Thermostat design differences

lidox19

lidox19

True Classic
I just bought a new old stock Wahler WTH181f 75degC thermostat. When comparing it with a Quinten Hazel QTH181 I have I noticed a few interesting design differences that may influence coolant flow.
Most noticeable was the difference in the size of the opening that the coolant flows through when fully open.
The QH181 is on the left and WTH181 on the right. 28mm vs 32mm.
The direction of flow is shown by the blue arrow. Note the lip arrowed in orange on the QTH that is not present on the WTH.
Any thoughts?

Thermostat flow.jpg
 
Integrale said:
I think your blue arrow is backward.
Click to expand...
As seen here (red box):
Thermostat flow.jpg



Regardless, I see your point about the potential flow capability. The diameter of the opening could make a significant difference. However I think there are other factors to consider. For example how far the diaphragm opens will also have a big influence on the actual size of the opening. And how much flow/volume needs to pass it; if the smaller opening is larger than the actual demand, then a bigger one won't really make a lot of difference (although I suspect this is not the case).

I really like Wahler T-stats. To me they are among the best. Just looking at those two, the Wahler appears to be a much better quality item in several ways.

Some VW's use the same style of T-stat and they are available in different versions from some manufacturers. A "high flow" model for example, with similar features as you point out. Some also have a narrow three-leg design instead of the fatter single hoop over the top (example below). While the VW application is slightly smaller diameter than the Fiat, it will work if a new rubber seal (O-ring/grommet) is used. I'd have to look up the part numbers, but that might be what you have. Is the OD the same on the two, and does it match the housing opening perfectly?

2034-160_1.jpg
 
Intergrale,
This is the coolant flow diagram from a fiat 128 stat housing that is the same as the X1/9 except the angle of the outlet to the radiator (7).
When fully open the coolant comes in from the rad through 3 past the thermostat 2 and out to the pump through 4. The stat is inverted with regard to my picture in the first post.


Coolant flow.jpg
 
lidox19 said:
This is the coolant flow diagram from a fiat 128 stat housing
Click to expand...
And that is correct, which is why I was thinking the one you have was intended for a different model of car. But I just did a quick Google search of the part number and it does come up as a X1/9 application. Interesting markings, but you were right. ;)
 
lidox19 said:
I just bought a new old stock Wahler WTH181f 75degC thermostat. When comparing it with a Quinten Hazel QTH181 I have I noticed a few interesting design differences that may influence coolant flow.
Most noticeable was the difference in the size of the opening that the coolant flows through when fully open.
The QH181 is on the left and WTH181 on the right. 28mm vs 32mm.
The direction of flow is shown by the blue arrow. Note the lip arrowed in orange on the QTH that is not present on the WTH.
Any thoughts?

View attachment 29301
Click to expand...

The stat on the right looks more like the OEM Savara stat.
 
After playing with T/stats endlessly some years back when trying to track down high cooling system temps, my experience was that it didn't really matter what brand / design was used (assuming you are using the correct temp range). I did end up putting back the OEM design (much more expensive than SuperStant, which was actually fine).

What mattered far more was correct water pump impeller-to-housing spec and water pump speed. In addition, ignition timing at idle will make a significant difference, along with correct fueling of course. This is all assuming also that you have a completely clear radiator & unrestricted coolant pipes, no leaks, etc..
 
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