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3/8" Fuel System-how much power?
I have read a number of articles on fuel systems, and am trying to get a better feel as to when upgrades are needed (as opposed to waiting until symptoms appear).
In my case, I have a 3/8" factory style pickup with 3/8" steel line running to a factory style pump (with return) and 3/8 steel line to a qjet. This setup has worked fine on a 420hp 455. But as always, performance upgrades trickle in here and there to make more power... As a general rule, how much power should this setup support? 450? 500hp? When is a good rule to start moving to a 1/2" system? It would be good to hear what some of your experiences have been! Thanks. |
On the dragstrip (not so much on the street) I started having issues at 400 hp with the stock lines and carter mechanical. Went to 1/2 pickup and lines. Didnt fix problem... sumped tank and electric pusher fixed it. So you are already doing better than i did with stock lines and mechanical pump. Fwiw
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You are probably aware of this but the dual quad 421 super dutys ran 3/8" lines back in the day.
I know nothing about quadrajets but for AFBs and Rochester 2bbls I've found that the seats under the needles in current manufacture sets are not large enough to allow the fuel pump to keep up with the engine on tripowers or dual quads when people want to go 1/4 mile drag racing; so I drill them out larger. That is the first thing I tell people to check when they start running out of gas part way down the track. But we're talking nostalgia type 400-600hp engines here and not all out modern racing engines. |
I had/have a fuel pressure guage i can see while driving. On WOT runs i could watch the pressure go down to <1lb. Then caput... were the super duty tanks baffled or sumped? The dual quad setups would have provided a much larger reserve than a single Qjet.
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Dual quad 409s also had a 3/8 line. With my old low 12s 109 mph car I added a Carter electirc just because back then. Sucked that $0.30/gallon leaded down pretty fast!
My TA's fuel history-factory pump factory lines good for stock motor. Stock rebuild but added a bigger cam 228/228 -ran out of fuel top of second. Wired and eltric fuel pressure gauge and watched it drop.Tried different mechanicals Holley /Carter helped a little but needed a helper mini AC Delco electric. We are talking a mild low compression 400 in the 14a Then added the 420hp 455 with the Qjet- not as bad but had to jump to a 140 Mallory to the Holley mechanical. Open headers-nope had to go 1/2 from the pump to the Qjet and dropped the mechanical. Still have the 3/8 pickup no sock in the tank and it is supporting 550+ HP no problems. |
Worthy debate. i favor using 3/8" steel lines into the 11's. Seems the total suite of Bend methods, pickup, pusher pump, mechump & carb Seat(s) orifice, & float setting(s) will call successful folks into question by the many that fuel-starve.
my 12.0 car has 3/8" steel lines. My 12.2 car has not yet repeated such perf & it may be the fueldelivery leaning out. Or maybe the next run will be 11's. I dunno. yet, 1/2" aluminum fuel line seems affordable & installs real nice with flare fittings. clamps help with reliability. |
If someone is starting fresh on a fuel system, i dont know why anyone would do a 3/8" line and a stock sender unless its a correct resto. The only time this should even be a debate is if everything is already in place but then my answer would be to just run it and see. You already have everything in place. See what happens. If you dont want any problems then upgrade to 1/2" line and a good fuel pump. RobbMc makes good extended tube sending units and Tanks inc. Makes a lot of tanks setup for walbro pumps with correct baffleing.
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I ran 114mph with a 3/8 line but with a half inch pickup and a Mallory 140 pump.Replaced the 3/8 line with all 8AN and picked up a solid 2mph plus.My current with a few combo changes runs 118 plus....but...the pressure still drops to 3.5 lbs at 1000ft...Next step is sumping the tank and moving the pump.
One of the best investments in any higher hp car is a fuel pressure gauge that you can see. |
My Trans Am ran 12.09 @ 113 With the 3/8 lines and a Stock fuel pump.
The sock was removed from the pick up. My GTO ran 11.79 @ 116 with the 3/8 lines and a Holley mechanical pump. |
Drag racing with good traction will require a LOT more attention to fuel delivery than street driving.
