Clearing up some of the myths regarding engine compression

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Re: Clearing up some of the myths regarding engine compressi

Post by Delanor »

A question I would like answered - does there seem to be a common cause of engine failure, for eg. is it down to a high mileage giving way to a specific problem such as worn seals, coked up seals, one or more bearing failure etc or is it a combination of factors?

Or to put it another way is there a common cause of failure?

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Re: Clearing up some of the myths regarding engine compressi

Post by Dadiodude »

I thinks that's one of the issues Del. there doesn't appear to be a common factor hence the long oil and premix debates. Some go early, some don't.

I still reckon we suffer more in the UK than elsewhere but that's just my opinion.
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Re: Clearing up some of the myths regarding engine compressi

Post by simonmd »

You probably hear more about UK cars because they had an amazing 8 year/100,000 mile warrany in the US. Most over there that had issues simply booked their car into a dealer for a rebuild and it's only now that the early cars are running out of warrenty.
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Re: Clearing up some of the myths regarding engine compressi

Post by Shagrington »

Bulldog wrote:6.9 at 250 rpm is the point that Mazda set to replace engines under warranty it's not the automatic failure point of the engine

Would I buy an rx8 that had 6.9 compression - nope

Would I replace my 6.9 engine if it was starting and running - nope

It's just a warning sign saying danger problems to follow
=D> I agree with this entirely.

As I see it, the lower numbers people might get from a compression test are an issue when it is a car they are considering buying, which is when we will offer the advice that it is below spec as far as what Mazda said was a pass or fail, so it probably wouldn't be wise to buy that car, especially if they are stretching their finances to buy the car in the first place. If they can only just afford £2-3k to buy a car they are unlikely to be able to pay that and more out again maybe just a year or two down the line for a rebuild, which is where Ben's comment about paying the going rate for a healthy car when buying one with low numbers is entirely relevant.

If they already own the car and get those test results, but the car is running fine and starting without problems it is just an indication to start saving.
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Re: Clearing up some of the myths regarding engine compressi

Post by SteveD »

pigman149 wrote:as steve d put it to me (ps glad you are on the mend mate) if i said i had 2 sets of tyres the same type size and price but one set was half worn, what ones would you buy.but then you have all the other things to think about that have been mentioned before how much you are paying in the first place.a £2000 car with comp of 6.8 in very good condition might be a better buy than a £5000 car with comp of 7.2 that is a bit tatty for example.
well that's now how i look at it :-)

Ahh saved me some typing, cheers Ian :-)
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Re: Clearing up some of the myths regarding engine compressi

Post by johnnybugs »

it has been one of the downsides of this club.

I bought my car £2500, without a compression test .... its starts hot or cold, drives fine.

If I decide to sell it though, and get a compression test done, chances are being a 53 plate its going to have compression in the 6's, and the car will only be worth £1k :wack:

At that point, I think I would rather part-ex it than let it go for silly money ... I dont mind a dealer screwing me over, you kind of expect it from them, but not a regular Joe
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Re: Clearing up some of the myths regarding engine compressi

Post by hayward rotary »

All this talk and fixation of 6.9 bar is where the trouble lies.
Until the RX8 Mazda never mentioned such a thing on all previous rotaries and also it was only an internal thing in as much as especially for people in the USA Mazda wanted to protect their rep.
Dont get swept along by the hype of anything below 6.9 bar being the death knell of the engine. It just isnt the case, as I have said with "Proper" maintenance the engine will last years even if below that reading.
For example, dont wait for 12,500 miles to change the oil, do it at the max mileage of 6000, and keep it topped up in between, use 10/40 mineral or Semi Synth(Fortified Mineral) and if you can change oil every 3000 miles which is even better!
We have even noticed that compression can rise a little if this type of maintenance is carried out.
Some cars have been running for quite a few years that we have worked on with this level of care and show no major signs of deteriation.
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Re: Clearing up some of the myths regarding engine compressi

Post by hayward rotary »

Regarding common reasons for engine failure it is most apparent that bearing wear is the issue over and above seal wear.
I say this cos if the rotor is moving in a wider orbit than it should due to bearing wear then there is no way that seals can seal as effectively thus a side affect of this is low compression.
Again bearing wear is caused by poor lubrication, hence the reason to carry out oil changes more often and use a higher viscosity oil.
Last edited by hayward rotary on Thu Feb 28, 2013 10:08 am, edited 1 time in total.
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Plan3tBob (Wed Aug 29, 2018 4:45 pm)
We warranty our rebuilt engines now for 30 months or 30,000 miles ,for Detonation, track day's and RWYB and We also build all types of race or road engines.
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Re: Clearing up some of the myths regarding engine compressi

Post by kopite72 »

Good stuff Carl and a great post,certainly opened my eyes to all this low compression malarkey
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Re: Clearing up some of the myths regarding engine compressi

Post by Nicknackwack »

Good thread !

Quoting Carl: "We have even noticed that compression can rise a little if this type of maintenance is carried out."

When I had my car recently serviced it was brought to my attention that my front rotor compression was 5.9 ~ yet the car performed perfectly with no starting or running issues. The view of MKG3000 was monitor the situation.

500 miles later ~ compression to the front rotor has increased to 7.2. Why ? As advised by MKG3000, the car needed a good long hard drive, beeping often to purge carbon build up as is caused by frequent town driving.

