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Fuel Distributor Rebuilds

I am sure that changing the thickness of the stainless steel diaphragm in a CIS fuel distributor will influence its performance.

Does anyone know what would happen if the thickness changed from 0.1mm to 0.2mm?

I would also be interested to know why the Cast Iron Bodied units on the Porsche have Stainless Steel diaphragms and the Aluminium bodies on Mercedes seem to be a reinforced Rubber.

Last edited by chris_seven; 08-09-2016 at 10:52 AM..
Old 08-09-2016, 10:44 AM
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Isn't the deflection of the diaphragms controlled by the central plunger and control pressure?

If you double the thickness you will increase the stiffness at lest 4x.

Why do you want it thicker? Material availability?

Stainless is chemically compatible with iron. Rubber probably better suited for aluminum verses stainless. There would be galvanic reaction with any water in the fuel. Good question though. Why rubber?
Old 08-09-2016, 04:02 PM
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I think I read somewhere the aluminum heads use a rubber diaphragm to allow the deflection needed to allow the piston to move enough to allow the fuel flow req'd.
Old 08-09-2016, 04:44 PM
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Chis, you could ask Larry at flowtech...he rebuilds these for Bosch too.
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Old 08-09-2016, 05:02 PM
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Quote:
Originally Posted by VFR750 View Post
Isn't the deflection of the diaphragms controlled by the central plunger and control pressure?

If you double the thickness you will increase the stiffness at lest 4x.

Why do you want it thicker? Material availability?

Stainless is chemically compatible with iron. Rubber probably better suited for aluminum verses stainless. There would be galvanic reaction with any water in the fuel. Good question though. Why rubber?
In simple flexure terms I think that the stiffness of a beam is proportional to the cube of the thickness so double thickness is 8X.

I would imagine a clamped diaphragm would behave a bit differently to this simple model but the stress analysis for this configuration would be tricky and doing FEA too time consuming.

I don't think that the stiffness of the diaphragm is part of the control system. The plunger depth is controlled by the air sensor and must be very free in its operation.

The springs that are used to control each piston must be much softer than the stiffness of the diaphragm and if the idea is to accurately control a differential pressure then in the limit maybe the diaphragm should be infinitely stiff?

The difference in stiffness between the Stainless steel and the reinforced rubber diaphragms would also be at least an order of magnitude and both types seem to work.

I suspect that the thickness of the diaphragm will influence the volume of fuel delivered for a given pressure as it changes the volume of the chamber and it is this aspect of the design I am struggling with. I am not 100% sure I have understood the operation of the distributor well enough.

The reason for my interest is due to the problems of sealing rebuilt distributors and the use of Permatex Indian Head Shellac which is hard to apply well although I am sure that other sealants may work.

I have been looking at the repair 'kits' available and none of them really deal with the sealing issue very well.

I have been looking into applying a vacuum deposited 'conformal' coating to the diaphragm which would mean that the use of any painted on sealer could be avoided.

Handling a 0.1mm sheet is tricky and a 0.2mm thickness would make this process much more straightforward.

We are looking at applying a 10 micron coating on each side.

I do realise that there are several 'expert' refurbishing companies that overhaul these parts but we want to try to produce our own solutions.

Last edited by chris_seven; 08-10-2016 at 03:59 AM..
Old 08-10-2016, 12:07 AM
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I wonder if an inkjet printer could be adapted to print a thinned version of Indian Head Shellac? Seems like it should be possible.
David
Old 08-10-2016, 04:18 AM
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I have never used the Indian Head Shellac but I would think that when dry it forms a very brittle film and may crack if it is sprayed onto the diaphragm and then flexed.

I have seen it applied to the body of the distributor and it clearly works this way.

The conformal coating application is a process carried out by a company that is close to our business and easy to make samples.

I have had some diaphragms cut with a gas purged laser and we will coat them in the next couple of weeks.
Old 08-10-2016, 06:22 AM
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I would think applying a bead around each cavity with fuel would be the way to go if using the Shellac. I have no experience with conformal coating, but i guess that's what they do to stainless shim head gaskets so should work.
David
Old 08-10-2016, 02:13 PM
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The coating on typical MLS gaskets is, I believe an FKM material (Viton?).

