Engine longevity / reliability / compression

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Naz
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Engine longevity / reliability / compression

Post by Naz » Sat Jan 29, 2011 6:10 pm

Engine longevity
Firstly, let us dispel any rumours that the RX8 engine needs a standard rebuild at any point in its life like the RX7 engines were (also mistakenly) reputed to need. It’s a different engine, do not make the mistake that as both are rotary they are the same. They are not. I know of a car in the UK at 120,000 miles, and others in the US on even more miles on the original engine.

Now, lets be honest here, the RX8 is a sports car. It therefore needs to be treated like a sports car. If it is not treated like sports car then it will more than likely fail on you at some point. The same would go if you bought a Porsche, Ferrari, Lamborghini etc. A sports car needs sports car treatment. Regular servicing with original or OEM standard parts as well as checks on oil and other fluids and belts will keep the car in tip top condition. Problems generally occur when cars have not been looked after.


Compression
The rotary engine as any other internal combustion engine needs to create compression to propel the rotors and therefore the car. There are many reasons your car may lose compression including broken apex seals and scored rotor housings.

How to tell if I have compression issues?
After much debate, it has been agreed that one of the tell tale signs of a potential loss of compression will be hot starting issues. Once the engine is at full operating temperature (about 30 minutes driving) switch the car off and then leave to stand for 15 minutes. If it starts up straight away after 15 minutes then there is likely to be no problem. However, if it takes more than 5-6 seconds to start, there is the potential for a compression issue.

What should I do if I have hot start problems?
Before you do anything make sure your batter is in good condition. It is also worth checking your starter motor. Mazda up-rated their starter motors to spin faster in the later models of the RX8. If you have the older model (Mazda will confirm) then it is worth speaking with them and they may be able to replace for you.
After that get a hot compression test done from a rotary specialist, be it a main Mazda Dealer or an independent rotary specialist. The compression test needs to be HOT, a cold compression test will not tell you what you want to know. Mazda specify that minimum compression is 6.9 Bar @ 250rpm; anything less than this then you have an issue.

This graph shows standard and minimum pressures at different cranking speeds.
compression graph.jpg
compression graph.jpg (81.99 KiB) Viewed 17615 times
What if I have confirmed compression issues?

a) If you have owned the car from new speak with Mazda. Hopefully you will still be covered by some kind of warranty and all being well with a little pushing and shoving you should be able to get a new engine, or possibly a goodwill gesture towards the replacement of an engine. Replacement engines from Mazda are circa. £5,000
b) If you are not the first owner, or are out of warranty:
a. Still have a word with Mazda, but you will need to have an impeccable main dealer service record and they may sympathise and offer a small goodwill gesture towards the cost of replacement.
b. Get an engine rebuild from a rotary specialist. These are currently in the region of approx. £2,250-£3,000 dependant on what other ancillaries you require.

More information is available in the members’ area
http://www.rx8ownersclub.co.uk/forum/vi ... =6&t=34957
http://www.rx8ownersclub.co.uk/forum/vi ... =6&t=32242
Last edited by Shagrington on Wed Nov 27, 2013 4:07 pm, edited 3 times in total.
Reason: Added compression graph
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Re: Engine longevity / reliability / compression

Post by warpc0il » Thu Dec 28, 2017 9:04 pm

The absolute figures can vary, depending on the exact equipment used.
The genuine Mazda kit has a long pipe that connects to the plug hole, with the pressure sensor at the far end.
Some aftermarket kits have a similar arrangement, though with a shorter pipe, while many have the sensors screwed (almost) directly into the plug holes.
These different arrangements vary the total amount of air being compressed, with the longest pipe showing the lowest figures, even if the sensors are all directly calibrated the same.

Variation can also be introduced by other factors, including;
- Oil Film; for the first test there should still be an oil film within the housing from the last switch off but, by the second test this will have been wiped/blow away. Thus the first rotor tested should give higher results.
- Starter Speed; unless the battery is fully charged and in very good condition, it's likely that the 1st test will be performed at a higher rotation speed than the 2nd. Mazda (and others) provide a formula to correct the readings to an equivalent of 250rpm, however, this is not an exact science and any significant difference in rotation speeds will produce a variance.
- Engine Temperature; the test is meant to be performed with the engine fully hot. However, if there is a significant delay between the first and second rotor tests, then this can impact the results.

One way to determine how much these last three factors impact the results is to test one rotor, then the other, then retest the first.
I've only done this once, so it wasn't much of a sample size, but the results were surprising, with the repeated test being 15% lower than the first.
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Re: Engine longevity / reliability / compression

Post by warpc0il » Thu Dec 28, 2017 9:09 pm

More reading here
viewtopic.php?f=4&t=50209
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