Case advice

[gsmith4965]

Which case is better foundation for a 2.7L track only motor? A early sandcast aluminum 2.0 with 90mm Ps&Cs or a 7R 2.7L magnesium case. I realize that either would need some machining to either accomdate larger disp (2.0) or for stengthening. I dont think it matters, but with either, I would look to use a 10.5:1 comp with 2.7heads and prob a GE40 cam.

Also, what are thoughts on using stock 'S' rods up to 7500 rpm? Can this be reliable with shotpeening and nitriting?

Thank you in advance for your advice.
Greg.

[chris_seven]

Greg,

I think that the sand cast cases are stronger than the mag and are less likely to have suffered pulled threads and other thermal damage.

Unfortunately they don't have piston squirters and I would have the cases machined to fit these parts as they will significantly help to improve reliability.

I would also carry out bypass mods and fit a larger pump

Nitriting or nitrating is confusing to me as here in UK as we don't tend to use this term. (Can anyone clarify ?)

Nitriding is a well established process and does need a specific grade of steel to be successful. Trying to nitride an unsuitble material will cause problems and can result in reduced fatigue lives.

It is possible to either plasma of ion nitride materials that are unsuitable for conventional nitriding but this is quite expensive.

The other common process is 'Tuftriding' which is a type of ferritic nitro-carburising and can be used for most ferrous materials.

Both of these process are used to improve the fatigue life of components by introducing compressive residual stresses into the surface of the component and work reasonably well.

Shot peening is also used to introduce compressive residual stresses and if carried out correctly is also quite effective. (Some don't agree but I believe that the body of evidence is subsantial and the process is well proven).

I don't think is it worth carrying out both processes. Peening will certainly help remove surface marks which could act a fatigue initiation sites and will also produce compressive stresses.

The nitriding or tuftriding is carried out at temperatures that will almost certainly cause the stresses introduce by peening to be relaxed and the main benefits will be lost.

I would magnaflux the rods to check for defects and if they are sound then shot peen.

[davidppp]

To add to Chris expert input:

S rods are bloody hard!

They will not really be easy to shotpeen without possible damage..they are already fully nitrided.

Kind regards
David