I never had any issues anyplace on the street with my old 428 engine, Continental converter and 3.42 gears. It was fed by stock pick-up, stock lines, and Carter HP mechanical pump. At the track it would nose over right at the top of first gear every single run. That problem was solved by putting a Holley red pump in front of the tank. That set-up was good to high 12's, then fuel delivery issues cam back when I installed my first 455 engine making 455hp. I got around that deal by bypassing the mechanical pump. The car ran mid 12's for several years, but I always thought it felt a tad "soft" up near the shift point. Everyone kept saying to put a Holley on the engine because the fuel bowl in the q-jet was too small, so I did, and it ran no faster anyplace. I went ahead and drove and raced the car for a couple more years, most runs between 12.40 to 12.60 at 108-109MPH, best ever 12.37 at 109MPH. Over one Winter I sumped the tank, installed a Comp 140 pump behind the tank, and 8an lines/fittings everywhere. The very first runs the next season were 12.0's at 112MPH, no other changes! I even put the big 850 Holley back in place and it ran about the same ET and MPH as the Q-jet. A few months later we were asked to do the KRE dyno/track testing, and the car went down to 11.70's at 114-115mph. I went on to install a 455 with even more power, and the car has went 11.30's over 120mph without the first hint of fuel delivery issues anyplace. This topic gets kicked around quite a bit, and I've seen a few folks post running really quick with stock fuel delivery systems, where most folks can't get out of the 13's trying to do the same thing. The size/shape of the tank plays a role here, as with most it's pretty easy to uncover the pick-up in the tank on hard launches. I would also suppose that how the fuel line is routed, subjected to any heat, and how many bends are in it play a role as well. Of course fuel pump location, and flow rate jump in there too, and how free flowing the filter(s) are would also impact the results. I would also imagine that even when we see pretty good results with stock parts, that those folks still may not be running as quick as they could with better parts. As I found out with my own car, it ran clear down to 12.37 at 109 and pulled right up to the shift point of 5500rpm's. However, when I upgraded the fuel system, the engine pulled noticeably harder at high rpms, and the tach whizzed past 5500rpm's so fast I actually over-revved the engine on the first two track runs......Cliff |
This is interesting. I always assumed fuel delivery issues would be obvious in a carb engine with stumbling, missing, etc. Thank you all for the great info!
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I have seen a 63 Chrysler 413 Dual Quad run 10.73s with a 3/8" feed line.
BEST CASE. 3/8" tubing is based on outside dimension so best case safe wall thickness (.042") times 2 = AN ACTUAL INSIDE DIAMETER OF .291" FOR THE FUEL TO FLOW THRU. Most fuel line is ~.062" wall so that would make the fuel line real Inside Diameter .251" to flow the fuel. Even with a decent pusher pump you still have losses due to pipe wall friction. 1/2" fuel line would be .500" minus .062" = .438" best case with thin wall steel tubing I personally would trust. Most 1/2" race fuel line used is aluminum so the wall thickness is going to be slightly thicker. Aluminum Tubing can be found in these wall thickness sizes: 0.035" 0.049" 0.065" 0.083" 0.095" So just because the label says 3/8" tubing does not mean it actually has that INSIDE DIAMETER for fuel flow. Tom V. |
To add what Tom said, the length and size of the supply line will limit the output of a mechanical pump. It's simply a lot easier to "push" fuel thru a fuel line than it is to "pull" it thru. Then there are the forces generated on the fuel in the lines and tank by hard acceleration.
This is most likely why many folks, including myself operate these vehicles on the street for many years with few if any issues, the IMMEDIATELY have troubles when you get to the track and find good traction. I like to see at least 8an for supply lines to any fuel pump, 3/8" or 6AN is typically enough on the pressure side for most street set-ups. I've ran 3/8" from the regulator to the carb deep into the 11's with zero issues, and that was feeding a q-jet with one needle/seat assembly.......Cliff |
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I have a tri-power on my 440 that seems to run out of fuel near 5600 rpm, even though I have a Robbmc 550 mech pump and 1/2" tank pickup, along with 1/2 aluminum line from tank to pump. Thinking of going with a sumped tank with an electric pusher, that only comes on at WOT via a micro switch mounted at carb. But if the carb inlets are such a restriction, this might not make much of a difference :confused: |
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The Rochester carb needles and seats have a inlet inside diameter of .117" (The typical Holley Double Pumper has a .110" needle and seat in each Bowl. The HIGH PERF needles and seats are .120"). So you have a Tri Power with THREE .117" needles and seats (FIFTY PERCENT MORE FUEL AVAILABLE) vs a typical Holley High Performance 4-BBL carb. I believe you have other issues, personally. Years ago I had a TRI POWER set-up fed by both a mechanical fuel pump and a Holley Blue (noisy sucker) electric fuel pump. The fuel circuit had a by-pass circuit where the mechanical fuel pump could draw fuel from the tank without it having to pass through impeller vanes of the Holley Pump when th3e electric pump was off. The circuit also had a check valve that closed when the electric pump was on. This kept the electric pump from just sending fuel right back to the tank vs forward to the mechanical pump. The pump was triggered by a on-off switch but today I would use a NOS trigger switch and turn on a high amp relay that supplied 30 amp current to the electric fuel pump. A Holley Blue Pump would be my LAST choice for the pump. Other Modern Holley Pumps or the Carter pumps should work fine. Tom Vaught ps My fuel pick-up was extended and was closer to the rear of the tank vs in the stock location. More like 3/4 to the rear. |
I do believe that fuel inlet seat diameters become more important when the fuel delivery system is marginal for the power level and vehicle performance.
