So I replaced my clutch with the same act street clutch with heavy duty pressure plate that I had before, after breaking it in 600miles I tried a launch with two other passengers and it slipped so bad I left a trail of white smoke and my car stank like burnt clutch for days. But when I accelerate while on the street I have no problem. Was it that fact that I had an extra 400 pounds of weight that made it slip with the passengers?

 

That would be an easy test. Call up those two passengers and go for a ride!

 

^and prepare to wear the clutch out more.

I'd try adjusting the clutch first. Maybe it's engaging at the wrong point.

 

You need thinner friends.

 

I don't think it SHOULD be slipping, but lets look at this logically.

1. Your car presumably is making more HP/TQ than stock correct?
2. Lets assume you've got roughly another 50 crank HP. That's roughly a 20% increase in HP over stock.
3. Consider that adding 400 lbs to a car that weighs (roughly) 3500lbs is like adding another 11% of it's total weight.

So you're increasing the power on side A of the clutch by 20%, and increasing the weight on side B of the clutch by 11%. And then we have to factor in that you probably aren't a professional racing car driver, the clutch may be out of adjustment etc. etc. etc.

You see how this could become problematic.

Don't forget that a clutch is a wear item. Better to have a burnt up clutch than a drivetrain that has detached itself from your car haha.