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what is the defining factor for R685 R686 R688 ?

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1 hour ago, ekennedy21 said:

Different engines

HMM? well I previously owned an "89 R688 with an Econodyne 4 valve 350, and currently own an"87 R686 with the same engine and according to mack and the vin the "87 was built that way.

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68"5" was the 237hp variant of the 672 cu inch 2 valve diesel,

68"6" was the 283 hp variant of the 672 cu inch 2 valve diesel,

68"8" is a non maxidyne engine, commonly rated at 350 hp, designation used both on E6 motors and E7 motors

6"90" was the maxidyne version of the E6 and E7 motors.

The E6 uses the same 672 cu inch block as the old endt676 motors of the R685 and R686. Probably minor internal block changes typical over years of running the same engine. The E6 used 4 valves per cylinders, whereas the ENDT675&6 used 2 valves per cylinder. So they have different heads.

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2 hours ago, Prowrench said:

HMM? well I previously owned an "89 R688 with an Econodyne 4 valve 350, and currently own an"87 R686 with the same engine and according to mack and the vin the "87 was built that way.

Your 87 is a maxidyne rated at 300 hp, whereas your 89 was a 350 hp econodyne. Same engine, different fuel pump. The Maxidyne makes 90%+ torque from 1020 rpms to the governor, whereas your Econodyne didn't. The Maxidyne's powerband can't be compared to any other brand's 300hp because no one else has one that has that big of a powerband. You don't need many gears because you're always making peak torque.

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Posted (edited)
5 hours ago, Prowrench said:

HMM? well I previously owned an "89 R688 with an Econodyne 4 valve 350, and currently own an"87 R686 with the same engine and according to mack and the vin the "87 was built that way.

I believe you have incorrect information. Joe is correct, your r686 was built with a maxidyne 300. Your r688 likely had a 12speed transmission and your 686 likely has a 7 speed.

Edited by ekennedy21

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1 hour ago, ekennedy21 said:

I believe you have incorrect information. Joe is correct, your r686 was built with a maxidyne 300. Your r688 likely had a 12speed transmission and your 686 likely has a 7 speed.

Ok the 688 factory built Mack 9 speed, the 686 according to two dealers Econodyne 350 and 6 speed  not looking to argue just trying to figure out what 685 686 and 688 are but i keep receiving conflicting info and the dealer doesn't know I emailed the Mack museum and haven't heard back this curiosity has  me chasing my tail

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4 hours ago, JoeH said:

68"5" was the 237hp variant of the 672 cu inch 2 valve diesel,

68"6" was the 283 hp variant of the 672 cu inch 2 valve diesel,

68"8" is a non maxidyne engine, commonly rated at 350 hp, designation used both on E6 motors and E7 motors

6"90" was the maxidyne version of the E6 and E7 motors.

The E6 uses the same 672 cu inch block as the old endt676 motors of the R685 and R686. Probably minor internal block changes typical over years of running the same engine. The E6 used 4 valves per cylinders, whereas the ENDT675&6 used 2 valves per cylinder. So they have different heads.

This is the most comprehensive response Thank you. It does conflict with what I've been told I researched my 686 before i purchased it and my local dealer said the vin indicates it was built with a Econodyne 350 Hopefully iI will hear from the Mack museum and get the VRI thank you again

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The 2 valve ENDT676 went away in the early 80s I believe, as the 4 valve heads were phased in.  Honestly not sure about the 87 "Econodyne 350hp 686" with 6 gears. To have a 6 speed it must have a hell of a powerband, much like the maxidyne. I know there were Econodyne versions of the 2 valves, I always assumed they had the same big powerband the maxidynes had, just with a few fuel saving tweaks.

My numbers are correct on the 685 and 686 up until at least the early 80s; they may have modified their numbering system when they introduced 690 to indicate maxidyne. That would leave the R686 meaningless, perhaps they repurposed the number. This is also around the time when the R became RD I'd guess...

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Up until the early 80s the VIN number would be something like my 79's: R686ST30072.   This would tell you which engine and typically suspension. 

ST was a six wheel tractor typically with 38k Camelback, though sometimes 44k

S would typically be 44k Camelback

SX would indicate extreme cameback ratings of 55k+. Make made 80k rears too, which I think just used a Ridgid beam instead of springs on the Camelback. I wouldn't want to ride that truck....

 

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There has been many threads done on decoding vin's and model #'s over the years.

I think one of the best breakdowns was done by 41chevy in the following thread from 2017. R model vin decoder. 

 

https://www.bigmacktrucks.com/topic/51420-r-model-vin-decoder/?tab=comments#comment-382830

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On 8/17/2019 at 12:30 PM, JoeH said:

68"5" was the 237hp variant of the 672 cu inch 2 valve diesel,

68"6" was the 283 hp variant of the 672 cu inch 2 valve diesel,

68"8" is a non maxidyne engine, commonly rated at 350 hp, designation used both on E6 motors and E7 motors

6"90" was the maxidyne version of the E6 and E7 motors.

The E6 uses the same 672 cu inch block as the old endt676 motors of the R685 and R686. Probably minor internal block changes typical over years of running the same engine. The E6 used 4 valves per cylinders, whereas the ENDT675&6 used 2 valves per cylinder. So they have different heads.

I think you are close but not quite. 

685=237

686= maxidyne 6 cylinder  with intercooling( tip turbine or cmcac) before e7.  This number was used on two and four valve engines 

690= maxidyne e7

688 = econodyne

an S after these number indicates tandem drive

These numbers are humbly submitted from my own experience/memory i welcome any correction .

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1 hour ago, 6368 said:

I think you are close but not quite. 

685=237

686= maxidyne 6 cylinder  with intercooling( tip turbine or cmcac) before e7.  This number was used on two and four valve engines 

690= maxidyne e7

688 = econodyne

an S after these number indicates tandem drive

These numbers are humbly submitted from my own experience/memory i welcome any correction .

I have a 1988 RD690S with an em6-300L

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21 minutes ago, JoeH said:

I have a 1988 RD690S with an em6-300L

You know whats funny so do i. What a dummy.   well i'll change my answer rd-690 is a maxidyne made after 1987.  seriously i have an 87 rd686 and the remnants of a 88 rd690  only big difference is 88 has a em6-300L and t2060 and the 87 has em6-300 and a  TRXL107

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20 hours ago, 6368 said:

You know whats funny so do i. What a dummy.   well i'll change my answer rd-690 is a maxidyne made after 1987.  seriously i have an 87 rd686 and the remnants of a 88 rd690  only big difference is 88 has a em6-300L and t2060 and the 87 has em6-300 and a  TRXL107

Is the 87 a 2 valve or 4 valve?

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1 hour ago, JoeH said:

Is the 87 a 2 valve or 4 valve?

4vh it is

 

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