I've also learnt that it's the cumulative combination of compression to both rotors that needs to be borne in mind when monitoring engine performance. Compression IMO can fluctuate depending on how you drive and look after your engine. Of course there are other factors but it makes a good starting point.
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Re: Clearing up some of the myths regarding engine compressi

Post by Phil Bate »

Hmm, that makes me wonder. I had a lower than expected result on the front rotor at Carl's place recently. However, directly before this I had a 4 hour drive up to Harrogate where the car sat for a week, then a 3 hour trip down to Newbury - both journeys being steady boring cruising on t'motorway.

Could the engine have built up a wadge of carbon in that time? I wonder what it would test at now, after the extended Italian tuning it has been subjected to since I've been home?
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Re: Clearing up some of the myths regarding engine compressi

Post by Nealb »

While these are all good points, how does it affect the guy who was asking about his rebuilt engine still under warranty (sorry, can't remember username).

His engine was still under warranty from a rebuild, but showing figures in the mid 6 range, and being told his engine was fine, and not to worry.

You'd agree from this it probably is, however would there not be an underlying reason why it was low(ish), and therefore not ok?
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Re: Clearing up some of the myths regarding engine compressi

Post by SteveD »

I don't think we can expect Carl to comment on that.

I will say that I have seen compression results on one of Carl's rebuilds, 2 years post rebuild and some very hard miles on it, and that was showing compression in the 8s.
Last edited by SteveD on Thu Feb 28, 2013 12:54 pm, edited 1 time in total.
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Re: Clearing up some of the myths regarding engine compressi

Post by Delanor »

Nealb wrote:While these are all good points, how does it affect the guy who was asking about his rebuilt engine still under warranty (sorry, can't remember username).
His engine was still under warranty from a rebuild, but showing figures in the mid 6 range, and being told his engine was fine, and not to worry.



I think he`s referring to Arzegum 27 who had his engine rebuilt by Hurley Engineering.

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Re: Clearing up some of the myths regarding engine compressi

Post by simonmd »

I think with that particular case, its fair for people to comment that the figures are too low and it needs looking at. That car should be in tip top condition engine wise and is still under warranty so he has every right to expect it to be investigated/rectified.

What this thread is designed to point out is that 'normal' cars who have lower than ideal figures DONT neccessarily need a rebuild and are not in danger of imminant failure. (He says with both sets of fingers crossed!!)
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Re: Clearing up some of the myths regarding engine compressi

Post by Cymruplayer »

Hi,

Would it be fair to say then that if your compression readings are below par but have no starting problems Hot or Cold and have had regular oil changes but using mazda recomended 5w30. Would switching to 10w40 would make a difference on the longetivity of the engine?

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Re: Clearing up some of the myths regarding engine compressi

Post by simonmd »

Well, this thread is about comp' figures, there are loads (prob too many) threads about oil type and effect. The change to 10w40 is, as I understand it, more relavent to the life of the main bearings. It's the oil they used as standard in the RX7 and it never had the bearing failure rates the early RX8s did, even though they were the same.
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Re: Clearing up some of the myths regarding engine compressi

Post by Cymruplayer »

Yea happy with that just wondering about engine longetivity and whether it's worth doing with low compression.

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Re: Clearing up some of the myths regarding engine compressi

Post by simonmd »

Personally, I would say yes, in conjunction with a bit of premix as well.
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Re: Clearing up some of the myths regarding engine compressi

Post by Cymruplayer »

I think i'll give it a go then as i'm in a similar position with not great compression figures but car runs fine and saving for a re-build. Even if it gives me another few pay days it's a bonus.

Thanks

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Re: Clearing up some of the myths regarding engine compressi

Post by Arzegum27 »

Would like to add that the points made here regarding regular maintenance and servicing were also echoed by Mick Hurley when we discussed my rebuild and compression test results.
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Re: Clearing up some of the myths regarding engine compressi

Post by Pettit Racing UK »

I can add the weight of Cams experience at Pettit to Carl's comments, and Pettit has well over 1000 rotary engine builds and counting.

Bearing failure is a big issue hence why Pettit Revomax oil is a 15w40, to help protect the bearing. We also recommend as frequent as possible oil changes. These are a much better idea than fitting Sohn adaptors etc! Back the regular oil changes up with Protec R

If your car drives well, makes good power and feels good, then don't worry about compression numbers, just enjoy the car.
The RX8 loves good maintenance, and using good fluids (including transmission fluid etc) regularly, is a key part of this...
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Re: Clearing up some of the myths regarding engine compressi

Post by Phil Bate »

Pettit Racing UK wrote:Bearing failure is a big issue hence why Pettit Revomax oil is a 15w40, to help protect the bearing. We also recommend as frequent as possible oil changes. These are a much better idea than fitting Sohn adaptors etc!
That might be a bit of an unfair stab at the Sohn adaptor - I don't think people that go to the trouble of fitting one are the kind that skimp on oil changes or quality. I like to think of my install as extra protection in addition to this. :)

On that note, do you have a video like the one you use to promote Protek R, but comparing it to sump oil being burnt?

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Re: Clearing up some of the myths regarding engine compressi

Post by simonmd »

15w40? Doh, another option to consider!! :roll: I did my oil when I got here (January) and have alot of mileage to do next month so will prob do AGAIN before I go. Thats prob' only 500 miles between changes but I think it's a good idea bearing in mind nearly half the old oil will have been left in the coolers.

I think most agree the main advantage of the Sohn adaptors is the fact that they can then use full synth in the engine, giving the bearings the maximum possible protection.
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Re: Clearing up some of the myths regarding engine compressi

Post by Dadiodude »

True but the real benefit of fully synth is that it outlasts mineral stock oil. If you're doing very regular oil changes much of that benefit is lost.

The major advantage of the Sohn as I see it is the cleanliness and suitability of the oil delivered by the OMP rather than the fully synth you can put in the sump.
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