True, But a flow test in a static (car not moving) situation will only confirm that the pump is working. You have to put the car in the operating condtion where the problem is occuring, and then check to see if it can maintain fuel pressure. Pressure is maintained when the inlet needle on the carb is opening and closing, indicating a full fuel bowl, or another restriction in front of the bowl causing the guage to read pressure. But, Just because you have fuel pressure in front of the carb, does not for sure mean you actually have fuel in the float bowl. The needle and seat in and of itself is a restiction. And with a Q-jet, we always like to install the biggest needle and seat available. An easy source is from Edelbrock, PN 1980. That's a .149 needle and seat assembly, and a must for high performance applications with a Q-jet. The typical Q-jet needle and seat is .125.. If we consider a typically Holley, with even just the .110 dia needle and seats, we can then get a better picture as to why a Q-jet is so difficult to keep full of fuel. That holley has .220 of overall diameter to feed fuel into the float bowls. So a more marginal fuel system will still be able to keep it full of fuel. Simple fluid flow dynamics.. less pressure is required to push the same amount of fuel thru a bigger hole, in a set time. And the typical 500 HP BBB, at 5500 rpm, requires 240 lbs per hour of fuel. In and thru the carb. But at 4000 rpm, even a 550 ft/lbs 455 only requires 178 lbs/hour. So a car with a marginal fuel system will go like crazy until the float bowl starts to drop, and then it will just kinda lay over. And it's pretty easy to just say "oh.. it does not want to make power past 5000 rpm... I had one motor on the dyno, that we tested a stock fuel pump on.. 540 HP Iron head 462 street motor... and with the stock pump, it lost 40 HP... and the amazing thing that hit me, was if you did not read the numbers coming off the Superflow, you would not think that there was anything wrong with the motor.. no pop, no sputter, sounded just fine. It just went lean at about 4800 rpm, and did not make any power. I recall thinking "I wonder how many of these cars are out there, that have this issue, and the owner has no idea".. I bet there are plenty.. JW JW
Using JW's numbers for a 500 hp engine and 6.5 lb/gal for gasoline (don't know the actual number, but think it's close) that is 2.5 pints in 30 sec. and that's needed @ ~4.7 psi fuel pressure and not with an open fuel line! I learned "pint in 30 sec" too, but that is with a little more stock-type engine, me thinks. As for fuel line size, it's all in the pump you use. More pressure=smaller fuel line requirement, all other things being equal. However, if you're using a mechanical pump only, where the pump is sucking through most of the fuel-line, then the line diameter becomes much more important. By the way, necking down at the tank from 1/2" to the stock 3/8 pickup is better than just running all the way to the carb with 3/8". It isn't better than using 1/2" in the tank too, but you get the idea. It's has to do with "line-loss" or "pipe friction" as it is called. Some of the physics of pumping liquids tend to defy horse-sense. If anybody thinks I'm full of B.S. then ask away and I'll do my best to explain further.
not having a dyno, cant you tell by looking at the plugs after a run to see if you are running lean???
You prolly told me already Jim, but will I need to upgrade the fuel lines on my GS? Right now it's 100% stock, which I gather is 3/8's line. I replaced the long supply line with a repo from Year One. I have to replace the rubber hoses @ the tank still since they are brittle. I think you had told me that the Stg1 pump I have won't cut it. I would rather use a mechanical pump so when you dyno it , can you find a pump for me that will do the job? Thanks, Brian
Good point, Jim. I'll bet the inertia created bt a rapidly accellerating car also tends to reduce pump capacity by forcing fuel back to the tank as well as toward the rear of the carb. :Smarty:
I think that perfectly explains why my car had the 60' and 1/8 mile times of a quicker car when I was using the Stage 1 mech. and stock fuel lines, but yet performed much more dismally OVERALL. ou: Sure did make a BIG difference with the new fuel system!:Smarty: Almost like adding NITROUS!
This topic has inspired me to ask a burning question, the opposite of the discussion topic: Why does mine run excessively rich at higher speeds (to the point of making black smoke)? Let me explain a little, I noticed this on the dyno as soon as it got up around 5,000 RPM's. I have a mild cam, stiffer springs and stock dual carters jetted as stock. I was running 93 octane and I used the thicker head gaskets, timing set to specs. On the first run, the black smoke literally billowed out of the right rear tailpipe. On the following runs, it was not as bad, but still was choking us. This concerns me.o No:
Here are a couple quick pics of the fuel system.. I will lay the whole deal out shortly on the TSP site.. with pricing.. just got to find the time.. turning wrenchs and pulling the trigger on the paint guns on these cars 80+ hours a week does not leave me a lot of time to update that site.. but it's "getting up there" on my priority list.. JW This would be the modified pickup..
The pump, with the 1/2 line exiting in the stock steel line location, and the stainless going up to the carb.
