Fuel System Design Example

Given information:

Let's say that you have a 1995 Eclipse GS with a bolt-on Stage II turbo kit, but is stock otherwise. Here is what you know about the engine and the turbo kit.

I-4 DOHC 16 valve Chrysler
Computer designed engine
The head design is pretty good at preventing detonation
9.6:1 compression ratio
Injectors are 235 cc/min rated at 43.5 psi fuel pressure
Stock FPR is on the fuel rail
Turbo kit manufacturer recommends the following fuel pressures
50 psi at idle [full vacuum]
60 psi at 0 psi boost
90 psi at 6 psi boost
110 psi at 9 psi boost
Approximate engine horsepower is 220 at 6 psi and 275 at 9 psi

You are worried about running 110 psi of fuel pressure and about maxing out your injectors. You wish to run bigger injectors to lower peak fuel pressures and lower the duty cycle of the injectors. You will keep 9 psi as your maximum boost. You only wish to buy injectors, FPR, pump, and a S-AFC.

Phase 1

Find out how much fuel you will need per injector. Assume a BSFC of 0.525 for this engine, based on its design. You have 4 fuel injectors. Your approximate maximum horsepower is 275. Your approximate fuel flow rate per injector is calculated below.

You feel that an 85% duty cycle will be acceptable. Remember that if you aren't using a S-AFC or similar device to trick the ECU into running a lower duty cycle, you must use a duty cycle of 100% for the calculation. Your approximate injector size is calculated below.

You now have a choice; you can use the 445.86 cc/min injector at 43.5 psi of differential pressure or a smaller injector at a higher differential pressure. Remember that they only make injectors in a limited number of sizes. So you can pick an injector size and calculate what fuel pressure will match your approximate injector size. You try a 370 cc/min injector. Use the fuel pressure equation to find what fuel pressure you would have to run with that injector. The equation gets reorganized as shown. You substitute the approximate injector size for the injector flow rate. You know the intake manifold pressure and rated pressure. You can solve for fuel pressure.

Running a maximum fuel pressure of about 72 psi is acceptable to you, so you continue on. You buy the 370 cc/min fuel injectors and plan to run them at 72 psi of fuel pressure at maximum load [9 psi boost].

Phase 2

Now you need to choose a fuel pump. Alot of the TSI, GS-T guys use Walbro pumps. They can be bought pretty cheap and work well if you use them properly. Using the pump charateristic curve below, you need to decide if one of the Walbro pumps will work for you.

I've drawn a vertical line up from 72 psi and drawn horizontal lines across from the 3 Walbro pumps. It looks like the 190 lph will supply 25.6 GPH, the 255 lph will supply 35.6 GPH, and the 255 hi-press will supply 47.4 GPH. But how much fuel do you need?

Using the above equation, it seems like you will need a minimum of 24.06 GPH. That pretty much rules out the 190 lph, but the 255 lph and 255 lph hi-press will both do fine. You might as well choose the 255 lph hi-press, since they are often the exact same price. This will give you some room to grow if you choose to raise your fuel pressures at any time.

Phase 3

Now you need to choose the FPR [fuel pressure regulator]. I previously listed the qualifications for a FPR. The Vortech S-FMU is a good choice as it is flexible. Lets say that you buy the S-FMU. You will also need to buy fittings and a fuel pressure gauge. Now you need to figure out what calibration disk to run and what to set the static pressure to. You will also buy a S-AFC to fine tune everything. I'll go over some basics on that to. Using the data supplied by the turbo kit manufacturer and the fuel pressure equation, you build a fuel curve that looks like the one below.

Now you need to build a new fuel curve with the 370 cc/min injectors. Match up the static pressure point first. The stock 235 cc/min injectors can supply about 276 cc/min at this point. Using the fuel pressure equation, it will take about 24 psi to match that. That is too low, especially considering that the idle pressure will be even lower. You are going to have to run a higher static fuel pressure and use the S-AFC to lower the fuel curve. Use the Fooling the S-AFC mod and set your low throttle point at 0 boost [static pressure point]. The idle point will take care of itself since idle and 0 boost will share the same adjustment percent [see the S-AFC instruction book if you don't see why]. Set your high throttle point at 9 psi [or slightly above]. If you run 40 psi fuel pressure and -22% at the static pressure point, that takes care of the idle and static pressure points. To actually set the static pressure, turn on the car and unplug the vacuum line running to the FPR. The FPR sees 0 boost and you can adjust the fuel pressure to what you want. Before installing the S-FMU, you will want to disable the stock FPR because you have a 1995 vehicle. Ok, now to pick the calibration disk. You plan on running 72 psi fuel pressure at 9 psi boost. Subtract the static pressure from the maximum fuel pressure and divide by the maximum boost pressure. That is (72 - 40) / 9 = 3.56. A 4:1 disk is probably as close as you are going to get. Vortech offers some disks that aren't on the website, you'll have to call to see if there is a more appropriate disk. But lets say that you buy the 4:1 disk. That will make your maximum fuel pressure about 76 psi. 76 psi fuel pressure and -22% on the S-AFC will work for the 9 psi boost point. From my experience, setting the S-AFC to about -22% changes the fuel flow rate by about that much. You can't really count on that, so start out rich like -5% and work your way down. An EGT, A/F gauge, and datalogger are great tools for tuning. The EGT and A/F are a must if you want to completely change your fuel system. Judging by the fuel curve in the graph above, it looks like you have accomplished your goal. That is, supplying the same amount of fuel as before, but with lower fuel pressures and lower duty cycles. The tuning stage will be difficult. You need to verify that all of you predicted fuel pressures actually show up [mount the fuel pressure gauge on a hose so you can see it while driving]. Tuning the S-AFC is not as simple as I have made it sound. There are other sites with good tuning methods for getting the right settings on your S-AFC. For example, there are a few here. If you do decide to "fool the S-AFC" and use a separate MAP sensor instead of the TPS [throttle position sensor], keep that in mind when you read other people's tuning sites.

Contributed by Corbin
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Cars Modifications Power Turbo Fuel_System Fuel System Design Example

Document statistics: Last modified on 2008-12-14 13:07:34 by Corbin