Clearing up some of the myths regarding engine compression

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Clearing up some of the myths regarding engine compression

Post by hayward rotary »

I hope in this thread to clear up a few points about compression results on RX8 and rotaries in general and allay many unfounded fears that people have over lower compression figures.

I have heard many times it mentioned than anything below 6.9 bar compression at 250rpm is an engine failure. Well I would like to say that is not really the case.

I know this might get suprise a few people but compression testing only tells you what the engine is like when cranking and not when running with gas behind the seals making good compression. Static compression testing by cranking is not the same as dynamic compression when the engine is running.

Over the years I have carried out so many compression tests that i have literally lost count so can speak from quite a bit of experience.

If you were to compression test every RX8 in the country I would be almost certain that 90% of them would have at least one reading below 6.9 bar and probably at least 50% or more of them all six chambers well below that. These are in cars where owners are happily going about in their daily lives enjoying their cars none the wiser to the state of their engines, why is that? Its simply because the car and engine are still doing exactly what they are designed for, running and starting well and making good power still inspite of what is perceived to be poor compression or lower compression.

I often see mentioned too that uprated starter motors are just masking the problem, but that also is not the case. If you think about it when the cars were new the original starter motor cranked the engine at around 250rpm but now they are old and tired they are often below 190rpm, so putting on a new uprated starter is hardly masking the problem it is resolving the issue of lack of speed to start the engine by using a better starter than original thats all.

We have had quite a number of customers who have even had compression figures in the 5bar range and their cars are still performing exactly as they should and I have driven them since I have driven alot of RX8's I can compare many cars. In some cases these people have been driving with low compression for a few years now too and given proper maintenance they have been able to keep there car going without the need for a rebuild, of course a decent start motor coils and plug leads and regular servicing are the keys to this.

I would like to point out that this obviously is not true in every case but largely from my experience car has to be pretty bad and mainly suffering from worn bearings along with low compression to make it an engine fail, and clearly have running issues despite adding all the right ancillaries, but this is much rarer than you might think. I have taken cars in that you would really think were on their last legs engine wise but they have been rescued without a rebuild and often are still running now after more than a year or two.

Also comparitively speaking if you look at the sheer number of RX8s on the road still that are happily bobbing around the ratio of good cars to bad is not that high.

Finally a bit of advice for those who are wanting to buy an RX8 I allways tell people to make sure it hot starts readily and there is no rumbly noise at low rpm from the gearbox area when in the car, if all is good in those departments then it should be a good car for a fair while provided you do proper maintenance.

I hope you havent got bored reading this but also hope that it reassures quite a few of you.

Best regards to everyone who reads this and I hope it proves helpful to a long and happy rotary car experience!
Carl
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Re: Clearing up some of the myths regarding engine compressi

Post by Dadiodude »

Thanks Carl, its nice to read a post that gives some balance to all this compression paranoia we all seem to suffer from. I have noticed in my relatively short time on the forum that failed bearings do appear to account for the vast majority of rebuilds rather than a failed seal or worn housing and always assumed that a compression test on an engine with knackered bearings and oil seals will always look grim anyway.

Out of interest, why did Mazda decide on this 6.9 figure as its now been adopted as the benchmark for failure?
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Re: Clearing up some of the myths regarding engine compressi

Post by kopite72 »

Great post Carl and a very interesting read,I think part and parcel of rotary ownership is plain old paranoia every now and again of which I'm as guilty as anyone else but its great when specialists like yourself and Ben come on here and post things like you just did to dispel any myths surrounding the engine.From my own point of view prevention is much better then the cure and if you look after the car religiously than I think its fair to assume you'll enjoy a happy marriage.Yes engines will give up the ghost and yes they will require rebuilds but I think if you did a comparison to pistons you'll probably find the rate of attrition of rotary versus piston is probably in and around the same.
I'm glad you mentioned the compression figure of 6.9 is not a fail and even if its lower than that it still may not be a fail,IMO too many people get too hung up on this figure and if its any lower are looking to rebuild even in your opinion its not necessary.if I had a comp test in the morning and she returned figures in the mid sixes,sure I'd be disappointed but after reading your opinions on comp figures I wouldn't be too disheartened as I know it probably does not require a rebuild.Interestingly rods engine was rebuilt at figure in the mid sixes(I think) and after reading your post I wonder was it really necessary?id be interested to hear what rod thinks.id also be very interested to hear what Ben thinks regarding your post,my own opinion is that he'd agree mostly with what you've said

