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Engine Specific Info and Questions => IDI Engine => Topic started by: KTZed on February 06, 2008, 04:45:03 pm
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Cummins 4BT forum but a nice writeup nonetheless
http://www.4btswaps.com/forum/showthread.php?t=2662
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Ok I stole this pic from regcheesman in another post, thanks reg.
(http://img516.imageshack.us/img516/418/boostpinro7.jpg)
Ok this pic is interesting. Theres a few questions I have about this system, hopefully someone will be able to help. Please correct me if ive made a wrong assumption.
This is the first pic of a pin ive seen where you can actually see the portion of the ramp that the "fuel pin" rides on. So...
In stock setting, is the horizontal travel of the fuel pin limited by the vertical travel of the boost pin (ie the spring pressure counteracting the boost), OR is it limited by the setting of the max fuel screw?
Obviously, more boost will mean more vertical travel of the boost pin, and therefore, more horizontal travel of the fuel pin, but only up to the point where it reaches the max fuel setting correct?
Id also like to clearly lay out what each of the fueling adjustments on the pump actually do to better help me in the tuning process and to help others getting started. Again, please tell me if im wrong about anything or if theres anything that should be added and i'll edit this post to prevent any misinformation for future searches.
Boost Pin
As you can see from the pic above, turning this pin will change the ramp that the fuel pin rides on and therefore the amount of fuel added in proportion to boost pressure. It will also change the initial amount of fuel the engine gets while in NA mode, before boost comes on. The overall vertical travel and position of the boost pin in relation to boost should remain unchanged. This means that the rate at which the boost pin travels from the min fuel position to the max fuel position is unchanged.
Max Fuel Screw
This will determine the max horizontal extension of the fuel pin. For example, it is possible that the boost pin will be pushed down to its smallest diameter but the fuel pin will lift off of the ramp due to a conservative max fuel screw setting. Conversely, if the screw is set for absolute max fuel, it may not lift off the ramp at all and your effective max fuel will be determined by the rotation of the boost pin, the amount of boost your running, and the setting of the star wheel.
Star Wheel
Turning this will adjust the spring pressure that counteracts the boost. Less spring pressure means the boost pin can move farther down and it will be moved easier by the boost pressure. Adjusting this will change both the amount of fuel added in proportion to boost pressure, and the rate at which fuel is increased in proportion to increasing boost.
"Smoke Screw"
Adjusting this screw will adjust the starting point of the boost pin. This will essetially adjust the amount of fuel while the car is in NA mode, before boost comes on.
*Note* The initial fuel setting will also be affected by the rotation of the boost pin as it is eccentric. Once the boost pin is rotated to your satisfaction, some juggling of the smoke screw and idle screw will probably be necessary to get a proper idle and off boost smoke level.[/img]
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:wink: :wink: :wink:
forgot where i got it
(http://img230.imageshack.us/img230/5681/veafchw4.jpg)
(http://img148.imageshack.us/img148/8600/hpim0985afpe1.gif)
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nice writeup, as far as I know your right on.
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Ok so from what ive read recently in a couple other posts, the max fuel screw actually takes the air/fuel ratio profile determined by the boost pin and star wheel settings and indexes the whole thing towards more or less fuel. I originally thought it acted as a stop for the fuel pin itsself and just limited the absolute max amount of fuel.
Also, it seems that the boost pin may actually bottom out at around 8psi. Is this due to the spring going into coil bind or is there a stop somewhere? I will hopefully get a chance to take my LDA apart soon and test the spring rate and its travel and try to determine how much pressure it would take to compress the spring. Anyone done this before?
Can anyone confirm or deny these points so I can update the OP?
ps. Great pic hillfolk!