Rotary Compression results

[hachiroku]

Could you kindly cast your eyes over the below compression test and give your opinion?
The numbers seem high enough, I'm just wondering about the difference between faces.
This is a test done around 5000 miles ago on a a car I'm looking at buying...

Front Rotor: 7.2. 7.7. 7.1

Rear Rotor:. 7.1. 8.1. 7.9

Normalized at 250rpm.

Test done by Ben Taylor @ Minisaki Rotary Lovers (Which is Glasgow I believe?).

[350matt]

All very healthy figures
the difference between faces is a little concerning but as the figure is still high I woudl hope that they will even out with some use and visting the beep area of the rev counter and a bit of premix in the fuel
how much is the car and how many miles are on it?

[SprintRX8]

I would actually disagree.
Those figures between the highest and lowest to me are bad.
Too much variation
I would get a Compression test done before you part with any cash.
5000 miles is a lot so it could improve and even its self out.
Or get worse.

This link is copied from Essex Rotary.

http://www.essexrotaryspecialists.com/c ... g-results/

[hachiroku]

Hmmm.... Thanks for the link, I've also read elsewhere that the maximum permissable difference between rotor faces is 1.4kg/cm, which puts those results within acceptable limits, but still a little odd to me. Error in the testing perhaps? Or beginnings of seal issue on one or more faces.
The car is a 2006 Evolve with 60k on it, so I'd expect some wear on the engine and would budget for a rebuild in perhaps the next 10k miles/2 years if it came to that.
From the MOT records It must have been SORN from 2016 to mid 2020 as there's no MOT's for that period..
Current owner says it's always been pre-mixed since he got it in 2020, but before that... Who knows? They're asking £2k but I wouldn't pay that much, it needs a few bits doing to it over the next year (arb droplinks and probably a steering column universal joint as they were advisories on a recent MOT).
I've had an 8 a few years ago, but that came with a recent and warrantied rebuild so I knew what I was getting with that.

[Nickp47]

£2k is a bargain imo - assuming it’s not a rust box

[Calcifur83]

Are you looking at the copper red one in Dundee by any chance??

[hachiroku]

Yes that's the one.
Any info on it?

[Lucky 8 Racing]

100% get a comp test done again. Those are high but not even. I’d be wanting low 6 and even rather than high 7s and all over the place.

[Calcifur83]

Yes that's the one.
Any info on it?

No, only that it was listed at £2400 a few weeks ago and has been coming down in price. It's a nice looking car. But a proper inspection would be needed I think. I don't realise my skills were rusting until my jack disappeared through through them!

[hachiroku]

Ha ha!!! Yes that happened to me on my old MX5 . Doh!!
Mazda must make their sills out of cheese.

Inspection in person definitely needed, plus test drive. At least I have some prior 8 experience to judge things by.

[DrewR3]

Quite high compression results for a 60k mile car. I would think that's already had a rebuild at some time in its life. For 2k though sounds very cheap and as stated above, if no rust issues id go for it and put the compression results out of mind. Drive the car and see how it feels. If you know how these drive then you can tell if you have a bad engine. Otherwise drive and enhoy until it goes.

[cib24]

Those results are fine. Side seals on this car are not spec’d by hand as Mazda only sells them in like one size uncut. That’s why compression is usually all over the shop compared to being very even on earlier RX-7s where you were instructed to cut them and hone them to spec. The results look good.

Expect rust. They all have rust. Don’t kid yourself. If it’s just surface rust on the chassis and abs subframes then you can deal with it.

[ChrisHolmes]

Those results are fine. Side seals on this car are not spec’d by hand as Mazda only sells them in like one size uncut. That’s why compression is usually all over the shop compared to being very even on earlier RX-7s where you were instructed to cut them and hone them to spec. The results look good.

Expect rust. They all have rust. Don’t kid yourself. If it’s just surface rust on the chassis and abs subframes then you can deal with it.

Incorrect. Side seal gaps are critical and must be checked and set accurately. Whilst Mazda sell a range of seal sizes to match the size code stamped on the rotor just about everyone buys the longest seals and cuts them to size. A fiddly time consuming task but it is essential for a good build.

[DrewR3]

Those results are fine. Side seals on this car are not spec’d by hand as Mazda only sells them in like one size uncut. That’s why compression is usually all over the shop compared to being very even on earlier RX-7s where you were instructed to cut them and hone them to spec. The results look good.

Expect rust. They all have rust. Don’t kid yourself. If it’s just surface rust on the chassis and abs subframes then you can deal with it.

Incorrect. Side seal gaps are critical and must be cirvird and set accurately. Whilst Mazda sell a range of seal sizes to match the size code stamped on the rotor just about everyone buys the longest seals and cuts them to size. A fiddly time consuming task but it is essential for a good build.

Given the mileage, good compression and this statement i would go with my earlier idea of its had a rebuild and has been built as chris just said with the sized seals specified.

[cib24]

I’m just saying from the factory you don’t need to cut the side seals. Just match the code with the rotor and be done. I think the engine is fine and really good compared to a lot of what you usually see with rebuilds Barry making mid 6s because the housings are too worn and the rebuilds themselves are suspect, with many cars for sale saying they had a “bronze” spec rebuild or some BS which is code for half assed. I would expect a higher quality rebuild to be cutting the side seals when you don’t have to for 80% or customers.