The basic sheet is only available in 0.2mm thickness with 20 micron coating of FKM no either side producing a total thickness of 0.24mm hence my original question.

I can vacuum coat a 0.1mm thick diaphragm with 10 micron either side.

I would like to escape from mucking around with shellac if at all possible but have never learnt what the original coating used by Bosch comprised of and how it was applied.

I am convinced that original diaphragms were coated as when you rebuild a distributor without a sealant they always seem to leak.

All of the units I have seen rebuilt used a complete layer but it is worth a try with a small bead.
Old 08-10-2016, 02:34 PM
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Nope........

Quote:
Originally Posted by boosted79 View Post
I think I read somewhere the aluminum heads use a rubber diaphragm to allow the deflection needed to allow the piston to move enough to allow the fuel flow req'd.


Boosted,

The plunger or piston movement for the fuel distributor has nothing to do with the type or thickness of the shim. The piston or plunger as people call it, is located inside a stainless barrel and never come in contact with the said shim (stainless or rubber). What needed deflection were you referring? Keep us posted. Thanks.

Tony
Old 08-15-2016, 02:36 AM
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Bad choice of words. Not the control piston or plunger, but the individual discs or buttons that control the fuel flow to each injector. The more fuel that is req'd, the more the diaphram has to move.
Old 08-15-2016, 11:00 AM
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A couple months ago I rebuilt the aluminum fuel head on my '87 930.
It was spraying fuel out of the #4 injector continuously if the pumps were running even if the motor was off and the metering plate was at rest in the middle of the AFM housing venturi cone. It would idle and run on 5 cylinders up to around 2000rpm when under low load because #4 was way too rich.

After replacing all the o-rings on the control plunger cylinder and the rubber diaphragm from a $69 Salvox rebuild kit off ebay the car runs perfectly smooth all the time.

The o-rings and diaphragm from the Salvox kit are ethanol tolerant and the ones that were in the fuel head were not.



A couple weeks ago I rebuilt a cast iron fuel head. I followed the instructions on the Salvox site and watched you tube videos people have made while rebuilding Mercedes fuel heads.


Old 08-15-2016, 08:45 PM
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I assume it sealed fine with the shellac?
Old 08-16-2016, 05:48 AM
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The top end of the motor isn't finished yet so I don't know if or how well the cast iron works yet.
Old 08-16-2016, 07:00 AM
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JF - does that 930 unit use both a rubber and a stainless diaphragm (looking at your first picture)? You show what look like two stainless diaphragms in the last picture.
Old 08-16-2016, 08:19 PM
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The aluminum lambda fuel heads have a rubber diaphragm sandwiched and squished between the upper and lower halfs of the fuel head and the older cast iron fuel heads have a single stainless steel diaphragm in between the two halfs.

No sealant is applied to the rubber diaphram in the aluminum heads, it all goes together dry other than a little oil on the control plunger cylinder o-rings so they don't get torn while pressing it all together and the stainless steel diaphragm in the older cast iron fuel heads needs a thin layer of sealant on it or it would leak fuel.

One picture shows the old and new steel diaphrams. You can see traces of old sealant on the old one. The old one also had small dimples from control pressure spikes which can alter the fuel mixture. That's why some of the old RUF 930 fuel heads had a pressure damper installed in the control pressure line to smooth out the pressure changes when on and off the throttle quickly.
Old 08-16-2016, 11:02 PM
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I would be really interested to know what other detailed differences there are between the design with the stainless diaphragms and the rubber diaphragms.

Using a rubber diaphragm would eliminate the need to seal the cast iron unit and using a material such a Contiflex which is a Viton/Nylon mesh would produce Ethanol resistant parts.

The coating on the stainless diaphragms is also very thin and we are trying to use a vapour deposited material to control the thickness so we can make coated diaphragms and simplfy the rebuild process.
Old 08-16-2016, 11:34 PM
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Interesting that the aluminum 930 FD seems to be from an 8 cylinder, perhaps Mercedes, model, with two of the chambers simply not machined out or something?

I wonder what is involved in choosing aluminum vs cast iron?
Old 08-16-2016, 11:47 PM
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The 928 uses an Aluminium Body
Old 08-17-2016, 06:42 AM
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