What is the firing interval for the 3.7L?

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k99jk99j

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I have heard that the firing interval for the 3.7L is 90-150-90-150-90-150?

anybody know if this is true? and if it isn't true what is the firing interval?

i know the 3.7L is considered and odd firing motor i am just wandering what the degrees are. mike
 

tjkj2002

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The 3.7 is a 90 degree V-6,though the 90 degree only is the angle the 2 cylinder banks are spaced.It's a 4 stroke engine so each cylinder fires once every 2 strokes,so 3 cylinders fire for every one RPM.The exact firing time is set a few degrees before TDC but that is a constant for every cylinder(called timing,not adjustable on the 3.7,only the PCM can do that) so no the 3.7 does not fire at odd degrees for different cylinders.
 

k99jk99j

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so no the 3.7 does not fire at odd degrees for different cylinders.

so if it is an even firing motor does that mean that all cylinders fire at 120-120-120-120-120-120, like a 60 degree v-6?

i have also heard the dodge 3.9L (firing interval) is similiar to the 3.7L?

maybe this explains why both have natural "rough" idles, because a 90 degree v-6 is not natural to fire at 120 degrees?

i can not find this info anywhere! allpar doesn't even give up the firing interval.
 
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tjkj2002

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so if it is an even firing motor does that mean that all cylinders fire at 120-120-120-120-120-120, like a 60 degree v-6?

i have also heard the dodge 3.9L (firing interval) is similiar to the 3.7L?

maybe this explains why both have natural "rough" idles, because a 90 degree v-6 is not natural to fire at 120 degrees?

i can not find this info anywhere! allpar doesn't even give up the firing interval.
You got it all wrong, a 60 degree V-6,a 72 degree V-6,and a 90 degree V-6 all have about the same firing interval,that 60/72/90 is just how the two cylinder banks sit in relationship to each other.They all fire one cylinder about every 720 degrees(+/- for correct timing),same goes for every 4 cylinder and V-8.Only 2 stroke engines fire different and they fire every 360 degrees.
 

k99jk99j

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They all fire one cylinder about every 720 degrees(+/- for correct timing),same goes for every 4 cylinder and V-8.

with all do respect thats not the way i understand it? because 720 degrees is 12 revolutions of the crankshaft required to fire all six cylinders.

wikapedia link:
http://en.wikipedia.org/wiki/V6_engine

cut and paste from wiki
[edit] Odd and even firing
Many older V6 engines were based on V8 engine designs, in which a pair of cylinders was cut off the front of V8 without altering the V angle or using a more sophisticated crankshaft to even out the firing interval. Most V8 engines share a common crankpin between opposite cylinders in each bank, and a 90° V8 crankshaft has just four pins shared by eight cylinders, with two pistons per crankpin, allowing a cylinder to fire every 90° to achieve smooth operation.

Early 90° V6 engines derived from V8 engines had three shared crankpins arranged at 120° from each other, similar to an inline 3-cylinder. Since the cylinder banks were arranged at 90° to each other, this resulted in a firing pattern with groups of two cylinders separated by 90° of rotation, and groups separated by 150° of rotation, causing a notorious odd-firing behavior, with cylinders firing at alternating 90° and 150° intervals. The uneven firing intervals resulting in rough-running engines with unpleasant harmonic vibrations at certain engine speeds.

An example is the Buick 231 odd-fire, which has a firing order 1-6-5-4-3-2. As the crankshaft is rotated through the 720° required for all cylinders to fire, the following events occur on 30° boundaries:


More modern 90° V6 engines avoid this problem by using split crankpins, with adjacent crankpins offset by 15° in opposite directions to achieve an even 120° ignition pattern. Such a 'split' crankpin is weaker than a straight one, but modern metallurgical techniques can produce a crankshaft that is adequately strong.

In 1977, Buick introduced the new "split-pin crankshaft" in the 231. Using a crankpin that is 'split' and offset by 30° of rotation resulted in smooth, even firing every 120°. However, in 1978 Chevrolet introduced a 90° 200/229 V6, which had a compromise 'semi-even firing' design using a crankpin that was offset by only 18°. This resulted in cylinders firing at 108° and 132°, which had the advantage of reducing vibrations to a more acceptable level and did not require strengthening the crankshaft. In 1985 Chevrolet's 4.3 (later the Vortec 4300) changed it to a true even-firing V6 with a 30° offset, requiring larger crank journals to make them adequately strong.

In 1986 the similarly-designed 90° PRV engine adopted the same 30° crankshaft offset design to even out its firing. In 1998, Mercedes-Benz introduced a V6 engine that not only had split crankpins, but had a counter-rotating balancing shaft between the cylinder banks to eliminate almost all primary and secondary vibrations, resulting in a very smooth-running engine.
 
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k99jk99j

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never mind i mis-understood? i thought you said only one cylinder fires ever 720 degrees.
 
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tjkj2002

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never mind i mis-understood? i thought you said only one cylinder fires ever 720 degrees.
That is true,every cylinder(only counting one at a time) in any 4 stroke engine,the 3.7 included,only fires once every 720 degrees which is once for every 2 rpms.When that 720 degrees is reached is different in almost every engine but is at a equal # for every cylinder.
 

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The firing order is Sequential 1,2,3,4,5,6:

The four strokes are:
1 Intake
2 Compression
3 Power
4 Exhaust

SO, what position are the other pistons in, when #1 is at TDC at fire, that is the interval you are looking for? Like, at what time does 2,3,4,5 & 6 fire after #1 does?

AS said, each piston fires on the 2nd revolution of the crank, but they fire in sequential order, so while #1 is firing, #6 is exhausting, something like that. You would have to see at what position 2,3,4 and 5 are in at that time...
 

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