So you are saying scavenging does not / can not impact overall flow? Me, physics and fluid dynamics have another opinion on the matter,..

 

This sounds pedantic. The airflow is the same, but it's not at the same velocity. It's literally "stuck" and building up between the blower and the intake valve since the exhaust scavenging is not pulling it into the combustion chamber as effectively and impeding overall flow. The engine itself IS flowing more air. The blower is not unless it's ability to suck air in is impeded.

 

With a screw or roots blower, the increased scavenging and/or reduced exhaust pressure can not possibly increase air flow through the supercharger itself, and without more airflow through the supercharger there is no more airflow through the engine. The supercharger takes inlet air based on the volume of the cavity, how quickly the cavity is moving, and the density of the air on the inlet side. The discharge conditions of the supercharger only have a minute effect on how much air it takes in (there is a slight increase in internal air slippage). I'm not saying headers/exhaust don't add power; just that they add power without adding airflow. You will get reduced engine pumping losses due to lower pressure on the piston during the exhaust stroke. You will get reduced compressor drive power and thus more at the flywheel due to lower discharge pressure of the supercharger. You will also get cooler engine inlet air temps due to less SC compression and lower cylinder temps due to lower exhaust pressure, which will result in more ignition timing. However, you won't get more air flow because that is determined by the size, speed, and inlet conditions of the supercharger.

This is absolutely not the case with turbomachinery used for compression (centrifugal supercharger or turbo) or naturally aspirated engines. The roots/screwcharger is a unique case.

Also when talking supercharged engine exhaust, keep in mind that turbocharged engines run very high exhaust pressures. I've measured NA and supercharged engine exhaust pressures in the 5-10 psi range, which is probably about where we are. I ran a turbo LT1 Camaro that measured 48 psi exhaust pressure at 20 psi boost and it was plenty powerful (9.9 at 146 at full street weight). So, we are far better off than that with even stock exhaust.

 

The engine can't flow more air mass unless the supercharger does first.

 

I disagree, that post is not consisted with fluid dynamic theory or practice. That lengthy explanation doesn't refute the simple fact that if scavenging is improved, overall airflow could (not going to try and say will, but most certainly should) be impacted. While headers power yield is mostly reducing pumping losses, scavenging can and does increase overall flow.

The increase in flow is not proportional to the reduction in backpressure vs a naturally aspirated engine, but it's there at a different ratio. Pressure rise across the rotors is fairly flat for PD blowers, however, the idea that reduced restriction from the relatively linear pressure source is a flawed supposition. Again, it's non-linear, but it's there. Power yields from exhaust are two-fold, pumping efficiency and increased flow, the only thing that changes is the proportion.

I leave it there, because I/m arguing against a theory that just doesn't make sense, no offense intended. Build an airflow model, it will disprove what you are saying.

No offense intended.

 

It does, the pressure rise across the rotors isn't very affected in that situation, so measured boost is virtually the same.

What you are saying would hold true if we were discussing water, the compressibility of air, and latent heat from compression changes the discussion.

 

Lts will sound bad ***

 

 

Agree to disagree,