911SC Carb Conversion Questions

[Porschestraße]

So I am in the midst of converting from 1981 Euro 911SC from injection to a carb setup, and I have a few questions on possible things I may and may not need.

I have the carbs already (Weber 46's) but need to clear up some others.

I know I need a different pump setup but through my research I read that when the 911R's ran the 46's they required two fuel pumps to acquire the necessary amount of fuel the carbs required, one at the tank and one at the motor, will I need something similar?

I have also read that a MSD distributed is required to achieve hot enough spark, can anyone confirm this?

Any and all info is greatly appreciated! I just can't wait to get this baby finished!


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[chris_seven]

It used to be a common practice to fit two fuel pumps to many old competition cars as there is this belief that pumps - particularly Bendix type fail on a routine basis.

I have never found this to be the case. A single Bendix pump will flow around 56 gallons per hour into an open tank and 35 gallons an hour at 2 psi.

The greater the pressure the lower the flow.

It seems unlikely that 2 pumps would be needed as even at 150mph on a prolonged run down the Autobahn 2 mpg seems unlikely.

I do believe that the 911R did have 2 x Bendix Pumps but both were in the engine bay.

I am a believer in fitting 'low' pressure pumps and not using a pressure regulator and have followed this practice with Weber Installations for more than 30 years.

If you are not concerned about originality there is a very simple low pressure/low cost Airtex pump that will do the job quite adequately and has a maximum pressure of 3.0psi.

We use them as a single pump on SCs with Zenith/Weber carbs developing around 250BHP.

We also use a pair of them on 2.0 litre Race Cars fitted with Solex 40PI carbs.

If you fit a Bendix/Facet you can change one of internal springs which will reduce delivery pressure and avoid the need to regulate.

I also realise that a number of people will disagree

The 911R also ran with a Marelli S121 Twin Plug Distributer and an Ignition system described as semi-transistoried but could have been an early CD type unit (I am sure someone will correct this statement)

Both coils were also in the engine bay.

I don't see why a standard Bosch CD won't work.

[wayner]

I have twin Weber 46 carbs and use a single pier burg pump (available form our host - search pier burg)

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[BK911]

The existing fuel pump will work if you use a pressure regulator.
Or you can buy a low pressure pump.
Your stock ignition will also work.

[Porschestraße]

Thank you all for the replies! I have been having issues with my current fuel pump, which is why I thought about replacing it all together, glad to know I can with just the one and be done!

As far as the distributor goes, is there any advantage to having a hotter spark, other than the obvious? Or is that only a necessary item if I were to be running a more race tuned motor? (Mine is stock setup, and internals)


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[prschmn]

46s are big for a stock engine-make sure your booster venturis are appropriately sized or you'll have a bog monster.

[SCOTITUDE]

46's a bit big, i had 40's on my 3.0 SC, no change in fuel pump, used PMO fuel regulator. Loved it.

[Jonny H]

Put one of our CDI+ units on it, it absolutely loves carbs. It will out perform the standard Bosch and the MSD right across the rev range. Have a look at the dyno test report on our website.