Author Topic: Engine Internals Question (Read 2307 times)

91 318i · « **on:** December 11, 2012, 08:44:44 PM »

So I just replaced my head. The shop didn't replace the valve guides so now I'm losing oil through my engine. Badly. Gonna call them they will be redoing it for free.
Anyways. I decided I'm gonna pull the whole motor while I'm at it and rebuild the motor. It's time anyways. It's gonna be a turbo build with a slight bore lower compression pistons for more room to boost.

My question is how what the stok crank is made of and how much boost it can really handle. My goal was at least 10 lbs. upwards of 13 or 14 would be ideal. I don't want to stroke it. I was hoping to use te stock stroke length. If the stock crank is strong enough to handle boost or if I need to find another crank. And what rods would be best to use.

There will be more questions to follow but that's it for now. Thanks

bmwman91 · « **Reply #1 on:** December 11, 2012, 10:07:24 PM »

The stock crank is very strong. Forged, fully counterweighted.

I know that a lot of guys have run 7-8PSI on stock M42's. You might want to drop the compression to 9.5 or 9.0 for a 15PSI build, although I think that you might be able to stay high if you intercool, do a good job with the head, put turbo-rated pistons in and tune it properly.

bmwman91 · « **Reply #2 on:** December 11, 2012, 10:07:25 PM »

The stock crank is very strong. Forged, fully counterweighted.

I know that a lot of guys have run 7-8PSI on stock M42's. You might want to drop the compression to 9.5 or 9.0 for a 15PSI build, although I think that you might be able to stay high if you intercool, do a good job with the head, put turbo-rated pistons in and tune it properly.

91 318i · « **Reply #3 on:** December 11, 2012, 11:36:24 PM »

Now what about connecting rods?
And I do plan on intercooling I feel like that's the only way to go.
Other thoughts were like you said just a small compression drop to around 9.5 or 9 with a small bore if any bore given the walls are already so close together. As far as head work I was gonna port and polish if that but use mild cams to suit the turbo

bmwman91 · « **Reply #4 on:** December 12, 2012, 12:18:29 AM »

The stock rods are pretty beefy. I wouldn't worry too much.

Make sure to have a 360 degree thrust bearing installed on the crank. The stock 180 degree one is poop.

bmwman91 · « **Reply #5 on:** December 12, 2012, 12:18:30 AM »

The stock rods are pretty beefy. I wouldn't worry too much.

Make sure to have a 360 degree thrust bearing installed on the crank. The stock 180 degree one is poop.

91 318i · « **Reply #6 on:** December 12, 2012, 05:47:53 PM »

So clearly upgraded pistons would be needed. And forged pistons are undoubtedly lighter than factory pistons. Does this mean I need to rebalance my crank? I can't remember from when I took auto shop classes