Scenicrusier using the twin 4-71

[Geoff]

As an independent shop I worked on a dredging boat that used the twin disc single output drive using two straight up 6-71 engines, a LH and  RH.  I had to rebuild the the left (LH) engine because the operators were trained to set the engine govenors by a pyrometer from each engine so that the exhaust temps ran the same.  That might of worked, but the left engine pyrometer wasn't working right so  they would adjust the left engine so hard that the stack would be blowing black while the right engine ran clean.  These guys weren't too bright, I had to take them outside and make them look at the difference in the exhaust color and they still wouldn't believe they were doing anything wrong because they trusted the pyrometers and that is what they were trained to do.

I finally convinced the owner the pyrometers weren't working right so he cut the pyrometer leads and let me adjust each engine to the same RPM's.  Problem solved, and this was the second time they had burned up the LH engine for the same reason.  The first shop that rebuilt the LH engine told them the same thing about the pyrometers and they didn't listen.

--Geoff

[luvrbus]

You are supposed to check the rpm after setting twins,triples or quads when you finish setting with a pyrometer when set right the quads won't vary over 2 rpms between the 4 done that many,many times without problems  

Seems like the first shop should have checked the leads and probe Geoff you always replace those on a rebuild according to the DD bible 

One thing I did learn about using a pyrometer on a set of twins you pull the valve covers to be sure both engines have the same injectors only took me 4 hrs to figure out one had 70's and not 60's like the tag said

I did see a 2896 Waukesa destroyed one of three on a drilling rig deck because the driller told a deck hand to check the engines and he didn't know the difference between C and F in degrees a pryometer is a dangerous tool in the wrong hands a great tool if used right    

[Jeremy]

I don't pretend to know anything about this stuff, but this issue of having to balance the two engines together makes me think of twin-engined kitcars, which are all the rage at the moment. These typically use two engines from high-performance motorbikes such as Honda Blackbirds or Yamaha R1s, driving through a custom transmission that links the engines together - but the bit which is relevant to this discussion is the fact that both engines still each retain their own gearboxes, specifically in order to allow the two engines to rev independently.

From what I've read about this kind of set-up, the driver has the choice of either operating both gearboxes together through a linked gearlever, or he can change gear in just one gearbox or the other - which would obviously mean that the engines would be operating completely out of sync with each other, and yet while still being connected together. As I understand it, being able to mix-and-match the power and torque output of the two engines like this gives better acceleration and performance than if both engines are performing identically (but presumably requires a very skillful driver).

I'm not suggesting that this is remotely relevant to buses (especially as the rpm of a sportsbike engine is 10 times that of a heavy diesel), but just thought I would mention it in passing

Jeremy

[HB of CJ]

Reason I ask is that I kinda distinctly remember hearing that kind of mill when riding in a Scenicruser going from Fresno to Bakersfield way back in the summer of 1970.  Four speed.  Kinda slow.  Flat land route running local.

First two gears sounded and felt kinda close, then a gap between second and third and a huge gap between third and fourth.  I asked the driver what the top end was and he told me about 72mph.....after 30 minutes.   HB of CJ (old coot

[Geoff]

The owner didn't want to spend the money for new pyrometers. 

--Geoff

You are supposed to check the rpm after setting twins,triples or quads when you finish setting with a pyrometer when set right the quads won't vary over 2 rpms between the 4 done that many,many times without problems  

Seems like the first shop should have checked the leads and probe Geoff you always replace those on a rebuild according to the DD bible 

One thing I did learn about using a pyrometer on a set of twins you pull the valve covers to be sure both engines have the same injectors only took me 4 hrs to figure out one had 70's and not 60's like the tag said

I did see a 2896 Waukesa destroyed one of three on a drilling rig deck because the driller told a deck hand to check the engines and he didn't know the difference between C and F in degrees a pryometer is a dangerous tool in the wrong hands a great tool if used right    

[B_K]

The owner didn't want to spend the money for new pyrometers. 

--Geoff

You must work CHEAP if a second rebuild is cheaper than new pyrometers! 
  BK 

[Geoff]

You must work CHEAP if a second rebuild is cheaper than new pyrometers! 
  BK 

The owner was a cheap  bastard.  Anyway, you can set the engines to run the same just using a tachometer if they are set up the same.  Its the speed of the engines that needs to be equal in a twin disc, single output drive.  That way you are putting equal torque on each side.  The pyrometers are helpful but not essential.

[wg4t50]

Have never seen two identical engines, boats, airplanes, nor diesels. Why tweaking one to match the other works good.
Dave M