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  #25  
Old 09-27-2013, 10:03 PM
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Originally Posted by sterno View Post

I haven't had to dig in to any problems on a Hemi (haven't owned one - yet), but modern engine computers usually store information about how they're running and adapt to what you feed them. The behavior you saw could have been from needing to let the computer "learn" that you were giving it higher-octane gas so the higher the octane the faster it learned. If so, you could probably shortcut that process by clearing its memory or forcing it in to quick-learn mode.
There is actually a published Star Case on the issue. Padgett is pretty close on the problem but it could be corrected with a programming update.
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Old 09-28-2013, 10:11 AM
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Re: Jeep feels like it is struggling around 1-2k RPMs

Only way to fix with a computer would be to richen the mixture and back off the timing a bit, both of which would impact MPG and possibly emissions.

The fact that the hemi needs two plugs per cyl firing at different times is an indicator because that is Not Cheap. Makes me wonder if the semi-unique "fishbite" is when the first plug misfires and all combustion if from the second, both at the wrong time and the wrong place.

Since I have a more modern engine, am essentially just making a few guesses. Hemi Engine Plugs Photo 3 has some good pictures of the head and suggests the old Chevvy trick for the straight plug small block heads (slant plug design reduced the need) of indexing the plugs to aim at the exhaust valve (hottest part of the cyl).

Ideally the spark would originate at the peak of the chamber but the key is in the piston design. Normally a design like this would make me wonder if there were really two small chambers formed by the quench but then you would need simultaneous firing not sequential (kinda like a twingle). Pictures on that site of a piston with a hemi-shaped recess does not support this though.

But apparently that is not what is happening "[using] dual fired plugs on each cylinder allows the firing to take place closer to top dead center, and then again when the piston is on the back side of the power stroke.” (from The new Dodge Hemi V8 engine in full detail - 5.7, SRT8 6.1, and 392 6.4 )

The result is a chamber with the initial firing over on the side instead of the center and secondary ignition (later) on the other side sort of like an inexpensive but very slow MSD.

Now on a Buick 3800, when the secondary ignition breaks down in lockup, the effect is pretty violent both in shake and loss of power. Possibly the second plug firing 90 degrees later ("Each cylinder shares a coil pack with another cylinder. Each of the two plugs on a given cylinder is fired by a separate coil") even though very much retarded, reduces the effect to a "fish bite".

The real answer would be a four valve design with a center plug but then the only "Hemi" would be the piston recess.

Sorry, when I get started, it is hard to stop,
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