We finally put on our AIM dash and have the data acquisition system running. One of the parameters we're logging is fuel pressures and we were surprised to see what it was doing. In fact we were so shocked we actually installed a standard fuel pressure gauge and visually confirmed it.

Fuel pressure was plummeting dramatically as soon as the car got under full boost. At this point we're making about 680whp with the car and are now running into severe fuel pressure issues. With a 5 bar FRP we should be right under 100psi of fuel pressure at 26psi of boost. The problem is that we're only at 55psi of fuel pressure in the rail!! How can this be? We put the stock pump on our fuel pump test rig and according to the results it should be good for about 700whp at 85-90psi of pressure.

We removed the factory sending unit and pump assembly to see what was going on. After pulling out the medusa-head like assembly we noticed the stock pump powers the 'jet pumps' that work to fill fuel into the factory fuel/surge cup that the pump resides in. Aha! The pump output splits in two, one to the 'jet' pumps and the other to the fuel rails. We put this whole setup on our test rig and now measured the actual maximum output of the stock fuel pump. Well now it flows about 25% less than before, leaving me feeling stupid. About a month or two ago I claimed the stock pump is good for 700whp and there is no reason to upgrade. Technically the stock pump is good for that but only if it's sole purpose was to supply fuel to the engine.

So what's it really good for, then? About 550-575whp to be safe, after that you'll start loosing rail pressure and the pressure differential across the injector drops, resulting in poor atomization and spray pattern. In our case we probably only had 25psi delta P across the injector, not good. you can actually see the power curve get crappy on the dyno at that point.

The good news out of all this? I think we found a great way to up the system to support about 900whp (or more once we test it) without an external surge tank and be cost effective. The factory surge cup is a great design and I think it'll work great in all forms of racing if properly supported. Our prototype fuel system should be up and running in a few days and be dyno testing early next week.

-Martin

here are some pics so you can see what the stock setup looks like.

The 'Creature'


Close up of the head of the 'Creature'. You can see the output from the pump Y's and one end feeds the two hoses that go down to the transfer pumps.


Our modified test rig to hold the calibration fluid for the factory fuel pump setup.


And finally the actual output from the factory fuel system as it runs in the car.

 

What do you think your results mean as regards the arguments of 5 Bar FPR and stock injectors vs. stock FPR and better injectors? If fuel pressure is an issue, wouldn't going to bigger injectors that don't need additional fuel pressure be the way to go? I'm interested in what you think, because I am about to do an 18G upgrade to my existing stock K24's in my GT2 amd was wondering which route is best...

Thanks for this interesting post..

P.S Waiting for details of your K24 billet upgrade... Come on already with the suspense..

 

Very cool. It's nice to see people to actual testing on things instead of just saying it works or does this or that in theory.


No no no, let's see the final release of those inlet pipes first!

 

Other peoples out there knew this a good few years ago already.

 

I'm sure other have found the limits of the stock fuel system, but I wanted to explain why and how the stock system works.

 

Thank u for that explaination.

 

billet 24s! and this, need I. Watching star wars on this snowy day

 

Without a doubt getting larger injectors is far better.

 

Informative post, good pictures.
Nice to here about the "creative process" and see the scientific process in action.

 

Eric,

Would you venture to guess what the pressure at the rail would be by adding a Bosch 044 to the stock system with 60lb injectors and a 3.8fpr?

 

I updated the post with flow data. As you can see from the graph the stock pump is actually really good as far as flow vs. pressure goes but yes, you could squeeze a little more power out of it by staying with a stock FPR and larger injector.

-Martin

 

Thanks Martin. Great info.

 

Aha, not that simple... how can I put this, you can't just simply add a fuel pump in-line to the system. We spent a few days testing different configurations to see what worked and what didn't. As a warning, don't just add an inline fuel pump to stock fuel system, it can possibly cause severe fuel starvation!

To answer your question though, the rail pressure would change with the boost you're running. With a stock 3.8 FPR and 25psi of boost for example, you should be at 80psi of fuel pressure if the pump is sufficient. A Bosch 044 is capable of flowing around 280-290 liters per hour at 80psi of pressure which should be able to support around 700whp on a 996tt engine. So if you're under that level you'll maintain fuel pressure, if you're making more, then rail pressure with start dropping.

-Martin

 

Yes AMS is new to the p-car scene, but if they were doing all this fuel system and turbo stuff 5-years ago or so things would be very different. Thumbs up on the top notch R&D. I will be interested in this fuel solution. Running gt-28's with all the bolt-ons (factory housings) and a 5-bar...car is starving at 520awhp running 91 w/1.3 bar on GIAC's Mustang dyno.

Anyhow nice job and keep it up!

 

Thanks for doing this kind of research that we mere car mortals don't have the knowledge or equipment to do. I am not at risk but appreciate your efforts.