Fuel Delivery - Get it Right or You're Screwed!
The most important part of a successful racing engine is the fuel system. (Carburetor, fuel pump, fuel pressure regulator) These three pieces need to work together to deliver the correct amount of fuel to the engine at all engine speeds. Everyone knows that the carburetor affects the air fuel ratio, but few realize that the pump, and regulator also affect the mixture.
No carburetor will work properly without the appropriate pump, and regulator. Most "carburetion" problems that I have seen at the track can be traced to one of two things. Insufficient fuel flow, or improper fuel pressure.
Fuel Pump
The fuel pump needs to flow enough fuel that the pressure seen at the inlet to the carburetor does not drop below the preset pressure level at high rpm. This means that the pump must be big enough to overcome a slightly clogged fuel filter, or anything else that may cause a restriction in the line. Choosing a fuel pump is relatively straightforward. It simply needs to flow enough fuel to get the job done.
I have had the best luck with the Carter 7lb. pump. This pump is big enough to supply fuel for any 12A application, and has proven to be very reliable. It is half the price of the Holley, and at least twice as reliable.
The stock fuel pump is barely adequate for its intended purpose. Do not even consider running an aftermarket induction system with the stock pump, two of them wired in parallel, or any other combination. This goes for the Facet as well. A large rotary vane style pump is the only thing worth running, and as I said before, the Carter has proven to be the best. (It is also the least expensive!)
Fuel Pressure Regulator
The purpose of the fuel pressure regulator is to supply the proper pressure to the needle and seat valve. The stock system does not use a regulator, and as a result, the full throttle mixture changes as the pump wears, as the fuel filter becomes clogged with use, and as the rpm increases. In other words, the fuel pressure at the carburetor is only correct when the pump and filter are brand new, and consistently gets worse over time. I have seen such a wide variance in pressure and flow rate that even a bone stock RX-7 could benefit from an upgraded pump, and adjustable regulator.
My current preference is the Holley or Carter low pressure adjustable regulator. They are the same regulator, only the name is different. This regulator is inexpensive, and can be adjusted from one to four psi. A fuel pressure gauge will also be necessary. The Holley regulator has two outlets, and a gauge can be threaded into the second outlet, or mounted remotely. Extra diaphragms are available at a reasonable price.
The small dial type regulators made by Purolator and a few other companies restrict the fuel flow at high rpm, and should not be used. The Holley/Carter is only a few dollars more, and is capable of flowing any amount of fuel a rotary could ever need.
Fuel Pressure Gauge.
You would think this would be the least critical part of the fuel delivery system. In fact, this has turned out to be the most problematic. In trying to track down inconsistincies in my race carbs, I suspected that the fuel pressure gauge on my test rig might not be accurate. I hooked up two other gauges and found that all three gave a different reading.
All three of these were 0-15 psi automotive fuel pressure gauges with prices varying from $12 to $58. To see what was really happening, I built a large manometer to accurately monitor fuel pressure, and found that all three gauges were off by a considerbale amount. The worst example displayed 2.75 psi when the pressure was actually .6! That's right...point six!
After much digging, and many phone calls, I sourced a 0-5 psi pressure gauge that was accurate, and reasonably priced. In the course of tracking down an appropriate gauge, I got a brief education from several gauge manufacturers.
The first thing pointed out to me is that you should always use a gauge that allows you to read the center 60% of the scale where the gauges are most accurate. A typical industrial pressure gauge may have an accuracy rating stated as 3-2-3%. This means the reading is accurate within +/-3% at the upper and lower 20% of the scale, and 2% in the middle. This sounds like pretty good accuracy, but here's the catch. The accuracy is rated as a percentage of full scale. In other words, a 0-15 psi gauge could be off by .45psi at the lower end of the scale, and still be within rated accuracy. If you are looking for 2 psi for instance, you could have a reading anywhere from 1.55, to 2.45 psi. Rated in percentage of the intended pressure, that is +/-22.5%. No small matter.
Right off the bat, the typical 0-15 psi. automotive pressure gauge is inappropriate for our use. Add to that the fact that these companies have huge amounts of money spent on advertising and packaging, and you can guess at the quality of automotive fuel pressure gauges. The gauge I stock is a generic industrial pressure gauge, and at first sight you can tell it is a quality piece. The best part is that it costs no more than most brand name automotive gauges.
Those of you used to Holley's, Weber's, or Dellorto's may think that accuracy at 2 psi is no big deal because you would never run the pressure that low anyway. My modified Nikki's run between 2 psi, and 2.5 psi., depending on the application
I recently had a customer dialing in a PRO7 motor on his engine dyno. My shipment of gauges showed up late, and so his initial testing was done with his dyno gauge. After receiving the new gauge, he found that his was off by .3 psi, and lowered the pressure accordingly.
Being a bright guy, he started the nights dyno session by doing a baseline run to make sure that he was getting repeatability. The motor was down 4 horsepower from the previous night. After talking this over with him, and scratching our heads for a while, it occured to me that he had lowered the fuel pressure. His response was "Yeah, but it was only three tenths of a pound, big fucking deal!" (And people joke about sailors, us engine builders are the worst!) After a bit of prompting, he went up a few jet sizes to account for the difference, and the motor made the same power as the previous night. FUEL PRESSURE IS CRITICAL!
To insure that all my carburetors have the appropriate fuel pressure, you must purchase a gauge to go with it. I am not being a hard ass, I am simply insuring that all my customers get the most from their modified carburetor. It does no good for me or you to skimp on this. If the fuel pressure is incorrect, the carburetor will not deliver 100% of its capability. This is bad for you and me, as I want all my customers to be as fast as possible.
So...The moral of the story is that the carburetor is only part of the equation. Get the delivery system right before you expect gobs of horsepower from your new induction system.
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