Forged pistons, why?

[Jacques]

Hi all,

I've recently had a talk with a friend, who says that forged pistons is exactly the same as normal cast pistons.

I am saying that forged pistons are used for a variety of capabilities:

 

a/ smaller tolerances.

b/ stronger.

c/ can take more heat and higher middle piston speed.

d/ lighter.

 

By using forged pistons, the engine can (without looking at other factors of course) take more rpm's, last longer before a piston crown is eventually torn apart and can take higher temperature, before it melts. A polished Crown can be on both types so I did not include that.

I am not in this looking at cooling oil squirts underneath, sodium filled exhaust valves etc, to do other goods, only the forged piston talk in itself.

 

He says it don't mean a jack s..t. An ordinary piston is as strong, can take the same beating, rpm's and temp. I dissagree.

 

Am I far off?

 

EDIT: I also think it is so, that a forges piston have a denser surface than a cast one, alowing to use less silicon to control heat expansion and thereby alowing for a more precise predictable and eventually shorter (lighter) piston. At least I think so

 

Kind rregards,

jacques

[Chillidoggy]

There's plenty of Internet information on the subject Jacques, but a forged piston is more dense, having been rammed into shape, but it does expand more than a cast piston, so clearances for them are generally bigger, and expand to the correct size once they've reached their operating temperature. I think Mahle are the best for thermal expansion.

They can rattle a bit when the engine is cold because of their higher expansion, and thus bore wear could be an issue for a road car. They do perform better under extreme loads than cast pistons, they are simply tougher, but they may be heavier due to their density.

I've used forged TRW pistons in my Cobra engine but honestly, unless you're going racing, I'd stick to the manufacturer's recommended fitting.

[Simon350S]

Cast Pistons in the early Turbo Esprit used to melt at sustained high power useage.

Improved cooling system helped and was a retro fit kit, but the forged Pistons were the inevitable way forward and they don't suffer the same issue. The later cars also had better block castings, better water pumps, and revised head cooling.....

If the cast Pistons were fine and no improvement were to come from using them, your SE would still be driving around with them!

You'll not win the argument with your friend. When someone has it stuck in their head like he seems to have, nothing will change his mind.

[Jacques]

Thanks, I thought that the high silicon in cast ones were there to control heatexpansion, but a high content made them softer, and not durable?

Also, I thought that forged ones can be made shorter because the uppermost ring can be closer to the top, and skirts shorter - lighter or at least counterbalance the heavy weight?

 

Kind regards,

Jacques.

 

I think the SE onwards have forges pistons because og high temp from turbo use?

Yeah, you're probably right Simon. He's quite hard to convince. It started from Laverda Engine talk. He's making some race engines, and use cast pistons. Mine id made with forged ones, and he said no no. I said yes, and we went on from there

Same thing about cars.

Fortunately I have written documentation on the Laverda engine But this then went toa cast vs. forged talk, hence me asking. I was sure that Lotus changed to forged ones from the SE onwards?

 

Kind regards,

Jacques.

[DanR]

Cast pistons come in all sorts. Different material compositions, drilled vs slotted oil returns, metal banded to control expansion, skirt variations etc etc. All done to try and get them closer to the strength of forged without the cost. A lot of these techniques are very successful and ultimately it comes to application.

 

For a high stressed engine, forged is best. In a lot of other applications carefully selected cast pistons can work just as well. There's also the benefit of tighter clearance so for a daily driver that can equate to less wear.

[MrDangerUS]

Jaques,

 

Majority of 86-88 Turbos had their boost restricted to 7.5psig by installation of an aluminum spacer ring between the halves of the wastegate. There was a good reason for that!
There was an issue in the early turbo cars, which had to do with the (early, cast) pistons crowning and causing excessive clearances causing hot blow-by contributing to the ring land failure resulting in broken rings and/or pistons. HCI upgrades and later Mahle pistons solved the problem.

Early turbo cars contained a lot of parts originally made for N/A cars that generated less heat and HP.

I think the nikasil piston/cyl, sodium valves and other improvements have helped our cars reliability a lot!

Edited November 9, 2014 by MrDangerUS

[Andyww]

Cast Pistons in the early Turbo Esprit used to melt at sustained high power useage.

 

 

I remember at the time there were some horror stories but I was under the impression the problems were caused by over-boosting when the wastegate stuck shut, which the early ones did quite often.

 

Are there known cases of pistons melting when boost is limited correctly?

[Simon350S]

One of the big issues was the cooling around cylinder 4 (from memory), this was cured with a revised cooling system at the same time as the internal mechanical changes.

When I first joined here the whole, S3T piston melting issue, was shouted about regularly, I recall being told it was a matter of when, not if! it was going to happen, and not to consider purchasing a car without factoring in the inevitable cost of a rebuild very soon.

Luckily that seems to have died down some what with the passing of time, that and a few of the early 'enthusiastic' members of LEF.

[Jacques]

So that seems to havebeen cures with a revised cooling system and the forged pistons? I think we can safely say now, that forged pistons are the way forward for performance as well as for longevity or safety.

 

The Nicasil coating I suppose was made to be a hard surface on liners if they were aluminium?

 

Kind regards,

Jacques.

[Andyww]

The forged pistons were mandatory for the higher compression engines.

 

But I have never heard of these alleged issues with the LC (other than wastegate-induced). These engines were very conservatively rated because they were worried about heat and mechanical stress. Lotus tracked the development cars extensively. My own 1981 car I tracked and the only issue I ever had were the brakes being rubbish.

 

The car I currently own is a 1982 , 76000 miles and running fine. (Tempting fate I suppose to say that!).