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Maxidyne And Thermodyne

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I was wondering if you guys could explain the difference between Maxidyne and Thermodyne engines.

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There are people here who can explain this a lot better than I will, but I'll take a shot -

Thermodyne diesels were a great leap forward in dependable diesel truck engines in the 1950's and early 60's.

With minimal care, it was hard to kill a Mack Thermodyne diesel.

As the US developed more and better roads, the need for maintaining higher driving speeds, while moving heavier loads became a real consideration.

The Maxidyne engines, particularly the early engines, are basically an evolutionary development from the Thermodyne diesels. They were developed in the early 60's, and became instantly popular for their dependable and economical power. (I don't know when the Thermodyne was discontinued)

Although physically they are very similar in size, the six cylinder Maxidyne develops a lot more torque much earlier

in the power band (the factory called it "high torque rise") than the older Thermodyne.

The increase in torque, and the broader useful operating RPM range make the engines popular yet today in a wide variety of truck applications.

The Maxidynes are very popular conversion engines in older Macks, again, because they are very similar in size.

The engines are also fairly available, and parts are reasonably easy to find.

Maxidyne horsepowers range from the basic 237 hp up to 300 hp for the two valve engines.

Whereas the older Thermodyne naturally aspirated 673's topped out at 185 hp, the 711NA at 211 hp,

and the turbocharged 673's could make 250 hp.

All of the Maxidyne engines are turbocharged. Only a couple of series of Thermodynes employed turbos,

and only one of those series was really good. Turbocharging is obviously a much more efficient method for force-

feeding air to an engine, but it does place additional stress on the engine internal parts. The Maxidyne was designes for turbocharging as a critical part of it's operating system.

As time went on, a four valve per cylinder head was developed for the Maxidyne.

That's too new for my limited knowledge.

Conversions are usually pretty straight forward from a Thermodyne to a Maxidyne, with the most common

challenges that I have heard coming from differing type clutch operators, differing clutch set-ups, and water

pump lengths. Space considerations can play a role too, when you are dealing with the intercooler box and piping

for the "tip turbine" higher horsepower Maxidynes.

Some consideration needs to be given to the transmission when converting to the higher power. "Drive it like you own it" is a saying that applies here. The older triplexes, duplexes and quadraplex boxes were not rated for the high torque Maxidyne engines, and some care needs to be taken, if you are thinking of using one. They will work, but the transmission can also fail from the increased power, if not treated carefully.

Hope this explanation helps.

Paul Van Scott

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well said, my friend!..I couldn't have said it any better...not that I knew any of that in the first place. But I now have a naturally aspirated 673, and I used to drive an F-model with a 300 and a 5 speed, so I know the difference-like night and day.

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The Thermodyne diesel operating RPM range was 1500 to 2100 RPM's. If you lugged them down below 1500 in a pull, you'd melt pistons, blow head gaskets etc, hence the need for Duplex, Triplex, Quadruplex transmissions to give enough gear ratios to keep the Thermodyne engines in that narrow RPM band.

The Maxidyne engines (ENDT675, ENDT676) had an operating RPM range of 1200 to 2100 RPM, lugging them down to 1200 RPM in a pull wasn't harmful, basically due to the fact that the Maxidynes had piston coolers* (nozzles which spray oil on the undersides of the pistons) to dissipate heat, so in an "on road" situation, a 5 speed transmission was sufficient to keep within the 1200 to 2100 RPM range. For on/off road use such as dump trucks or concrete mixers, the 6 speed transmissions were used with the Maxidyne engines. The 6 speed was a 2 stick transmission, a 5 speed with an extra lo gear in the compound for off road use in severe conditions. Some early 6 speed transmissions (TRDXL1070) had an overdrive ratio in the compound which you shifted into once you were "tached out" in 5th gear, these used commensurately lower rear axle ratios compared to the other 6 speeds (TRXL107, TRXL1071), which were direct drive 1:1 ratio in high gear.

The ENDT 675 and 676 Maxidynes were the same basic engine block as the 673/711 Thermodynes and are nearly a bolt in swap for a B model, with the exception of the differences noted by Paul in the previous post.

*The ENDT 673B and ENDT673C (turbocharged 673's)also were equipped with piston cooling nozzles, even though they were Thermodyne engines.

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