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What is the firing interval for the 3.7L?


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Register to remove these ads. It's free! I have heard that the firing interval for the 3.7L is 90-150-90-150-90-150? anybody ...

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Old 09-17-2009, 10:34 PM   #1 (permalink)
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Default What is the firing interval for the 3.7L?


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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
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Old 09-17-2009, 11:55 PM   #2 (permalink)
 
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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.
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Old 09-18-2009, 07:54 AM   #3 (permalink)
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Originally Posted by tjkj2002 View Post
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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Old 09-18-2009, 09:53 AM   #4 (permalink)
 
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Originally Posted by k99jk99j View Post
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.
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Old 09-18-2009, 01:24 PM   #5 (permalink)
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Originally Posted by tjkj2002 View Post
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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Old 09-18-2009, 01:34 PM   #6 (permalink)
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never mind i mis-understood? i thought you said only one cylinder fires ever 720 degrees.
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Old 09-18-2009, 11:12 PM   #7 (permalink)
 
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Originally Posted by k99jk99j View Post
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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Old 09-19-2009, 09:53 PM   #8 (permalink)
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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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Old 09-20-2009, 08:09 AM   #9 (permalink)
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Originally Posted by Powerslave View Post
You would have to see at what position 2,3,4 and 5 are in at that time...
a picture of a crankshaft would be nice, but i have not seen one and any liberty forum in the last 4 years.
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