Stumped On Compression Test Results - Desire Forum Input

[coredata]

A bit of history: I currently own (and love) a '05 White Grand Touring RX8. Prior to my '8 purchase I knew I wanted a '7. I am finally in a position to own two vehicles; I found a '7 for sale locally, and have had a prepurchase inspection performed and a compression test. The chassis is a 1993 with 190k+ miles on it; the motor was rebuilt 17k miles ago. I am completely happy with the overall quality of the vehicle (other than what follows), but am stumped on how to evaluate the compression test results. I would greatly value some input on the results of the compression test! (I apologize if this was not the most appropriate place to start this thread -- if I am in error please advise). Compression Test Results: (performed at a Mazda dealer) Rotor 1: 680, 680, 580 @ 274 rpm Rotor 2: 680, 700, 680 @ 275 rpm Numbers in kPa Idle vacuum when warm: 16.5-18.5 inches (averaging around 17) I have bounced these off a good friend and colleague of mine and he has advised that the one low number on the front rotor combined with two high numbers could be indicative of a corner seal problem, and that overall (I understand the 93 shop manual calls for 690 minimum) the engine is 'tired' and due for a rebuild soon. Unfortunately the history of the last rebuild is quite fuzzy -- a receipt exists, but no documentation. I have confirmed that it was performed by a local mechanic (not a shop), and it was not ported beyond stock. Again I would greatly value any input or commentary on the above numbers and where the engine is in its lifecycle. Also, any links to information on how to correctly understand the above numbers would be fantastic! I have done a lot of googling, but mostly what I find is 'if its below this number its rebuild time', and I just want to confirm/deny this with some educated input from the gurus on rx7club These numbers seem to be on the brink of 'OK' and 'rebuild time', from my own research. As much as I would like to have a 'project car' (ultimately thats what this is about), I want to enjoy it while it turns into a project -- I dont want to outright purchase a project (if that makes sense?). Thank you very kindly, cd

[j9fd3s]

its really hard to say whats really going on.

if compression is even but at the minimum, with a fresh "rebuild" it might need housings or something. new seals on warped/chattermarked rotor housings, will run a long time, but compression will be low overall, for example.

another possibility is that the tester, or person doing the testing isnt right. i remember we compression tested a car with 3 different testers in the shop, and came up with 3 completely different readings.

if its smooth and pulls good vacuum id say you're ok for a while

[DaleClark]

The vacuum it's pulling is strong. If you can find someone that is real familiar with FD's in Houston (there's a bunch of guys in Houston, BTW) to take a look at it in person, they can probably give you some input.

Biggest thing is the car starting hot and cold - it should have a strong "buh-buh-buh" sound when cranking, fire easily, and not smoke. If cold first thing in the morning, you may get a little puff of smoke from condensation or the oil metering system, which is OK. If it ever seems like it struggles to start, that's a motor getting older and more tired. If it starts nice and easy, no big deal.

Compression test numbers are really hard to judge. I normally just use a compression test as a go/no-go on an engine to determine for a fact if it has a broken apex seal or the like. Engine health is moreso indicated by how well it starts and idling vacuum.

Dale

[alexdimen]

That's a range of compression values between 85 (which is borderline shitty) and 100 psi.

The odd thing is that the vacuum indicates an engine that would have excellent sealing conditions.

I would have gotten the car checked out by a rotary shop rather than Mazda. They have a tendency to not know what the **** they're doing.