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Re: Clearing up some of the myths regarding engine compressi

Post by simonmd »

I think this should be made a sticky. Its great to have a post like this that we can refer to in the future as the amount of speculation on this and other forums is rediculous. I happen to know my cars figures are in the mid, high 6s but she is faultless, ALWAYS starting cleanly and quickly when hot and pulling like a train on the road so i'm not bothered. The way I look at it is its an 8 year old car with over 50k miles on it so it's not going to be 100%.
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Re: Clearing up some of the myths regarding engine compressi

Post by Rotary Revs »

You cannot argue with the logic whoever you are :-)

I too have driven cars with 'low compression' that perform very well and give no cause for concern.

Rather than saying to a customer that owns an rx8 that has compression readings in say the mid 6's 'you need a new engine' I would simply say 'consider putting some money aside I case problems do start'


I think a good example of this been put into practice has been with Vic_Cambridge who had low compression diagnosed by project rx8 8 - 12 months ago, the results were quite bad (mid 5's) but the car started and performed perfectly.

Vic chose to carry on using the car but begin planning for a rebuild that may or,may not come. Then when the car began with start problems rather than messing about he sent it straight to me, we carried out a rebuild and I found the apex seals worn, front housing a nasty mess and a couple of not so bad but not pretty bearings. He now has his car back and (I hope) is very happy with it again [-o< :mrgreen:

If I were in anyone else's shoes this is more how I'd look at compression figures. below 6.8 - more of a 'there's nothing wrong now but there could be in 6, 12, 18, 24 months and I'm not gonna get caught out by it' ... If the day never comes happy days I'm going on a fancy holiday :-D

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Re: Clearing up some of the myths regarding engine compressi

Post by kopite72 »

True Simon,I also like Carl's explanation of the faster starter motor NOT masking compression,when you think about it,it makes perfect sense.Fitting a faster starter motor just does exactly that and does not affect compression one iota!!
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Re: Clearing up some of the myths regarding engine compressi

Post by kopite72 »

Sound advice Ben =D> its a fair point you make tht if the comp figures are mid 6's are lower than start thinking about squirreling away a few bob every month just in case,but as you say it could 1 or two years or possibly longer before she gives up the ghost.I think people that have borderline comp figures after reading this post will rest a little easier and to be fair to Carl I think he has to be commended =D>

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Re: Clearing up some of the myths regarding engine compressi

Post by Rotary Revs »

Ah now on hat I'm sure Carl will say that in certain cases a faster start will mask problems

I stripped an engine on Monday that has done 101000 miles, the newest version starter bolted to it had gotmitmthat far nothing more! It's had a few 1st's. The engine ran tho I wouldn't say faultlessly I did not expect to find what I found!

Destroyed front bearing that had gotten so hot I (no joke) for the 1st time ever had to use an angle grinder to get most of the front cover assembly apart and because of some fancy tapping from me even had to cut off the end of the (already nackerd I might add) eshaft :shock:

It got worse! The rear rotor bearing had also slipped, tho I suspect not recently.

Nowmim not saying this engine had compression issues and the starter was covering that but I will say that without the faster more powerful starter it wouldn't have turned over let alone started :shock:

There's defiantly a grey zone where a 300 rpm starter will keep a dying engine going, whipping a dead horse if you like :-)
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Re: Clearing up some of the myths regarding engine compressi

Post by kopite72 »

Eh that was my point Ben!! it just turns over the engine but doesn't effect compression?!!
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Re: Clearing up some of the myths regarding engine compressi

Post by TenToSeven »

Rotary Revs wrote: I stripped an engine on Monday that has done 101000 miles, the newest version starter bolted to it had gotmitmthat far nothing more!
<snip>
Nowmim not saying this engine had compression issues and the starter was covering that but I will say that without the faster more powerful starter it wouldn't have turned over let alone started :shock:
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Re: Clearing up some of the myths regarding engine compressi

Post by Dadiodude »

Who mentioned whips? :kinky:

What I take from Carl and Ben is that the only people fixated by 6.9 are club members. For everyone else they just enjoy their car and don't worry about it so long as it starts and runs well. When it doesn't they seek help. Perhaps the obsession with compression figures rather than a cars actual performance and condition has in part contributed to their reputation and low value. I'm not saying don't compression test a potential purchase but perhaps its not the be all and end all that we thought. It's possibly no more informative as dropping the oil and listening carefully to the bearings. Anyway, I feel better have read what the guys have said.
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Re: Clearing up some of the myths regarding engine compressi

Post by Rotary Revs »

Well yes and no lol

An engine will produce the same compression at 3000 rpm wether its got an old starter or new starter.

