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Throttlebody configuration

ObdTuner gives you the option to define the relation between the throttle pedal position and the throttle valve opening. This table allows you to define more or less agressive reaction on throttle pedal movement.

The table is also used by the ECU when idling. Pedal index cell 0 to around 14 are beging used during idle.

When converting to the 65mm throttlebody from the LE5 engine, the amount of air that can pass through the throttlebody increases. When no changed are made to the throttle pedal to throttle valve relation table, this can affect idle (especially with non stock camshafts), engine start and cruising (very small pedal position changes can result in lots of extra air going into the engine).

I did some searching to find a way to convert the throttle valve opening to the open area it translates to for a given diameter. One formula I found was the following:

$$A(\alpha) = \frac{\pi D^2}{4 } ( 1 - \frac{\cos(\alpha)}{\cos(0)})$$

A = area
D = diameter
Alpha = throttle angle in degrees

Rewriting the formula so it can be used to get the throttle angle for a given area and diameter you get:

$$\alpha = \cos^{-1}(\frac{-4 A \cos(0)}{ \pi D^2 })$$

When comparing the most commonly used throttlebodies (58mm, 65mm and 68mm) you can see quite a difference in area vs. throttle angle.

I've setup a spreadsheet and done some calculations to translate the base factory setting of the 58mm throttle body in the ECU to the other 2 throttlebodies. In the graph below you can see how the opening relates in a percentage of the opening of the factory throttlebody. The X axis contains the ECU table cell id as found in ObdTuner.

When zooming in on the the part of the table that is being used by the ECU for idling, the differince is significant.

Using both formulas I used the spreadsheet to translate the percentage of opening for the 58mm throttlebody in factory configuration to the area for the throttle angle. Then I used the area to translate it back to the percentage of throttle opening this requires for the 65mm throttlebody. This gives the folling data.

Factory throttlebodyInstalled throttlebody
58mm z22se65mm LE5
ECU table indexopening % of 90°angle of throttle plateopen area (mm2)angle of throttle plateopening % of 90°
00.00%0.0°0.0mm20.0°0.00%
10.30%0.3°0.0mm20.2°0.27%
20.60%0.5°0.1mm20.5°0.54%
31.00%0.9°0.3mm20.8°0.89%
41.80%1.6°1.1mm21.4°1.61%
52.30%2.1°1.7mm21.8°2.05%
63.60%3.2°4.2mm22.9°3.21%
74.70%4.2°7.2mm23.8°4.19%
86.50%5.9°13.8mm25.2°5.80%
98.60%7.7°24.1mm26.9°7.67%
1010.90%9.8°38.6mm28.8°9.72%
1113.50%12.2°59.2mm210.8°12.04%
1216.60%14.9°89.3mm213.3°14.80%
1319.90%17.9°128.0mm216.0°17.74%
1423.50%21.1°178.0mm218.9°20.94%
1527.20%24.5°237.5mm221.8°24.23%
1631.20%28.1°311.0mm225.0°27.78%
1735.10%31.6°391.5mm228.1°31.24%
1839.00%35.1°480.5mm231.2°34.69%
1943.00%38.7°580.1mm234.4°38.22%
2047.10%42.4°690.7mm237.6°41.82%
2151.40%46.3°815.4mm241.0°45.60%
2255.80%50.2°951.6mm244.5°49.45%
2360.00%54.0°1089.1mm247.8°53.11%
2464.20%57.8°1233.4mm251.1°56.75%
2568.40%61.6°1383.8mm254.3°60.38%
2673.00%65.7°1554.8mm257.9°64.33%
2777.50%69.8°1727.6mm261.4°68.17%
2881.90%73.7°1901.0mm264.7°71.91%
2986.20%77.6°2073.8mm268.0°75.53%
3090.50%81.5°2249.3mm271.2°79.12%
3195.30%85.8°2447.2mm274.8°83.09%
32100.00%90.0°2642.1mm278.2°86.94%

Using this table as the basis, I changed the throttle configuration in ObdTuner. I changed the last couple of cells so the throttle valve is actually opened 100%.

The result is quite apparent. Engine start, idle and cruising is different. Although it is a matter of taste, I quite like the fact that I can use the full range of the throttle pedal to control the throttle valve. It gives more control and small changes during cruising (as when driving over a bump) don't result in a jolt from the engine.

The spreadsheet I used can be downloaded from the link below. As I don't own Microsoft Office or anything like that, it is made in Gnumeric. I exported the sheet to excel format, but I don't know if the function is still correct.