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

[casioqv]

Originally Posted by 745 TurboGreasel

Did you have to do much trimming on the uprights by the radiator?

I had to cut out an opening for each of the intercooler outlets- a section of sheet metal about 5"x3". My tiny cordless dremel managed in little bits over two days with long cool down periods...

The appeal to me is that this already has an engineered mounting system for the 7/9 series... although that might be overrated because I had to do a lot of cutting, bending, and trimming to make everything actually fit.

[jbg]

Hey Tyler, how're the towing modifications going? I think you're close to leaving for your trip ... if you haven't already.

[v8volvo]

These may be too late to count now, but a few suggestions in case:

- Low gear, elevated engine speed, light load, low EGT are the best recipe for keeping cylinder head temperatures under control. I drove my 745 TD over the Grapevine last July, did it around midnight, took it easy and had no problems. I found later in that same trip climbing up other passes that I could make the needle start to climb by selecting a higher gear and putting my foot down further to maintain the same speed. 3rd gear at around 3200-3500 rpm with no more than 3-4 psi of boost on the gauge kept it in check even when ambient temps were over 105F. Pushing any more than that in those circumstances led to issues.

- Cooler thermostat is a huge factor and adds greatly to thermal stability and engine's ability to exhaust heat. 80C Wahler t-stat, part # 035 121 113, is the best one and makes a major difference in engine's ability to transfer heat to the rad.

- I think your premise that the engine will not suffer as long as the coolant does not vaporize is flawed. I think devoting a lot of energy to knowing exactly how hot it the motor is and how hot it is able to get so that you can run as close to that threshold as possible is also not a great approach. For one, we don't know 100% that the head can't be damaged as long as the coolant is liquid. We do know that the head doesn't like to get much hotter than normal operating temp and that headgasket failure threshold conditions are unusually easy to achieve with this engine, relative to others. For another, your ability to really know cyl head temps is limited since you're only measuring at one (stock) or at most a few points -- temp could be higher elsewhere, up to and beyond the boiling point, even while according to your measurements you're maintaining a >5C distance from your calculated limit value. Your decisions could be affected by incomplete or inaccurate information, and something unexpected might happen at a point you thought would be safe. Also in terms of unexpected happenings, to keep the coolant liquid requires running system pressure as high as possible, which involves increased risk of a component rupturing. Combine the intention to run the motor at temperatures well in excess of target normal operating temp -- leaving a limited cushion for reaction if something happens that causes temps to suddenly rise further -- with the necessity of high pressure and its affiliated risks, and you are inviting sudden disaster in a big way.

On that point, don't forget too that even if your information is perfect in terms of accuracy and resolution (which it cannot be), when you're on the highway with traffic nearby, a situation could start to develop and your ability to respond to it might be limited by the circumstances around you. If you're going to run close to the edge and leave yourself very little time between the point when the alarm bells go off and when the situation gets out of control, then you'll be putting yourself in a position where you might have to make the decision to sacrifice the motor in order to avoid an accident, which would be a shame. In a laboratory sense it's a cool idea to push the absolute limits but your environment in this case will not be especially well controlled. I share the interest in experimentation, but if you're hoping to have the car survive this one, then rather than approaching it with the goal of running right on the line between hot and too-hot and trying to marshal sufficiently precise information to know exactly where that is, I would focus instead on trying to maximize the safety margin by aiming to keep it as cool as possible. The safety of car and passengers/other drivers alike (as well as the success of the trip, of course...) hinges on having maximum time to react to any signs of trouble and aiming to back off well *before* your situation starts edging towards the danger zone.

I usually operate with the assumption that anything outside the range of normal operating temp (i.e. exact middle on the gauge -- let's say up to 100C) is not safe for the motor. With that strategy I've driven mine on trips through the western US under many different weather and terrain conditions, usually with success -- but not always, even using this very conservative approach. The margin is pretty slim and what's only a little too warm in something else can be way too warm in one of these. I think they're completely solid as long as you keep the temps absolutely in check, but you have almost no headroom to play with. My sense is that 120C as an alarm point could be already way past the point of no return -- I'd be backing way off way before then -- but I'll be interested to hear what happens....

[casioqv]

I leave in 2 days (sunday morning). My planned car modifications are done so I'm focusing on putting brand new brakes and wheel bearings on the trailer.

Originally Posted by v8volvo

These may be too late to count now, but a few suggestions in case

Great advice, this is a lot of good stuff for me to consider, and it did come in time. I always appreciate your advice, I recently purchased a 2001 golf TDI for my wife, and reading everything you wrote in the past about the 01m was very helpful.

Originally Posted by v8volvo

Low gear, elevated engine speed, light load

The relationship between EGTs and engine temps isn't something I thought about a lot before, but I can see how that will be critical. I will definitely use the high rpm/low throttle technique. In the past I have always used full throttle up grades.

Originally Posted by v8volvo

80C Wahler t-stat, part # 035 121 113

Why is the 80C better than the 87C? Won't they both be full open all the time up a grade? I currently have a brand new 80C BEHR, and a brand new 87C Wahler but was planning to go with the 87C Wahler. Should I put the 80C BEHR in instead?

I haven't collected a lot of data, but it seems my fuel economy has been much better in the past with an 87C vs 80C thermostat.

Originally Posted by v8volvo

you're only measuring at one (stock) or at most a few points -- temp could be higher elsewhere

That is a good point. I do have a very accurate digital temp gauge mounted to the head itself, and I've found with my infared thermometer that this usually represents the highest temp I can find on the exterior of the motor. However, I suppose I shouldn't expect the hottest part of the motor to be the exterior surface!