The rx8 has a 10:1 compression, even an engine that is showing 5 KGf at 250 will likely have near 10 KGf at 7000 rpm, but it will also have compression near 6.5-7 at 300 rpm

There's a graph on here somewhere that shows how compression is affected by the speed te engine is turning over :-)
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Re: Clearing up some of the myths regarding engine compressi

Post by Rotary Revs »

:lol: it's my iPad and it's a nightmare lol
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Re: Clearing up some of the myths regarding engine compressi

Post by Rotary Revs »

There's another point tho. I think the fixation with compression started when people were looking to buy cars, would anyone here, even now after you've read this buy a car for the normal going rate of a good car if the compression was below 6.8?


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Re: Clearing up some of the myths regarding engine compressi

Post by kopite72 »

Depends how low Ben,after reading Carl's post I wouldn't dismiss it whereas before I wouldn't even have considered it, obviously I'd get the advice of a specialist but if he was of the same opinion as you and Carl regarding comp then the answer is yes
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Re: Clearing up some of the myths regarding engine compressi

Post by Rotary Revs »

Well that settles that then :-)
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Re: Clearing up some of the myths regarding engine compressi

Post by kopite72 »

Lol that's just me Ben I'm sure there's many guys on here who'd have a completely different opinion!! something tells me they'll be along shortly!!!.......
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Re: Clearing up some of the myths regarding engine compressi

Post by Dadiodude »

iPad? We though you were using a phone.

Bloody giant thumbs lol

Of course not mate but what we see these days are potential new owners coming on here asking for opinions/advice on a £2000 car they've seen. Almost always its the same "check coils, check compression, budget £2500 for this and £500 for that". Back when a decent 8 was £10k fair enough but now they're a lot older and cost about the same as a decent TV I don't think its as reasonable. What it does is resign a lot of decent cars with a few years left in them to the trash can. If the view was that (on a cheapie) if it starts on the button and runs well with no evidence of bearing noise then for the money its OK. Of course a future rebuild may be required but if you applied the rules we apply to the 8 to any other car you'd have a similar situation as all cars have their weaknesses.
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Re: Clearing up some of the myths regarding engine compressi

Post by Rotary Revs »

Which is a very good point of view and one then we should adhere to :-)

If you can get 2 years out of an 8 that only cost 2k then you got a bloody bargain! :-)
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Re: Clearing up some of the myths regarding engine compressi

Post by Bulldog »

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
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Re: Clearing up some of the myths regarding engine compressi

Post by Shaunzrx »

Its all now as clear as our many concerns have been with Oil haha Lol.... 8-[
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Re: Clearing up some of the myths regarding engine compressi

Post by Shaunzrx »

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
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Re: Clearing up some of the myths regarding engine compressi

Post by simonmd »

Rotary Revs wrote:Which is a very good point of view and one then we should adhere to :-)

If you can get 2 years out of an 8 that only cost 2k then you got a bloody bargain! :-)
:whathesaid: Damn right and its exactly how I look at it. My car is a late 2004 with just over 50k miles and looks, feels like new and I bought it for £2500. Yes, I knew the comp was in the 6s (between 6.5 6.9) but I had done alot of reading and knew this wasnt the end of the world. Sure, if I was after a track car that I was going to mod mechanically, I'd not have bought it but for my useage, i'm confident I have got a bargain. How many cars costing 2.5k will have people pointing and smiling in admiration?

One of the main reasons RX8s are so cheap in the first place is all the missconceptions regarding reliability, etc. Unfortunately, that means alot of you will have lost a ton of money in depreciation but for new users like me, it's worked in our favour.
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Re: Clearing up some of the myths regarding engine compressi

Post by pigman149 »

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 :-)
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Re: Clearing up some of the myths regarding engine compressi

Post by simonmd »

A fair point, a good respray can easily cost as much as a rebuild.
Jobs done: 10w40 oil, Gearbox oil changed for synth, Diff' oil changed, coils replaced, Cabin air + K&N panel filter fitted, Red Brake calipers, Black Alloys, Fitted Aux/USB adaptor to stereo, Numberplate lowered, de-badged front, DRLs, Plastidipped badges and strakes black, Tein springs

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