seasicksteve

G'day RX8 Club,

I had a low compression rebuild on my 2004 RX8 last October here in UK.

I've noticed its got harder to start progressively since I picked it up from the garage - with a new uprated starter motor that they installed for me.

I've just had it tested for compression at another garage (at an independent who has been rebuilding rotary engines for 8 years) and the results (average per rotor) are, disappointingly :

Front rotor 5 bar at 205rpm
Rear rotor 4.8 bar at 196rpm

I've gone back to the company that rebuilt the engine and they say the rpm is too low to be meaningful and I need either a new battery or starter or both as the test should be done at 300rpm.

I thought the lines on the graph on which the test figures are plotted take account of the rpm. I've seen one or two other graphs on the web where tests have been done at 200rpm and no-one has mentioned the test is flawed.

I don't have access to Mazda own workshop manual guidelines - not sure if anyone else does - so would be very grateful for any views on this before I go back to them.

RIWWP

iTrader: (2)

1) This is posted in the Series 2 subforum, since you have a 2004 you have a Series 1.

2) There is a normalization calculator that will tell you what the 250rpm value is. Any rotary shop should know this

3) You need 3 values for each rotor, just 1 is worthless. Any rotary shop should know this.

4) Do a search for foxed ca RX-8, workshop manual has a link to the tester calculator

seasicksteve

Sorry, thought I posted in Series 1 - doh !

I've got the spikey graph which plots the 3 readings but I think the machinery averages the figures out. 3rd one slightly lower than the rest. I'm seeing the guy tomorrow - should I insist on the 3 seperate readings per rotor ? Is that in case one is substantially lower than the other 2 ?

Thanks for confirming the rpm the readings were taken at were ok.

RIWWP

iTrader: (2)

Ah, understood. Averaging the numbers is fine for quick reference as long as the independent numbers are actually known. Part of this is that an engine with healthy numbers in the 7s or 8s can still fail if the separation between any two faces on a rotor is too great.

The RPM definitely shows that you need a new starter. That is really slow. Your engine is having trouble getting enough compression to start for sure. That doesn't automatically mean that your engine is failing though.

Here is the official calculator from the OEM dealer network: Foxed.ca - Rotary Compression Calculator

A 5.0 at 205rpm normalizes to 5.7
A 4.8 at 196rpm normalizes to 5.6

Your engine is still failing.

seasicksteve

Yes, the garage that did the work are telling me I need a new starter or battery but, as you say, that is not going to sort my compression problem out.

I had a new uprated starter 4 months ago. I've done 1200 miles in the car since then and started it about 50 times. Is it possible the starter needs replacing already ?

Or could it be the battery which I have not changed ? I'm nor sure what Ah and CCA the standard battery should be, and what limit it can be uprated to ? Any particular makes preferred ? Any help greatly appreciated.

RIWWP

iTrader: (2)

If your starter is that new, then it could be a weakening battery that was used to drive the starter for the test? Usually they hook up to a backup power source to be sure they don't run down the battery, but, maybe they didnt?

I honestly don't have the exact info on our battery specs.