It was I that told Scott there was a market for the smaller pump pulley; when he asked me how small I told him 4.9" to 5” diameter, i.e. enough to overdrive the pump by 10%. 10% was not a number I pulled out of the air, it was based upon the amount Ford engineers over-drove the pumps installed in the 351C powered Fords and Mercurys. So as promised here’s the information I have to offer on the subject.
Historically speaking overheating during stop and go driving is the cooling system problem that plagued the Pantera, especially the Pre-L versions. The overheating problem was more or less resolved when DeTomaso/Ford replaced the vertical radiator tank baffle with the horizontal baffle in April 1973. The 1974 L model Panteras could idle all day long without overheating, with the air conditioning operating, on a warm day. Most owners would agree however the Pantera's cooling system performance during stop and go driving is perhaps best described as "fragile". I do not advocate extensive modification of the cooling system; I do however advocate a direct approach that identifies the cooling system’s primary deficiencies (if any) and corrects them.
I have come to believe there are 3 primary deficiencies in the Pantera’s cooling system:
•Coolant leakage around the OEM radiator’s vertical baffle allowed coolant to bypass the radiator.
• The Pantera’s cooling system has never had a functional vent system for automatically removing air trapped in the radiator. When air is trapped in the radiator it reduces the radiator’s effective surface area and in extreme cases impedes coolant flow.
•The coolant flow rate is insufficient at low rpm as a result of the coolant pump being under-driven and/or the coolant system plumbing being restrictive.
DeTomaso/Ford resolved the first deficiency in April 1973 by modifying the radiator, replacing the vertical baffle with a horizontal baffle. The second deficiency can be resolved by installing a drain cock at the top of one of the radiator tanks and manually bleeding air from time to time, or by performing the vent system modification I have detailed elsewhere.
What I intend to do is explain the factors that lead me to believe the third condition is a primary deficiency. Bear with me, as most of you already know I can be a bit verbose at times.
Fords and Mercurys powered by the 351C were not plagued by the same cooling system problems as the Pantera. The Fords and Mercurys which were contemporary with the Pantera were not equipped with modern high flow rate style cooling systems, their cooling systems have more in common with the Pantera’s cooling system than not. Examining the differences between the Ford/Mercury cooling systems and the Panteras cooling system may help identify the deficiencies in the Pantera’s cooling system.
One difference between the cooling system of the Pantera and other 351C powered vehicles is the diameter of the pulleys which rotate the coolant pump.
There were at least two different diameter crankshaft pulleys (basic part number 6312) used on 351C powered Fords and Mercurys for driving the coolant pump; one was 6-5/8” in diameter and the other was 6-3/4” in diameter. The variance in circumference is 20.81” to 21.20”.
Although there was several coolant pump pulleys (basic part number 8509) used on 351C powered Fords and Mercurys the pulleys can be lumped into two general categories: the smaller ones used for cars equipped with air conditioning and the larger ones for cars without air conditioning.
Most of the smaller coolant pump pulleys vary in diameter between 5-15/16” to 6-1/8”. The variance in circumference is 18.6” to 19.2”.
Most of the larger coolant pump pulleys vary in diameter between 6-7/8” to 7”. The variance in circumference is 21.6” to 22”.
In conclusion, the coolant pumps in air conditioned Fords and Mercurys powered by the 351C were over-driven by approximately 10% whereas the coolant pumps in 351C powered Fords and Mercurys without air conditioning were under-driven by approximately 4% or less.
The Pantera was equipped with a 5-1/2” diameter crankshaft pulley (17.3” in circumference) and a 6” diameter coolant pump pulley (18.8” in circumference). Therefore the Pantera’s coolant pump was under-driven by 8%. I am not sure if the coolant pump pulleys were the same diameter in all Panteras, the pulley I measured came off a 1974 Pantera-L.
The Flow Kooler coolant pump has a unique impeller design they claim improves the low rpm pumping rate of their pumps. Pantera owners who have installed the Flow Kooler pump report improved stop and go cooling system performance as a result. Does the improvement noted by owners indicate the Pantera’s coolant pump may be under-driven a bit too much? I thought so.
A quote from the DeTomaso Macchine da Corsa book, page 102, from an interview with former group 4 Pantera driver Marcel Schaub: “Bertocchi also installed a smaller pulley on the water pump, which helped increase the pump speed and lowered the water temperature. I raced chassis 2873 for ten years from 1974 to 1984. I won three Italian and one European championship.” Guarino Bertocchi believed the Pantera needed a smaller coolant pump pulley, at least for racing.
Another difference in the cooling system between the Pantera and other 351C powered vehicles is the diameter of the plumbing between the radiator and the engine. When a valve is partially closed on the inlet or outlet of a centrifugal water pump it reduces the pumping rate of the pump at any given speed, therefore significant restrictions in coolant system plumbing should have the same effect upon a centrifugal coolant pump. The Fords and Mercurys were equipped with a 1-1/2” hose between the coolant outlet and the radiator inlet; whereas the hose between the radiator outlet and the coolant pump inlet was 1-3/4”. The Pantera was equipped with 1-3/8” OD plumbing in both directions (roughly 1-1/4” ID).
The coolant exits each 351C cylinder head through a ¾” square hole; the coolant exits of both heads have a combined cross sectional area of 1.125 square inches. The cross sectional area inside the Pantera’s coolant pipes is about 1.96 square inches; so although the coolant pipe between the engine’s outlet and the radiator’s inlet is 1/8” smaller in diameter than it is in Fords and Mercurys, it seems as though it is still large enough.
The 351C coolant pump inlet has an inside diameter of about 1-1/2”, or a cross sectional area of 2.35 square inches. Fords and Mercurys connect the pump inlet to the radiator outlet via a 1-3/4” hose, whereas the Pantera uses tubing with an internal cross sectional area of 1.96 square inches. I suspect this is where a flow-rate reducing restriction occurs.
I have heard reports from a few original owners that the hose connecting the under-car plumbing to the coolant pump suction would collapse as the car sat idling. Does that corroborate my suspicion that the pump suction plumbing is too restrictive? It seemed that way to me.
John Taphorn has written of experiencing improved cooling system performance during stop and go driving after reducing the restrictions in his Pantera’s cooling system plumbing. See Pantera International magazine issue 116, POCA Profiles and elsewhere on the internet.
To summarize all that stuff, the Pantera’s coolant pump is under-driven by a greater amount than 351C powered Fords & Mercurys which had no air conditioning! Although the Pantera’s a/c condenser is not located in front of the radiator as it is in Fords and Mercurys, the a/c compressor does increase the load upon the motor when it’s operating. This would point to the need to under-drive the Pantera’s coolant pump less than non-a/c equipped Fords and Mercurys. The restriction in the coolant plumbing between the radiator outlet and the coolant pump inlet also points to the need to increase the diameter of that plumbing OR spin the coolant pump faster to compensate. That fairly well sums up all the factors and information that was on my mind when I made my recommendation to Scott.
Please note that even though the Pantera’s OEM coolant pump pulley is 6” in diameter; IPSCO chose to manufacture their standard coolant pump pulley 5-1/2” in diameter. They obviously believed there was a need to spin the pump a little faster.
If an owner has already installed a Flow Kooler or Stewart Components coolant pump I would hesitate to recommend the 4.9” pulley; for they have already accomplished the same thing accomplished by the smaller pulley.
I am curious about learning how well the Pantera’s cooling system performs during stop and go driving with the 4.9” pulley in combination with the OEM radiator fans. So if anyone goes that route I’d love to learn about your experiences.
I also plan to purchase a 4.9” pulley for 6018.