Difference between Detroit Diesel and EMD?

Hello! I am new to the forum and joined to ask a question about engines. It seems to me from an outside observer that EMD engines and Detroit engines are almost identical, except for size. I know EMD engines have a fabricated block and Detroit engines are cast, but they are both airbox two-stroke engines made by the same company. Are there any other differences or similarities between the two?

Biggest difference that comes to mind is EMDs have individual ‘power packs’ with cylinder head, liner, piston, etc that can be replaced easily and quickly on each cylinder.

So functionally one is designed for quick and easy field maintenance on individual cylinders. The block can be repaired using field tools rather than swinging the engine or slapping Belzona on it and hoping. That’s what comes to mind to me.


Heritage, lineage, history.

EMD came out of the Winton engine line that became Cleveland and are medium speed engines. Detroit Diesel came out of the General Motors line of small high speed diesels.


I was going to say RPM but Steamer beat me to it. When someone mentions Detroit Diesel my brain automatically recalls the 71 series & the old 645 for the EMD. But both EMD & DD have differences within the same brand depending on what model of EMD or DD that you are asking about. Another thing would be the purpose for which DDs & EMDs are normally used. Unless you work on a large vessel or rig you won’t see an EMD used as an auxiliary generator or an 71 series DD used as main propulsion unless it is a smaller vessel. Lastly, ones green & the other is grey but they both leak like a son a bitch after a decade or two.

Handy feature of EMD which Detroit 71 does not share is the ‘snifter’ valve, each cylinder has one, and when opened, the cylinder looses compression. Barring over made easier and more precise.

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Worked alongside my chiefs on the rare occasion to replace a pack in an EMD. I wouldn’t call it easy, but much easier than the latter. Had 16’s and 20’s. Detroit 71 series were our generators. And yes, the EMD’s made a mess leaking oil, but hard to kill. I always took it easy on the throttles when I could on the EMD’s. Chiefs were quite alright with that.


Geez, you guys are making me feel old … haven’t you guys had the joy of working on the old Cleveland 268s and 278s?


Not on purpose Steamer. All I could do to remember . All I did was help lift and hand him tools. My chiefs were dear to me and had a hard job. I’m not driving when we are broke down. Now that’s common sense, to help your crew. Some may debate that until the cows come home. “Ain’t my job”. Fuck em. Let’s get her rolling again. We will figure out the rest later.

I worked on an old YOG converted to a processor with an old 8-278A. Hercules DC gensets.

For those who are interested. The grandfather of the EMD engine who so happened to be a one time marine engineer just like his father.


Other than both Detroit’s and EMD’s are diesels which by their very nature have similar components (pistons, bearings, crankshafts, etc), and were once both divisions or a subsidiary of the same parent company…there aren’t much in the way of similarities.

parts may not fit but supercharged 2 strokes puts them in a unique group
( along with ship engines)
Will any engine ever have a production time span and long as a Detroit or EMD?
Both still being made ( I think Detroits to order?)
Love to see their common rail cam injection go full electronic

The intake & exhaust flow is opposite between the two. Detroit has the supercharger and intake in the valley and conventional exhaust manifolds, EMD has a scavenging airbox and ‘hot V’, the exhaust manifold is in the valley.

Emergency stop on the Detroit is a flapper that shuts off the air into the supercharger, EMD has a cam down each bank that disables the injectors.

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Thank you all for the responses! Interesting, I thought Detroits scavenged as well. Is that way Detroits can use a regular turbo whereas EMD has to have the clutch driven turbo?

I guess I worded that carelessly. The flow is the same, just a different layout. Both use airbox bottom cylinder port intake scavenging and exhaust valves but the Detroit exhaust goes outboard of the engine into manifolds like a car, EMDs go into the V.

Some EMDs use a gear drive turbo that overruns the clutch at about 75% load and becomes a true turbocharger, some are roots supercharged. All Detroits are roots supercharged in the valley but some have a turbo in series too, compound boost for extra power.

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Not really in series, the DDs use a bypass system that unloads the blower to reduce the power it consumes.

It is the same concept as the EMDs, beyond a certain power level (around 65 percent) there is sufficient exhaust gas mass and temperature to reduce the mechanical load imposed by the blower (DD) or drive the compressor (EMD).

Interesting! Out of curiosity, why did EMD choose to go the gear driven turbo route, rather than having the compound boost setup? It seems the compound boost would be a simpler design.

I think the compound boost is more complicated, especially with the size of an EMD. Detroits just have a turbo pretty much slapped on.

I thought the bypass blower was a later option on the Detroits and not on all models. It would definitely increase efficiency though.

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Simpler to mount roots blowers in the same space as the turbo? I suppose it might reduce the size of the turbo but more moving parts, etc. It’s pretty simple as is, engine driven blower until mass flow is sufficient to run as a turbo and parasitic load of the engine driving it drop off. Really an elegant solution.

And a story just to contradict myself - in one of my attempted shoreside jobs a long time ago the ME/NA company I worked for competed for and was given a MARAD grant for an R&D project to try using more efficient free running turbos on an EMD. A PTO was used to drive a hydraulic pump which ran a hydraulically driven blower for low load operation. Questions were did using BBC come on line sooner and remain effective in a larger section of the load range. Would depend on the vessel service of course too. The test case was a tug on the rivers somewhere I can’t remember. Must have been running hooked up for long periods. I can’t remember the results and can’t find any MARAD R&D reports on their website. Obviously it never came to anything but would be interesting to read over the results. Those were the days when MARAD actually funded R&D.