It's insane, I was in a backstreet tyre place this morning getting my fronts changed and I was talking to a guy with no specific Subaru knowledge and he asked me "is it the 2.0 or the 2.5?" I told him it was the last of the UK 2.0's (widetrack Blob) and he said "oh good, no headgaskets and rebuilds for you then".

I couldn't believe it, I wondered if it was just SNet scaremongering but it seems everyone knows of their reputation!

 

...everyone except Subaru and their dealers...

 

Oh they know alright!
They just choose to play dumb,like most manufacterers would.

 

2.5 been around in Legacy for a while in NA version, must be something to do with running a Turbo on Impreza.

 

+ 1

 

correct

 

We've had a number of Hawkeyes in with this issue, and it always seems to boil down to the same thing.
The headgaskets don't "blow" or at least not in the old fashioned sense. They are metal gaskets and tend to maintain their integrity. What they do is leak!
Whenever we've pulled these apart, we find an obvious staining compromise on the "fire ring" which is the sealing area between the cylinder and the head.
The rad caps on the header tank have a one way valve in them. If there is any pressurisation due to either normal water expansion with heat, or pressurisation from combustion gases leaking into the cooling system, then water will pass from the header tank into the expansion tank. When the engine cools, the vacuum then created inside the block will pull water back into the engine from the expansion tank via the valve.
If water is then blown out from the expansion tank, when the vac sucks what's left back into the header tank, the expansion tank will be low on water.
It's my view that Subaru have had some operating issue during the build, possibly as daft as not having the head bolts tight enough!
A complete cure is to replace the head gaskets with Cosworth ones, fitted with ARP head studs.

Broken piston ring lands are another issue theoretically, although if any water ends up in the cylinder, the resulting hydraulic lock could easily break a ring land.

That looks like this;



We've seen them come in with both issues, put it that way.

 

should you always be scared owning a subaru then? as they seem to just implode lol

i take it if either of these issuse happen its not classed as a catostophic failure? and can be sorted easily ie not a whole new engine

 

Funny how its normally piston #2 and #4. With the damage on #4 piston when mine went pop the fracture was longer on the top than it was on the bottom, like this
\ / indicating the fracture was caused while the piston was going downwards. While this can be caused by hydraulic lock or detonation it can also be cause by poor assembly. If the rings were not compressed enough and caught the top of the liner during assembly then cracking in the ringland in this manner can be the result. I know for a fact i had no det issues or hydraulic lock.
Now you mention that during assemble Alan that some heads may not have been torqued up enough. makes you wonder whether some of the Subaru robots are in need of re-calibrating?

 

Just started a poll/thread on the 2.5 hg and or ringland failure... probably not the first person to try it but might prove interesting

https://www.scoobynet.com/scoobynet-...lure-poll.html

 

All the cars we've fixed have stayed fixed..

Incidentally, we see much worse issues on other cars, some of it plain stupid like Audi running some expensive diesel engines with a bike chain driven oil pump. Crap and no mistake.

 

It makes you wonder indeed! The gaskets are ok, and so are the block, heads and bolts, so why do they leak? We never give the heads more than a light surface, and a light clean of the block is all that's usually needed.

 

only the 2.5 they fobbed off on the u.k market the 2.0 are fine

 

I don't understand why this tends to worsen under boost, surely this just affects pressure on the exhaust/inlet, unless boost has some affect on compression?

 

the heads lift

 

And how does boost create more lift if it isn't affecting compression?

 

it does affect compression

 

Does it? How?

 

By cramming more air into the cylinder. Thats what a Turbo does.

 

you need ladybirds "my first engine book" this may explain how one works.........

 

1 bar is double the ammount of air inside the cylinder of a n/a car, thats why the compression ratios are lower for turbo cars to allow for this

 

The compression ratio remains the same?, but the amount compressed obviously increases, increasing the pressure inside. Have I got that right?

 

correct

 

yes and this is what lifts the heads up.

 

Its more about cylinder pressure than compression ratio. The ratio is calculated by many engine factors like swept volume ect. Cylinder pressure would be about the volume of mixture in the chamber being compressed.

 

It's about cylinder fill.

 

that what my misses says

 

I'm getting that now thanks Alan.

 

well off boost it would be 1 bar atmospheric although i think its actually slightly more . I believe modern normally asperated engines run slightly higher than 100% vol. But if your boost was 2 bar than you have doubled it. Basicly turning a 2.5 into a 5 litre in volume.
Edit. could be triple at 2 bar of boost, cant remember. You need someone brainier than me to work it out for you.

 

Bike engines tend to run 11:1 or so compression ratio and some rev to 18k and beyond. I've often wondered how much can be played with on the compression front but then if you're practically doubling the pressure under boost then I suppose it's quite risky.