I had Cosworth pistons in a mini engine, I melted them :-)

andyjd

[Edited by ukhuskynox - 3/2/2002 6:01:30 PM]

 

I know that the non STi use Cosworth forged pistons as they are better than the standard items, but I've read that even STi's (forged pistons as standard) use Cosworth items?

Just wondering why (sad aint I lol)?

 

Really.

I guess what I am asking is - why do people who own top of the range Imprezas i.e. 22b's, STi's, use Cosworth pistons? Are these better items than the original items? Or is it cost related?

Just curious, not having a dig.

 

You talking about modded engines I assume?

STD STI pistons aren't that great.

I reckon most people use Cossie pistons purely through ease of availability.

 

Thx Craig. Yes I meant modded or even just rebuilds...most seem to use Cosworth pistons.

I wonder if these 'Cosworth' pistons manufactured by Cosworth Engineering or are they merely Cosworth specification pistons.

 

Cosworth pistons are made by a subsiduary of Cosworth Engineering. They have the advantage of being strong, reasonably light, and readily available.

As CraigH said, both standard turbo, WRX and STi pistons are not perfect, and tend to be somewhat expensive to replace. Given the other costs of rebuilding an engine, people generally upgrade the pistons while they are at it.

However, if you are buying a car and it claims to have Cosworth pistons, ask yourself why somebody felt like the expense of an engine rebuild in the first place?

Duncan

 

The Subaru pistons are NOT forged.

They are called Heploylte, which use a stronger grained metal for strength, these pistons are still cast, then cleaned up for balancing. They are only good for 18PSI continuous boost any more and they melt.

True Cosworth pistons are tried and proven for weight and strength, that is why so many people use them. The Cosworth forged piston expands quite a bit with heat, so you have to run more tolerance, making them very slappy when cold.

 

18 psi - is that for the standard UK Turbo as this would be about 1.25 barrish

Given I've been running around 1.5 bar, 1.4 bar held, which is over this amount I cant see its true - though I've only done this for about the last 40,000 miles.....

I just need them to melt then I can ditch the engine and put in a nice 2.5litre one ala christianR

 

Submanz

I can understand that cast pistons are not as strong as forged however I cannot see why they would melt if used with the correct fuel ratio at 18psi.

Another example of wrong and misleading information me thinks

cc

 

The subject of forging and casting for pistons is one of the great debates within engineering. In simple terms, it is easier to get a high strength and light weight piston via forging, but casting is less expensive and can give superior resistence to stress.

In Subaru terms the standard pistons are OK for a standard car, but are too heavy for serious high revving applications, particularly given the strength of the standard rods.

Duncan

 

Duncan has got this spot on.

The only reason for going to forged is the weight saving, so reducing stress on rods and bearings at high rpm.

There are numerous cars in the UK that use the standard cast pistons and still get well over 300bhp, running at over 20psi. This has everything to do with the quality of the mapping and nothing to do with luck.

 

Pete,

You're not seriously saying that Cossie pistons for instance are no stronger than the std ones are you?

 

Indeed that it exactly what Pete is saying. As a generalised rule, pistons only fail when too hot, and no matter how much boost you run, if you fuel it correctly, the heat is manageable.

The load on reciprocating components goes up by the square of the speed. Rods are the likely failing point on an impreza engine, and tuned engines with high torque outputs and high revs are likely to throw rods.

Lighter pistons reduce the load on the rods significantly. Cosworth pistons have a better design around the gudgeon pin too.

 

Craig

What do you mean by strength? Often a heavier piston is physically stronger, but causes a number of problems;

• reciprocating mass (as above)
• worse heat transfer (hence why a number of Cosworth engines employ oil cooling of the underside of the piston crown)
• worse expansion properties (and our beloved slap!)

The bottom line is that the standard Impreza pistons appear to be OK for a car provided it is not revved too hard, the mixture is kept correct, and the power is kept to "reasonable" levels. For the type of power that you achieved, and in particular the rev limits used, forged pistons were essential.

Duncan

PS Whilst a number of Cosworth engines use forged pistons, the F1 turbo engine of the late 80's used cast pistons that weren't made in house

 

Am not gonna disagree cos you both sound like you know what you're talking about and I very rarely do

With regards to high revving, is it then safe to say that as STIs by their nature rev v high anyway, as soon as reasonable mods and higher boost is used, then the std pistons are the weak link?

I only ask, as I know of at least 1 well known dealer/engine builder/motorsport prep co that often when rebuilding STI engines keep the std rods but replace the pistons with Cossie versions.

I would guess that as the pistons are now lighter the loads on the rods are less?

 

What he said is partly true about the weight, however forged materials are a lot stronger. Why you you think that demascus sword blades are superor/stronger, because they are forged.

What makes the same metal stronger? It is the desity of the grain the grain of the metal has been increased.

If you split a cast material the grain will be very crystalised, quite large, the forged equivalent will be still crystalised but far far less.

 

Sorry that should have said split.

Steve you have obviously been lucky, mind you the cars I am talking about are track raced so are running that boost most of the time.

Dan

[Edited by submannz - 3/5/2002 12:58:43 AM]