Engine blown??
 

Hey guys,

here's what happened. My friend raced a porsche on the highway last week and suddently under WOT the engine made a strange noise and started running like a loan mower.
He stopped and thought maybe a hose popped off but after opening the hood everythings seemed to be fine.
He then drove the car to the local mazda shop 30 miles away. He had a compression test done which showed 4 bar on both rotors and all chambers. He also tested weather the coils are working. The fuel delivery also seems to be ok.

He had the car mapped for 0,9 bar with standard twins on a street ported engine with stronger apex seals. But when he raced that porsche he raised the bosst to 1bar. Knock was never above 20 (under boost)on the PFC.

Now what I like to know is, whether it's possible that an apex seal is blown even though the comp test shows 4 bar compression:scratch:? The engine was built in the US some years ago but he only put around 10k on it.

thanks for reading.

4 BAR is extremely low compression but if every face is even and the car starts fine then i doubt it is the engine and just an improper compression test.

have your friend find the MAP sensor at the firewall near the brake booster and follow the hose from it to the intake manifold behind the throttle body. reattach if necessary.

Which seals? What is vacuum at idle?

Tanks for the replies so far. I think I forgot to mention that ever since he shut the engine down we havent been able to start it anymore.

So we can't evaluate what the vacuum is at idle. The engine was built by Rotorsportsracing and is a stage 2. I think they used two piece apex seals but I don't know what make.

He also said that he was running an afr of 12 to 13 which got lower to 11,5 under boost. Maybe that was a little to lean. Unfortunately he has no egt gauge in his car.

The map sensor connection is fine. We checked every hose under the hood and everything is ok.

11.5:1 AFR is richer = 11.5 'parts' of air to 1 part fuel.

VS

14.7:1 AFR = 14.7 'parts' of air to 1 part fuel (heading leaner as more parts of air are in the mixture)

Air:Fuel ratio. First number is quantity of air in the mixture, compared to 1 part fuel. So it richened to 11.5:1 right before something went wrong.



Anyways...
4 Bar compression is awful. 1 Bar = 14.50 PSI . So 4 Bar is 4x14.50 PSI= 58 PSI.
If he has only 58 PSI on all faces of both rotors, his compression is too low for it to fire up and start, as Karack hinted earlier. Something has caused a huge loss of compression on both rotors.

I think minimum compression PSI to run is something like 95 PSI?? (Dont quote me) but 59 PSI is too low.

Its usual for one rotor to die, as a seal issue is confined to one rotor housing in the event that a seal fails. But it sounds like he has 2 dead rotors, which is unusual to happen at once.

I would double check MAP line connection, coils/plugs are firing, fuel system is in check, then do the compression test again. If you get low 4 Bar compression readings again, then the engine should come out and be disassembled and inspected.
Dont just put new seals in it and put it back together without finding out what caused this to happen in the first place. Or history could repeat itself again..

IIRC, Mazda's limit is 85psi

difficulty starting will begin around 75-80psi of cranking compression @250RPMS if compression is even. if you have one dead seal and 100+ on a single rotor but say 30/30/90 on the other rotor this will also have difficulty starting.

^ Figured I was sort of ballpark on minimum compression...

But 59-60 PSI at 250rpm cranking speed is not going to do it. If both rotors are showing such low compression (assuming you're doing the compression test properly) then hell, I would say its time to unbutton it and have a look inside. If it destroyed seals on both rotors, I would be checking that seal fragments didnt exit the exhaust ports and damage the exhaust turbine wheels of the turbo(s).

It would pay to know what caused it to happen, so you can fix the underlying cause while its in pieces.

Thank you very much Sir for teaching me what afr means:lol:

maybe my statement about the afr values was slightly confusing. I just wanted to know if an afr of 11,5 under boost may be to lean and could have caused the low compression figures.

It is also common knowledge that 4 bar on each face is not enough to get the engine started but we were just wondering why it is so low on each chamber and on both rotors. We first thought that mazda did an improper compression test but if so why can't we fire it up.

My friend is going to take off the turbos this weekend to take a look through the exhaust ports. But what if the seals seem to be visually fine? Is it possible that the seals just bend (all at once??) and cause these low compression figures because of the lean conditions under boost?

Once again the engine has only seen 10k! :scratch:

11.5 is not a lean condition under boost.

-Geoff

depends on what these unknown seals are. 11.5 isn't lean but it is borderline for that boost. the seals could have warped and is causing blowby which is causing the low but even figures.

softer seals which can warp should be a little more on the rich side to keep the walls wet and promote seal to wall/rotor cooling. just pulled apart an engine that was running a little lean and had all warped seals on the rear rotor, these were atkins seals also which i thought was strange since they normally do not bend!(new material?)

https://imageshack.us/a/img820/5957/picture034i.jpg

i think they're out of spec...