Detroit Diesel question

[Open-d]

Slight Thread Drift......
But I am really enjoying the YouTube Channel Dangar Marine from Australia.
There are several really good videos involving the rebuild of his Detroit Diesel (and assorted projects on his steel trawler)
Today's entry is the final adjustments and getting the 471 started (again) after the second rebuild.

I enjoy that too, and like to hear the mechanic talking, but the host just can't shut up long enough for the mechanic to finish the sentence, so it is very difficult to clearly hear what is being said.

 

[Mako]

I am cost conscious . Boats with Detroits seem to be less expensive.

Mopar-man Bill, I was just thinking about your opening statement. False premise. I really doubt that boats are priced lower because they have DDs in them. It's just that you are looking at more affordable boats, which are going to be older perhaps 50's-70's, and DDs were much more prevalent back then.

 

[Mako]

The DD mechanic diagnosed it as unburnt fuel "ends" due to too low exhaust temperature. He had me put all of the engine cooling water into the injection elbow to put more back pressure on the exhaust and raise the exhaust temp.

I've always heard that DDs were sensitive to back-pressure, hence the caution to carefully choose mufflers and watch out for long exhaust pipe runs. Did you notice any consequences to making that mentioned modification?

 

[moparharn]

Mako, I wold say you are mostly correct in your comment. The same boat with different engines would be worth more, but it is likely that those engines were a re-power and thus the value is more. The most likely alternative to DD in planning hulls from the 80's and early 90's would be 3208's. In my area 3208's are preferred to DD's and would command a premium, marginally. A planning TT from the 80's versus a non planning TT of the same age with Lehmans or natural Cummins would see the pricing higher on the non-planning hull all things being equal and I realize we are talking apples and oranges. So, in my search boats with DD's are less expensive and I want to know if I can make them act like a lehman or natural cummins. Can I have my cake and eat it too? I think DD's are the coolest engine ever made (next to the Hemi of course). They are also, in my opinion, loud, leaky, smokey, and getting harder to find mechanics for. I would absolutely want a 71 series DD.

Mopar-man Bill, I was just thinking about your opening statement. False premise. I really doubt that boats are priced lower because they have DDs in them. It's just that you are looking at more affordable boats, which are going to be older perhaps 50's-70's, and DDs were much more prevalent back then.

 

[JimC]

I've always heard that DDs were sensitive to back-pressure, hence the caution to carefully choose mufflers and watch out for long exhaust pipe runs. Did you notice any consequences to making that mentioned modification?

Our exhaust run is 4 in pipe with one 90 deg turn after the injection elbow and an 18 ft +/- straight run to the stern with no muffler, so minimal backpressure. Adding the extra water flow to the exhaust helped the problem and pushing the rpm up to 1200 ( now that we have better sound insulation on the cabin floor) has pretty well cured it. We haven't noticed any other consequences

 

[JimC]

Mako, I wold say you are mostly correct in your comment. The same boat with different engines would be worth more, but it is likely that those engines were a re-power and thus the value is more. The most likely alternative to DD in planning hulls from the 80's and early 90's would be 3208's. In my area 3208's are preferred to DD's and would command a premium, marginally. A planning TT from the 80's versus a non planning TT of the same age with Lehmans or natural Cummins would see the pricing higher on the non-planning hull all things being equal and I realize we are talking apples and oranges. So, in my search boats with DD's are less expensive and I want to know if I can make them act like a lehman or natural cummins. Can I have my cake and eat it too? I think DD's are the coolest engine ever made (next to the Hemi of course). They are also, in my opinion, loud, leaky, smokey, and getting harder to find mechanics for. I would absolutely want a 71 series DD.

moparharn, re your closing comments on loud, leakey, smokey and harder to find mechanics for:
Our original 6-71N was all of the above and it was a 2 valve head and a chain drive oil pump so it was likely WW2 war surplus.
The 471T is a total rebuild on a good block by a very experienced DD mechanic. It is not quiet but a lot quieter than the 6-71, it is not leaky as long as you don't overfill with oil or coolant and it doesn't smoke at all on on start up. As far as the mechanic, I agree, they are getting harder to find. We are lucky to have a very experienced one and it is a privilege to work with him.

 

[Lepke]

a chain drive oil pump so it was likely WW2 war surplus.

I don't know when it ended, but they were still selling chain drive 671s after the war. I suppose they could be engines left over from cancelled contracts when the production ended. I've found that low block engines are usually chain drive. I've got 2 1947 low block engines with chain drive oil pumps. Seems to work.

 

[JimC]

I don't know when it ended, but they were still selling chain drive 671s after the war. I suppose they could be engines left over from cancelled contracts when the production ended. I've found that low block engines are usually chain drive. I've got 2 1947 low block engines with chain drive oil pumps. Seems to work.

Lepke: My understanding is that Grey Marine typically used chain drive oil pumps in their marinization (if that is a word) of the 6-71's they produced for the war effort. I wouldn't imagine they would stop production as soon as the war ended As far as I could find out the one we had was bought new in the 1960's when a surplus lot of them apparently came on the market. If it had been looked after it would probably still be in there. It had badly warped head, which we redid but the block had helicoils in it and there was a leak of oil into the coolant. We had to pull the block at that point and decided not to put it back in. Glad yours are still going well.

 

[Open-d]

The specific fuel consumption of two stroke is higher than four stroke that wont change.

The question with buying a high output engine is how hard was it run. There must be some data somewhere showing the prop demand fuel consumption of different detroits at the same output.

It is generally not the HP you have but the HP you use that determines fuel use.

Why is that. Ive always wondered.