It seems to me that modding an rx8 (for "power") is pointless anyway, unless you have money burning a hole in your pocket. IF you're modding the car to make it look, sound, or handle like you want, then so be it but if you're trying to make it a "fast" car you're probably wasting your time.

You didn't fail the test. Your numbers are just average and not strong or great. Bottom line you're out there with everybody else on this. I'd guess 75% of the people on this forum would test in the same range if they had it done today.

 

Your starter rpm is not going to change significantly from test to test unless there is a battery voltage issue at work. Since you already know your approx. rpm there's no expectation that it would change with subsequent tests, unless you also change the starter or battery.

IT is also not that critical. a 10% change in cranking rpm does not necessarily indicate a 10% change in compression readings, probably more like 5% from my experience. So you'll still be within about 5psi of "true" test values even if your cranking rpm is 10% off.

Knowing this you can use a piston tester to gauge the highest compression value in each chamber, which is usually good enough for a test when the condition of each apex/side seal is not in question (in other words when you are not testing for a "blown" engine).

 

IF you're under 6.9/98.6 you're supposed to qualify for engine replacement. None of your values are equivalent to or higher than that value, so I don't see why you won't qualify. Unless you can't provide proof of maintenance history.

 

Well the "modding" part is more for reliability, I only listed the mid pipe cause my cat is failing (rattles) I would like to do the omp adapter mod so I can run straight premix from it. But I know it will void my warranty. I mentioned the ignition because mine has 2 failed coils already, and everyone here seems to love the bhr one. I see no point in buying another cat when I'm not tested for it. All the power I want is there.
I think he had something hooked up to make the starter spin faster, cause that was the fastest I have ever heard the starter spin, although it could also be because the throttle was open and he had the plugs removed?

 

Yes, any engine spins faster when the load of compression is removed from other cylinders/chambers. In a rotary you remove one plug from each chamber at the same time, but the test is being done on only one chamber, so the net cranking rpm increases because the untested chamber is unloaded.

 

i had a little chat with some folks at ricart mazda and asked them why my comp. test was so low, and that the readings are low enough for a new engine.. they basically sent me away.. saying that a carbon build up could of caused the low comp. readings which i wasnt sure about.. and that most rx8s that fail the comp test are in the low 4's and im in the mid 6's.. they told me that i do not have a good enough problem to be qualified for a new engine.. and that i would need to go through a deflooding procedure where they clean the carbon inside to accurately diagnose the problem.. so do my rx8's comp. test results qualify for a new engine? now are they ripping me off?

 

Again your numbers are low enough for a new engine yes. You are under the min requirement for compression. Go to a different dealer. You can try seafoaming the engine yourself first if you want to push for better results but why would you want to pay for a compression test. You need to contact Mazda North Americas corporate offices and speak to someone there. You should get a refund on your money and a new engine put in.

 

Carlos at mazda has told me that he does not have the power to determine if it has passed or failed.. but i have already been told from another dealership that the readings have failed so tomorrow i shall call carlos back and see what happens after that

 

Note sections D2 and F below:



Mazda numbers are measured in kg/cm^2. You can google it if you don't believe me, but you multiply that value by 14.2 to obtain the psi reading. The FSM shows both.

 

i completly believe u.. im just confused with all the bullshit ricart is telling me, this should help me prove that it has failed.. ill be going back monday to get a new cat. then ill make sure all this is taken care of

 

Hate to bring this back up but, I think I copied the cranking rpms down wrong, I now have a scan tool that does live feed.

Also on another note the Autozone 1.4kw starter does not perform much better than the original weak one. But I also see they now offer the 2.0kw starters, and I have my eyes set on one...

In the video the fuel pump is disengage via holding the gas pedal to the floor for the first 2 cranks. I get 180-185rpms cranking, I can't see gaining 40-50rpms from just removing the plugs....
I do plan on one day removing the plugs for an official number but this will do for now. Hard starts are still very far and few, now that I've driven the car more, I'm noticing the hard starts
after about an hour from shut-down, leading me to think I may have a leaking fuel injector. Cold starts always take the same amount of time.

And just so we are on the same page I'm defining "Hard-Starts" as taking 4-5 sec to start. It always starts first crank though.

One last thing, using the scan tool for rpms and a standard compression tester, should net me a very close number to what Mazda machine gets right?

http://www.youtube.com/watch?v=Ld8_dCsLISI

-Shawn

BTW, I still haven't changed the 2 coils that tested out of spec yet, I've been really lazy lately lol

 

Welcome to the new engine club!

 

Just coming out of left field here, but a couple of thoughts from a newbie:

In most engines, doesn't carbon build up actually increase compression?
With carbon build up in the combustion chamber, there is less area for the air/fuel mixture to be stored, so that effectively increases compression...

So technically, if the engine is badly carboned up, and if you decarbon the engine, shouldn't the compression reading then be even lower?

Next question is pretty simple:

If you're driving around with dead coils, won't that increase the carbon build up on its own? More incentive to replace those coils on a regular basis, I would say.

Just my thoughts...

BC.

 

Last I know oh Rotary engine is completely different than piston engines.

you have a lot to learn.

 

More Carbon = less room for air/fuel = less powa

Or something like that....

 

Engine theory is still the same when it comes to combustion chamber volume.

You have a combustion chamber volume of xx. Lets say 65cc. Compression ratio of 10 to 1 means that 650 cc's is being compressed into that 65 cc combustion chamber.