And the line we use for a Holley carb.. certainly could do it cheaper, but I prefer this type of line setup.. Most of these parts (except for the pump and pickup) are available from Jegs, although we do use Areoquip industrial stuff for the 1/2 hose, due to the fact it's a bit tougher and has a higher temp rating than some of the performance stuff they have. Beware that putting the ends in the push lock -8 hose can be quite a treat.. I bought a $400 tool to do it from Areoquip, since I put all the lines together, so it's easier for you guys just to "bolt in and go". Ok, I guess that is enough of the infomercial for tonight, but hopefully you guys can get some ideas for your system from the pics. We have the HT kit available now, and will be putting the ragtop kit together here shortly. Price is yet to be determined, I just have to get the time to sit down and figure it all out.. in the $650-$750 range for the whole deal from tank to carb with a Holley.. Q-jet somewhat less due to the difference in the engine lines. JW
Jim, It looks like the fuel line "downstream" of the pump is -8 and "upstream" is -6. At what HP level do you need to make the line on the pressure side of your pump -8?
I wanted to add what I did here for reference. I have the RobbMc 550HP mechanical pump and I did not want to use a pusher pump. RobbMc recommends 1/2" fuel line for anything over 450HP without a pusher pump. He also says to use a 100 micron pre-filter. I did some searching and 100 micron 1/2" filters are tough to find. I used a NAPA #3299 temporarily until I get the RobbMc pre-filter. I modified my pickup like JW with a difference. The fuel gauge sender is attached to the 3/8" pickup line and I didn't want to deal with having to reattach it so I left that alone and drilled out the 1/4" return instead of the 3/8" like he did. I drilled to 5/8" which is the OD of the 1/2" soft copper tubing I used. I silver brazed the copper to the steel and used soft solder(plumbing) for the other copper to copper connections. Maybe you can use regular plumbing solder for copper to steel, I never tried. I cut the sock off and if I ever need a return line I will use the old 3/8" pickup tube. You have to use something flexible at the end of the copper so you can bend it when it and the float goes into the tank so I added a short piece of the socketless tubing. Still, it was a very tight fit getting the sender into the hole. I didn't want to reduce the size of any part of this less than the ID of the hose so I chose to slip the hose over the copper tubing. Getting the 1/2" ID Aeroquip socketless hose over the 5/8" copper took some learning and some work. I found that if you boil the hose in water, you can slip it over something with a 5/8" OD and it will stretch and remain that way after it cools. If you try this with something hollow, like pipe, the hose has a tendency to cut or rip. I used a 5/8" swaging tool. It took about 3 tries in the hot water to get the depth I was looking for. Then I boiled the swaging tool with the hose on it for 2 minutes just for good measure. I used a fish tape to pull the hose through the frame in two stages. The first stage was from the rear to the break in the factory lines and the second was from there to the pump. I decided to locate the filter up front so it would be easy to service when I have the front of the car up for oil changes. Like JW, I removed the forward portion of the factory lines to make room. The fitting that threads into the fuel pump has an ID quite a bit less than 1/2". I drilled it to 27/64. Much more and I think it would be too weak. Being right at the pump, I don't think there would ever be a problem with it being too small. I shouldn't have to worry about running the Holley dry anymore.
Andy, I'm going to be doing something similar, although I'm using braided house a'la the Summit Racing kits that they sell. I'm having a friend modify a single-line pickup that I bought from Rock Auto to use a 1/2" diameter tube. He's done the same thing with his Chevy II that runs in the 10's on nitrous. Although he continues to use his 3/8" pickup and then runs both the 1/2" and 3/8" into a y-block and then a single line out. He's also running an electric fuel pump back near the tank too and I'm going to run the RobbMc 1100hp mechanical. No pusher for me either. So what does your system look like after it exits the pump? I've got an expensive Russell 150 degree swivel coming out of the pump that will lead into more braided line into the filter and then into a Mallory dead-head regulater with two outlets - one for line into the Holley. I also bought a new gas tank for this too, so everything involving fuel will be new. My original plan was to have the new tank sumped, but as long as it feeds the motor, I like the idea of having it all tucked away out of sight.
Gee, just hours after my post, I see RobbMc is taking care of the pickup/feed tube issue: http://www.robbmcperformance.com/products/sendingunit.html The price is more than modifying one yourself, but if you can't (or don't have someone who can) modify a factory pickup - it seems like a great deal.
Steve i just did the same thing to my sender i drilled out the 3/8 line used a 8an 90* bulkhead fitting to a flare nut 1/2 copper tubing worked great i'll post pic's tomorrow.
Bigger mechanical fuel pump: http://www.robbmcperformance.com/products/buick550.html Much bigger mechanical fuel pump: http://www.robbmcperformance.com/products/buick1100.html Bigger sending unit with 1/2" pickup and return: http://www.robbmcperformance.com/products/sendingunit.html
