Some s50b30 crank questions.

[SA E30]

Good day.

I've just been wondering, how high you can really rev the motor till the crank becomes the weak point ? ... a Motorsport or Forged crank is just out of the question far to pricey.

and yes I know it doesn't help to rev it much higher than 8000rpm on a stock bottom end, but I'm not going to be keeping the stock bottom end, forged rods and pistons.

Also there is loads of talk here in South Africa, that at our altitude in Jhb, these cars dont perform when a set of aggressive camshafts are fitted, they seem to loose power... can't even keep up with the stock ones.... anyone seem to know why ? only thing I can think of is the dynamic CR drops to much, with the bigger overlap ? ... but yeah thats just a shot in the dark.

[SA E30]

bumpity bump :P

The list of mods I'm thinking about for his motor are.

Forged Pistons
Forged Rods
Shrick Camshafts
Shrick Valve Springs and Retainers
s50b32 head
vanos delete
stand alone management.

I'm also wondering if the std oiling system could stand up to 9000rpm

[MauiM3Mania]

Also there is loads of talk here in South Africa, that at our altitude in Jhb, these cars dont perform when a set of aggressive camshafts are fitted, they seem to loose power... can't even keep up with the stock ones.... anyone seem to know why ? only thing I can think of is the dynamic CR drops to much, with the bigger overlap ? ... but yeah thats just a shot in the dark.

Higher altitude, less oxygen. As I understand it, FI does better with high altitudes.

I can go from sea level to 10k feet in about an hour (never timed it) and it does get doggy anywhere from 4000' and beyond.

[mdok]

Forced induction is definitely the ticket at altitude, and I'm not even a fan of FI

Yes, dynamic compression is an issue, raise the static compression up. Barometer tells roughly what percentage loss of air is, relative to sea level. I have quite a lot of experience with different engines at sea level, 2500, and 5500 feet. At 5500, I'd run at least a full point up, ie, 13.5 instead of 12.5. The fuel we have here in California will generally work with 11.5 - 12.0 (or more) WITH cams with moderate overlap. It is def okay with 215+ lbs cranking, typically, sea level, with decent overlap, and a good chamber. How each engine feeds itself air varies, so a given compression ratio and cam timing gives dif results based on volumetric efficiency at cranking speeds.

The biggest single mistake people make with uprated cams is not having enough compression - they put them in stock compression motors and the dynamic comp falls so much from overlap that the engine has to have the shit spun out of it to get some pressure. Hence, a good bit of the reputation of hot cams robbing lowend. Well developed motors have equal or better lowend to stock, with lots added on top, they still feel cammy from the fast rise once they start to add substantially, when the cams "hit". Cranking compression gives a good idea of what you can get away with. Hot cams knock back the cranking number considerably; I've seen people put cams in with 25 degrees more duration at 1mm lift, and lose over 25lbs right off. Add some compression ratio!

I tend to use more cam than people can believe, and also more compression. The owners are amazed at the broad powerbands. Match your parts, they offer much more that way. Current race engines generally have broad powerbands for what they get at peak.

Having said all that, altitude robs you big, no matter what you do, Nat Aspirated. Use every trick you can, just remember, when you drive down from Jhb, you'd better look for some serious octane at the coast, if you should take the high comp route....or advance the intake and retard the exhaust to get more overlap.

Good Luck!