Im putting edelbrock alum heads on my 340. The mopar perf. engine book recommends I use the 8 bladed water pump for better cooling with aluminum heads. This doesnt make sense to me. Wont I run cooler with the aluminum heads to begin with?? I dont want to waste the horse power if I dont have to. My stock 340 with out the below hi po parts ran on the cool side.
Thanks.

Engine specs.
340
TRW 10.5:1 .030 over pistons
Edel. alum heads, open chamber 65cc
air gap intake
TTI headers
Comp cam XE285HL 241/247 duration .545 lift
3 core radiator

On a big block, the 6 blade is a Hemi impeller and the 8 blade is for the V8's with a/c if that helps? I think sometimes on a bigger HP motor, they want to slow down the flow with less blades on the impeller to give the cooling system a chance to work?

The 8 bladed impeller is for better cooling. Thats why they are on the A/C cars. The prob. is they also rob horsepower because theya re pushing more water. My question is, is anyone running alum. heads on a small block with a 6 blade water pump with no over heat issues. Ive never run alum. heads before. Again, the mopar perf. engine book recommends the 8 blade with alum heads for better cooling.

Originally the 8-vane pump was for non-A/C cars with the water pump driven at near (.95) engine speed, the 6-vane pumps are for A/C cars that have the pump/fan driven at 140% of engine speed. The extra fan speed on the A/C cars was to improve cooling at idle and could cause an 8-vane pump to cavitate at higher rpm's.

The Hemi also got overdrive pulleys in '69 and used the 6-vane pump from then on.

A non-A/C car with .95 pulley ratio should use the 8-vane pump.

John is right on, as usual. As time has gone on, most of the later Mopars (small blocks) all went to the smaller pump and the overdriven (1.3-1.4) pulleys to help keep them cool at lower speeds with mechanical fans. If you have overheating at idle, it is the way to go.

As far as water flow through the engine goes, they are designed to flow the same, although I would expect the 8 blade to do a touch better at low rpm and the 6 blade at high rpm, by a small amount.

One thing to check for sure is that the pump is not using a GM or other aftermarket impeller in it, regardless of the # of blades. The impeller diameters are listed in some of the repair manuals, so it is fairly easy to check.

I run a Flow Kooler water pump on my 340, it has 6 blades with a anti-cavation plate riveted on the impeller. It's supposed to flow 35% more coolant at lower rpms. I have a 2 core aluminum radiator with a viscosus fan, it runs 165* on a 160* thermostat.

The pump I bought was a MoparPerf. 6 blade. I think I will rivet the flow kooler disc on for good measure. If all fails, I will take the 6 blade pump off and get an 8 blade.

Here are the original, factory, impeller sizes from my 1970 factory service manual. As I said earlier, you may find that the impellers being used are not Mopar sizes.

small block:

without air--4.38" dia.--8 blade--.95 drive ratio

with air--3.70" dia.--6 blade--1.30 drive ratio

big block:

without air--4.38 dia.--8 blade--.95 drive ratio

with air--3.50" dia.--6 blade--1.40 drive ratio

If your impellers vary from this, you will have to take a best guess at the right drive ratio.