My view, take it or leave it but that engine is good if those comp results are legit.

[hachiroku]

Thanks for all the replies, and the Info.
I'll have a good look at the car and make a decision based on what I see.
I'll update with the news, good or bad

[SprintRX8]

Knew I’ll find it eventually.
Not the one I originally found from Mazda but does give you the details.


The workshop manual for the RX-8 states the “standard” compression as 120 PSI (8.5) at 250 RPM, and the minimum as 98.6 (6.9) PSI at 250 RPM. Standard difference in chambers should be within 21.8 PSI (1.5), and difference between rotors within 14.5 PSI (1 (kgf/cm2). Readings outside of those levels, i.e., lower compression readings or greater difference in readings between rotor faces or between rotors, indicate a 'less-than-healthy engine' that probably requires rebuilding.

its quite possible to have 1 rotor knackered and the other fine - in some cases the healthy rotor will 'carry' the other until it too fails or the dying rotor simply will not fire at all, if in this case you have unknowingly become accustomed to the 'change' the only way of finding how your engine is is to either test drive another car and compare or have a compression test!

Also i should add that in most cases if your engine is already on route to failing and the cause cannot be rectified without a re-build - re-building the engine asap is recommended as 'whipping' a dead rotary will cause excessive wear within the rotor housings and side walls and these in turn would then need replacing ... basically fix it early (rotor housings and walls are not cheap!!!)

[13Black]

... Standard difference in chambers should be within 21.8 PSI (1.5), and difference between rotors within 14.5 PSI (1 (kgf/cm2) ...

Soooo ... the engine is fine then, because it's within those specs ... ?

Chambers max allowable: 1.5
OP's largest chamber difference: 1.0 (67% of max)

Rotors max allowable: 1.0
OP's rotor difference: ~0.5 (~50% of max)

[SprintRX8]

... Standard difference in chambers should be within 21.8 PSI (1.5), and difference between rotors within 14.5 PSI (1 (kgf/cm2) ...

Soooo ... the engine is fine then, because it's within those specs ... ?

Chambers max allowable: 1.5
OP's largest chamber difference: 1.0 (67% of max)

Rotors max allowable: 1.0
OP's rotor difference: ~0.5 (~50% of max)

Still within specs yes.
But a great bargaining tool if your buying.
It’s going to need a strip down at least.
May be okay for a while but who knows.
Not so good for the seller.

[350matt]

8.5 bar you say ......

I don't I have ever seen a figure as high as that on any test thats been reported on line here or in the states....

very rarely you get an 8.0 bar but I suspect 90% of the cars driving around are in the high 6's to low 7 s

[cib24]

Results are fine. Sprint RX8 is off his rocker.

If you like the car, buy it. These cars are cheap as chips still anyway.

[Torque23]

When I looked for ages for a good RX8, I have not come across a single engine with such high compression figures like @350matt already mentioned. Best I have seen is mid or high 7's even on fully rebuilt engines. Perhaps worth another compression test at a different place for repeatability?

[cib24]

When I looked for ages for a good RX8, I have not come across a single engine with such high compression figures like @350matt already mentioned. Best I have seen is mid or high 7's even on fully rebuilt engines. Perhaps worth another compression test at a different place for repeatability?

Yes, that’s fair to get another test but if it starts hot and cold and pulls hard to 9k it’s probably fine mechanically.

My recently purchased RX-8 showed 6.7-7.0 compression results on all 3 faces front and rear. It’s previous two owners were 65+ years old. I’ve put 1,000 miles on it since I took over ownership and have been redlining it 5-10 times per drive. I tested the car last Saturday out of curiosity at Rotor Torque, and the car showed 7.1, 7.5, 7.2 and 7.6, 7.3, 7.4.

Point is just drive it. Don’t worry about compression too much if it runs well.

[ChrisHolmes]

Those numbers might just reflect one of the seals sticking due to a build-up of carbon, as a result of the engine being statically run at idle, or the car just being driven too gently, not making good use of the rev range.

In which case, all it might need is "An Italian Decoke" - drive gently until the engine is good and warm, then give it the beans.

Note that this isn't an excuse for speeding, just drive one or two gears lower, for the same road speed, as you would in a normal car.

[Torque23]

My recently purchased RX-8 showed 6.7-7.0 compression results on all 3 faces front and rear. It’s previous two owners were 65+ years old. I’ve put 1,000 miles on it since I took over ownership and have been redlining it 5-10 times per drive. I tested the car last Saturday out of curiosity at Rotor Torque, and the car showed 7.1, 7.5, 7.2 and 7.6, 7.3, 7.4.

Point is just drive it. Don’t worry about compression too much if it runs well.

Very good point and I am actually in the same situation as you. Got the RX8 about a month ago, owned by an older person for last 15 years and the car saw very little use in last 10 years. Initial compression test showed 6.5 to 7.1. I have now double changed the oil to 10w40 Millers Trident and have completed ~1000 miles already. I was thinking of getting another compression test at the same specialist in a month or two to see what difference it has made. Good to know yours have improved noticeably. I am premixing with Idemitsu rotary lube and redlining it a few times on the 20+20 miles commute every day.

Having owned Italian cars and motorbikes, I am used to the "Italian Tune up"