Intake runner sizing..

[Mark Boxsell]

>
>"Why use RPM sensing? The tuned length should change with air temp, 
>pressure, etc."
>
> No, the character of the curve you get with a given intake geometry 
>changes with RPM only. Air temp and pressure can be considered DC 
>offsets for that curve.

Correct. The speed of sound in air is hardly affected by changes in
temperature due to atmospheric factors.  A tuned exhaust however is a
different ball game!

>
>"I would assume that greater air flow would lead to  higher MAP values
>and the ECM would adjust the fuel mixture  accordingly."
>
>Greater airflow does not necessarily relate to greater MAP pressure 
>because most MAP sensors are located in the plenum of an intake. If 
>you improve the airflow downstream of the plenum, the MAP sensor will 
>never know it. Furthermore, a nicely tuned intake will not affect MAP 
>yet can really boost engine performance, particularly at low rpm where
> airflow is low and corresponding flow losses are negligible.
>

The pressure waves travel so fast that measuring it with a ("normal") map
sensor is out of the question. Anyway that's what you have a VE table for.

                                regards,
                                                Mark Boxsell
                                                MRB Design.

VE = Volumetric effiency.