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I read in some magazine or website, i cant remember exactly. But it said that the H.O. engine required premium gasoline. I was just wondering if this was true, as this is the engine i would like to get in my dakota whenever i get around to getting it. Could it be because the compression ratio is higher or something, would appreciate some feedback, thanks guys. And how do you guys like this engine in the dakota
 

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The 4.7L HO is calibrated on premium fuel but has knock sensors to reduce the spark advance if lower octane fuel is used. Maximum horsepower/torque is on premium but you can run regular if you want to it will just make less power/torque.
 

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Frequency vibrations characteristically produced by detonation (typically 6-8kHz). The knock sensor produces a voltage signal that signals the computer to (momentarily) retard ignition timing until the detonation stops (ie no load with pedal)

The engine has to knock first,before the senser retards the ignition.Detonation very early in the compression stroke is usually the silent killer that goes unheard by the driver. This form of combustion usually results in rotating assembly failure by attacking the connecting rods and bearings. Detonation that comes nearer to TDC or slightly after is usually heard as pinging and will likely result in the burning of the piston or the lifting of the ring land from the piston. In the never ending quest to reduce emissions, the distance from the top ring groove to the top of the piston has been reduced on the 4.7 to only 1/8th of an inch, making these areas susceptible to excess wear and heat.

One of the drawbacks of having the top ring so close to the combustion chamber is that it reduces ring life.If the distance is less than about. 0175" to .020", the top ring runs hotter and requires a larger gap .The top land is also weaker which makes it more vulnerable to cracking if there's detonation.

Compression of 4.7 is 9.0:1 where as H.O. model is 9.7:1
I would say run 91 grade up. :gr_patrio
 

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PVO said:
Frequency vibrations characteristically produced by detonation (typically 6-8kHz). The knock sensor produces a voltage signal that signals the computer to (momentarily) retard ignition timing until the detonation stops (ie no load with pedal)

The engine has to knock first,before the senser retards the ignition.Detonation very early in the compression stroke is usually the silent killer that goes unheard by the driver. This form of combustion usually results in rotating assembly failure by attacking the connecting rods and bearings. Detonation that comes nearer to TDC or slightly after is usually heard as pinging and will likely result in the burning of the piston or the lifting of the ring land from the piston. In the never ending quest to reduce emissions, the distance from the top ring groove to the top of the piston has been reduced on the 4.7 to only 1/8th of an inch, making these areas susceptible to excess wear and heat.

One of the drawbacks of having the top ring so close to the combustion chamber is that it reduces ring life.If the distance is less than about. 0175" to .020", the top ring runs hotter and requires a larger gap .The top land is also weaker which makes it more vulnerable to cracking if there's detonation.

Compression of 4.7 is 9.0:1 where as H.O. model is 9.7:1
I would say run 91 grade up. :gr_patrio
Great information that I'm sure everyone here will appreciate. Thanks.
 

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Discussion Starter #5
thats an awesome answer PVO, thanks for your input
 

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No problem guys....i use to own a Dakota,a 1998 model.The other problem i see is if people change the plugs to the wrong heat range/type plug for this motor.A too hot a plug and (pre-ignition) may occur.The ultimate cure for weak pistons are Ross pistons.Below is a Ross on the left and a stock on the right.4.7L Ross 11 to 1 double dome piston next to stock, notice the pin location for strengthening the weak upper ring lands. Notice the very thin top ring land on the stock piece.
 

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Ya but won't that screw us Cali guys who have to bend over and grab are ankles for emissions? I was told the pistons have such a thin design for emisions reasons?
 

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Yes.... moving the top compression ring closer to the top of the piston as in the OEM,
this reduces the volume of the dead space between the piston and cylinder wall above the top ring that traps unburned fuel and contributes to incomplete combustion.

On the Ross pistons i wouldnt suggest the compression ratio as pictured,they do have Stock ratios available......As for emmissions,we are still intakeing the same amount of fuel from the pcm tables,using the same fuel injectors at the same lbs per hr and the same camshafts,lift and duration and same timeing....the problem would lie in that space per cylinder on the ross pistons compared to stock... during an emmissions test and what they allow per year, model,engine configeration as it ages....per each state emmissions guidelines.A motor in good shape,with healthy coil packs and new plugs,and a yr or 2 old,running stock compression ross pistons... i think would pass in most states.Most engines cylinders today are very efficient in their burn characteristics,the intial head and wall design.

I also would (not) suggest a tear down of a new H.O. Motor just for pistons.A used 4.7 would be a nice canidate.
 

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Hmm...interesting besides just increased engine life is there much of a performance benefit of stock ratio ross pistons?
 

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Benefit would be being able to run a Supercharger at max boost,or running nitrous at a high shot say 100 or 150...both of these mods there would be a concern with piston top cracking on the thin OEM stock land pistons,compared to a forged piston with more meat on the top..You could do a H.O. Motor 9:7:1 ratio but you would have to make sure the camshaft valve springs were updated,so on a high rev,you dont get a smack of the valve after a (little time on the engine),the valve springs wont hold on a high rev and you can get a valve float,there is room inside the cylinder on stock pistons and H.O. camshafts installed where people have gotten away with it,but the domed pistons you wont get away with it cause your now much closer on the stroke.

Also running a greater pcm flash,with more timeing than what kennebels optimizer2 puts out,you could probally talk to BGChrysler and tell them to max timeing,without concern of cracking a piston if it knocks under a heavy load with forged ross pistons....4.7 loves lots of timeing advance and fuel.
 

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It would be a PCM flash and when you increase timeing to that degree,you still would need the higher rated fuel with its ability to stop detontation.Even a blower needs a timeing retard system.
 
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