Now, if you have carbon build up in the combustion chamber, that takes up part of that 65 cc's of space meant for the air to fuel ratio. Lets say that takes up 10 cc's of space, forcing 650 cc's of air and fuel mixture into 55 cc's of space. That in effect raises the compression ratio to 11.8 to 1.

This makes for harder starting, and increases the chances of detonation, which is bad for a rotary engine, and only mildly annoying for a piston engine.

Now, I understand that most carbon build up is causing the apex seals to stick into positions that can lower the compression, but can't it at the same time cause higher compression readings if the build up is in the actual combustion chamber port on the rotor?

Hence the reason why I'm asking, so that I can have more clarity.
If you guys don't know the answer, it would be nice if someone who does might answer it. Still makes it a legitimate question.

So what I see a possibility is one of two things:

Combustion chamber deposits are increasing the compression, and if cleaned off, might then lead to lower compression readings on a retest.

Another possibility is that combustion chamber deposits are causing the apex seals on the rotors to not seal properly, causing the low readings, and if cleaned away, will increase the compression readings.

A last possibility I can think of, but don't even know is possible, is that the combustion chamber deposits are increasing the compression ratio to the point that when the air fuel mixture is compressed, the volume is too much for the combustion chamber, which then forces its way past the apex seals, leading to loss of compression that way.

So, can anyone who understands engine theory, in a rotary application tell me which one of my three thoughts above is the most likely one in this instance?

Or, can you point out where in my thinking I am off base?

BC.

 

U couldn't fit 650cc of air/fuel in if 10cc of it is carbon. It would be 640cc going in.

 

The carbon buildup in our engine clogs the ports. Its hard on the intake port only slightly but the exhaust port can get pretty built up. This causes the exhaust gases to recirculate in the engine. Those hot gases are not only larger molecules taking up more space (1600 degrees - 2000 degrees) leaving less room for fuel mixture. The more the exhaust port gets clogged up the less time the exhaust has to get out of the chamber. The carbon also causes the apex seal to freeze up ever so slightly. This can sometimes lead to it getting stuck, or not floating as freely as it should. Once that happens well you get the idea. Stuck apex seal leads to incredibly less compression and less power.

So you have multiple things going on here. Exhaust ports shrinking. This causes the exhaust gases to recirculate in the combustion chamber. Which in turn leaves less space for fuel mixture, and because the molecules are so hot they take up a substantial more amount of room in the chamber. And the apex seals/side seals not sealing properly due to buildup.

 

Its the combustion chamber that shrinks in size, not the amount of air fuel mixture that is being sucked into the combustion chamber.

Whenever you see the picture of the face of the rotor, you see that little cup that is on each face. That is the combustion chamber, that the air fuel mixture is being compressed into, and that the ignition flame from the spark plug spreads through.

That, from what I can read on the forum, and from the pictures that I have seen, is where the bulk of the carbon seems to build up inside of.

If that cup (plus the amount of space between the rotor itself, and the rotor housing) is 65 cc's, then the 10 cc's of carbon build up decreases the total size down to 55 cc's. When the rotor passes the intake port, the engine is still trying to ingest 650 cc's of fuel and air (at wide open throttle, at high rpms, in a perfect world, before the ECU changes the amount of fuel due to the O2 sensor readings). Hence the increase in compression ratio.

Hopefully that helps explain things a bit more clearly, and I apologize if it wasn't clear in my last post.

BC.

 

How will it still try to ingest 650cc's? The carbon isn't going anywhere.

Lets say max volume is 650cc then compressed to 65cc's (like you stated)
If there is 10cc's of carbon built up there will only be room for 640cc's at max volume (intake stroke). Naturally Aspirated engines work off vacuum, as the combustion chamber expands it sucks the air/fuel in.... It will not suck that last 10cc of air/fuel due to the room being taken up by the carbon.

Even If the apex seals were unaffected by carbon you still wouldn't gain any power because you have less air/fuel going in.

RIGHT?

 

He just made every word bold/arial for no reason ...

 

DocBeech,

Thanks for the bit of info.
That does make sense, and was the extra piece of information I was looking for. Interesting that the bulk of the carbon build up is taking place in the exhaust ports leading to the headers and the cat converter.

I'll have to keep that in mind.


I always post in Arial Black on every forum that uses vBulletin.
Just a preference.

Don't believe me? Look at my couple other posts on here, and then search my user name for other forums I'm a member of, and see what I use on those forums.

I understand that you're a fellow NYC native, and we have a reputation to uphold, but seriously, why in the world are you being confrontational with me? I thought from the posts that I read of yours on here that you were one of the better, more productive forum members that gives decent advice, and that the other guy was the silly post crashing troll...

Eh, whatever.

BC.

 

I'm sure your referring to me as "the silly post crashing troll" , and all I have to say to that is....

That's what you get for thread jacking me!

Come outta Left-Field somewhere else and go make your own damn thread about "Carbon build-up" and all it's wonders of increasing your compression.

 

It is customary here to use the default font, size & color for a normal posts. You could use variations of this to emphasize a point being made. You should make an exception to your preference for this forum.

 

simply because the information is everywhere, but you insist on your "oh piston works this way so it should be the same" ... when it comes to situation like this, this is how I usually react.

I mean we all start new at some point, but at least get "some" of your facts straight. Search is ur new friend ?