cjb80

Could someone please explain to me when you'd want to replace the fuel pump and when you'd want to change the injectors, and the difference between the two. In all of the forced induction options available it seems that everyone is using the stock injectors but replacing the fuel pump.

Why would you do one but not the other?

Also, what the hell does duty cycle mean??

Chris

ultimate lurker

Chris,

To truly understand the fuel system picture requires examining how everything works together.

Let's start with the injectors. Injectors are usually rated at a particular delivery rate. This is often in lbs of fuel per hour, or in cc/minute. For example, all Honda B-series engines (Integra, Civic Si, etc.) used 245 cc/min injectors.

Now, note that the injector rating is at a particular fuel pressure (usually 44 psi) Injector delivery is proportional to the fuel pressure. More pressure means more fuel delivered (to a point). You can usually determine how injector delivery will change with pressure by using this formula -

new injector capacity = [(new fuel pressure/rated fuel pressure)^1/2] * rated capacity

The downside of running higher pressure on injectors, at least pintle type (like most, if not all, Honda injectors), is that the injector opening time is longer which alters the delivery characteristics. Not a big deal at WOT when we're running rich, but it can be problematic at idle. Excessive pressure can cause the injector to lock in one position.

Oh, and duty cycle represents the percentage of time the injector is open. Usually running more than 80% duty cycle is not recommended.

Getting fuel to the injectors is the job of the fuel pump. Fuel pumps are also rated for delivery at a particular pressure. However, if you ask a fuel pump to deliver fuel at a higher than rated pressure, its capacity goes down. More pressure at the injectors means the pump has to work harder to push the fuel, so capacity drops.

Now, here's the kicker. When you go to forced induction, you need more fuel to match the increase in airflow to the cylinders. A car running 6-7 psi of boost will need, on average, about 40-50% more fuel than a normally aspirated car at WOT.

You could just add 50% bigger injectors. But now you've got a problem when you're not running under boost. The ECU will try and compensate by pulling out injector pulsewidth, but that will affect your full throttle mixture too and you might go lean.

Another option is to put a rising rate fuel pressure regulator on the car. When not under boost, it behaves normally. But when the pressure side of the FPR diaphragm sees boost, it increases fuel pressure by a multiple of that boost. For example, 1 psi of boost might net 7 psi more fuel pressure. If an S2K normally runs 50 psi at WOT (0 vacuum) normally aspirated, it would then run about 99 psi of fuel pressure at 7 psi of manifold pressure. This would equate to about 41% more fuel delivered. A bit lean perhaps, might want to try a regulator with an 8:1 ratio which would get you 106 psi and 46% more fuel. This approach works decently. It doesn't necessarily give you an optimum mixture between 0 boost and maximum boost (with the 8:1 setup I described, we'd be 2% lean at 7 psi, but 2% rich at 3 psi), but under vacuum and at full boost you're in good shape. Its also cheap and easy, so most FI kit manufacturers use it. The downside is that now you're asking the fuel pump to go way beyond its rated pressure, which means capacity drops a lot. Thus, you need a beefier fuel pump. www.hondata.com has a good report on what happens to fuel pump capacity as delivery pressure changes. You'd also be surprised how much hp a stock Honda fuel pump can support at its rated pressure (often 2-3 times what the engine normally delivers).

The ideal way is to put a boost sensitive control system in place. An ECU such as one from Hondata or Zdyne (I use the latter) allows the ECU to add injector pulsewidth as boost rises. This allows you to often retain the stock fuel pump and pressure regulator and just go to bigger injectors. This is what I have done on my supercharged CRX. I went to much bigger injectors (450 cc/min vs. 245 cc/min) and kept my stock pump/regulator combo. Works great despite making 70% more power than stock.

Hope that helps.

UL

Shamu

UL -- you are truly AMAZING!

BTW, you mentioned you were running with a ZDyne CPU...could you please provide more info to this as I am also looking at aftermkt. CPUs for the S2K.

Thanks in advance!