Lower CR Pistons?

[DesioMedia]

Hi,

I am not new to building engines but new to the m42. I want to turbo my 318i. the consensus around here seems to be that you need to lower the CR to ~8,5:1. On my old honda dseries (I know I know) I ran 280 hp @ 15psi on a 10.8CR engine w/o any det issues. This was properly tuned on a brake dyno on w/ 93 oct pump gas and cooler plugs. Im looking for ~250 hp on the m42. Why does the CR need to be so low? I would run a e36 knocker.

On a slightly related side note, why does changeing the piston hight change the CR. Logicaly, if the same ammount of air was to enter the cyl the air would be compressed more. However, it seems to me that less air would enter the cyl thus keeping the CR the same. Simply, the change in piston height would either raise or lower the ammount of air in the cyl not change the CR.

[gearheadE30]

The CR needs to be low so that it doesn't detonate, although if you are running knock sensors tuned correctly it probably isn't such a big deal.

The different CR pistons have a different height, as you know. Approxemately the same amount of air enters the cyliners regardless of CR. Thiws is based on the space in the cylinder and whether or not the intake can supply enough air. The difference comes when that air has to be compressed. Air will fill the cylinder at about atmospheric pressure-natural aspiration. When the air is compressed, however, the difference in piston height increases its effect because the area of air is so small in the first place that even a small difference will change the CR

Hope this helps, it is a bit difficult to explain

[Tor Arne Kaasen]

dont go whit a hige CR. use CR. 8,5 - 1 is the best for 14,5 psi off boost 93oct . the knock sensor will drop hp when nok is detected. Fuel sulply is important to. to lean = knock.
Lower comp whit thikker gasget/lower piston will ruin your squis effect.
Use custum turbo pistons whit disch in it.