Crank pulley effect on boost?
 

Figure you guys will be of some help, i'm doing a custom supercharger set-up on a BMW with an M62 purchased from here (which is mainly why i signed up).

I'm doing a junkyard set-up and couldn't find much for a pulley that would work, a Dodge Durango one practically bolted up, but it's huge in comparison to the 9psi pullies.

Sooo, does the size of the crank pulley have a huge effect on boost?

I'm not looking to hit over 12psi.

Yes. It will affect how fast the belt moves, which will affect how fast the SC pulley turns, which affects boost.

12psi? :shock:

What's the compression ratio on that motor?

9.2:1

12psi would be the max i want, i actually want to keep it under 10, but with this pulley i doubt it's an option, i know it'll effect boost, i want to know by how much though, a general ratio.

Say, 1 inch bigger diameter crank pulley = 1psi.

I know there's the internal gearing factor and supercharger pulley size that come in to play, but i know there's formulas for it. I actually have one, but i don't know the number for the internal gear ratio.

Is there a larger pulley junkyard available i could use? I'm not sure how it bolts to the supercharger, maybe something from say a water pump or alternator in a junkyard?

9.2:1 with 12psi? BOOM.

Even with iron heads, and I bet it's aluminum. KaBOOM.

We have no idea what your crank pulley diamter is, so we can't help you. Sounds like something for the BMW forums.

There is no internal gear change in a supercharger.

This section of the Forum is for 3800 supercharged or turbo (or nitrous) topics. I'll move your topic to the Lounge for you, but I can tell you that you're way in over your head on this one. Nobody in their right mind would stuff 12psi into a BMW with that compression ratio unless they had a 5k budget to START with.

If you can tell us the effective diameter of your crank pulley and what pulley is on the SC, we might be able to guesstimate the boost. But we'd need to know your engine displacement as well.

12psi isn't shit for a BMW's engine. =\

4-500whp is usually what people shoot for on the stock engines, 600whp has been done a few times with head bolts.

Anyway, best i can do for measurements is a picture. I'm using this car for a learning experience, i have a spare motor tucked away for it. I've turbocharged a motor so i have the idea behind boost, but i want to learn the supercharger way, i've had alot of fun with this project so far. I'm asking about crank pulley relation because you guys seem like you'd know a little more about superchargers, BMW guys tend to stick to turbos (for good reason), but i felt like a different approach to forced induction for the learning experience, not the end result.

The pulley shown gives 9psi, the red line is roughly the size of the pulley i have installed.

https://i10.photobucket.com/albums/a...2_5/pulley.jpg

Well over 12 psi is my guess. That red circle is a lot bigger then the crank pulley.

Compression ratio and cylinder head material have a whole helluvalot to do with boost. I gaurantee you the guys doing this are doing alot more to their engines than you have planned, but seriously, I don't know jackshit. So don't listen to me. Sounds like you have it all figured out, so go for it.

In fact, just like everyone else here, it looks like you can make all your calculations on SC drive ratio from a picture of a crank pulley.

Can we get serious here now?

Cylinder head design, quench area, engine cooling systems, timing, how the boost is applied, are all important as well. If BMW guys are running 12 psi regularly without issues, then follow their advice. An m62 seems like more work though then other boosting methods, turbo, CSC, etc..