2.5L crankshaft ..... colour

[Pisobiker]

Hi guys.

Think I at least catched your curiosity on this....

Well it sounds odd but checking out and comparing the 86.4 stroke cranckshaft (2.2L) and the 95.5 stroke one (2.5L) I noticed a difference in the forgings colour:

While the 2.2L one has a brownish color, the 2.5L one is much darker, with some very evident purple reflexes particularly noticeble in the machined parts of the counterweights. It is just like the purplish colour metal gets when tempered or heavily heated up, but in a very uniform pattern all around.


Any ideas what this is due to? Does anybody know of different forging materials among Audi crankshafts?


Marco

[audiman]

Not sure Marco, but it sounds like the 2.5 crank got heat treated to a much harder spec. Would this mean it would be more lkely to crack? I have little knowledge in heat treatments, so im not gonna comment too much as I may be badly wrong LOL

[Pisobiker]

Yeah Tom, heat treatment was something I was thinking at too ..... but purplish colour (if produced by heat) normally starts showing when heating metal up to very high temperatures, I think much higher of the ones normally needed for a stress relief process .....

I hope you're wrong on the "strenght" issue.... I'll need an even stronger than stock S2 crankshaft for my "stroker" application


Marco

[Stick Insect]

Hi Marco,

Nitriding and more recently nitro carburising will affect the external appearance of metal but it is more likely to be a grey colour the appearance of external colouration would lead me to believe that the crank has been hardened and then stress relieved to reduce crack problems. I wonder if the diesel crank is considered a high stress/performance item?

One is Audi and the other Volkswagen?

David

[diesel des]

Ahhhhh! i had a long reply writted and lost it. This will be cut down version sorry. Drive me mad computers do..

Anyhow, the tdi crank is VERY strong. It has a hard job to do in even a stock tdi with 15psi boost and 19.5:1 CR( gives about 155 bar peak cyl pressure) never mind a tuned one with 30psi and 18:1 CR(180+ bar).
A high power s2 bends its rods at say 450bhp with only 25psi ish boost and 9.3:1 CR. This is much lower than a tunned tdi deals with with its CR almost 10 yes TEN points higher....
The crank is stronger than the s2 item as are the rods. Just look at a tdi rod compared to the s2. The crank is similary beefed up.

I no my tdi (4 pot)stuff. I have a mrk2 golf with a 200bhp 4 pot tdi. i have never bust the crank. I no of someone that has bent tdi rods at about 250BHP (35psi boost with lots of diesel and methonal injection!) I have bust pistons, and head gaskets but the crank is very good. i now no that 180 bar is the safe max they last for. I have the sae design paper for the race tdi engines and they use stock crank and rods..

[Pisobiker]

Thanks Des.... this makes me feel better !!
I also guessed it should have been stronger but I didn't know for sure, not having had feedback from anybody.

I also know Tdi conrods are much stronger. Cannot quantify numerically but it is just evident. I have a set for sale (see "for sale /trade" section) which, as a matter of fact, I would have used for a high output engine if I didn't need longer than 144mm for my stroker project.


Marco

[diesel des]

No problem. Here is a pic of a bust tdi piston.