Pleiades

What is the pros and cons of using forged cosworth pistons in preference to the STI items ?

I have been told that the cosworth pistons are "a competition part and as such are noisier than standard". How noisy ?

A brief search of this board has revealed issues with excessive noise on startup and possibly piston slap !!!!

Are these issues due to incorrect bore clearances which I understand are different for cosworth and STI pistons ?

I've also been told that cosworth piston is superior to the STI item and they are significantly cheaper.

Any advice and experiences good or bad would be appreciated.

Adam M

Cosworth, and similar forged pistons, expand more than the standard pistons, so need a bigger bore clearance. They also tend to have a shorter side skirt, and this allows a little more lateral movement.

There were some problems with the early Cosworth pistons, issue 1, but these have been rectified. I believe they are up to issue 3 now.

However, most of the problems can be sorted out by running a little less clearance, than Cosworth would recommend. This is ok, because the spec' was designed for a "racing" engine, which would run much hotter.

If you run a clearance of 2.5~3 thou, it will be a "little" noisy when cold, but almost as quiet as standard, when warmed up.

Cosworth pistons are far stronger than even the STi pistons.

Are you changing the rods too.

Mark.

Pleiades

I would like to use the cosworth pistons and have the noise, slap etc within the levels of the standard STI pistons. Is this possible ?

I had planned to have the standard rods lightened, balanced and shot or glass peened. What do you think ??

[This message has been edited by Pleiades (edited 05 August 2001).]

Pleiades

I'd forgot about the front pulley, cheers.

R19KET

I'm going to kill Adam....he keeps using my laptop, and leaving his I.D on......

So, It is possible to get pretty quiet results from Cosworth pistons, but if you try to run the same bore clearances as standard pistons, you're asking for trouble.

I was out in a car tonight, with Cosworth pistons, and it was very quiet once it was warmed up. The owner reckons it's quieter than his standard engine !!!!!!!

As for rods, it really depends on what you're planning to do, regards boost, etc'.

If you're planning on running a lot of boost, I would advise uprated rods. If you're going to keep things pretty low (up to about 1.4bar) then the standard rods should be fine. It would be worth seeing if uprated rod bolts are available. I'm not certain if ARP do them.

DON'T lighten them. This will make them weaker. Get them dressed, balanced, and shot peined. This needs to be done with the pistons. They actually weigh, and match the pistons, and rods, as pairs.

I assume you're getting the crank, flywheel, clutch cover, and front pully balanced also ?.

Mark.