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Author Topic: 8V71T advice  (Read 2203 times)
RickB
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81 MCI 9 smooth side 8V71 Allison 754




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« on: February 18, 2012, 07:30:11 AM »

Clifford and gang,

Some things never change... Like the sun rises in the east and my never-ending search for a turbocharged motor for my bus. So, I found an 8V71T for a really good price that according to the owner was rebuilt with the last 5-7k miles. No documents because he did the rebuild a good number of years back and has rebuilt basically every piece of equipment that his old road construction company so obviously I will have to be diligent in determining whether it is really rebuilt or not. (think every Church bus in existence on ebay since the beginning of time... ALL FRESH REBUILDS! but no documentation)

The PO used it in a lowboy for transferring road equipment within a 30 mile radius of his house so there's a good chance he's being honest. My motor, although it starts in a millisecond, has begun to show some wear, blue smoke after air building idle for a half mile but once that clears she runs awesome. But that smoke and oil usage is evidence she's not getting any younger.

So, here's the catch, he says they used the lower compression two piece pistons when they rebuilt this 18 to 1 instead of 22 to 1 and so this motor tends to let out some blue smoke at startup that clears up after warmup. He said it doesn't leak, doesn't use oil and that it has brown tag 65 injectors. So, is that true of the lower compression pistons, that they smoke a bit at start up but that is not a sign of wear? Also, I'm thinking around 325 HP is that realistic? The recent posts about the 50 DN's make me a little warmer to the idea of running a belt driven alternator which would mean no bell housing change hopefully.
Remember I have an 8v71 in my bus now so mounts, bell housings, are all at my disposal. The motor hasn't been run in 5 years and I don't want a rerun of the 8v92 I bought a few years back so I would have my mechanic pull the injectors, make sure they are good and probably put some lubricant in the cylinders to prevent the ring damage that I had with the 8v92.

I'm reluctant to post here again because frankly I got sick of the flamethrowers and politics/religion crap and I won't be here long if that stuff starts again. It seems I'm not alone in believing that a computer and an opinion should not be used to insult people personally.

So, with that in mind,  I'm not asking whether or not it's a good idea to repower my bus. Just looking for some advice if I choose to make this move.







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bevans6
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« Reply #1 on: February 18, 2012, 07:51:06 AM »

Well, the normal pistons in a turbocharged 8V71T are 17:1.  8V71N pistons are normally 18.7:1.  I don't know of any 8v71's that ever had 22:1, which don't mean they didn't...   Wink.  I attached links to the old DD sales brochures below.  Brown tag N65 injectors are not particularly normal in an on-the-road 8V71T, but they are on the list in the document, which gives power curves for 65, 70 and 75 injectors.  Basically the injector sets the maximum power and the curve, the more fuel you put in, the more power, in layman's terms.

Smoke is smoke, and dark smoke on startup could be indicative of a worn engine in my book.  My 8v71T with N80 injectors and 17:1 compression smokes for about two revolutions on cold startup, it's so clean you can't tell it's running by looking at the exhaust.  The question for me is how worn, is the smoke oil or fuel, is the starting aid set right, is the rack run right, is the buffer set right, is the gap set right...  All those things can have an impact as it starts to life.  Turbo engines have a starting aid that is supposed to limit the amount of fuel injected while cranking to let the lower compression start easier (I think that's how it works, anyway).  If it is set wrong for too much fuel, it could make smoke on startup.

Take a close look at the rear of the engine and how the accessories are mounted - air compressor, power steering pump, alternator.  the MCI engine is pretty specific in how those things have to work, so check if there is a power steering pump drive in the left hand (looking at it from the back) position.  If there isn't the rear engine plate has to come off to install one.  At the front, look at how the front cover and engine mount works.  the MCI cradle uses a big round boss for the front engine mount so you may have to swap the front cover.  If you have to swap the thermostat housings to make the coolant plumbing match the bus then the front engine plate has to come off for the left hand thermostat housing.

Check out my project page on converting an 8V71T to an MCI bus engine:  http://www.busconversions.com/bbs/index.php?topic=21836.0

Here are the links to the sales documents:  http://www.powerlinecomponents.com/literature/detroit_diesel/brochures/8v71na_automotive.pdf
http://www.powerlinecomponents.com/literature/detroit_diesel/brochures/8v71t_automotive.pdf
http://www.powerlinecomponents.com/literature/detroit_diesel/ddc_brochures.htm

Brian
« Last Edit: February 18, 2012, 08:02:14 AM by bevans6 » Logged

1980 MCI MC-5C, 8V-71T from a M-110 self propelled howitzer
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Oonrahnjay
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« Reply #2 on: February 18, 2012, 08:06:37 AM »

Rick, I can't offer much because I have absolutely no experience with the DD 2-strokes, but in general, we're in the same position -- we have aging engines that aren't pushing us into an "emergency -- gotta do something immediately" mode but will need investment in the medium-term future. 

