TR2/3/3A Water Pump Housing Bypass

[Tropical TR]

This topic has been discussed in some way many times and there are different opinions about the bypass with a sleeveless thermostat: to block or not to block, partially block, or it doesn't matter

However, I understand at TR4A time the Triumph engineers opted for an un-sleeved thermostat with a reduced bypass passage in the housing. This means a constant, although reduced, flow of coolant via the bypass, which some TR3 folks approximate using the pipe plug with a drilled hole.

If the TR engineers found that appropriate for TR4A, why would that not be right for TR3?

If one accepts the need for such a fix with a plug, would the drilled hole size equal to the TR4A bypass not be best? Does anyone know the TR4A bypass hole size?

I know discussion of this topic can get "heated".

 

[Geo Hahn]

I used a 3/16" hole on both my TRs - I believe that is smaller than the reduced openings in the later thermostat housings.

You (like me) live in a place where we are not often desperate to get the heater quickly working on a cold morning. If you were using the car as a daily driver in Indiana (I have) then getting the coolant heated up might be more important that optimizing cooling efficiency on a hot day.

For many years I drove with that hose totally blocked (a short piece of broomstick) and it was fine, but adding something that allows some flow seemed prudent.

 

[CJD]

As long as you understand the purpose of the bypass, you can do what you want with it. It allows circulation through a cold engine that would otherwise have none when the thermostat is closed. The reason for cold circulation is to allow the engine to warm up more evenly, provide heat to the cabin as soon as it is available (vs. when the thermostat opens), and prevent cavitation of the pump vanes. Cavitation tends to wear the metal of the vanes.

If you don't rev a cold engine and don't care about immediate cabin heat, then you can completely block the bypass with no ill side effects. Adding a restricted bypass accomplishes the same thing as the original design, except the larger the restriction, the more you are decreasing the engine's hot cooling efficiency.

Note: from what I have seen of the currently available "shroud" type thermostats, I am convinced you get a good bit of bypass all the time anyway. The sleeve is not that tight in it's bore, so you know there has to be a constant flow to the bypass hose anyway. If that's the case, then totally blocking the bypass in a hot location would likely improve your cooling over the factory set-up.

 

[TR3driver]

The factory "restriction" is much larger than that, roughly 5/16 by 5/8.

IMO it's important to always have _some_ flow even with the engine cold, as otherwise you can develop localized hot spots around the exhaust valves that can lead to cracking the cylinder head. So to test the extreme case of no bypass flow at all, I drilled a 5/16" hole through the thermostat backing plate. It did lengthen warm up time (as expected), but had no noticeable effect on my overheating problem. Once I discovered that the radiator was the source of my problem and had it recored, I went back to leaving the bypass fully open (with a TR3 housing and the larger passage as on the left above); which works fine for me.

 

[Tropical TR]

Thanks. I understand and agree with all of you, and would not completely block the bypass. But in summary it is reasonable to say that the less bypass restriction (or larger hole) there is, the more compromised the cooling can be because more water is being diverted from the radiator. Yes?

The TR engineers must have thought the smaller unrestricted square bypass passage was the best compromise in the engine's final evolution.

 

[Geo Hahn]

...it is reasonable to say that the less bypass restriction (or larger hole) there is, the more compromised the cooling can be because more water is being diverted from the radiator...

That is my understanding. Recall that the engineers were building cars for all climates and FL, SoCal, AZ, etc allow us to consider some things that would be less than practical elsewhere.

 

[TR3driver]

But in summary it is reasonable to say that the less bypass restriction (or larger hole) there is, the more compromised the cooling can be because more water is being diverted from the radiator. Yes?

Well, sorta. The question is just how much "compromise" we are talking about. Slowing down the coolant movement through the radiator only reduces overall cooling if there is enough air flow that the coolant exiting the radiator is significantly cooler than what is entering it. If leaving the bypass fully open only reduces cooling by .01%, is that enough to worry about?

To put it another way, a chain is only as strong as the weakest link. In my 40 years experience with TRs, I've not seen any indication that coolant flow through the radiator is the weakest link. Maybe it is for some people/cars, but not for me and mine.

The sleeved thermostat had its own litany of problems, including being very slow to open and a tendency to close due to increased coolant flow at high rpm. And as John said, it never fully blocked the bypass, just restricted it a bit.

 

[Geo Hahn]

...I've not seen any indication that coolant flow through the radiator is the weakest link...

Me neither, in fact I suspect air flow is more of a factor. I've added an air dam, have the shroud in place and run a tropical fan all for that reason. But I also used the partial by-pass block just in case it helps. As a famous physicist may or may not have said 'It is my understanding that it works even if I don't believe it'.

 

[CJD]

I've found cooling issues to always be an accumulation of small (and large) details. Like:

Leaves and bugs blocking the fins.
Air not ducted properly to the radiator.
Blocked cooling tubes...either by dirt or by previous shop.
Scale in the engine and radiator.
Suction hoses sucking down under power.
excessive/improper bypass
fan and fan belt.
Efficiency of the radiator design (like a crank hole that blocks 15% of the coolant tubes?!?)

If you are having problems with overheating, then all the above need to be addressed. I do agree that airflow appears to be a biggy with the TR's...but all of these matter. If you don't have a problem, then you can afford to let 1 or 2 slip a bit, like Standard did with the tiny, partially blocked radiator!

 

[Tropical TR]

Thanks for the good discussion, gentlemen. I drove my car from northern to central Virginia into 15 year storage in 1998, and it was boiling over when I got it there. As this full restoration is finally nearing completion, everything that can contribute to overheating has been rebuilt or replaced, including a re-cored original radiator and a Macy fan. What remains when the time comes is proper setup and tuning.

Using a modern thermostat, I just want to be on the safe side concerning the bypass. If all goes well, I won't have to worry about boil-overs again...even in the Florida summers. Your comments, as always, are very much appreciated!

Regards,

 

[karls59tr]

Me neither, in fact I suspect air flow is more of a factor. I've added an air dam, have the shroud in place and run a tropical fan all for that reason. But I also used the partial by-pass block just in case it helps. As a famous physicist may or may not have said 'It is my understanding that it works even if I don't believe it'.

What is your air dam made of and where does it bolt on to? I was thinking of fabricating one myself.

 

[Geo Hahn]

Pretty simple, just a piece of plywood:

Uses existing bolts (I think they are the ones for the crank guide braces):

I make a couple of spares while I have the saw set up as they also make a nice curb feeler for concrete parking lot stops.