Nitrous Fuel Pressure
Rich Schuler, via CarCraft.com: I have a question about your article I just finished reading ("Nitrous-plate Shootout," Nov. '05). It states you used a common 0.063 nitrous jet with 6 psi of fuel pressure and let the company figure out the best fuel jet for its tune-up. My question is, how did you get 6 psi of fuel pressure? Was it flowing? If so, what size jet was it flowing through? I hope it was flowing the 6 psi through the jet the company supplied, but it is not stated and would make a big difference in the tune-up.
Jeff Smith: That's a valid question, Rich. It also brings up a good point about dynamic versus static fuel pressure. We were at the track the other day and ran across a guy fiddling with fuel pressure on a typical "dead-head" fuel system. A dead-head system utilizes either an electric or manual fuel pump that plumbs one line directly from the fuel tank, through the pump, and up to the carburetor, perhaps through a pressure regulator. To accurately determine fuel pressure, this type of delivery system must be monitored under load, such as at wide-open throttle (WOT) down the dragstrip. At rest, this system will indicate a higher fuel pressure because there is very little fuel being used compared with WOT on the dragstrip, which demands much more fuel.
A full-flow, or return system uses a pump and a regulator but is designed to allow any fuel not used by the engine to be returned to the tank. If you are using an electric fuel pump in a return-type system, merely energize the pump, set the pressure at the regulator, and you're done. The pressure regulator will automatically compensate for any additional fuel used and maintain the fuel pressure very close to where you set it.
We used a full return-style system on our nitrous test so that fuel pressure was always maintained at the 6 psi limit for all the different fuel jets used. Had we been using a dead-head system, this would have required minor changes to the dynamic fuel pressure to maintain that 6 psi nitrous fuel pressure. If you want more information on designing a full-flow fuel system, we covered this topic in the Dec. '05 issue ("Fuel Delivery System"). In a dead-head system, changes to the fuel-jet size would change the fuel pressure dynamically. Realistically, however, these changes would probably be minimal. On a full-flow system, changes in the fuel jet would not affect fuel pressure unless the jet was so large that the delivery system could not maintain the pressure.
SSG Michael Ascott, U.S. Army: I am looking for anyone who can give me some advice on what to do about the lack of handling with my ride. I have a '64 LeMans post model. I have replaced the stock powerplant with a strong 455 and TH400. I didn't think the weight difference would cause that many problems for me because of the fiberglass Goat hood, but it drives like a bus. I just upgraded to a 12-bolt rear out of a '69 4-4-2 and added the rear sway bar and a set of springs from a station wagon, so I think the rear is where I want it. I have a variable-ratio power steering box that I will install as soon as I can find a pump and bracket in the boneyard. My question is, what can I do to stiffen up the frontend that won't require stripping everything to the frame and make me feel every pebble in the road? Also, by swapping to disc brakes from a '70 A-body (I think it changed the location of the wheels) and the 12-bolt that was 1-inch wider, did I ruin any chance of this car ever handling correctly?