Bain Racing Podcast brought up some interesting questions

First of the podcast was one of the best. Jake is a master at his craft and everytime I hear him speak, I realize how little I know about induction design.

Andre had made a comment that he would rather tune an NA engine with Speed density rather than an ITB engine (assuming with a throttle based load axis). I am not sure as why this would be the case. I keep hearing and seeing online that an ITB engine should be TPS and everything else is speed density. Every time I ask why, I always get, "its just the way it is".

I have not been able to find any reasonable explination as to what or why this is the case. Let me ask this question, Take a piston engine, No VVT, No boost, No variable runners or cam switching, If at WOT, 6000rpm, 99kpa MAP, my VE is X(pick any number you like), If everything stays the same besides MAP, say down to 80kpa, 100% TPS, 6000rpm, what is my new VE, X+?, X*?

While the VE is the same, the amount of fuel injected should be proportional to the change in the mass of air. With 19.2% less air, you would expect the fuel delivery to be 19.2 % less. if tuning alpha-N (TPS as the load axis), then a baro (or airbox pressure) sensor would typically be used to compensate for the mass of air, with a trim of -20% for 80 kpa (0% for 100 kpa).

So in Speed density terms, the fuel model/fuel trim/baro correction should account for all of the changes happening for MAP at a given RPM range, correct?

A buddy of mine in the US who is very accomplished with Motec platforms tunes NA ITBs using TPS as the main load axis with MAP as a fuel table comp. Throttle position represents load while MAP represents the density of air entering the cylinders.

Yes, the MAP changes represent density (along with air temperature), so the Speed Density tuning uses that as the load axis, and the fuel table could be the VE at that load / RPM (air temp is included in the air-mass calculation).

The reason that ITBs are tuned using the throttle position, is because:

1 - It's difficult to get a good manifold pressure signal since there are multiple separate manifolds - and just connecting them together and logged the pressure at high sample rates will show the pulses of each one. Some ECUs are capable of using only a single runner, and sampling at a specific crank position to get a steady MAP signal.

2- At low throttle positions, they are very sensitive to changing the air-mass. So the hard=to measure MAP is easy to measure with TPS.

On the ignition side you are even more challenged. What you want to use as one axis is cylinder fill. In the case of TPS for load this is not always a good choice since baro changes affect the fill.

Nigel, that technique is useful for Mx00 (hundred series) ECUs, but with the M1 we use the Multi-Throttle Tuning method, where the VE table is a single entry of 100%, and the Throttle Position is used to estimate a "Manifold Pressure Signal", relative to airbox pressure. This allows most other systems (like ignition Timing) to with a more accurate estimate of the actual Manifold Pressure.

I just finished tuning a 1969 Datsun 510 with ITBs using this method, and it can work very well.

David,

If the VE at WOT for a given RPM is mechanically fixed and the compensation for the air density is handeled by the ideal gas law, Baro and Air temp compensations, why are the MAP based fuel tables not just flat in the vertical axis?