Determining Actual WOT of TB's and its application.

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Location: Lincolnshire Coast

Determining Actual WOT of TB's and its application.

Post by Charlie54043 »

Hi Davrianites,

A spin off of MattnRach question about damping the MAP sensor, has been the facility to determine Actual Wide Open Throttle (WOT) at various RPM.

If you think of an engine as a simple air pump, then “Actual WOT” is the throttle opening at which the cylinders are full of air at atmospheric pressure and therefore the TB's are not restricting the volume of air being pumped, further opening the throttle will not pump any more. This is Actual WOT and in my installation at 1200 rpm the TB's are only open 25% when the cylinders are at atmospheric pressure.

Knowing where Actual WOT is, can be useful when setting up the fuel and timing maps.

To find Actual WOT, do runs from low rpm with the Data logging running, at your chosen rpm briskly open the throttle to Mechanical WOT (MechWOT).

Afterwards inspect the log and the point at which the MAP reaches atmospheric pressure and opening the throttle further makes no difference, is the Actual WOT position for those revs.

It can be the seen from the MAP trace below that at 1550rpm even though the throttle is opened past 45% the MAP is no longer increasing as it is already at atmospheric pressure, this is the Actual WOT for 1550 revs.
WOT at low revs2.jpg
Top graph: red =AFR, White = Throttle position, Bottom graph: green = revs, red =MAP

Increasing the VE fuel table values above this throttle opening will only result in a richer mixture and wasted fuel.

A similar procedure can be used to determine whether the TB's are correctly sized for maximum power.

Again, do runs at MechWOT with data logging running but from 5000 to Max rpm. Afterwards inspect the log, but this time look for the point at which the MAP drops by 1 to 2 Kpa from atmospheric. This indicates that the inlet system is starting to restrict airflow.

The MAP trace below is taken off the vacuum line directly under the butterflies and as the revs pass 6000 the MAP can be seen to drops by 2 Kpa. This indicates the TB's are slightly undersized.
WOT at 6300 rpm2.jpg
Top graph: red =AFR, White = Throttle position, Bottom graph: green = revs, red =MAP

Fitting larger than 4 x 36mm TB's MIGHT produce more power but the amount of air they would pass at the very small throttle openings near idle, would make it difficult to map the fuel correctly....not a problem for a race car, not so good for the ultimate road going sports car!

The facility of determining the actual WOT at various revs is also useful when writing the ignition advance table.

The general rule for the “home tuner” is to increase the ignition advance until it detonates ( pinking? ) then back off by a few degrees, this will give close to maximum power and efficiency.

As the Actual WOT is now known, the advance values above this throttle opening can be increased so when out mapping and detonation occurs at MechWOT, the throttle can be immediately backed off to Actual WOT, retarding the timing with little possibility of damage to the engine.

I have circled an example of this in the spark table below (previously both values were 25 degs BTDC)
Over advance ign values.jpg
Over advance ign values.jpg (235.28 KiB) Viewed 1664 times
In previous “hill tests” where the car was accelerated up a reasonably long hill at MechWOT ( What in Lincolnshire! ) it was found that the carburetted engine started to detonate at 28 degs BDTC and 3000 revs. So the 2800 rpm, 75 %(130 tps) and 100% (170 TPS) throttle opening values were increased as in the table above and another “hill test” carried out to see when detonation occurred.

The engine started to detonate at 29 degs BTDC and 3000 rpm so the previous two table values, plus 62% (105 TPS) (Actual WOT)/2800 rpm were set to 26 degs BTDC to prevent detonation. This proceedure was carried out for the remaining rpm points

I hope this is useful if you are setting up your Electronic Fuel Injection.


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Re: Determining Actual WOT of TB's and its application.

Post by matnrach »

Looks like a good method although I certainly would not advance the ignition at high speed until it detonates as this may cause damage. Depending on the engine you may go over the point of maximum torque anyway.

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Re: Determining Actual WOT of TB's and its application.

Post by edgedj »

good thinking and I'm certain it's right as your graphs prove. One thing that occurs to me on my 28/36 inlet with the throttle at 90 degrees to the inlet ports, maybe my setup will flow better with the throttle half open than full because the angle of the butterfly is pointing air into the inlet tracts at 45 degrees but WOT allows air to hit the base of the manifold and perhaps slow the flow?


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