.....15 years ago I rebuilt a 225 cu in slant 6. The bores were enlarged and I installed larger pistons. As soon as I rebuilt the motor I started to have an over heating problem. I had to keep the radiator very clean to keep the car drivable. Recently I rebuilt the motor again, and the bores were enlarged more. The over heating problem got worse. I installed a new, larger radiator and that helped. However, it finally occurred to me that when the bores are enlarged, the motor is putting out a little more power and heat at any given rpm, however the water pump is not going any faster.
.....My question is: are different sized water pump pulleys available for the slant 6 motor? A smaller pulley would make the water pump go faster at any given rpm.
.....I would appreciate any replies, thanks.

turning the water pump to fast can cause the coolant to move to fast through the engine which can result in a hot engine. your problem seems like to much timing or a lean carb? what are you running?

The factory only offered one size (diameter) of water pump pulley.

I have run some extra large bore Slants and never had overheating problems with those engines. (3.465 on up to 3.504)

When rebuilding a SL6 engine, I find it important to get the water jackets completely clean so I use muriatic acid (swimming pool acid) to remove every bit of scale and get the passages spotlessly clean.
Shops that "hot tank" or bake-out the blocks will usually leave crap in the passages. Once running, any left over junk will quickly clog a radiator.
DD

Thanks for the 2 replies.

Over the years I have tried changing the ignition timing and the valve timing. I think at present I have them about at factory specs. During the recent rebuild I cleaned the water passages out with muriatic acid. There was still some crap left on the bottom surface of the water passages after using the acid but not on the sides. Over the years the problem of overheating has bedeviled me, and I have tried about everything, I think. There is no obvious manner to set the carburetor richer. I think the idle adjust screw doesn't have much effect at highway speeds. I did try making the mixture richer by restricting the air intake by taping up the outside of the air cleaner.

I installed a new, larger radiator and that definitely helps. Also it helps noticeably to have the heater working. I imagine that pumping a little more water through the radiator would have the same effect as having the heater going. The new radiator I installed is actually for a 69 or later Dart (couldn't find a new one for a '65). It has a threaded opening on the top tank which has a plug in it. I don't know what that opening is supposed to be for, but I was thinking of buying a 12 volt pump and using it to pump water from one of the heater hoses into the radiator using the currently plugged hole.

As far as my theory about the motor with bigger bores producing more heat at a given rpm, I'm sure that is right, but I suppose it would not make enough difference to notice. In the example given above a little arithmetic shows that the volume of the new larger bores would only be about 2 % more. I don't know what the size of the bores in my motor are, but the machine shop said they could only be enlarged one more time.

Thanks again, and if you have any more thoughts I would be interested to hear them.

The heater core is basiclly another radiator the reason it runs cooler with it on is there is more air running over more area of cooling fins. I don't think more water flow is going to have the effect your looking for but if it works let us know.

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Even in moderation i am excessive!

There is a thermal switch that is used for the EGR system on newer models that goes there. It allows vacuum to pass once the engine warms up, thus allowing the EGR to function.

Everything below here is opinion, and I'm no expert here.

I don't think your going to solve your overheat problem by pumping water through there. Where heat transfer and fluid flow go, there needs to be a minimum flow to avoid a laminar condition, but extemely turbulent flow resulting from excessive flow rates will create the "Hot Spots" Doug mentioned. Some engines seem to have characteristic overheating problems, while others don't.

I have a theory on this that has to do with water passage variations in the manufacturing process and/or modification by those who have touched the engine at whatever point in time.
Most of the heat that we need to shed is at the head, or at the top of the bores. Below the combustion area, the amount of heat present isn't going to amount to much. The bottom end is cooled by the engine oil.

Getting the jackets as cleaned up as possible is the first step. The next would be to verify that the passages in the head are clean, and of the correct size. Too small/too large/plugged up, can have serious effects.

I have two engines that have never had overheat issues. Even standing on them chasing crazy old drag racers across mountains and deserts at insane speeds.

Perhaps those that have had issues can investigate what's going on with coolant passages on their heads, and we can compare notes. I'll be pulling down last year's motor for a bump in compression, and perhaps a long rod bit of foolishness. I'll look everything over while I'm there.

CJ

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trying to restrict the air intake to richen the fuel mixture will make the engine work harder and hotter, you have to think of what is restricting the engine and causing the over heating problem. check your timing marks and total advance with mechanical and vacuum, is the cooling problem at idle, cruise or under load?

I doute your block can only be bored one more time. The machine shop is probably used to only boring engines to .060 thou and the slant six can be bored to around .100, and sometimes even more. So your is probably at .040 thou now.


It really sounds to me like you need to put some bigger jets in your carb now. The changes in todays fuel can cause a engine to run lean, plus your engine is a little bigger and may even have better flowing exhaust which will also demand more fuel.


Jess

Thanks for the suggestions and comments.

With the newer and bigger radiator I put in, the over heating problem is only on a hot day at highway speeds.

It seems my theory about bigger bores having a noticeable effect is not correct, but here is another theory of mine. Please shoot it down, if it seems impossible.

The water pump that is sold for my '65 Dart with a 225 cu in motor is the same pump sold for all model cars of whatever year that have a slant 6 motor, either 225 cu in or 170 cu in. Possibly the water passages were changed in the slant six motor over the years. Possibly the water pump for my motor was designed to pump more water than the pumps used in some of the other slant six motors. If that were the case, it could explain the matter. I'm sure a put in a new water pump when I rebuilt the motor the first time. At the present time, there are various brands of water pumps available for my motor, but they are of only two different designs. One design has an impeller with 6 blades and the other design has an impeller with 8 blades. I'm not sure, but I believe that when I replaced a 6 bladed pump with an 8 bladed pump it helped. Both the 6 bladed and the 8 bladed designs are distinctly different from the original water pump as shown in the factory service manual. The original pump had a bowl shaped disc with round holes in it attached to the back of the blades of the impeller. It's hard to see from the picture how many blades the impeller was made of, but perhaps 12. Maybe that original water pump was more efficient in pumping water through the cooling system.

What temp thermostat are you running?

Are you running antifreeze at the right mixture?

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Aggressive Ted

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74 Swinger, 9.5 comp 254/.435 lift cam, 904, ram air, electric fans, 2.5" HP2 & FM70 ex, 1920 Holley#56jet, 2.76 8 3/4 Sure-Grip, 26" tires, 25+MPG

Uhm may sound daft but have a look at your lower radiator hose does it still have the steel/stainless steel spring - Dan tells me newer hoses does not have this - it must be there otherwise the hose gets collapsed by the suction of the pump if it not there make one from stainless wire - the spring must have the same diameter as the id of the water hose

Caused me to recon a radiator without need mine was rusted and collapsed

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Regards,
Fanie Gerber
It's never junk, it's just a part you're not currently using

http://www.valiant50.co.za
Just say I own a few Mopars

Theoretically, the only reason why there'd be enough negative pressure at the lower hose to collapse it is if the radiator's overly restrictive of coolant flow, either because it is clogged or because it is underspec.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Dan I Agree

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Regards,
Fanie Gerber
It's never junk, it's just a part you're not currently using

http://www.valiant50.co.za
Just say I own a few Mopars