carbs and fuel pumps... Pressure AND flow...

jpnovak · 08:12 AM

This is meant to be an informative thread for those who are converting a later CIS engine to carburetors. This should work with Zeniths or Webers. It will also have the same implications when non-stock fuel systems are used. This starting sentance should have all the key words to pop up in a search for the conversions.

When I installed the 3.0 in my ST replica it was converted from CIS to webers. This was a simple operation. I chose to use a fuel pump from my FLAP that was rated at 3psi. I plumbed it inline to feed the carbs with a PMO style manifold and return loop. I used a pinch on the return loop to keep the pressure up. I should have paid more attention to the clues that it did not work as planned.

clue #1. Car takes a while to build pressure when key on.
Clue#2. Car is hard to start
Clue #3. Car takes a while for all throats to equalize and run smooth when cold.
Clue#4. Car looses power at 5K rpm and feels like it sputters at 6K.
Clue #5. My pressure pinch was all the way closed to get the required pressure.

What does it all mean? I was not getting enough fuel FLOW!

Just because a carb is receiving the right pressure it does not mean the pump can keep up with fuel requirements. After a long search I found the correct Bosch roller (rotary) style fuel pump for an early carbureted 911. I installed it on Sat. afternoon. Yesterday I took the car out for a spin and it never ran better.

I noticed that the fuel pressure is steady and constant within 5 seconds of turning the key on. Startup is immediate with a short lag to balance cylinders. throttle response is fantastic. No cutout or loss of power to redline. Teh engine has a completely different personality above 4K.

Moral of the story. Don't just pick a fuel pump on the pressure requirements but also consider the implications of flow.

btw here are the specs for the fuel pump for a carbureted 911T Bosch pump. Thanks to Derek Murray for posting this. I found it in the archives.

EWPurdy · 06-06-2005, 08:09 AM

Jamie,
What fittings does the Bosch LP pump have on it? Hose barb? Threads?
Thanks

jpnovak · 06-06-2005, 08:13 AM

Hose barb fittings. I think it is 10mm supply. 8mm delivery and return.

EWPurdy · 06-06-2005, 08:37 AM

Thanks.

buster73 · buster73's Garage

If you convert from CIS you can keep using the CIS fuel pump, with the PMO style return loop and line pinch to adjust the pressure. Works great and you have all flow you need.

rw7810 · rw7810's Garage

jamie - Where'd ya find the Bosch pump? I'm having some of the same symptoms since a rebuild. I'm using the same Facet pump that I ran on the pre-rebuild.

jmz · jmz's Garage

...been using the Holley pump. It's OK but needs replacement from time to time.

JZ

jpnovak · 06-06-2005, 08:02 PM

Bob,

I found the pump on ebay for $30. It was labled as a Bosch pump, not Porsche. I just cross referenced the part number as listed above. I was using a small Facet like solid state pump. That was the type that was giving me problems. I know that the solid state pumps (like the holley) are prone to failure. The Bosch pump is so simple they hardly go bad. Best of luck on your search.

btw, these Bosch pumps are available from many sources new or rebuild. Prepare to open up your wallet though. Mucho Dinero.

bavaria911 · bavaria911's Garage

Jamie.

After reading your thread, I copied the data sheet and as I left work in Wurzburg Germany, I stopped in the local Bosch and had them price me the two pumps....

The T pump new runs 515 euros
The E/S pump new runs 600 euros.
Both are in stock !

The dollar vs euro is $1.25 to 1 Euro.

They told me a rebuilt E/S pump is only 100 euros cheaper.

Vr,

Bavaria

jpnovak · 06-07-2005, 09:31 AM

I concur, they are expensive but easily found new.

That is why I was excited to find the used version at the price I did. There is virtually nothing to wear out on these pumps. Just the o-ring seals which can be easily replaced in about 5 min with a screwdriver.

}{arlequin · }{arlequin's Garage

cool thread. thanks for the heads up

Formerly Steve Wilkinson · 06-07-2005, 10:10 AM

Why all the fuss? As Buster73 said, a CIS pump works just fine, road or track, with Webers or PMOs. At least mine has for the last 8,000 miles, on an engine that dynos at nearly 290 crank hp. Just bought a brand-new one from Pelican for something like $275 (precautionary changeout--nothing obviously wrong with the current pump, which I'll keep as a spare).

Stephan

jpnovak · 06-07-2005, 11:41 AM

Fuss? I never said a CIS pump would not work. I am referring to application driven low pressure pumps.

Let's say you bought a roller sans fuel pump. You may not have a CIS pump. What about engine swaps? There are many other reasons to need a fuel pump for your carb setup.

Nitrometano · Nitrometano's Garage

You need to set the pressure to 3 1/2 psi.

RoninLB · 06-07-2005, 10:39 PM

Quote:

Originally posted by Formerly Steve Wilkinson

Just bought a brand-new one from

I'm with Steve on this one.

The install of the CIS is more bullet proof. The install of the Bosch low psi pump ports isn't the greatest. If I had to use the low psi pump [I gave a brand new one away for free a few months ago] I'd glue the hoses on. Maybe I'm over serious but once the pump is installed it's not like I look at it often. It may be years before I'm in there again.

all above a prejudical rant.