When you do a compression test with rotaries..it's either good or bad?
You cannot have a low compression.
It's either you have compression or no compression???

I have full 17inch/vacuum at neutral and I'm selling the car...the buyer wants a compression check...Should I be worried? If everything seems to be pulling fine and the engine has 99,000 Kilometers??

 

compression on rotaries can lie about engine condition though.

 

Yes the buyer want to do a full inspection on my car...should I be worried?

 

FALSE.

You can most certainly have low compression without having no compression. In fact, my motor is being rebuilt now because of low compression, but the engine was not "blown". All numbers were pretty even but were below spec.

 

How much vacuum did you have for a low compression engine?

 

At what RPM? 17 sounds about normal to me..

 

16-17 normal idle, 18-19 idle with accessories on

 

did you do the compression when engine is hot?

 

Tyler, not to pit one person's word against the other, but I had the same worry/concern too a few years ago. Before I bought my FD, I had it compression tested, and this is what they said (it was a Mazda dealer in Miami... hey, I didn't know much, nor did I know about the forum lol):

Standard is 7.0k kg f (farads?).
Differential limit = 1.5 kg f
Rotor Differential limit = 1.0 kg f

They ran the test 3 times

1) R1 = 5.5, R2 = 5.8
2) R1 = 5.6, R2 = 6.0
3) R1 = 5.7, R2 = 6.2

RPM 265 (?)

Note: Rotor (1) Chamber (1) is at 5.5, which is min.
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So...a few months later, when I was at PFS, I asked them about this, and they said it not to really worry about whether the numbers were high or low. The important thing is how close the compression of the two rotors are together (ie don't worry about differential limit...just about rotor differential limit). I'm all ears if someone can clue me in as to which way to go here...

 

So..what did those numbers represent to mazda? Was your engine good or normal?

 

this is a confusing topic.I thought 9.0 was perfect compression and under 6 meant that a rebuild was needed.When i bought my last fd with 27 k miles i had it tested and all readings were between 8.2 and 8.5. The car was still running strong when i sold it with 51 k miles.

 

The ROTOR differential was fine... the front and rear rotors were consistantly within 1.0 kg f from each other. But, the limits of compression for each rotor were NOT. Norm was 7.0, and variance was 1.5. So upper limit for norm would be 8.5, and lower limit would be 5.5. One of the tests showed one rotor at 5.5, so it was said to be "at minimum factory specs." Basically, the guy told me it had low compression, and was on it's way out. PFS tells me that's not the case. In fact, PFS told me you can have a motor blown w/ an 8 or 9 even, if the difference between rotor 1 and 2 is great (like one at 6.5, and the other at 8.5, etc). There's a thread about this somewhere, but again, w/ no email notifications, and the search function takin a crap, I can't find jack on this forum...

 

it is quite confusing! can somebody tell me something that is not confusing? I head somewhere that you either get compression or none...

 

I really thought that having 15-20 inch/vacuum was a good sign of a healthy engine....and the car seems to pull quite constantly however it does idle weird ...600-750 up and down but only sometimes. been told PFC would sold this.

 

Sorry...I of all ppl don't have answers. I just have lots and lots of questions Hopefully Rynberg or someone else can clarify things for us a bit here.

 

ramy, your results are good b/c the +/- tolerance is small (#s close together) but the #s themselves are/were low. Rynberg is correct. Vacuum at idle is a good measure of engine health, but compression #s are *the* measure of engine health.

aznraver, does your car start right up when it is hot/heat soaked?

 

Here's what I know, real engine rebuilding experts can chime in if I'm wrong:

*if you have low but even compression numbers, it means that you don't have any blown seals, but that the engine is wearing out -- either the seals are worn down, the housing are worn down, etc. This was my situation, and to be frank Ramy, it sounds like yours. The engine will run decently, but fuel mileage starts dropping, the car becomes harder to start when running hot, the low-end power and throttle response suffers, etc.

*anything else other than even numbers all above 6 kg/cm^3 indicates a blown motor

 

hey howcome you guys know my name????
and yes my car starts easily anytime in any condition

 

I said Ramy to FDNewbie, is your name Ramy too?

If your car starts easily when hot and runs good, I wouldn't worry about the compression results. Make sure they are done properly.

 

aznraver, I think only two names were mentioned. Ramy (mine) and Tyler's. My guess is your name isn't Ramy too If your name's Tyler, it's a coincidence... I was referring to the other Tyler posting here hehe

Rich & Tyler, so basically my seals are good, but the engine's taken a beating huh? I figured as much. Tyler, I already mentioned to you a rebuild was on the way, because the numbers are low. The good thing is (thank God), the engine still pulls hard as hell, and fires up no prob always. Fuel mileage does suck tho (always been like that). I guess it's just not bad enough to show symptoms (yet).

 

Yeah, you could very well get another 10-15k miles out of it, I probably could have gotten another 10k miles out of it too...maybe. As the apex seals get worn down, they become easier to break as well.

 

my name is Ramy khung

 

I don't remember what the numbers were, but a Mazda Dealer told me that my compression was low and the engine would need a rebuild very soon.

That was around 8 years ago.

 

Get outta here...lol. Where ya originally from?

Haha...yea I figured as much. It's been over two years and about 20K miles for me, and that's on a highly modified FD.

 

i'm asian with a lebanese name