question about boost/cylinder pressure

[SMS]

OK, time to clear this up in my head. I have seen it posted here, that the boost you see in a turbo engine is the air pressure behind the intake valve waiting to go through the combustion process. Is the cylinder pressure raised when running a turbo? Or, is just more air being put through, and cylinder pressures are the same? I have heard it said, " boost doesn't blow head gaskets, detonation does". Is this true? High boost with no detonation will not blow head gaskets? Thanks

Steve

 

[ijames]

More air/fuel mix is stuffed into the cylinder with boost, and the combustion chamber pressure is higher with boost than without. That's how you make more power. However, detonation causes very short pressure spikes that can go up to 10 times higher than the normal combustion chamber pressure, and that breaks stuff. Normal combustion is a smooth process where the pressure rises as the piston comes up towards tdc, rises more after ignition, then drops as the piston goes back down. That mostly all happens over about a 1/2 turn of the crank. It is always possible to go too far and find some mechanical limit, even without detonation, and bend a rod or lift a head or push a gasket, but detonation is usually the cause - a pressure spike from detonation goes up to max and back down in about 1/20-1/100th of a turn of the crank so it is like hitting everything with a hammer while you are already pushing on everything with the normal chamber pressure.

 

[BlackBandit]

Also, a simple way to think about boost vs flow (flow puts more are in the cylinder and hence makes more cylinder pressure and more power) is with a water analogy where water is the air. Picture a pressure washer and a garden hose. The pressure washer is running about 1000+ psi of pressure and the garden hose is running maybe 20. If you put them both is separate buckets (cylinders) and let them run the hose will fill up it's bucket first even though it is running way less pressure. That's because it flows better.

Apply that type of thinking to our cars and you'll see why stock turbo guys can run 25 psi and get to the low to mid 12's and other guys with ported heads and big turbo/intercoolers can run 19 psi and push a motor to low 10's or high 9's. Once you get that it doesn't take a rocket scientist to figure out who's making more power and is therefore pushing their parts harder. Think about this with what ijames said it should all be pretty clear. HTH. james