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911st 911st is offline
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Join Date: Jan 2004
Location: Sacramento
Posts: 7,269
The following it based on my opinion and from some of the info I have learned for many on this board. Thanks! However, I often make mistakes on my way to better understanding.

What is the limit of the stock FD? It has long been accepted that about 340whp/400fwhp as about the limit with stock FD and WUR. See Bruce Anderson’s book and discussion on modifying 930’s.

Most of us know there is more fuel by lowering on boost Control Pressure (CP). This lets the sensor plate advance further. This is how the Andial Fueler, Rice Fueler, D-WUR, BL-WUR and other’s have accessed this potential.

From my limited experience, aggressively lowering CP on a stock FD will add about 1 point or about 8% more fuel at peak output. Thus, about 370whp or 450fwhp or so should be supportable with a stock head and lowered CP.

However, I believe there is more HP than this if we can get the sensor plate to advance further. Lowering CP more than 1.9 bar dose not seem to accomplish this and I believe the sensor plate stalls. The Rennsport Systems metering arm hinge mod might to the only mod I have heard of that might be accessing this potential.

It seems a stock FD is specified to flow about 3000cc/min of fuel. A turbo might need as much as 600-650cc per 100hp. If this is true, the stock fuel head should be able to make about 450-500fwhp on boost which is more than we are getting.

Lee Rice of the Rice Fueler that was popular some 10 years ago once told me that even with a very low CP on boost, he could still push down the air sensor with a screw driver when on a dyno and flood the motor!. Sounds like something fun to try!

Thus, there may be another 50 plus HP that we are not accessing if we can get the Sensor Plate to advance all way. This may or may not be true and many do not believe this is so.

There is also more fuel to be had from the stock FD by just shimming the heads pressure regulator for higher System Pressure (SP), so long as the fuel pumps can support doing so. This also should effect the slope of the AFR curve and require CP adjustment to keep cruse AFR from getting richer than intended, just like the HF FD dose.

Anyway, once we have gotten all there is to be had with lowering on boost CP, then we need a High Flow – Fuel Distributor (HF-FD) if we want to go further. (FlowTech HF-FD?)

The HF FD mostly gets its increase by increasing the Differential Pressure (PD) between the upper and lower head by adjusting the height of the internal orifices and increasing the spring tension around the orifices. With the higher PD more fuel passes to the injector as the regulator function between the upper and lower part of the heads is reduced. Same principle as is when changing the spring tension on an adjustable fuel pressure regulator.

As noted above, the more we increase fuel from the FD the more we need to play with WUR and CP to reinstate our part throttle AFR’s and the more we are going to create more lag potential.

Thus, one might think of only having the head built to make as much fuel as one expects to need.


How much more fuel can we get from a 930 FD?

Talking to Larry of CIS Flowtech, he can get a bit over 10% from a non Lambda fuel head before the diphragm 'oil cans' and the flow rate is not controllable.

On a Lambda style FD the diaphragm is more compliant and he can potentially get up to 30% more fuel over stock if really needed . With this he removes the Lambda function.

Thus, with a non Lambda HF head we can probably support to about 425whp /500fwhp.

However, with an extreme build HF FD based on a Lambda style head, it might be possible to support something like 500whp/600fwhp.

Remember, we have to be able to manipulate control pressures (CP) to make this work.

Again, Larry at CIS Flowtech mentioned a significant number of HF owner’s had drivability issues as they did not know how to set up the WUR to compliment the head.

You better know what you are doing to try to go this far. It is not just the part load AFR curve that is effected but the on boost AFR curve.

I will say the best potential for controlling a HF-FD is with the D-WUR. This allows tuning CP by load and rpm. Next would be the BL-WUR and enrichment delay.

The fun question is, can we push the fueling limit further?

Last edited by 911st; 01-26-2010 at 01:19 PM..
Old 01-26-2010, 01:02 PM
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