Electric Fuel Pump in Series with a Mechanical Pump

The stock mechanical fuel pump (and lines) will feed a 2180 with 48IDA's and 42/37 heads making power to 6500rpm from a standing start to the top of 4th gear. This is possible (in part) because of the IDA's large float bowls (which can be modified for even more volume) and the fact that on the street (or even a standing start drag race) you're usually not at full throttle for more than 12 or 15 seconds at a time. In longer full throttle situations (competition) you may find the engine emptying the float bowls and having to upgrade to a higher volume electric pump and larger lines earlier.

 

Most electric pumps are "pushers" and work best when mounted close to the gas tank. I don't know how a stock pump would react to higher fuel pressure on the inlet side; I'm guessing it might increase the volume nominally, but I don't know anyone who's tried it, and it's just that, a guess. Al

25 gph is the fuel consumption for roughly 300 HP. A normally aspirated VW engine won't hit that. The stock pump can feed a big NA VW engine just fine.

Running two pumps inline won't do you any good, at best the electric will override the mechanical's valves making it pointless. Hooked up the other way, the mechanical will starve the electric. Use one or the other, not both.

300 HP...please show me the arithmetic in BSFC terms. I (and many others) have run series fuel pumps on big V-8 motors...the electric pushes through the mechanical just fine. The issue is does anyone have an example of a nice manifold that allows running the 2 together in some configuration. I have IDF's not IDA's and they do have decent fuel storage in the bowls. But...if you have electric pump(s), just shut them off at 3000 Rpm (in 4th gear)and watch (and listen) how quickly the motor runs dry !

 

  ReV

Well, I have had some experience with a failing mechanical pump on a 2110cc and it was leaving me with a vapour lock and giving me low response at 65 mph. ... finally it quit and I installed a rotary electric ... and the car could now do  75 to 80mph... go figure, the higher speed was a symptom of the low pressure of the mechanical unit. 

I never looked back after that... and was glad to have the block off plate for that manual pump. Ray

With a quality electric pump (I'm not Empi/Facet ******** pump fan) you can also add a toggle switch to both cut 12v to it quickly in an emergency (fire/accident) and also turn it off as a poor man's anti-theft device.  

"R":

 

I've been on here for ten years or so and have never heard of anyone running a dual mechanical/electric fuel pump setup on these engines (or our Subaru variants).

 

If you have extensive experience with these on other, V8 cars, then I would fall back on that experience and do what you've done in the past.

 

And then let us all know how it works out so we can learn something, too.

 

Thanks.....

HP = (Flow gallons/hour) x (6 lbs/gallon) / (BSFC lb/hp·hour)

 

300 hp = (25 gallons/hour) x (6 lbs/gallon) / (0.5 lb/hp·hour)

 

Yes, that was my point, you make the mechanical pump superfluous doing that. Just use the electric one.

Wolfgang and Gordon,

 

 I am lazy...I have an older style (German) mechanical pump on my Pat Downs motor...and I don't feel like running secondary fuel lines or blocking off the mechanical pump...I knowthe motor runs out of kraftstoff on top end...so the question is/was how do I use both devices in tandem to supply the motor

 

  ReV

3.5 PSI, 30 GPH fuel pump $52.95

 

Fuel pump block off plate $4.95

 

7/32 hose barb (assuming stock 5mm fuel hose) for connecting the inlet hose and outlet hose of mechanical pump together (use hose clamps too), or you can just replace the whole hose instead of splicing.

 

Other than a bit of wiring for the pump that's all you need to remove the mechanical pump and use a higher capacity electric pump. In my opinion, there's no reason to over complicate this by adding more points of failure.

Rev- Not that it can't be done, but I've been involved in hotrodding aircooled VW engines since 1974 (I was 17 and we put a Holley 2 barrel, header and an 009 on the 1300 in my first car, a '66) and, as Gordon and Justin said, for more fuel the mechanical pump is pulled and replaced with an electric. If the stock pump is truly at it's limit and it isn't enough it may be time to install a bigger fuel line as well.

 

It's accepted in the hobby/industry that the stock pump is adequate to the point where the car is running mid/low 12's in the quarter mile (with IDA's). Drag racing is the frame of reference/activity for most VW owners that race.  I've never compared float bowl size between the 2 carbs (IDA's vs IDF's) so I don't know if this applies to IdF's as well.

 

I'm guessing you're running 48 IDF's. Have you upped the ball/seat size?, as this can be the cause of insufficient fuel delivery. I do know that 48IDF's are usually supplied with 2.0 ball/seat assemblies, and have read that they will support 85hp/carb. 2.5's are available and are said to support 120hp/carb (info courtesy of John @ Aircooled.net). 

 

Hope this helps. Al

Hello Al,

 

 I have had a great deal of experience with Webers, Mikunis,Dellortos etc. on all matter of Italian iron and race cars.

 

I have a trove of old parts including  complete set of jets / emulsion tubes etc. for the DCOE series.

 

And I use Grose - Jet balls so I can run essentially any fuel pressure I like, and can afford a degree of inefficiency in operation, as in “how come you have a 70 gal/hour pump in this application”…because I already have several of these pumps and don’t need to buy any.

 

The problem is that running a single electric pump means that when it fails and (they always do) you are stuck !

 

Having a mechanical back-up is very desirable or vice-versa.

 

So I will have to figure something out...

 

  ReV

Then you have way more experience than me. I'm bowing out of the conversation and will watch this thread to see what you come up with...

Some people carry a spare electric pump just in case, usually a relatively easy swap.  Maybe easier than trying to figure out how to install both mech an elec.  Elec do tend to fail but some are more reliable than others.  

^ x2. It's $40 or so for a rotary pump (buy a Carter from the interweb). I just carry a spare and I'm done.

 

Or... you can just leave the mechanical pump connected as you want, and fill up the crankcase with gas when the ethanol in your fuel rots out the diaphragm in your high-quality Brazilian mechanical pump (you know, the one that won't feed your 300 hp engine).

 

It's your car. 

Bruce ...to clarify...this is a spyder not a speedster...but where are you mounting the electric pump...on a spyder it's usually under the gas tank and changing it is a big production...

 

  ReV

Say Stan...help me out here...I have been to Brazil and driven VWs on "ethanol"...why would a Brazilian pump set-up to run ethanol...not work with ethanol in North America

 

  ReV

Finally, we get the reason for your inquiry. Redundancy is a whole different discussion.

 

First, you'll be getting rid of the mechanical pump anyway. You've already stated it's already inadequate for your use and if the electric pump dies it can have problems trying to suck through the inline dead electric pump (depends on pump type).

 

Fortunately, it's very easy to have dual redundant electric pumps. You just connect the inlets and outlets of the two with Y fittings.

Tank -- Y fitting -- Pump1 -- Y fitting -- to regulator/carbs

                  ᒪ---- Pump2 ----ᒧ

 

The check valve on the outlets of the electric pumps should prevent reverse flow through the pump that is off (if not it's easy enough to add a check valve to each outlet). Then you just have to wire in a toggle switch to select power to pump 1 or 2.

ReV,

 

Well... since you asked for help...

 

I've been to Brazil a few times as well- I was there for 4 months in the early '80s and few times since.

 

The álcool Fuscas were not known for their reliability. Ethanol eats rubber parts (here or there). When a mechanical fuel pump diaphragm ruptures, it fills the crankcase up with fuel because the tank is higher than the level of the fuel pump. One of the Fuscas we drove "back in the day" had this happen.

 

If you want to run two pumps, by all means, run two pumps. IMHO, you risk a good deal for no real return.

 

That's my help.