Fuel Delivery

Facts & Figures for Mechanical Pumps, Filters, & Lines

There are three main components needed for a basic fuel system. However, getting facts about the fuel pumps seems to be the biggest difficulty. The other two items, lines and filters are not quite as bad in this respect. Ideally, a system should have a regulator as well. However, many mechanical pumps have sufficient control built into the diaphram spring. If this cutoff pressure is below what the carburator's inlet can handle, all is well and the additional regulator can generally be dispensed with.

ON THIS PAGE:
Pump Performance
Pump Fit Issues
Lines and Fuel Pick Up

Pump Ratings & Performance

An excellent solution can be had be switching to an electric fuel pump located near the fuel tank. However, because I am a little paranoid of electric pumps in an collision, and because of the extra load they place on the electrical system, it has been my preference to stick with a mechanical pump.

Below is a list of the mechanical pumps available for the Chrysler LA 273, 318, 340 and 360 cid engines. The advertised numbers provide little insight because what is unstated is the test conditions they were made at. As a rule of thumb, the pressure given is the cutoff or maximum pressure the pump will produce. Maximum pressure occurs when the flow out is zero, so its only useful in deciding if a regulator is needed. The flow advertised is ususally the maximum. Maximum flow only occurs if the pump was emptying into an open bucket and had no resistance to drawing on its supply. What we need is to know if there is sufficient fuel delivery through the rpm range the engine is going to operate with a realistic pressure or resistance in the inlet and outlet fuel lines. Fortunately, Holley is now showing such information, in addition we sent three pumps to Ryan Brown for testing.

Fuel Pumps for Chrysler LA engines

Brand & Model	Part Number	Number of Valves	Stated Press. Limit	Tested Press. Limit	Advert. Free Flow (gph)	Max Flow (gph) @ 4.5psi	Measured Maximum Flow @4.5	Port size	Weight
Holley
chrome	12-838	2	7.5		80		failed	1/4 npt	2# 4.5 oz
80 gph	n/a		7.5		80 @2000	50 @ 2000
110 gph	12-360-11		6.5-8		110 @5000	70 @ 7000		1/4 npt	1# 8.2 oz

Carter
stock			5-7		15 @ 500
Super Street	M6902	3	6		120			1/4 npt
Competion	M6270	3	7.5	7.5	80		70 @ 6000	1/4 npt	1# 9 oz
Mopar Perf
is a Carter M6270	p4007040		7.5		80
	p4876074
	p4529368	6	9.5	7	90		71 @ 6000		3# 7.5 oz
with Edelb	bottom plate							3/8 npt	3# 9 oz

Pressure Limit = Cutoff or Maximum Pressure, zero flow.
Advert. Free Flow is the advertised maximum which is free flow, or in the case of the stock pump simply the shop manual specification which is not maximum because it is only at 500 rpm. (Flow will go up with rpm until the internal design restrictions choke it off.)
The Holley Chrome pump was not new. Ryan Brown could not get it to work well enough to produce consistant numbers.

Maximum flow rate at 4.5 psi on the outlet seems to be a good number. How comparable Holley's test procedure is with Ryan Brown's is hard to know, but they both provide a useful comparisons. For a truck or street machine, having maximum flow at 7000 rpm is rather pointless, and what would be more important is to see if the flow was sufficient in the engine's working range. One way to know how much fuel is needed is to calculate the horsepower of the engine through its rpm range. Then apply the formula for fuel required based on hourse power.
1lbs/hr of fuel required = horsepower /2
gal/hr needed = (lbs/hr /6)
Finally, before making a final decision, use a good safety factor because fuel consumption could be higher than estimated or the resistance in the flow might be higher than estimated resulting in less fuel actually making to the carb bowls than estimated. This is even more true during extended high G accelerations such as as drag strip launches.

Max Required is based on horsepower for a 340 bored .060 with Competion Cam's "Magnum 280" cam.
Holley Data from Holley Website product info. They also have good pump tech.
Carter and Mopar Performance pump data from testing at Ryan Brown Racing. The MP pump was new while Carter pump was not.
Interesting to note that the Holley 80 peaks before or at 2000 rpm. (This model is not avail for Chrysler small blocks.)

Clearly the MP pump would be the choice if lots of flow was needed. Also it would better handle a restrictive fuel system and it had a reasonable cutoff pressure that would not require a regulator if used with a Holley carburator. However, it has drawbacks in weight and in fit. The latter is described below. For this reason, we are going to try a Holley 110 pump which has an advertised curve that appears smoother than the tested curve for the Carter 6270. Ryan mentioned that the new Holley Kieth Dorton series pumps he has tested are generally pretty good. Anyone dependent on a mechanical pump for racing should consider sending their pumps for testing. Only drawback is the delay in packages going across the US/Canadian border.

Fit Issues with the Mopar Performance 6 valve pump (p4529638)

Interference problems occur using the six valve Mopar Performance fuel pump and water pumps with passenger side inlet. Note that the inlet sweeps back, but it still clearly possible to fabricate a solution to avoid contact and not restrict flow. The casting nub just barely touches the top of the fuel pump housing and can be ground back to clear.

The other problem is that the inlets and outlets are 180 degrees opposite each other. When the inlet faces toward the fuel line from the tank, the outlet faces right into the coolant hose. However, the Edelbrock bottom plate offers a solution using gentle 90degree tube fittings. In the photograph the alternative inlets and outlets are shown. The ones not used can be simply plugged.

Lines

Many of the a-bodies used 5/16" fuel line. A good way to minimize the resistance in the fuel lines is to size up. Getting good 3/8" diameter replacement lines is not quite as simple as we'ld all like. First, the lines need to be made in such a way that the bends do not pinch or get oval and reduce the cross sectional area. Second, the clips and fittings need to be changed as well. Third, and perhaps most important, there is not a good source for 3/8" fuel tank pick up for these cars. The pickup is probably the most critical place to increase the fuel line diameter. For the new fuel line, three brands were tried. Only on the third try was the product satisfactory. Inline tube provided a line that was specifically for the '67 and had very little loss of diameter in the bends, the others both had noticible distortion in the bends, and one did not fit the '67 frame in the wheelwell. The first was returned so I don't know if it would have fit.

Serious high horsepower or high G cars should also consider going to braided aircraft line. At least AN-6 will be needed, and many prefer -8. This gets into a whole new area of hooking up AN hose and fittings to a stock fuel tank, or getting a fuel cell.

To convert the fuel sending unit required a little work. At least in a '67 A-body Plymouth, the original pick up is a different design than the usual replacement units. As best possible, the replacement unit was made more like the '67 when the 3/8 tube was installed. The sending unit was disassembled to get it as far from the soldering as possible. The it was reassembled
and riveted to a bracket.

Detail of modified unit. Sender housing barely fits into tank hole when attached as shown. It had to be squeezed down a little in a vise, and the back plate of same filed down to match.

Home:'67 Barracuda..one curve to another

More Tech:
Improving Forward Lighting & Electricals
Ignition Wire Comparison
Handling Theory for late-'60s to mid-'70s Mopars

Disclaimer:
If you take advice, or copy something I have done, you are on your own. I'm just trying to be helpful. You need to make your own decisions, and know about or discover the risks before barging ahead on any endevour.
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