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911st 911st is offline
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Join Date: Jan 2004
Location: Sacramento
Posts: 7,269

Good heads up warning of the need to rebalanced the WUR / control pressures with a HF FD.

I knew we would have issue with the AFR curves with the first head I talked Larry into doing for Brent930. It flowed 17% more fuel at all points along the metering pin travel. No issue getting idle right, cruse was a little issue, however first on boost was the biggie. We tried getting control of it with the WUR but could not get the first boost enrichment where we needed it. He set his WUR to delay enrichment till a higher boost level but it blew through and it only delayed enrichment a small amount. He adjusted the on boost enrichment CP higher and could make improvement but lost the high rpm enrichment we needed. We ran it w/o any enrichment and got a near perfect AFR curve except it went lean at the top but what a nice on boost AFR through the mid range even without lower CP on boost. That was when I suggested using an rpm switch to clamp the boost signal until much later in the RPM range.

Imagine my surprise to come back to the boards 5 years later to see such a large adoption of the HF fuel distributor and the MSD switch/ solenoid enrichment clamp.

We got some 17% increase if fuel with I think just readjustment of the spring tension and the height at the internal orifices along with higher system pressure (which was not a big secret at the time) without any machining or hard mods.

I have no idea what he and Stephen came up after the first head to get more fuel except for modifying the Lambda head to work like a non Lambda head. That was brilliant!

My dream at the time was to try building a HF FD that kept the Lambda function and dial it in with a 25% duty cycle at the frequency valve. Then bump it up to 75% to get even more fuel than the non Lambda head. Unfortunately, my turbo head was changed to a euro head and I could not find a reasonable C2 Turbo core at the time on the cheap.

Have you confirmed any of the other hard mods like machining to the slits or any other? Stephen or Larry did say they had not done anything to the pin but not sure about the other.

If the slits are larger in gross than the internal orifice that could just accelerate fuel delivery without adding to the total. If the orfice can flow more, such a mod might help w a restriction.

A perfect taper on the slits it could tune and fatten up the later part of the pin travel so that we might not have to play w the WUR.

Seems system pressure and reducing restriction from the fuel regulation function around the orifices is what has gotten most of our increases. Getting more might require a larger opening in the orifices if they are the bottle neck. Maybe then the injectors would become the bottle neck if they are not already.

However, I could be wrong on this but that is how I learn.


Many experts have told us fuel is not the ultimate restriction, air is.

It was not until I started thinking about your comment about the turbo snorkel restriction that I stared thinking about this and start to understand that we are approaching the air flow limits of the CIS. Appreciated!

Do or anyone know how large the exit on the metering assembly?

It seems that will set the limit of how large of Turbo intake we can go up to and only if we do not have restriction in the path way to the turbo.

Unless come up with some type of air bypass around the metering system that should set the ultimate limit as Turbo efficiency drops off quickly when it tries to suck air through a to small straw increasing the pressure-differential about the compressor wheel.

Anyway, back to the topic at hand. It seems the injector has up until now, not kept increases in fuel we have found at the head from making it through the injectors.

I am worried that at the higher flows, if the injectors are near becoming a restriction, they may contribute to unbalanced fuel delivery. If the 3.6 injectors are bigger they might be a good insurance. Your encouragement of doing the injector balance test is a very, very good idea.

Old 01-06-2010, 12:18 PM
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