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carguy10101010

Engine Layout Choices and Tradeoffs

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Was looking for some feedback on the pros/cons of using differing engine layouts, particularly in regards to the resulting torque curve as well as the peak power/torque numbers. 

 

The impetus behind the question are the specs of the new 2.8L I4 duramax diesel at the LA auto show (in the Colorado ZR2 concept).  GM is reporting that it will be putting out 181hp at 3400rpm and 369 lb ft at 2000 rpm.  Chrysler's current 3.0L V6 diesel puts out 240hp at 3600rpm and 420 lb ft at 2000rpm. 

 

What are the advantages to GM choosing to use a 2.8L I4 diesel instead of a 3.0L V6 diesel here?  Displacements are very close but the V6 is producing 60 additional peak hp at only 200rpm higher.  Its also putting out an extra 50lb of torque at the same rpm.  With similar displacements and block materials I am going to assume that engine weight is very similar, with perhaps a slight advantage going to the I4.  Does the I4 have substantially less internal friction?  If thrown into the same vehicle is the I4 going to be able to return significantly better fuel economy numbers? 

 

The only efficiency advantages of an I4 engine that I can  think of are easier turbo plumbing, a *tiny* bit less valvetrain friction and a *tiny* bit less piston ring friction.  Is there something big I am missing here?  I am really curious as to whether there is a good engineering case for GM's engine choice here, or if the choice to the similar displacement, less powerful, less torquey I4 was a business $$$ based decision. 

 

 

As an aside the I4 uses a timing belt instead of a chain, which is an additional turn off.  Changing timing belts is a pain (at least for me) and its expensive to pay someone else to do it. 

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Great point, would love to get feedback from some of the stronger engine heads here on this also.

 

Course very possible that GM did this on purpose to allow the 3rd party peeps to sell upgraded tuner chips that gets you 300HP and near 500lbs of torque. :P

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I think a main reason is the I-4 is already adapted to non-us GM pickups based on the same platform.

 

If this is true that is fine, but it would be a solely business decision with no real engineering input into it.  That would be disappointing, but its the reality of the world we live in.  I was hoping however to generate somewhat of a discussion on the pros/cons of using a similar/same displacement and valvetrain layout but either choosing a small V6 or a large I4.  Discussion could probably be expanded into a large V6 vs small V8, etc.

 

Given the performance numbers that Chrysler is getting from a 3.0L V6 diesel and that GM is getting from a 2.8L I4 diesel it seems hard to justify using the I4 given how badly it is lagging its competitor.

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For engines with similar displacement, the one with lower cylinder count is more efficient than the one with higher cylinder count. The reasons are obvious - less heat and friction losses.

 

For this comparison, there is more than it meets the eyes:

 

  1. We (at least I) do not know the turbo boosts of the Baby-Du/Di and Venturi.
  2. How much is GM holding back in this engine through compression ratio, etc.,
  3. How much room the engine accoutrements have to bring it to similar level of power as the Venturi.
  4. Torque and HP curves with peak RPM for the engines.

 

Remember massive diesel engines like Cummins SX-15 family, have only up to 625 hp at max RPM of 2,000 and a torque rating of near 2,050 lb-ft from a 15 liter engine.

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I suppose I am also curious if the I4 duramax is really benefitting from DOHC instead of SOHC.  Its probably redlining at ~4k rpms.  Do the DOHC really do much to increase airflow at such low rpms?  Are forced induction engines more sensitive to having DOHC for maximal airflow?  Does the better control over intake/exhaust valves result in a real performance difference over such a low usable ropm range?  Is the extra cam really doing anything for performance here?  I'm sure its adding some parasitic friction to the drivetrain even if its not adding to engine performance.

Edited by carguy10101010

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