Originally Posted by v8volvo

I usually operate with the assumption that anything outside the range of normal operating temp (i.e. exact middle on the gauge -- let's say up to 100C) is not safe for the motor

I agree with you, but in that case the D24T may not be suitable for towing at all. On other motors (such as Volvo red blocks) a good condition cooling system will never let the motor exceed 100C.

My experience with D24Ts is that they can't even get the fan clutch to engage solid until about 110C, and aren't seriously throwing off heat until close to 120C. Hopefully that will change now with my giant oil cooler, and 100% dialed in cooling system. I've done some full throttle runs up long grades without a trailer but with the A/C on in over 100F heat, and I currently can get it just to 100C but no hotter.

Before my cooling system improvements I have ran this motor to 118C without problems, but I may have been pushing my luck.

Per your advice, I will (instead of pushing the limits) go as slow as I possibly can, and stop as often as possible to let it cool. I will be going on a Sunday morning before sunrise, so I should have virtually zero traffic. Climbing at very low speeds in the rightmost lane is considered acceptable anyway on the grapevine.

I have blown head gaskets twice in the past on D24s, but both times there was a total loss of coolant, rather than simply getting warm with the coolant remaining liquid... and both motors had fiber gaskets. My current motor has an MLS gasket, and also has a giant 1.5 gallon coolant reservoir with a float sensor in the very top of the range.

[casioqv]

Here is the elevation profile for my upcoming trip:



The grapevine is steep but once I pass that it's nearly flat. Also I'll only be going north where the uphill is less steep. I'm not too worried about the downhill, as I'll go very slow at high RPM and likely won't even need to use the cars (brand new) braking system. The braking system on the trailer is also brand new and rated for about twice the weight it will be controlling... and most importantly everything has brand new high boiling point fluid.

[745 TurboGreasel]

Dont rev so slow as to bog on shifts, struggling to regain speed makes as much heat for much longer than a higher RPM pull and a smooth shift. Make the most of your inertia.
CA 17 I can pull in 5th, 3000-3400 even on a pretty hot day. If I get blocked, I have to be careful getting back up to that speed, even on a much cooler day, and ike I said earlier, as long as my EGTs are under 1200, never a problem.
My fan mostly seem s to kick in before the gauge starts to rise, I feel it about 1/8 mile(I think) into most of the steeper high speed hills here, pretty distinctly drop maybe 5HP.

[casioqv]

Almost ready...

[v8volvo]

I definitely advocate for the 80C t-stat over the 87. When combined with a healthy radiator, in my experience this seems to resolve or improve a great majority of the serious issues with the way the D24 cooling system operates in conditions of heavy loads and high temps. I'll give some of my theorized reasons for this when I have more time, but I wanted to post this while you still have time to put the t-stat in. Suffice for now to say that whatever theoretical ideas I have about this are things I have tried to come up with, after experiencing the results, to explain the *reality* that it does help, rather than the other way around.

The Behr should be equal to the Wahler in quality -- I'd put it in there and carefully make sure you get all the air bled out!

[v8volvo]

The relationship between EGTs and engine temps isn't something I thought about a lot before, but I can see how that will be critical. I will definitely use the high rpm/low throttle technique. In the past I have always used full throttle up grades.

Want to absolutely avoid full throttle and boost for sure. As 745TG said and I have also experienced, you can cruise at moderate steady-state load safely for a long time... but if you spike the load and EGT suddenly it can make a big difference in engine temp even if only for a few moments. If your strategy is high gear and full throttle up a grade then I'm not surprised you're accustomed to watching the temp gauge rise routinely. I'd aim for a balance point that lets you stay on the road but keeps gauge in the dead middle, and plan to back off as soon as it starts to rise, if you want the most conservative possible approach.

I have blown head gaskets twice in the past on D24s, but both times there was a total loss of coolant, rather than simply getting warm with the coolant remaining liquid... and both motors had fiber gaskets. My current motor has an MLS gasket

It's no problem to blow the gasket on a motor that still is full of coolant... ask me how I know. Get it hot enough where the head starts warping or the bolts stretch and you're there -- you start with a small combustion gas leak and pretty soon after that the motor suddenly shoves about 3 gallons of coolant out the top of the expansion tank -- game over. Doesn't help that the hotter the motor gets, the less ignition delay there is after start of injection, so you effectively get a little more timing advance when you're approaching the limits for heat -- increasing peak combustion chamber pressures and making gasket failure all the more likely. The MLS gasket is not indestructible either; I have had to replace them before. Doesn't change the rules in terms of what is acceptable and what is too hot, only means that it's not subject to failure just by being in the motor long enough to erode from normal use.

[casioqv]

I installed the 80C BEHR even though I can't think of a reasonable mechanism by which it could help. At this point I'd resort to anything that might give me a better chance. As an engineer I know theories are usually missing critical details about reality.

Originally Posted by v8volvo

If your strategy is high gear and full throttle up a grade then I'm not surprised you're accustomed to watching the temp gauge rise routinely.

My usual strategy is high rpms + full throttle... this is a pretty under-powered car by modern standards and that's what it takes to keep up with traffic and keep my wife from asking me to buy a faster car. I typically drive hard and keep it over 3,000rpm. My feelings are that a properly built and maintained car should easily maintain 100% engine output without overheating... and if a car can't it's cooling system needs to be significantly upgraded.

However, 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.

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.