On a supercharged motor...

[jstein]

is the mach index of the intake valves as much as a concern as on a NA motor? Also, how does a supercharger effect the peak torque curve compared to a NA motor? Any links or calculations for these? thanks!

 

[patprimmer]

Firstly, I don't actually know what a mach index is.

Secondly, your question on superchargers is so general, that it would take a large manuscript to answer it.

For a roots type blower, they are positive displacement. Approximate boost can be calculated if you know the displacement per revolution of the blower vs the displacement of the engine per revolution, with correction factors for drive ratio and VE's of both.

The VE of a roots blower falls of sharply as the rotor tips reach supersonic speeds.

Screw type blowers are also positive displacement.

Centrifugal types are not positive displacement, but their manufacturers generaly publish flow maps.



Regards
pat

 

[jstein]

Thanks for the replys. Pat, I want to know if the velocity in the port becomes too high(.6 mach is optimum in NA motor)in a superchared motor if this is less efficent.For example, one head has a highflow high velocity, smaller port area. The other head is just bigger everywhere, bigger port area, bigger valve, etc. Now supercharge them and compare dyno graphs. Which one makes more power and where? We are talking a race motor, not good mileage.

 

[patprimmer]

The bigger head will normally make more power, wether supercharged or normally aspirated, but it depends on the whole package, and what RPM you want to run.

Positive displacement superchargers make it less sensitive, as no matter what you do, it will make a lot of HP, but the big heads will still make more.

As I said earlier, there is a lot more to be considered, and volumes of air pumped and consumed, pressure and temperature changes, parasitic losses driving the supercharger, drive train stress, increased weight, impact on aerodynamics if it sits on top of the motor, fuel avaliability vs requirements.

When you supercharge, you increase the density not the velocity. This changes as the valve opens and the greater pressure difference then increases the velocity, but as the cylinder is charged, the velocity drops off, but the higher density is still there. This is a dynamic situation and not at all cut and dried.

If you want to pick my brain, make it easy and tell me the full story. I don't have time to play 40 questions.

You know, details of what type of raceing, their rules and regulations, type of fuel, type of engine, types of superchargers, type of car, expected life of engine, high or low budget, will you change cam and pistons and fuel supply to suite the supercharged motor etc etc etc.

Regards
pat