Compression
 

Can someone explain the compression figures given by dozens of people on this forum as to what their engines compression is, and then look at the official MAZDA compression graph and explain the difference in what the graph says and what THEY say?

https://www.rx7club.com/forum/attach...&stc=1&thumb=1

Try this thread and the graph on the second page: https://www.rx7club.com/forum/showth...ht=compression

https://www.rx7club.com/forum/attach...hmentid=153468

https://www.rx7club.com/forum/attach...hmentid=153467

how about a readable graph

just to throw something else into the mix, here's a page out of the Rx-8 manual on compression tests:

Mazda numbers are calibrated to their compressor tester only.
Compared to the hack-job piston compression testers that's normally used, you will get lower readings...thus, the lower numbers than Mazda spec.


-Ted

Hailers, I am not entirely sure I understand your question. But in reference to the second thread and Kick7ca’s claim that his compression was 9.2 kg/cm2 that value is off the chart. Since we do not know the cranking speed when measured it does not tell a whole lot other than it is beyond the FSM table.

As you know from the FSM graph, the acceptable minimal values are linear to the cranking speed. Each increase in cranking rpm increases the minimal acceptable compression by .015 Kg/cm2. No? Eyeball from table – 200rpm = 5.25 kg/cm2 and at 300 rpm = 6.75 Kg/cm2 thus: 1.50/100 = .015 Kg/cm2 per rpm?

Using his example of 9.2 the compression would be kaput at a cranking speed of 463rpm.
His value 9.2
300 value 6.75
Variance = 2.45
Thus: 2.45/.015 = 163.3 plus the 300 limit equals 463 rpm. If he got a 9.2 at 463 rpm cranking speed it is at the lowest acceptable limit. Not sure I explained my reasoning correctly.

However, if your base question is does the often cited value that 85 psi indicate the need for a re-build? The obvious answer is it depends on the cranking speed of the test. Thanks for making me think this through.

Yeah, Jadoolin, I'll re-read that.

I think I'm being misunderstood because I threw in that *other* thread when all I wanted was to put the charts/graphs in my thread.

I'm being completely misunderstood.

Most people on this site will give you figures of 120psi, 115psi, 110 psi etc for a S4 or S5 engine. Now look at the chart for the S5/S4. Nowhere on that chart do you see those figures on the chart.

I'm not talking about the chart ARGHX put up for the RX8, and I do thank him for showing that chart because I've never seen it before.

So, the bottom line is, how do you square the readings given on this forum with the S4/S5 chart????????? You can't. At least I can't.

EDIT: This chart https://www.rx7club.com/forum/attach...hmentid=153467 You see 120psi on that chart? 115psi on that chart? 110psi on that chart?

EDIT: And by the way, I can turn my fuel cut switch off and crank the engine while watching my Fluke88 set on rpms, and if memory serves, with a good strong battery, I see about 250rpms cranking with one bottom plug out and the compression gauge in the other lower hole.

I don't know but when i was doing searches on compression in the first gen section there where a few well known guys in the first gen section saying that the reading in the fsm is what is spec but can be more. If you look throught the first gen section and search for compression they will have a few threads talking about the same thing you are.

Hey Hailers, I know I no longer have the chart, but plenty of folks can atest to it. My compression on a stock 10thAE, 117k miles, was ~105 in the front rotor and ~108 in the rear rotor. Those numbers are also off the chart. That was done with a Mazda Compression tester. The kind that gives out a graph so you can see every pulse.

I wonder the same thing HAILERS. There's either a large cranking discrepency, or the FSM simply lists the lowest possible acceptable reading graphed versus RPM.

I believe so because you need to worry about is just low end side.

Actually, I'm about positive that's the case. They list it as the tolerable limit of the engine specifications, they don't happen to list anything higher. However, since they mention max acceptable varience, i'd assume you could safely gather adding 21 to 85 would lead you to believe the engines are capable of getting over 106.

http://1300cc.com/compress.jpg

I've decided its a useless chart. It's in the S4 AND S5 manuals. Now a 87Turbo has a compression ratio of 8.5:1, a non turbo 9.4:1 and a S5 has a turboII ratio of 9.0:1 and non turbo 9.7:1.

Junk chart.

I don't think it's totally useless, I think it's meant to show you the LOWEST possible compression they will consider tolerable, and give you a graph of it verse RPM, obviously this is going to change depending upon the engine/etc, but then again the compression ratios pretty much just increased over time, so they didn't really see a point in changing it as if you're lower than 85 it's screamin for a rebuild or new engine regardless. I think it's Mazda being lazy on their part (except for the spiffy rx8 one!)

I also feel that the graph illustrates the lowest acceptable vs rpm. But now let me ask a possibly dumb question. Why would a lower compression be acceptable at a lower cranking rpm? Not sure I understand why higher or lower rpm would change the acceptable compression.

Volumetric efficiency is the likely culprit. But that image I posted above clearly states that 85psi is the minimum compression, thus proving that the graph is indeed meant to just show a minimum compression correction versus RPM.

Ah. SonicRat was posting while I was writting my last post (10:49 and 10:50 times), so I didn't see his chart. Now that I have never seen and explains things better. Much like the answer Icemark made to another iquiry I had made on this same subject. I just missed that chart in the S4 manual.

I'm done. Question answered.

It's in the technical specifications section, not exactly anywhere near relevent material!