For decades now folks have bad mouthed the quadrajet for small bowl capacity. I've found that the real problem when using one is only having one needle/seat assembly, not how much fuel the bowl holds. Carburetor with larger bowls provide more reserve capacity when your fuel delivery system isn't keeping up, no doubt about that fact, but they still need to be kept full on hard runs, as the tune applied to them is based on a specific amount of fuel I in the bowl(s). Sucking the bowls completely empty produces engine shut-off, but sucking them really low changes the A/F ratio with any given tune they are using. I saw this happen many years ago with my last 455 engine making 1hp/cid. It ran 12.40-12.60 most track days with my 1977 Q-jet or 4781-2 850 Holley DP carb. With either carb the engine felt "flat" up near the shift point, but never nosed over or died out. On really hard runs, if I pushed shift points up near 6000rpms in low gear (well past peak HP for that engine) it would shut down, quickly recover, then make the rest of the run without issues. I tried .135", .140", .145" and .149" fuel inlet seats and none of them corrected the issues. Since the engine never showed any serious running issues to 5500rpms, I continued to drive and race the car for several years, and was quite successful with it. I was quite surprised (see post #10 above) at how much the car picked up in ET and MPH when I upgrade the fuel system. I also went on to test .135", .140", and .145" fuel inlet seats in my q-jet after the fuel system modifications, and they made no measureable difference in ET or MPH. So the cure to the fuel delivery issues was not going to a larger inlet seat, but putting more fuel up to the carburetor instead. Adding that to what Tom said, I am able to support over 550hp in a 3600lb car to low 11's over 120mph with one .135" inlet seat and a carb with a pretty small fuel bowl. I would suspect that having three .117" diameter seats would be PLENTY to support at least that much power, if fuel delivery to the carburetors was adequate.....FWIW.....Cliff |
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I feel it should rev past 6000 easily with the 252/262 @ .050 sft cam that I have. |
I'm not surprised that any type of fuel delivery system or type of needle/seat assembly used would pass a "static" test (engine or chassis dyno) with flying colors, even at or out past 500hp.
Do the same test at the track on a pretty hot/humid day with a 1.60 or so 60' time and see if it passes the same test? If so, fuel delivery is adequate as are the size/type of needle/seat assemblies being used......Cliff |
Some people love the disc needles and seats.
I like the normal needles and seats as you know if they actually seal the needle to the seat (using the float). Personally I think that a tank mod might be in the future. It is for me anyway. Not an ad for these guys as I don't currently own one of their tanks, (but plan on buying a tank (and straps) in the near future for my 64 GTO). http://www.tanksinc.com/index.cfm/pa...rod/prd417.htm The tank I am looking at is the new 64 GTO EFI tank with the internal baffle around the EFI Pump. I plan on buying the parts and converting my car to the in tank EFI pump vs the Noisy, Hot Running external pusher pump I currently have. EFI fuel supply is very critical. The tank people had to do their job to make the pump feed properly with efi so I think the baffle system and the feed to the baffled area must be a pretty good design. Plus the Tank holds an additional gallon of fuel vs the stock tank. I can run the car with a carb(s) and proper regulator and later just switch to the EFI Engine stuff without messing with the tank later. FYI Tom Vaught |
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