Regarding Fuel Injector Improvements for Normally Aspirated Stacker Engines
We were recently asked about fuel injection nozzle design. It got me thinking about several responses throughout our technical manuals and other racing experiences.
In one respect while a fuel spray into the intake port may interfere with air flow, it is fuel vapor that burns, not liquid. There is not enough time for the fuel droplets entering into the cylinder to fully vaporize and burn in the brief combustion event. Changing the fuel delivery to another location or fuel flow method may increase air but at the expense of vapor formation. I saw several cases where a reduction of fuel vaporization dropped power.
In another respect to examine the speed of the the fuel coming from the injector to see if it was below, at the same, or above the port velocity. Port velocity for the air and fuel mixture is in the 200 to 400 feet per second range for a normally aspirated engine. FYI: In highly supercharged engines, it is upwards of 1,000 feet per second.
For a 410 cubic inch sprint car engine, the velocity of fuel coming from common 0.032 inch diameter nozzles at 85 psi fuel pressure is about 149 feet per second. That is below port velocities when the intake valve is open for peak flow. Keep in mind that port velocity is lower at low valve lift. For a conventional 4 cycle engine, it is stopped about 75% of the time over 2 revolutions when the valve is closed or near closed.
Fuel velocity would be increased by higher pressure from smaller nozzles & jets. Several fuel injection manufacturers and flow bench services report that there does not seem to be any power advantage with fuel pressures over 100 psi in conventional stacker injection systems. I am not sure whether there is a fuel velocity issue or a vaporization issue that plateaus after 100 psi fuel pressure.
The intake valve slams shut interrupting fuel and air flow. The turbulence from that is dramatic. In fact, when the intake valve opens it is difficult for the air flow to be re-establish to anywhere near atmospheric pressure without ram tuning. I saw pressure measurements in the port where full atmospheric pressure is barely ever achieved around the valve in normally aspirated racing engines due to the impulsive interruption of the air & fuel column from the valve.
In another respect in our methanol book, chapter 22, p. 209, the same size engine was set up for very high fuel pressure by Don Jackson Engineering. We examined the setup using Pro-Calc. A nozzle with 0.020 inches diameter (with a small main bypass jet) for a 410 sprint engine provides 500 psi of pressure. For that smaller nozzle, the fuel velocity was 383 feet per second or approximately the port velocity. That match in velocity may have something to do with the greater power level reported from that combination in the methanol book. Unfortunately that high of a pressure is beyond the rating of many fuel pumps.
In another respect both Mickey Thompson and Kinsler experimented with compressed air shooting into the injector. The goal was to increase vaporization and port air flow. But that was not something that was pursued. It would be a rules violation in many classes where stacker FI is popular.
In another respect I believe Waterman designed a special nozzle a long time ago for increased vaporization in Top Fuel. Shirley Muldowney was reported to run a set in Top Fuel. We hooked one of those nozzles up to a garden hose with about 60 psi of water pressure, and compared it to a conventional nozzle of that time. There was a dramatic increase in vaporization from the special injector.
In another respect very high dollar Formula One racing engines commonly use injectors located directly above the bell mouth of the inlet tube. They are centered and shoot in the direction of air flow. That is the best for their application. If there was a better way, they would do it.
In another respect Hilborn tried injectors further up the ram tubes and lost power. One of the small block stacker manifolds locates the injectors on the inside of the throttle body assembly, pointing into the port at the bend from the throttle body into the cylinder head. Others are on the outside with flared tips. They both work quite well.
In another respect Gene Adams converted his drag race FI setup from a stacker to a tunnel ram. His setup picked up power at every engine speed. So the tunnel ram looked like the way to go for that application. On the other hand, a couple of our book customers report that stackers with a nozzle located close to the intake port make the fastest ETs in sprint cars, over tunnel rams as well. So one works in one application. Another works in another application.
In another respect the Earl Gaerte’s sprint car team reported that moving the injectors closer to the port made better ETs in sprint cars. Moving them away from the port in some designs did pick up dyno power though. They just did not work in the racer.
In the final respect given time and investment in injector design, companies like Kinsler put a lot of development into their injection systems that are run today with current nozzle and throttle body assembly designs. Gaerte put a lot of development into the sprint car engine package with current nozzle designs. Vern Tomlinson, formerly with Hilborn, reported in our methanol book, chapter 14, p. 120, an optimum nozzle location close to the port. Vern said that is best for the nozzle designs that are common today.