Towing Mods: how hot can we safely get a D24T?

[v8volvo]

tomorrow morning I'll be shooting for 4000rpm in 1st gear with light throttle if I can manage it safely traffic wise, although I probably have enough power and cooling to do the climb in 2nd.

1st gear is pretty short. I'll wager you will do OK in 2nd or perhaps even 3rd on flatter stretches depending on how it seems to do and how warm it is outside. I don't know whether going up to really high sustained engine speeds will help or be detrimental. I think the reason somewhat elevated revs help is that you can keep the car moving while keeping the load on the motor light, and simultaneously spinning the water pump and fan at a good rate of speed -- I think you achieve those things satisfactorily by about 3000 rpm. Going further beyond that I think the goal is vehicle speed more than cooling effect, assuming you can do it without making boost and EGT rise significantly. My non-scientific sense is that around 3300-3500 is a good spot, or at least that seems to be where mine was happy; much above that, it gets difficult to keep boost down and also you're getting into an inefficient combustion range where to my mind it seems possible that heat rejection per unit of power produced might be greater than it is in the midrange. That's a conjecture, though; I'll be interested to hear your results if you try spinning it faster.

If you see this soon enough, what are your thoughts on the altitude compensation system on the injector pump? I'll be going high enough to trigger it, and it's operational still. If advanced timing with a hot engine is a problem should I bypass it? My timing is set to the factory 0.85mm.

Dunno, I generally run mine with the wires jumpered at the barometric switch to keep the system from engaging, but I also run more static timing than you do (~0.95mm). I don't think we have a way of quantifying how much advance that system adds in terms of mm of plunger travel at TDC; however, I got to be present while a D24T VE pump was being run on a calibration bench recently and watch the various timing advance mechanisms in action, and we concluded that the altitude compensation solenoid adds about a degree at low rpm and a degree and a half in the upper end. Hard to reach any concrete conclusions without seeing a lot of engineering data about how combustion temps and actual start of ignition are affected in this engine by the combined factors of altitude, ambient temp, engine temp, and start-of-injection timing, but my tentative hypothesis would be that at the timing setting you are running and the way you're planning to drive it, it won't make much difference whether it's engaged or not. In high-load situations where the duration of the injection events is relatively long, insufficient timing advance can significantly increase EGT, so if you were going to be hammering it up the hill then you'd probably want to be sure it was connected and working, but since you're not planning to be leadfooting it that effect probably will not be a major factor. I would probably leave it connected and see what happens when you cross 3000' or wherever it kicks off -- maybe make a little jumper wire so you can disable it quickly by the roadside if you don't like what it does?

Makes us begin to appreciate the challenge that Bosch/VAG had in tuning these mechanical VE pumps to perform under varying conditions... why the TDI was such a huge step forward. Everyone that builds "m-TDIs" with mechanical pumps, I can never understand -- I guess they assume the direct injection is the main advantage they have, but I think their electronic feedback control is at least as important in making them run as well as they do.

Good luck! Objectively speaking attempting this with a 30yo car known for cooling system trouble and fragile cylinder head is nuts -- hopefully you're well aware of that -- but it sounds like you could succeed in making it work out. Let us know how it goes.