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Does the MQB Fuel System Defy Physics?? AKA Why do I need a HUGE Fuel Pump

rao

Passed Driver's Ed
Location
IL
Get comfy, this is going to be a fairly long post.

Before I bought my A3 I spent a lot of time researching the platform because I knew I was going to be making some changes to the car :)

One thing that I could not understand was why the fuel system inthe MBQ seemed to be defying the laws of physics. People were building 1000 hp fuel systems for cars that were making 500 hp and the systems weren’t large enough. What was going on? Why was a Hellcat fuel pump that easily supports 700+ hp in a Mopar struggling to support 500 hp in a VW? Why were vendors trying to sell my $2k brushless fuel pumps coated with fairy dust? Why did my last few cars reliably make over 700 hp with a single cheap Walbro fuel pump and a Boost A Pump?

Answers were hard to find but I knew the answer was going to be fairly simple because it usually is: It turns out that the issue is pressure.

Once I had MPI on my car that added the ability to see and log the low pressure fuel pressure. These cars run at a typical fuel pressure of about 100 psi, which it turns out is pretty high.

For reference carburetors use about 3-4 psi of pressure, early Throttle body fuel injection systems use about 15 psi, Multi-point fuel injection systems (think Fox body Mustang or any car after 1996) use about 43.5 psi or 3 Bar, and newer systems use 58 psi or 4 bar. The service manual spec for the Mqb is 87 psi or 6 bar minimum (6000 mbar) and the cars will typically see 7100 mbar or around 100 psi.

The MQB has a “returnless” system with a mechanical fuel pressure regulator inside of the fuel bucket - it is a very simple device that you will get to see up close and personal if you open up the fuel bucket.

The MQB also has Fuel Pump Control Module (FPCM) which takes a digital signal from the ECU (if only it were analog this would be a lot easier :) ) and uses it to control the duty cycle of the fuel pump, which makes the fuel pump work less at idle and cruise and work harder under load; these are very common and they are used to keep the fuel cooler to fight evaporative emissions.

So why is pressure the issue?

Not surprisingly, It turns out that it takes a lot more work to provide fuel at a high pressure than it does at a low pressure. Simply physics there.

Because the fuel injection world started with a 3 bar pressure; that is what everyone uses to rate their pumps. For example, when you see an Aeromotive 340 pump; it is rated to flow at roughly 340 lph at 43.5 psi and 13.5 volts (13.5 volts is also standard and approximates the voltage most cars see while running), the rating drops to 225 lph at 80psi and the chart stops at just a little over 200 lph at 90psi; it is not rated to produce over 90psi. That is why if you installed one you would have all sorts of problems. Also the higher the pressure and the higher the work, the higher the amperage draw from the pump. This presents a problem because the FPCM can only handle so much amperage draw before it melts.

Here are the popular options for the MQB (the Walbro 305 is the pump that APR uses and is in the PR “Stage 2” system):



Look at that current draw!

Because of the current draw, you see people dropping the duty cycle of the fuel pump so that the FPCM doesn’t fail. That is a problem because you aren’t going to get near the capacity of the fuel pump running it 80% or lower duty cycle.

There are 2 solutions to the amperage problem.

The inexpensive solution is to ground the fuel pump directly do the chassis and not through the FPCM as it is from the factory. Some people say this works great and others say it will destroy your car and your life. I haven't tried this but it is certainly easy to do.

The other solution is the Torqbyte PM4 controller; this acts as a substitute FPCM and gets power directly from the battery. I use the Torqbyte CM5-LT controller for my methanol injection and it is excellent and I have used it on several cars. This definitely works but is pricey $650 for AWD cars $699 for FWD cars.

I have a new modified fuel bucket with the APR pump in it and a Walbro 450 high pressure pump waiting to be installed in the bucket on my bench but for now I am using a Kenne Bell Boost A Pump with the factory fuel pump and it is working very well. I like the Boost A Pump for its simplicity and reliability. Plus I had 3 in my garage already :)

I cannot find detailed specs for the factory fuel pump but it must be pretty stout.

I have a DBV2 V1 Turbo running 33 psi with MPI and have no fueling issues at all on pump gas (with or without meth) and no issues on E85 although the MPI rail pressure is starting to be a concern once I get above e70.

I hope this is helpful.
 

vw671

Autocross Newbie
Location
San Diego
Torqbyte + 525 FTW


 

Hoon

Autocross Champion
Location
Rhode Island
People have all sorts of issues because they don't want to spend the money on a Torqbyte.

Buy once, cry once, move on with the setup.
 

Hoon

Autocross Champion
Location
Rhode Island
I personally don't like dual pump setups. The next step over this for me would be the Iroz or PagParts brushless setups.

I have a Pagparts billet basket w/ 525 being delivered soon.

It's going to Railroader first for testing on his car because of some fuel system issues he's had.

Not a cheap piece, but I look at a bulletproof fuel system as an insurance policy.
 

vw671

Autocross Newbie
Location
San Diego
I have a Pagparts billet basket w/ 525 being delivered soon.

It's going to Railroader first for testing on his car because of some fuel system issues he's had.

Not a cheap piece, but I look at a bulletproof fuel system as an insurance policy.


That's a nice piece for sure...I might get it if run into a fueling limit.
 

Hoon

Autocross Champion
Location
Rhode Island
I thought that Railroaders issues were from the exhaust flap not opening on one side?

Might have been a factor but didn't fix it.

Car is down about 100whp right now so he's trying everything. I strongly encouraged him to give the PPT LPFP a try because I still think he has a cavitation issue.
 

aaronc7

Autocross Champion
Location
Clovis, NM
That's for the post... MQB fuel system was a little bit of a learning process for me too. I'm used to the more basic systems so this whole PWM pump thing was new to me. 100 psi is quite high pre-HPFP, I wonder why they did that.

Stock pump + BAP seems like a good option for a lot of people who aren't doing a crazy hp setup but might still want or need a little headroom. I run a 450 + BAP on my C5Z and never had an issue.

I don't want to derail too much but can you provide some details on the Torquebyte WMI controller and some setups you've done with it? I read thru the product page and it looks pretty good for the price point. Do you run a FAV? Overall looks pretty similar to aquamist setup..but could probably cobble something together for quite a bit less $$ if you knew what parts to get. I run AEM on my C5Z and it's fine I think for a centri s/c setup where boost builds linearly and overall the fuel control strategy is a lot less advanced (open loop at WOT).
 
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