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When the weather breaks, I'll be installing a new 255lph fuel pump kit from Racetronix. I'm thinking of adding an adjustable fuel pressure regulator.
1. Is this a definitely should do, a good option, or unnecessary? (Usage: car is rarely raced, used mainly for pleasure cruising but for that occasional WOT run I want to know fuel is supplied.)
2. Product suggestions e.g. which one?
3. Where is it typically mounted? (95Z)
4. Are any changes to the stock regulator on the fuel rail required or recommended?
Stock FPR is adequate for most applications up to 500 (flywheel) HP. They are generally very accurate. There is limited need to adjust the fuel pressure, as long as it is within GM specs. You cannot “tune” with an AFPR, at least not with a closed loop tune.
One practical use is to increase the flow rating of the fuel injectors. The LT1 operates at 3 bar (43.5 PSI) differential fuel pressure. The regulator is vacuum compensated maintain the differential pressure (= rail pressure minus manifold pressure, where both pressures are measured in “gauge” pressure). By increasing the fuel pressure to 4 bar (58.0 PSI) the flow rating of the injectors will increase by 15.5%. A 36 #/HR injector would flow 41.6 #/HR. Not all types of injectors are happy with higher pressure. Additionally, increasing the system pressure will decrease the flow capability of the fuel pump. This can be determined by looking at the pump's flow vs. pressure curve.
Since the LT1 mounts the FPR into the back end of the driver side fuel rail, unless you want to make significant alterations to the fuel rail and hard line, you have to buy a direct replacement. One of the more popular AFPR's is made by Aeromotive. In spite of that company making some excellent fuel handling equipment, the LT1 AFPR has been a disaster for 20+ years. They are not reliable. The first question I ask when someone is having fuel system problem is “what brand is the FPR”.
I run dual Bosch 205 LPH high pressure pumps. The second pump only operates when the nots system is armed. You don’t want continuous high flow when you don’t need the extra fuel. Just builds up heat. The fuel flows through -6AN line, up into the engine compartment where a “Y” fitting splits it into two -6AN lines, one into the back of each rail. The rails are now running in parallel, not in series. Less pressure variation between the cylinders in each rail. The fuel returns to the fuel pressure regulator via a -6AN fitting welded to the diagonal crossover pipe at the front of the rails. a Weldon AFPR is mounted under the cowl, and maintains 58 PSI In the rails at all times. The fuel pressure is not vacuum compensated, and that is accounted for in the tune. The 64 #/HR Bosch injectors flow 73.9 #/HR. Duty cycle at 800 HP is between 75-80%. Fuel returns to the tank via -6AN line.
People question why I use only a -6AN feed line. Size was selected based on balancing frictional pressure loss in the supply line vs. sudden loss of fuel pressure when the car launches. With launches in excess of 1G, the mass of fuel in the supply line exerts a force toward the fuel pump equal to the mass of fuel in the line (x G). That force, spread over the small area of the pump discharge results in a sudden reduction of fuel pressure.... exactly the time when you don’t want a sudden drop in fuel pressure. I didn’t figure this out. The shop that built the engine showed me data from logs on their 6-second Pro 5.0 car and others.
I second Injuneer's views on the Aeromotive piece. They may have eventually fixed the design for all I know, but years ago I went through two of them on my LT1 that failed within a year. Diaphrams failed and leaked. Unacceptable at their price-point. It was a nice looking unit though. I finally realized I didn't need one on my mildly modified LT1.
I have been lucky so far with my Aeromotive AFPR, but have definitely read horror stories. My biggest issue has been the O-rings with how long the car sits between running. The sizes do not seem to be standard and they are not documented anywhere I can find. I ended up going through a local speed shop with a connection to Aeromotive who got them to put together a generic set of O-rings that would replace everything on any regulator they make.
To the point, if you don’t have a specific thing you are looking to do with pressure, the stock FPR is fine.
Fred, do you have pics of your rails? Im still figuring out what I want to do with mine. I had been thinking I was going to buy a fitting to have welded/indexed into the driver side rail for the AFPR, then an AN fitting on the passenger side. I had not figured out the cross section line yet and hadn’t thought about how well the rail in series impacted per cylinder pressure.
The -AN male stub is welded to the crossover. I didn’t do any of this work.
I would have been more involved in the build, but I dropped the car off at Second Street at the end of December. They pulled the engine/trans and put the chassis in indoor storage, then had to wait almost three months for Callies to run a batch of the 1-piece RMS crankshafts. By that time, I was in northern Italy working on an unexpected six month assignment. I spent Valentines Day flying from JFK to Milan. I got back in August, just in time for them to email me the engine dyno sheet for the full hit of nitrous.... 762 HP @ 6,000 RPM / 752 lb-ft @ 5,200 RPM. I missed all the fun.
People question why I use only a -6AN feed line. Size was selected based on balancing frictional pressure loss in the supply line vs. sudden loss of fuel pressure when the car launches. With launches in excess of 1G, the mass of fuel in the supply line exerts a force toward the fuel pump equal to the mass of fuel in the line (x G). That force, spread over the small area of the pump discharge results in a sudden reduction of fuel pressure.... exactly the time when you don’t want a sudden drop in fuel pressure. I didn’t figure this out. The shop that built the engine showed me data from logs on their 6-second Pro 5.0 car and others.
That is some impressive stuff. If I had a hard launching car losing pressure on launch, I don't think it would ever enter my mind of the fuel itself going back towards the pump on launch to be the cause.
02-20-2021, 06:11 AM
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