Callum Ferguson

iTrader: (1)

There seems to be some concern over V5 heads being cracked. Is this due to a fault in the castings, ie their design, metallurgy, etc or is it through use or abuse? Have they improved with the later versions like the JDM V7-9? Any thoughts?

EMS

Interesting! I saw quite a few cracked heads on the MY99/00 models, but all of those heads came from broken engines. I don't know if the cracks could be due to knock.

Mark.

Callum Ferguson

iTrader: (1)

Thanks for the reply Mark. I noticed you posting on a head related thread on NASIOC regarding port sizes. Have all the heads you saw cracked been the larger port MY99/00? What was the O/D of the socket you used in the photographs? Have you looked at opening up the small port Euro V8 heads to match the JDM V8's?

Bob Rawle

Typical of the heads up to MY00 as they tend to micro crack between valve seat and plug. Doubt the later ones are any better. Both mine were like this when I rebuilt, just replaced the bare castings. Not really visible with the naked eye but can propogate into something serious. cause ... seems a "sort of" design issue but cylinder temperatures would significantly contribute.

cheers

bob

dowser

Sorry - off topic: Would this cause intermittent det around peak power? I started experiencing it after a 38 degree ambient track day in summer....

What's the best way to test my new (secondhand) heads?

Richard

David_Wallis

look between the spark plug hole and the valve seats..

Best way is to have them crack tested.

David

Bob Rawle

Richard no it wouldn't, suggest consider the spark plug performance and/or oil vapour contamination. Also if peak power was in the 6000-6800 region then it would be a bit flaky around there.

bob

EMS

The cracked heads I´ve seen were all MY99/00 heads, so they also had the bigger ports. But as stated before the cracks are between spark plugs and the valve seats, so I don´t think it has something to do with the port size.

I´ve used a socket of 28 mm O/D for the pics.

I am going to use the small port heads on my own project, as for me torque in a broad rev range is more important than big HP. I assume the "small port" heads should be fine at 360 - 380 HP on a 2.5 liter engine. I will do a litte porting but only little smoothing and matching work.

Mark.

Callum Ferguson

iTrader: (1)

Many thanks for the input.

It would appear that there's lots of cracked heads around but with the concensus that there are heads available without cracks or "relatively" inexpensive new bare heads as Bob has used. I say relatively because my valvetrain components will not fit so these would also need to be purchased. The fact that not many V8 heads have shown up as cracked could just be down to not many of them being pulled apart yet.

One of the US tuners has flow tested the smaller ports after a full port job to find that they would flow 360cfm - or the same as the large port heads - so one option may be to go this route given that I already have the heads?

Going slightly off topic I am interested in the valve sizes. I'm told that the V5/6/9 heads have the same valve part numbers but that the V7/8 (Euro spec only?) have different / smaller valves. Can anyone provide the two different valve sizes? I understand that billet valves are available if required.

Cheers,
Callum

EMS

Not yet, But I am going to remove the heads in a short time.....

Mark.

Callum Ferguson

iTrader: (1)

Hi Mark,
It has just been suggested to me that there may only be 0.5mm between the small & large valves? No figures on actual diameter yet though. I guess this would be more than enough if you were machining out lightly damaged seats but it may not have a huge influence on power figures. Maybe recutting the seats & flowing them could easily have as much effect?
It will be very interesting to hear how you get on with your heads

Callum

David_Wallis

There are billet valves available, I use them in my car.

You can cut multi angle valve seats etc and allsorts of other fancy things.

David

LeoneTurbo

As far as I know all DOHC Subaru engines (four bangers at least) have 36mm intakes and 32m exhaust valves.

However, not sure about the latest engines... Someone?

Callum Ferguson

iTrader: (1)

Hi David,

I guess I spoke to your billet valve supplier earlier today

Why did you choose to go this route? I imagine they could be lighter, stronger, flow better, etc but did you have a specific reason/s that you are willing to share?

Thanks,
Callum

Pavlo

The billet valves are just available. They are a normal replacement item for a non subaru engine as far as I know. Personally I think the Sti ones are better for a number of reasons, depending on exactly how you sort the cam clearances.

Paul

GTJ

I’ve seen cracks in my cylinderhead (MY99) as well, during rebuilding the engine (the engine had have a big-end bearing problem). The cracks are between the spark thread and the inlet valve seat. Some small cracks could be seen in the outlet valve direction (longer path to valve seat, not yet grown to the maximum length, clear in what direction the crack grows). The cracks could be found at all cylinders (after 67.000 km, including 27.000 km PPP)!
See the picture:



Of course I was concerned about the cracks and did investigate them. With a röntgen-machine, used at an aluminum casting facility, it was clear that the cracks where only at the surface and started from a porosity hole (in fact, the surface of the head was full with porosity’s caused during the casting process).
I strongly believe that all cylinderheads have these porosity’s.
The cracks will not grow any deeper, because the stress will reduce to zero.
If you have a closer look at the spark thread, it is almost logical that a crack will start here as well. In fact they should have made a small room or machined the thread to a less crack sensitive object.
The cracks will start at the surface where the stress is high (high stress @ small section = between spark and valve seats).

Solutions:
1) Leave it as it is. I’ve done this.
2) If you really want to stop the cracks (they will stop themselves however), carefully push them together with a strainer (?) and hammer.
3) An other drastic way is: shot-peen the head (this is also done with the WRC’s cilinderheads, now you know why). With shot-peening the porosity will close and some extra pressure stress will be put in the surface, this will help to decrease the stress during combustion. This is a specialist job though, and not usable for al places on the head surface.

