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Author Topic: 6v92 vs 8v71  (Read 9792 times)
mccarlk
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1967 mci 5 8v71 auto




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« on: August 04, 2008, 08:17:35 PM »

hi i have both engines for my mc5a, what are some of your pros and cons , i dont care much about the differance in fuel, i just want the best performance, thanks   oh ya the 871 is a 1980 model,and the 692 is a 1978 model if that matters
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NJT5047
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« Reply #1 on: August 04, 2008, 08:33:07 PM »

All things being equal, the 6V92TA wins hands down over an 8V71N.   Now it the 8V71 has a turbo....that's going to equalize things.  If no turbo, no comparison. 
Go with the 6V92TA.  It's a fundamentally better engine design. 
I'd go with a later 6V92TA.    HP could range into 350 HP.  Also common in 270, 300, and 330.
Pay attention.  There were 6V53s out there too.  You DoN'T want one of those.
The early 6V92s had less than the best blocks castings.   Find an engine from an 80 model or later.  Avoid the 'Alpine green' blocks and look for a silver...or purple!  Purple was an in-house NJT rebuild.  The NJT folk did good work. 
JR
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JR Lynch , Charlotte, NC
87 MC9, 6V92TA DDEC, HT748R ATEC

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TomC
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« Reply #2 on: August 04, 2008, 09:50:01 PM »

Here's the difference- the 8V-71 is 568cu in.  The 6V-92TA is 552cu in-so that 16 cu in isn't going to make a squat of difference with the two set up the same.  For instance a 6V-92 with 90 injectors will be out performed by a 8V-71 with 70 injectors (both turbocharged).
Personally-if you're going to keep the manual transmission, keep with the 8V-71-it is more reliable and you don't have to worry about the cylinder liners leaking since the 71 series has dry liners.
If you're going to install a HT740, then go with the 6V-92TA (because of length space on a 5).  With the up graded injectors, you can get with 9G00 (100) injectors 370hp and 1050lb/ft torque.  But with the 8V-71 with 90 injectors you can get 450hp and 1350lb/ft torque.  I have a turbocharged and air to air intercooled 8V-71 in my transit with sedate 9G75 injectors for 375hp and 1125lb/ft torque and love the performance.  I get 5-6.5 mpg.  Good Luck, TomC
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Tom & Donna Christman. '77 AMGeneral 10240B; 8V-71TATAIC V730.
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« Reply #3 on: August 05, 2008, 08:28:39 AM »

Keep in mind that we're talking about an MC5.   I'll wager that trying to cool a big humpin' engine in an MC5 is going to end up costing more (and require more engineering)  than the engine swap.  I won't wager much....just a little bit.  Smiley
HP equals heat.   Heat dissapation costs big $$$$$$$$$$$     Wink
Another thingy is that there are very few 8V71Ts around.  Probably have to build one from an "N" model....$$$$$$$$$$
Tom, for a reference point, advise "MC" how much you paid to have your 8V71 turbo'ed?   Roll Eyes
If looking for a high horsepower engine, an 8V92TA also has similar dimensions compared to an 8V71.
Couldn't cool it either though.  Not without re-engineering the coach cooling system.   
Changing out to a 6V92TA will require cooling improvements too...just not as radical. 
What's in your wallet?     
JR
 


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JR Lynch , Charlotte, NC
87 MC9, 6V92TA DDEC, HT748R ATEC

"Every government interference in the economy consists of giving an unearned benefit, extorted by force, to some men at the expense of others.Ē

Ayn Rand
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« Reply #4 on: August 05, 2008, 11:12:47 AM »

The turbo job including the enlarged radiator, custom made air to air intercooler, bigger muffler and air cleaner, then on the engine itself, bigger injectors, bypass blower valve, custom exhaust to facilitate the turbo, new oil pump- $12,000.00.  Compare that to an engine swap, and I think you'll see it isn't that bad.  Throw in a complete over haul and you'll still be under $20,000.  I priced out changing my engine/transmission to a Detroit series 50 with the HD4060-changing the bus from a V drive to a straight drive since my model of bus came either way, and I was looking at around $35,000.  Even with the increase of fuel mileage from 5-6.5 to 8-10, I couldn't justify it, so the turboing job was next best.  Good Luck, TomC
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Tom & Donna Christman. '77 AMGeneral 10240B; 8V-71TATAIC V730.
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« Reply #5 on: August 05, 2008, 02:07:14 PM »

