I have to chime in on this one. I’ve calibrated a lot of TB air flow models and there are a couple big points being missed here.
Compare the areas of the two TB’s.
65mm: 3.14 * (65/2)^2 = 3316.625 mm^2
74mm: 3.14 * (75/2)^2 = 4415.625 mm^2
The relative difference in area is about 33%! This is like comparing apples to watermelons.
Don’t get me wrong; I still think that this comparison is interesting, but the only conclusion I can draw from the data is that the 65mm TB is somewhat restrictive when compared to the 75mm part. For further study, I would REALLY like to see how an SN95 75mm stacks up against the FOX 75mm since the 65mm hangs in there so well!
Like someone mentioned earlier, a bigger TB will always give a more aggressive throttle response because you are allowing more air to flow into the engine for the same pedal position. The down side is that driveability may suffer since you have a more coarse/rough modulation of the air flow with the pedal. (This is especially noticeable in creeping traffic/parking lot driving/normal launches.)
The comments mentioned above regarding forced induction are correct as well. Since the increased boost argument is vague, I will put forth the two possible conditions:
1) Pressure measured upstream of the throttle, post supercharger – If the boost pressure increased after a TB swap, you just lost air flow into the engine and added a restriction into the system.
2) Pressure measured directly in the intake manifold – If the boost pressure increased after a TB swap, then you just removed a flow restriction and are getting more air flow into the intake manifold.
3) Turbo engines can’t really be evaluated this way since there are too many variables at play - waste gate.
In all of these cases, forced induction and n/a, the air flow measurement from the MAF would be the best criteria for proper evaluation. RWHP is a rough technique due to all of the other parameters involved (i.e. spark, lambda, temps…).
Consider the scenario where a new TB is installed on a car, it increases the air mass flow by as much as 5 to 10% at WOT, but the RWHP went down. The flow increase could have pushed the ECU to move to the next load breakpoint in the spark map, where the spark is not calibrated properly for this load point for any number of reasons. Or on newer cars, the increased air flow could have pushed the ECU into component protection fueling faster than before and the engine is running pig rich during the dyno run.
I’m not trying to beat up on this thread. I think it’s interesting and I wish I had access to a chassis dyno for my own personal research projects! My only criticism is that a more scientific approach needs to be taken before general statements are made from a part comparison like this.
My two cents, Mike
