Compression Results

[am5622]

Newbie here. Just wondering if there are any hard and fast figures for compression results?

Does the engine need a rebuild when it drops to a certain figure?
What figure should I be looking for when buying one?
What is the lowest figure you would buy one at?

Basically, I have no idea what the figures really mean?

Cheers

[am5622]

Just to add, I\'ve seen some people boasting about mid 6\'s, and some people boasting about 8\'s

[Arnie_O]

Hi buddy
I guess the first question should be what car are you looking to buy?
Compression figures vary between versions of engine.

[am5622]

Looking at this at the moment. It\'s a 96 RS

http://www.torque-gt.co.uk/car_details/?act=productDetail&productId=683

[am5622]

I\'m guessing there\'s no magic figure, that you know it\'s going to need a rebuild, so just trying to judge what relevance the figures are!

[Will66]

Lower figures are generally worse.

Compression figures indicate the pressure in each chamber on each rotor.  So you should get three for each rotor.  Ideally you want them all the same but inevitably there will be small differences between each chamber and across the rotors.  The tolerances for the RX8 motor (I am not 100% certain the RX7 is the same but it gives you an idea) are minimum compression of 6.9 kgf/cm2, maximum difference between chambers (on the same rotor) of 1.5 kgf/cm2 and maximum difference between rotors of 1.0 kgf/cm2.

Mazdas specification is a bit tight however and many cars run around with compression in the 6 kgf/cm2 range without an issue and do so for many miles.  Generally, anything above an 8 indicates a new/rebuilt motor, 7s and 6s are generally healthy and 5s and below means a rebuild is imminent.

[am5622]

Awesome, just the answer I was looking for, cheers

So if I keep an eye out for 7\'s or high 6\'s, I should be good for a while!

[Will66]

You should be provided you look after it well!

[HaywardRotary]

In an RX7 if it is a stock original engine with never having had a rebuild It will have pretty high compression figures as the engine in the FD RX7 was built from factory with 3 piece seals and therefore does not really give a good indication of wear as up to the last they can read quite high still.
Has it been rebuilt?
At 61,000 with no rebuild proof it might be an issue.

[m1tch]

The useful figure is the balance of the pressures, if say the engine is for example 7,7,5 then although 2 of the faces on that rotor are good there is an issue on one of the seals.

I usually go with something like this:

8s = new engine after bedding in
7s = good, strong engine
6s = engine is ok, its used but still fine
5s = best start thinking about a rebuild

If you look after the engine then it should last ages, its usually the old fueling or cooling system that damages the engine rather than anything internally wrong - overheat it or run lean on boost it usually causes expensive issues.

[Prof]

It\'s probably worth adding that you won\'t be able to use a standard compression tester for this, but most of the specialists on here can get the test done for you for a smallish fee - it takes no more than about half an hour tops.
 
The test also shows the RPM figure that the engine was turning over (the test is done without the engine running, but being turned over by the starter, and sluggish startes give lower RPM figures and hence lower compression readings - there is a table / graph in the service manual that shows how to scale the numbers up or down to the standard test speed of 250 RPM.  The ratings are different for each class of engine - so 13B (NA) 13BT (Turbo FC), 13B-RE (COSMO), 13B-REW (FD) and 13B-MSP pr RENESIS (RX8).
 
Happ hunting :-)