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Location: Sacramento
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HF Fuel Distributor/ Head. How much CIS HP? CIS FlowTech

How much fuel or HP we can support with CIS will always be a question. Yes, EFI may be a better solution but some of us just love CIS and would not think of changing.

330HP stock
400hp with fat CO adjustment.
450 w lowered control pressure
500HP with HF, non lambda FD.
600hp with HF, Lambda FD set for extreme flow
700hp w max HF plus 8 bar system pressure
900hp+ w custom HF 8 cylinder Bentley head.


Larry at CISFLOWTECH ( 251-929-3771 ) is still modifying 930 Fuel Distributors (FD) to flow more fuel if anyone is interested.

For what it is worth, there is nothing magical about what he dose but he has perfected it and dose it well. It is always a good idea to think of having even the stock head rebuilt at this point to ensure accurate fueling and having the head set for more flow dose not add much to the cost.

He did the first HF head based on my ideas and request some 7 years ago for Brent930. We are lucky IA adopted it and popularized it as a very successful product.

Larry mentioned that a significant number of HF owner’s have had drivability issues as they did not know how to set up the WUR to control the head.

Anyone thinking of going this direction needs to know that manipulating WUR control pressures is key to drivability and successfully getting more fuel from this approach.

For example, if one can get 14.5/1 AFR at idle and cruse with a stock system, a HF FD that flows say 15% more fuel, once idle CO is readjusted to say 14.5 AFR, is probably going to be at about 12.3 AFR on cruse which is to rich unless it is a race car.

To get a proper cruse AFR with a HF-FD, base control pressure (CP) will have to be increased. This will slow the rate the sensor plate advances and bring the cruse AFR back toward a proper setting. Think of it this way, if the FD flows 10% more flow, we need to slow the metering progression 10% to keep stock AFR’s on part throttle.

However, most do not think of it but doing this will bring a bit of a negative effect on lag as it will present more resistance at the air sensor/metering plate and will create more fight with the turbo's effort to try to suck in air.

Thus, it is probably a better approach to exhaust all efforts to get more fuel from the stock FD before adding fuel by modifying the FD.

Last edited by 911st; 01-26-2010 at 01:06 PM..
Old 01-26-2010, 01:01 PM
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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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Of course we can get more (at least I believe so)!

We can get more from any of the above by bumping System Pressure.

Along with lowering Control Pressure this is how most 930 tuners got more fuel in the past.

This seems to be supported with some info another turned up that when Volvo was racing its CIS Turbo motors it ran System Pressure (SP) in the 8 bar range. We run in the 6.2 bar range.

It seems, bumping CP from 6.2 to 8 bar should be able to bump fuel flow about another 14% +/-.

If so, this could allow us to possibly approach 700 fwhp range. It should be noted that this would require two higher pressure pumps in series.

Thus, I suspect about 700fwhp or so is about the fueling limit of a modified CIS system.

How much air we can get through it is another story. .

I understand there are other potential factors and I would be off on this but this is what I believe to date.

There is however, one more step that could be taken for the truly crazy.

Larry tells me the Bentley / Rolls V8 is the only FD’s that can flow as much fuel per cylinder as the 930. If one could source such a FD and have Larry modify it, we might be able to plumb in on or two extra injectors before the throttle body for maybe another 33% fuel and get to almost 1000hp fuel delivery potential but that is just crazy, crazy thinking.



Something I wish someone would explore is reshaping the AFR curve by reworking the cone area around the air sensor plate. The factory designed this to move quickly off idle and then to begin to stall at about stock peak TQ levels. With modifications we blow thought this pretty quickly and go fat to fast and then lean.

Changing the cone around the air sensor plate on the CIS Turbo Volvo race car was noted as a way to change their AFR’s for different tracks. We should be able to do the same and get a good AFR curve without add-on’s.

I think a stock looking WUR/CIS 930 cold be built with no electronic add on's that can support up to 700fwhp and have near ideal AFR's by reprogramming the metering cone so the air sensor plate advances correctly at higher air flows.

In further support of this the 1994 CIS Turbo has a different cone profile supporting that this is an effective way to tune ones AFR’s.


I am not a real expert but this is what I believe so far.

I appreciate any addition and corrections.

Last edited by 911st; 01-27-2010 at 08:24 AM..
Old 01-26-2010, 01:05 PM
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Oh boy, your brain must have ADHD syndrome over CIS : ), but it's ok.

I really enjoy reading you mindstorms mostly because I'm a 930 noob and just learning how this engine works and your writings are IMO good 101 material mostly (after possible mistakes have been edited).

One thing I have not seen explained in deep sofar is using those extra injectors for extra fuel. I've read they do not work like they should and are considered even dangerous because of non equal flow of fuel per cylinder? If the spray is nicely atomisized I would think it would follow the airflow pretty closely and also HELP those cylinders that have most air (leanest). This tought came from thinking of water/meth injection as those systems use high pressures and special nozzles to have nice mist of w/m. I'm thinking if the extra injectors would have nice enough mist (wonder if the cc would be enough) it could actually work richening the lean cylinders?

The stock injectors have more like spray so different extra injectors would be needed and I dunno if any are suitable for high volume mist.

I would not use water/meth for extra power but for cooling and safety. It could be used for more power but if it stops working 100% the engine would be in trouble. Safety features (leds and triggers) with w/m kits are nice but I would't trust them with my engine.

Last edited by smurfbus; 01-27-2010 at 12:15 PM..
Old 01-26-2010, 11:51 PM
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In my opinion there are several things going on that impact additional injectors.

Many think it is because the manifold is not conducive to fuel running through it. That may be but I do not really accept this.

I am more inclined to believe it is the timing of the EFI injector events.

If it fires once per rpm or even twice per rpm the burst of fuel it going to go to the cylinder or couple of cylinders that are on the intake cycle at that instant. The other cylinders will not get as much of the extra fuel and run lean.

I suspect one can overcome this by using saturation injectors that are only run at 100% when on. One could stage two different injectors.

Using an 8 cylinder CIS head with two extra injectors would fit that. The injectors could even have valves on them that are trigger when needed. However, on such a CIS system they would not have to be staged. They can be adjusted around by increasing control pressure so the metering system advances slower.

A secondary opportunity is where they are located. The further from the throttle body probably the better as long as the fuel dose not run down to the hot turbo should it pool

The last factor I suspect that may effect some cylinders going lean and burning through is the exhaust. With an unequal length exhaust tubes to some cylinders at certain times may end up with different levels of residual exhaust pressure when the exhaust valve closes. This could effect how much air is packed into each cylinder on the intake stroke thought the amount of fuel remains the same. That might create a variation in temperature and effective AFR per cylinder.

At least that is what I believe so far.

Last edited by 911st; 01-27-2010 at 08:09 AM..
Old 01-27-2010, 07:57 AM
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