Kind regards,


Gert-Jan

Callum Ferguson

iTrader: (1)

GTJ - Many thanks for the info - very useful illustration of the problem. I have spoken to several people who have done as you have and left them alone. No-one has yet said that they have encountered a failure such as an inlet valve seat coming unstuck. The heads should be off my '02 car next week after 11k miles so it will be interesting to see whether they are cracked or not.

callum

[Edited by Callum Ferguson - 1/2/2004 6:35:12 AM]

GTJ

Callum,

please tell us when you've seen the cylinderheads.



Gert-Jan

dowser

I've just checked my spare set of phase2, no cracks I'll post piccies once the valves have been ground in too, but knife edging is sharp enough to cut fingers with!

Richard

BlackR

I had an engine rebuild on my STI5, there were cracks in both cylinder heads - one of which failed the pressure test and was replaced. Both were in the same place, between the spark-plug and inlet valves. I was informed by a reputable Subaru engine rebuilder that these cracks are common - especially on STI engines, and are normally no cause for concern.

APIDavid

iTrader: (4)

We've built loads of these engines that have this problem and have NEVER changed a head yet for this crack problem. As stated above , much more scientifically than I could, the cracks are small and insignificant.

And in our opinion are there from very early age [ if not actually made like that!! LOL ] and are not in the least bit threatening. Of course we pressure test and 'Ardrox' them and all appears OK.

Never heard of a crack related failure I.E. valve seat coming loose and falling in. It could happen, but so far not know to me after well over 300 engine rebuilds logged in the last 3 years.

So not a concern to us, but I do see the point about wanting it perfect. I just think that if you do buy new heads, that after a small-ish mileage you'll find them cracked just like your old ones.............. Lets not make IM that much more profitable.

David API Engines.

PS I'd be far more worried about the compression ratio after fitting 2 litre heads on a block with 25 percent more swept volume.

Callum Ferguson

iTrader: (1)

GTJ - I will pass on the info on my heads once they are off - soon hopefully.

Richard - I look forward to the pictures of your ported heads.

API - Thanks for the input David.

The web that's cracking looks very narrow so it's not surprising that small fractures are appearing. Sounds like as long as they are superficial, as in the majority of cases, then they can be ignored.

Paul, Mark(EMS) & anyone else who's interested

Just to move the discussion on I've been reading a little about heads today, one of Bell's informative books, and I wonder if anyone has any thoughts on this...

Bell suggests that the inlet port to valve diameters should be in the ratio 0.81/0.83 to 1 & that the inlet valve size for a 625cc/4valve cylinder should be between 38 & 40mm.

Assuming that the inlet valve diameter is 36mm, as stated by LeoneTurbo, & looking at Mark's pictures on the NASIOC with the 28mm O/D socket in the inlet port it would appear that, for a 2.5, both the valves and the ports are undersized by some margin. For a 500cc/4valve setup the MY99 ports almost look too large and the V7/8 STi Euro heads too small.

Anecdotally, I cannot help thinking that the MY99 heads had nothing wrong with them. I had a 270 bhp/99 car that went very well through the bottom end & midrange where ports that were too large would, in theory, not have worked very well? I am just about to try and get 400+ out of ports that are at least 4mm smaller

I understand this is just theory, practice/finance will dicate that I've got to use what's here

Bob Rawle

I know that the Leicestershire police Scoobs suffer with cracked heads regularly causing poor running, misfiring and generally bad behaviour, RCM look after those cars when they go wrong so I've seen some of the heads as they come off, its not true that they don't cause a problem but ... it is true that the degree or not would depend on mod level and use/abuse.

bob

EMS

Valve size of 36.0 and 32.0 confirmed for the MY03 Eurospec STi!

Nice light buckets (is that the right name?) between valves and cams! (a bit expensive if valve clearance has to be adjusted........)

Mark.

The Fixer

Callum, the 0.83 ratio is the one used on my factory Ducati race engines. I would however be not too keen in increasing inlet valve size as the cracking may become worse.

Have a read on here with regard to port sizes & velocity, efficient filling of combustion chambers etc.

Click Here

Conrad

LeoneTurbo

Told you!

To take this discussion to another level, does anyone have information regarding camshaft specs of the various STi versions? Would be nice to compare lift and duration in order to select the best cams for suitable application (high revving EJ20 or midrange EJ25 turbo).

johnfelstead

When upping the engine size of an otherwise unchanged engine layout, you normally find what would be considered as a wild cam on the smaller engine would be relatively mild on the higher capacity engine. So i think trying to use the 2.0 litre cam specs as a basis for tuning phylosophy on a 2.5 would be a mistake. You are better working with the requirements of the 2.5 in terms of rpm range and flow requirements.

Callum Ferguson

iTrader: (1)

Conrad - Thanks for the link - interesting stuff

I had no intention of trying to increase the valve sizes but as for the ports that could be another matter

LeoneTurbo - try this NASIOC thread for some cam info - maybe it would be interesting to have something similar on SN..

NASIOC Cam Info Thread

John - I agree with your thoughts. It just seems that there is still a little bit of uncharted water and a lack of GDB engine parts which will let us utilise AVCS to it's full potential; whatever that may be