha). But in my case, I've always used premix, and my engine has less than 8K miles. I have had fuel starvation in a corner before, and literally felt the fuel cut, so i could have damaged a seal. But how does this make it so I'm just down on power as opposed to having a blown engine? A chunk of the seal is gone but didn't destroy anything on its way out?

I thought under deceleration that there was still combustion? The spark/coil don't just shut off do they? And there is the same amount of fuel per revolution that you have at idle being injected, right?

I see ZERO for injector duty cycle on my commander under hard de-acceleration. It's usually brief, recovering when rpm reaches something resembling idle. Particulary brief between shifts, tending last longer with engine braking.

^ I agree the fuel is shut off. I do see this as a problem for people who ONLY use premix. Even if there is no combustion the air is being pumped through the motor and that can wash away oil residue. I believe this is more important at higher RPMs due to centrifical force pushing the apex seals against the rotor housings with more force scraping away the oil faster.

The thread you are talking about really made me think and now it does make sence to keep the omp and premix as well.

One thing that makes me believe that the injectors are shut off is the actual tone change when you let off of the throtle. Also when coming to a stop in second gear just coast with the RPMs from 2000 to below 1500 and you will feel an abrupt difference in the amount of engine braking and I believe that is due to the injectors starting to fire again.


John

For those of you running a Power FC, the fuel is shut off when the throttle is released and rpms are higher than the "F/C" settings under the rev/idle menu.

"why our engines sometimes loose compression slowly"

by "our" it almost seems like you are including me. please count me out of the "our."

if a rotary is properly built, tuned and fixtured (meaning correct subsystems) it will GAIN compression over time.

losing compression means bad things are going on inside.

heat does the damage.

generally compression is lost due to apex seals warping slowly from excessive heat.

deal w the heat, keep the motor interior carbon free and you will gain compression slowly over time. my motor gained 10% comp over a four year span.

Everything wears out over time. While Howard's engine gained compression over time, it was certainly due to components wearing together to make a better seal. However, that engine (like any other) will eventually experience more wear on those same components, and lose compression. That's life, nothing lasts forever.

Sorry Howard, you are counted in ;)

Of course it did it probably had 5 to 10k miles on a fresh build with 200 dyno pulls which is like daily driving and taking your car to the track for a day.

Get that car on a road course 10 or 15 weekends a year and run it for 3 or 4 years and get back to everyone about the increase in compression :D

I'm pretty sure I'll be sending my engine to you when it goes out...hopefully that's not for awhile though :)

So will everyone else :nod:

I bet he's already backed up big time.

"That's life, nothing lasts forever."

sure, given my age i can assure you i am well aware of that.

while nothing does last forever, some things last longer than others.

i agree w Fritz's re road racing. it delivers tons of heat.
you can offset almost ALL of the heat. (see my thread/ 3rd Gen section... the FIX)

for my 200 mph Texas Standing Mile attempt (Oct 25) i figure i need 600 SAE rwhp for 30 seconds. while that doesn't sound like alot more time than the typical 10 second trip down the quarter mile it is many many times more challenging for the powerplant.

consequently i will not be running gasoline... i will be running on 100% methanol. the motor will need the immense cooling of meth.

most of us are not planning on making 600 rwhp but ALL of us need to use a number of methods to readjust the interior heat levels within our motors. if you do remove the heat you will go a long way to maintaining apex seal integrity and support compression.

as to my 4 year motor... i had over 23,000 miles on it when i decided to break out my digital micrometers and just see what was going on... there was nothing wrong w the motor, i just wanted to take a look. during the 4 years it had gone from 385 MM HG to 410-415 and the last reading was the top compression tick.

i do believe it had to do w the Atkins apex seals marrying w the rotor housings with absolutely no warpage. (interior heat management)

i do run nothing but Mobil 1 5-30 and i add 4 oz of ZDDP at each oil change (3K miles).

there was almost no interior carbon existant. most motors i disassemble have huge amounts of carbon, perhaps as much as a tenth of an inch coating the rotors.

here's a rotor from my 09 motor. this rotor has not been cleaned

http://a.imageshack.us/img220/3706/58558352.jpg

notice no carbon inboard of the sideseals... again, most motors have the same amount of carbon inboard of the side seals as outboard! you want no leakage past the sideseals and if they are sloppy lots of bad things happen.

so, yes, nothing last forever, but mine is going to last longer than yours.

you remember the story of the two guys sitting around the campfire when a hungry Grizzly shows up. one guy starts putting his track shoes on and the other says, "what are you doing, you can't outrun the bear..." his buddy says, "i don't have to outrun the bear, i just have to outrun you."

speaking of bear i am going to be off the net for a few days in the northern wisconsin wilderness... probably w my track shoes on.

howard

You forgot to say neener neener neener ;)

For the same reason you slowly age!

Hmm some good info in this thread lol

Chudson, maybe your car is feeling slow because of the weather??

I know there is a substantial difference between doing a couple of pulls in this weather than in the fall and of course winter weather.

Yes, weather always impacts how the car feels to most people, personally ive seen it more on early/mid 90s cars.

Ive always seen it as the 8 Bit Ecu of that time cant do much to compensate for weather/condition's overall.

Can used housings potentially cause the loss of compression with such things as unnoticeable warpage? Is it just overly cautious or a determining variable?

Howard, did you use new housings in that 23k engine?

"Can used housings potentially cause the loss of compression with such things as unnoticeable warpage? Is it just overly cautious or a determining variable?

Howard, did you use new housings in that 23k engine?"


most of what i personally run (including the 4 year build) and build is used as far as housings which now retail for over $700 each.

the good news is that good used housings are greatly underloved as far as being able to deliver most of the hp at significantly lower cost.

they need to be straight. they need to have not shrunk as to width, they need to have decent frictional surface.

you can help your housings maintain their shape by running 10 or colder plugs. hot plugs retain heat around the spark plug boss and eventually various things happen... all lethal

the plug boss distends and eventually cracks

the plug boss heat warps the apex seal in the longitudinal center

the retained heat from 9 or hotter plug eventually shrinkes the width of the housing at the plug area and below. a 2 thou shrinkage and the housing is toast. i just had to junk two nice looking housings because they were way past this.... all because of heat.

manage the heat, don't loose compression.

off to the woods for a couple of days.

howard

What is the heat range of the stock plugs?

7's and 9's, a lot of people run 9's all around.