Performance wise, they are about dead even.  Otherwise, each type has its advantages and disadvantages.  I think the V8 sounds better than the V6, but that is an abstract.  Good luck.  Smiley Smiley Smiley
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mccarlk
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« Reply #6 on: August 05, 2008, 02:57:33 PM »

thanks for the info, what engine will climb the mountains better?
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Barn Owl
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« Reply #7 on: August 05, 2008, 03:11:58 PM »

I have put several hundred miles in a GM 4106 with both motors. I own one with an 8V71 and my fathers has the 6V92. Hands down, I like the 6V92 better in all aspects. I also like the sound of the six better than the eight but that is personal preference (My fatherís has a smaller muffler so it sounds sweet, and the sound of the turbo spooling up is just plain cool). However, the difference could be in the health of the engines but I donít have another 8V71 to compare mine. I also think the turbo might have a lot to do with the performance especially when climbing hills and the lack of black smoke. I have N70s and my father has 90s or 95s I donít quite remember. I used to be a die-hard traditionalist for the 8v71, but not anymore. I might add that in addition to out performing mine, he gets better fuel mileage.
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« Reply #8 on: August 05, 2008, 03:24:00 PM »

Good question as to which mill would climb hills better.  My best guess here is, that in a Bus Conversion, the 6V92 would have a slight advantage.  Too many different variables, like trannys, gearing, engine settings, etc..  Smiley Smiley Smiley
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JackConrad
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« Reply #9 on: August 05, 2008, 03:28:07 PM »

thanks for the info, what engine will climb the mountains better?


At higher elevations, definately the 6V92 compared to a non turbo 8V71  Jack
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makemineatwostroke
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« Reply #10 on: August 05, 2008, 03:42:02 PM »

FWIW the torque on a 6v92 is 957 lbs and on a 8v71N 318 hp it is 818 lbs but the 6v92 has a faster torque rise and with a 8v71 TA set at 350 hp it will be 1038 lbs
« Last Edit: August 05, 2008, 03:57:45 PM by makemineatwostroke » Logged
Lin
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« Reply #11 on: August 05, 2008, 05:17:24 PM »

It seems from most of the posts, the raw engines are going to be pretty similar.  It all depends on turbos, injectors, and cooling.  If that is the case, what are the specs on the two engines you have?  As I understand the posting, you already have the engines and were not necessarily looking to buy another.  Further, if you are looking to use the engines as they are without putting much work into them, you might want to get them tested to see which comes out on top.
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mccarlk
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1967 mci 5 8v71 auto




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« Reply #12 on: August 05, 2008, 06:36:25 PM »

the 692 has the 90 injectors, and the 871 has the 65s, as for cooling i dont mind upgrading to have a turbo motor, seems that it will climb the hills better, the coach is only 35 ft so im hoping for fairly good performance, i will be rebuiding what ever engine i use, that way i will be starting fresh, the trans is already rebuilt,
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« Reply #13 on: August 05, 2008, 07:56:41 PM »

Well. If you're going to rebuild your next engine anyway, and don't mind spending a few thousands of bucks, the least expensive way would be to turbo your 8V71N.  The injectors pose no issues...replace them with whatever you want.  The end result would be good. 
But, upgrading the cooling system will be necessary. 
An uphill race between an 8V71N v 6V92TA would favor the the 6V92TA considerably. 
However, a turbo'ed 8V71 will easily compete against a 6V92TA. 
Still, 6V92TAs are everywhere...and not too expensive.  An 8V71T will have to be created.  They are rare.  The military uses them...and you may be able to find a new or reman surplus military engine.   They turn up on the 'net often.   
At altitudes, a turbo engine is a definite plus.   
Have you considered an 8V92TA?  If you don't mind upgrading the cooling system, an 8V92TA would be a huge improvement in performance and durability.  Build yourself a veritable "sports" bus.  A relaxed 400 HP.   
An 8V92TA would be almost a bolt-in as far as dimensions go.  Exhaust plumbing mods will be necessary. 
I'm not familiar with the engine room of an MC5, but you may wish to verify that you have overhead space for the blower mounted turbo on these engines?  They are pretty tall.
Good luck!   JR


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JR Lynch , Charlotte, NC
87 MC9, 6V92TA DDEC, HT748R ATEC

"Every government interference in the economy consists of giving an unearned benefit, extorted by force, to some men at the expense of others.Ē

Ayn Rand
edroelle
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« Reply #14 on: August 06, 2008, 06:01:54 AM »

I would choose a turbo engine, for mountain performance.  If you want an automatic, the 740 is strong and can fit behind a 6V92 in an MCI 5.  You must go to a small, weaker Allison transmission 64X behind an 8V71 in an MCI 5.

Ed Roelle
Flint, MI
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