Just working on the principle that this is basically a hobby, I'm looking to use the time as my engine ages to arrange to repower my bus (see the separate thread) with the best engine I can find at the lowest cost.  For me (and probably for most of us), any cost difference between an engine that will run fine for 100K miles versus a 300K one is just money lost.  But we can't be buying someone elses problems, especially when we have time to look, compare and consider.  Something about an engine that's "'really good shape' but I can explain the symptom that might make you think it's not", just don't give me the warm and fuzzies. 

Can't tell you anything about how much the swap you're talking about would be worth to you versus what it will cost, but something about this tells me that it might be a good idea to *very carefully* investigate this engine instead of passing on it and waiting to see what the Universe sends you in the future.
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Bruce H; Wallace (near Wilmington) NC
1976 Daimler (British) Double-Decker Bus; 34' long
6-cyl, 4-stroke, Leyland O-680 engine

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« Reply #3 on: February 18, 2012, 08:11:20 AM »

Rick, hold out for a 8v92TA better setup for you set it at 404 hp no heating problems and better fuel mileage than a 8v71T and IMO  a better engine

good luck
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RickB
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« Reply #4 on: February 18, 2012, 08:21:46 AM »

Brian,

Very informative post thanks. With my current setup 8V71NA and the 60 injectors I'd be surprised if my HP rating is above 275. They seemed to adhere to the "better safe than sorry" ideology when they put these in buses with cooling issues. I was surprised by the actual compression ratio's I thought they were higher than that. But the fact that they guy I was talking to was uninformed as well makes me pretty gun-shy about this. He's supposed to send me some pics today so I can see if the bell housings are the same minus the alternator. I am still using my bus air so I'm wondering if I will be able to find an alternator that would run my AC that is belt driven.
Oonrahnjay,
The cost of this motor as it stands is less than two tankfuls of diesel in my bus or I wouldn't even be looking at it. The 10k rebuilt motors are not on my radar as I would rather have fuel than a motor anyway.

Clifford, I had the 8v92 of my dreams 6 stinkin' miles from my house and I blew it! They were asking 10k for a 1984 MCI (with a documented remanufactured reliabilt with 60k miles on it) and I thought they were firm in price. Well, I was wrong, they sold the entire bus for 3k. Oh and just to add insult to injury it had a rebuilt 740 from Allison with it as well. Still kicking myself over that one. All the mounts, right bell housing, everything was literally right there. So, do you agree that blue smoke at startup is always a sign of wear?

thanks for the input guys I appreciate it

Rick
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Oonrahnjay
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« Reply #5 on: February 18, 2012, 08:26:37 AM »

(snip)  Oonrahnjay,   The cost of this motor as it stands is less than two tankfuls of diesel in my bus or I wouldn't even be looking at it. The 10k rebuilt motors are not on my radar as I would rather have fuel than a motor anyway.   (snip)   

Yep, that's a big factor.  If I had access to a suitable engine for my bus for that money, I'd be considering it strongly too.  Good luck with how this works out for you.
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Bruce H; Wallace (near Wilmington) NC
1976 Daimler (British) Double-Decker Bus; 34' long
6-cyl, 4-stroke, Leyland O-680 engine

(New Email -- brucebearnc@ (theGoogle gmail place) .com)
luvrbus
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« Reply #6 on: February 18, 2012, 08:42:11 AM »

Rick blue smoke is not necessary a sign of wear could be a small problem or a major lol,I don't have any idea what compression ratio the guy had he could have had the 22:1 those are getting hard to find now days but 23:1 is common in a 53 series I always use 18:1 or 19:1 in the 8v92TA engines never broke a crank yet lol go for you a 8v92 TA low horsepower no comparison a 8v92 set on a low 365 hp to a 71 series pumped up to 365 hp just my way

good luck
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RickB
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81 MCI 9 smooth side 8V71 Allison 754




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« Reply #7 on: February 18, 2012, 08:47:11 AM »

Well pictures really do tell it all. That thing was a rusty boat anchor. So glad I didn't drive all the way down there to see it.

Keeping my eyes open for one though.

Looks like another summer of avoiding the rockies for this guy. I don't think I'll ever get used to folks flying by at 70 mph when I doing 20-25 tops with two kids in the bus. We almost got plowed into twice last time we did any mountain climbing that is one of the main reasons I'd like to make a change. Just pure safety.

Rick

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bevans6
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« Reply #8 on: February 18, 2012, 08:57:31 AM »

Too bad you aren't closer to Artvonne - he seems to have a 8V91 and nothing to do with it...   Grin

Brian
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1980 MCI MC-5C, 8V-71T from a M-110 self propelled howitzer
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Cary and Don
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« Reply #9 on: February 18, 2012, 09:23:33 AM »

Is there any way to tell what the compression ratio, 17:1, 18:1, 19:1, is without tearing into the engine?  What is the difference in the engines that have the small inspection holes compared to the large inspection holes?

Don and Cary
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luvrbus
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« Reply #10 on: February 18, 2012, 09:34:09 AM »

The large inspection covers are for the older model dry 8v71 the small inspection for later models 8v71 which have water around the dry bore where you install the liners DD changed in 70's because of the older models breaking liners.

They do have a gauge to check the dome on the piston for the compression ratio that you use through the inspection ports or you can do it with a ruler pretty easy to tell if you know what you are looking at

good luck
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