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Has anyone had their 12A engines compression checked.

Rotechniks did my car at 35081 miles,results were,

Front rotor
6.7,6.9,6.6 at 2450 rpm

Rear rotor
6.8,6.8,6.7 at 2320 rpm

This check was done in 2004 & cost £24.50 + vat.

Does anyone know what excellent readings are & what poor readings are.
What readings indicate imminent engine rebuild!

Cheerio

*i have not info other than what I've read, it may be totaly rubbish*

I've a feeling I've read that for FDs 8s are totally brand new, 7s are good and 6.5 or lower are not far from re-build. BUT the 3mm apex seals in the 12a, vs 2mm in the 13b, don't seal as well at lower RPMs?

Like Cliff I've heard that consistency between chambers is a good sign

syebba wrote:Has anyone had their 12A engines compression checked.

Rotechniks did my car at 35081 miles,results were,

Front rotor
6.7,6.9,6.6 at 245 rpm

Rear rotor
6.8,6.8,6.7 at 232 rpm

This check was done in 2004 & cost £24.50 + vat.

Does anyone know what excellent readings are & what poor readings are.
What readings indicate imminent engine rebuild!

Cheerio

I,VE AMENDED THE ABOVE POST SO IT'S CORRECT.
When i read my compression report,it had readings taken at 245rpm & 232 rpm,i thought this could'nt be right so i assumed they meant 2450 & 2320 rpm!

Rotary engines before compression testing should be warmed up & then compression readings taken using a special mazda compression reader at CRANKING SPEED.

Even wear accross the rotors is considered a good sign as Cliff pointed out.

Still can't find Mazda's spec's for the readings,but there's lots of bodged results using incorrect piston compression testers on the net!

syebba wrote:Has anyone had their 12A engines compression checked.

Rotechniks did my car at 35081 miles,results were,

Front rotor
6.7,6.9,6.6 at 2450 rpm

Rear rotor
6.8,6.8,6.7 at 2320 rpm

This check was done in 2004 & cost £24.50 + vat.

Does anyone know what excellent readings are & what poor readings are.
What readings indicate imminent engine rebuild!

Cheerio

Had the Elfords engine compression tested today and the results were quite low....


rear rotor....






front rotor...





it's not the end of the world as the car drives nice and starts well plus I have 3 other 12a's in the garage including an Elford engine with a recent documented rebuild so when this one eventually gives up the ghost, I'll just drop that one in.
Interesting to know though!!!

Many thanks to Keith (Trind-FD) for offering to bring his expensive Mazda testing kit and doing some free compression tests ( even if my car came last in the results table!) Top man!


I have it from 2 top rotary engine builders that fresh rebuilt motors comp tested will give you 10 plus on all faces,once the motor has run it will give you low 9's to high 8's all the way round.
so new engines around 9's.
7's are fine then around 6's means its getting tired.
The rear rotor normally goes first.
They both also say, don't worry about a bottom compression figure.... as long as it starts ok, just run it.... you know when it's knackered when it won't start.

Mine is quite low on compression according to the readings above and yet pulls like a train and did so even before the turbo went on it.
.

Had mine done at Ian's on sunday.

Rear.


Front but battery was getting low.


Car has done 65k miles on original engine so par for the coarse.
Thanks again to Keith for doing the tests .

As some may know, I have a soft spot for the fb, being the current owner of two, soon to be one
I have very favourable rates for fb owners when it comes to compression testing,
Was a pleasure testing keiths & Ians cars.
And although it's nice to have high readings, as long as it starts hot and cold, I wouldn't worry about a thing.

Keith.

I wonder what the difference is between a NA 12A engine and a 12A turbo due to the lower compression ratio. And when a car starts getting harder to start due to wear- does this usually show up when starting cold or starting a hot engine?

I would expect a slightly lower comp figure from the lower comp turbo rotors, but not much. I would expect a bigger difference from fitting single piece apex seals ie carbon race seals. No idea what the numbers would be.

Alex, it's hot starts where the low compression shows up first.

My 12a that lost compression first gave me a hint that it was starting to go by taking just a few more turns of the starter than usual to get going after stopping for short periods, like getting petrol. Over a few weeks this got worse till it was 50/50 whether it would start after getting fuel and would take 2 or 3 goes to get it to run. At this stage it would still hot start fine if I stopped and started it immediately, for example if it stalled.

Another few weeks went by and by constantly adjusting my megasquirt settings I could start it at the petrol pump within 30secs or so, but was feeling a bit touch and go. It was about then that it started struggling to re-start after a stall. Then one day I pulled over to take some photos and when I tried to re-start it, it just wasn't having it. Took me 1/2 hour of waiting till the engine was cool enough to start it, that's when I took it oof the road till I got a new engine, was just too much of a liability.

For reference I did a compression test shortly before it fully died, only with a piston tester so not super accurate, and was reading approx 3.6 bar on all faces.