2.0TD into 1987 VW Quantum Syncro Wagon

[Nevadan]

Originally Posted by v8volvo

Regarding your questions about coolant flow:

Here are my thoughts: the non-turbo D24 engines look the same as the back of your 2.0TD, with no outlet from the back of the head. There is an outlet from the back of the block for the heater circuit, but that circuit is designed to be able to be closed some of the time (heater output on a Volvo 240 with a D24 is controlled by a heater control valve). So, as produced from the factory, there has to be an accommodation for there being no coolant outlet from anywhere on the backside of the engine, either the head or the block, yet for the engine still to maintain temperature and balanced coolant flow. I think they therefore accomplished balanced coolant flow throughout the engine in the usual way, by using precisely calibrated, and varying, orifice sizes for the "steam holes" (i.e. coolant passages) in the headgasket.

If you examine the headgasket you'll notice that the coolant passage holes towards the front of the engine, closer to the water pump, are very small, practically pinhole size. As you work back towards the rear of the engine, you see the holes get progressively larger. The holes surrounding the rearmost cylinder, and especially behind it at the very rear of the engine, are by far the largest of all. This I think is intended to prevent the problem you describe, of the front two or three cylinders near the WP getting all the coolant flow while the rear ones get nothing. Instead it regulates the flow through the front cylinders and forces much (or presumably an equal) amount of flow to run back to the rear of the engine, through the back of the head, then back around through the head to the upper radiator hose outlet, or the internal return through the block, depending on what the thermostat is doing at that moment. I assume the exact size of these holes in the headgasket is the result of a very careful fluid flow engineering calculation and probably lots of testing. It probably also takes into account the design of the cooling and HVAC systems of the vehicle applications the engines were meant to be used in, and accommodates the possibility that a heater control valve could be closed.

I have an old D24T headgasket floating around that I'll take some pictures of later to show what I'm describing.

So, all that said, I think the question of coolant flow through the rear of the engine probably won't be a problem in this setup, regardless of how you build it. This coolant distribution question is really a challenge that has to be solved in any inline engine, and the longer the engine the greater challenge, since the water pump is way up at the front and there's a long distance to the back of the motor. But obviously there must be good and well proven solutions for it, otherwise long inline engine designs would be considered troublesome, which is the opposite of the real world situation (I-6 is dominant in every heavy duty application, etc). My belief is that the headgasket's careful design is what does it. Bottom line is I think you won't have to do anything for it to work as it should, unless you actually observe a cooling difficulty once it's in the car.

Beyond coolant circulation within the engine, though, there's one other issue that is relevant to reliable cooling performance with this design, and this is the one that has been of greater concern to me. It's the ability of the wax pellet in the thermostat to "read" the engine's temperature. I think that can be problematic due to the way this engine is configured, since the tstat is positioned at the inlet of the water pump, rather than the outlet from the head like in some designs. The WP draws coolant past the tstat from multiple sources, including the heater return. The coolant that flows past it therefore could include some coolant that has passed through a heater core and has been cooled down significantly in the process, thus causing the tstat to "think" the coolant in the engine is colder than it actually is and begin to partially close, even if the real cooling demand is still very high. One of my old goals that I have never really figured out is how to return the heater circuit somewhere else rather than the tstat housing to avoid this. But this is only a real problem in the bitterest cold weather. Not so much if the heater is not in use. Anecdotally, all the headgaskets I have ever blown in D24T engines have been while climbing mountain passes in subzero weather, which is why this issue interests me. It's a bit of a paradox that *cold* weather is a threat to effective engine cooling with this particular design, more than hot weather even. Still want to work more on it, someday.

But that's not really the question you're asking, so maybe it's a separate discussion for another place and time, rather than in your build thread...

Look forward to seeing the next phases of the project, looks like you have it moving along at an efficient pace.

Excellent summary of the cooling system on these IDI engines. Going wide open uphill when it's subzero. When I had my Volvo D24T I could go wide open uphill in the summer and it never even got hot.

Fluid flow engineering and head gasket holes answers the question for me. There's a "whitepaper" on these 4 and 5 cylinder IDI engines that says something like 40 gallons per minute gets pumped through the system (I'll find it, I think it's even on this site) so, with the somewhat larger holes near the 4th and 5th cylinder that's enough flow to move a lot of water through the system. I was thinking more in a low flow scenario, where the water's slowly pumped into the block and then goes right back out the hole in the head, and that's not going to happen. Maybe at idle but even then it's doubtful.

I'll inspect the QSW plumbing tomorrow and see exactly how it's plumbed.

Just a thought on a solution to keeping the thermostat wide open: install a pushrod in the tstat cover (on my version it's an elbow) so you could manually force the tstat to stay forced against the block.