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Old 05-09-2019, 06:11 AM   #1
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DD Compression check

The following is from a site for boats that use DD engines.

The "quick" compression check is primarily for DD , but the concept is universal.

https://www.samsmarine.com/forums/sh...pression-check.
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Old 05-09-2019, 10:03 AM   #2
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Detroit's, being 2 stroke, are completely different animals than any other diesel. Even "cold" (ambient temperature over 40 degrees F) if they take a full second to light off you probably have compression problems.

They are so much easier to start than a 4 stroke they are not comparable
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Old 05-10-2019, 06:41 AM   #3
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"Even "cold" (ambient temperature over 40 degrees F) if they take a full second to light off you probably have compression problems."

Down near freezing most 2 stroke DD require a cranking technique , an alternate fuel (ether) or an air or block heater to light off.

The heavy engine construction absorbs much of the heat of compression when cranking , so compression ignition is difficult.

As a bus converter I know of lots of folks that needed to use a BBQ heaped with coals for a couple of hours under the engine to get underway every AM if below zero F. Block heaters all night work best.

Many bus companies like Greyhound , just idled them all night in the cold, as did OTR truckers.
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Old 05-10-2019, 08:33 AM   #4
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" They are so much easier to start than a 4 stroke they are not comparable "

I'm surprised you say that. Four stroke Cummins and cats I have experienced fire right up. At least here in FL.

IIRC there were some Volvos in the past that shut off fuel for some short period to warm cylinders before lighting off.
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Old 05-10-2019, 07:55 PM   #5
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Try Detroits.

I won't speak for super cold weather, I avoid that at all costs. But in temperate climates mine start in half the time of any 4 stroke diesel I've ever been around.
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Old 05-10-2019, 09:34 PM   #6
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Mine crank every time, I have two 6-71s in my big boat and one 6-53 in my pushboat, if you get any smoke on start up you have compression problems
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Old 05-11-2019, 03:08 AM   #7
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I've been running DDs, mostly 71s, since the early 1960s. In weather above freezing they start fine unless needing a rebuild. But in cold weather block heaters do make for easier starts and speeds up warming. In trials the engines last longer and get better mileage if allowed to warm completely before put under load.
In my current boat, IL671s, the engines were well past needing a rebuild, but started fine with a block heater in below zero F weather. Now with a good rebuild I just touch the starter and they usually fire with first pistons reaching TDC.
I like the reliability of a mechanical engine with no electronics.
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Old 05-11-2019, 07:34 AM   #8
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One thing to consider is that the built-in compression ratio on marine 71's varies between models. I think 19:1 for non-turbo, and 17:1 for turbo. So the turbo ones tend to be harder to start and make more fuel smoke at a higher ambient temp.

Second that about block heaters. The are really beneficial on these two-strokes. Without them the breeze can go through the open cylinders and cause condensation and rust on cylinder liners. Don't need arctic wattage (1000-1500W), even 250W will do, just need engine above outside dewpoint temp. 10F warmer will do the trick.
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