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Any horror, or good, stories about 8 stack injection

October 9 2017 at 9:21 AM

john vermeersch  (Select Login johnvermeersch501)

I got a Ford crate motor with an off idle stumble, customer thinks its a balance issue....lookin for any input !!! TIA John V

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(Login 32bantam)


October 9 2017, 10:40 AM 

Hey John,

What system/engine do you have? Is this a modern EFI unit or a mechanical set-up put on a crate engine? Either way it is very common on those units I've worked on to not have the butterflies sync'd properly. It is also common that shaft rotation is wrong so one side doesn't seat correctly. Many units are opposed shaft rotation for equal flow per bank but it is common to see them assembled where the butterflies on each bank park on the same side and rotate parallel to each other. If the casting isn't machined for this the backward side bleeds more air at idle and doesn't brake fully on release. Then the way the air enters the port affects fuel distribution through the range so that side often runs lean. Some units use an under/over linkage for the opposed opening shafts which means you can set them for opening rate or opening distance but not both at the same time. Generally not a huge issue on a drag car but still not equal at all times and a compounding problem on a roadrace or street setup. Others use a jackshaft system that solves this but it can suffer similar issues if split shafts are used. Think motorcycle carbs where they are linked together with a single drive but must be indexed. I've found this to be the problem on several units as the idea is to install the intake and then set each shaft after torquing the manifold while making sure the intake bores are still parallel. Due to this they are often not shipped set-up, the installer just cranks the intake down which binds the shafts and butterflies resulting in premature wear creating more leakage and inconsistent operation. Yet it is most common that the links are simply not adjusted or even worked loose.

I'm not sure if this pertains to your problem but linkage and adjustment is where I start and it seems the last place the shops before me look usually blaming the electronics, barrel valve, nozzles etc.

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(Login kleandan)

It depends on the system adjustability.

October 13 2017, 7:59 AM 

There are variations in the 8 stack systems. I am talking aftermarket stuff, nothing vintage.
The older systems used dual stack throttle bodies that were conjoined by a single throttle shaft.
This single shaft system between two butterflies requires physical bending of the shafts and butterfly valves to adjust them to the Nth degree.
The bends required to perfect the sync are so subtle and difficult to get perfect that basically you can not get them perfect.
This is one reason they altered the systems to include split shafts between all 8 throttle body butterfly shafts on more recent variations.

If the system has adjustable couplings between each throttle body you can now micro adjust all 8 butterflies.
This added adjustability greatly increases the fine tuning ability.

The 8 stack systems are race pieces and not really made for street use. On the track the ability to smoothly transition from idle, to cruise, to WOT is not of much concern.
Watch some vintage race footage. Things like the Monterey Historics or Goodwood Festival of speed and listen to the cars with 8 stack systems. They pop and buck and snap and crackle between on and off throttle transitions.

The 8 stack systems look awesome and can produce some great power but they are hyper critical of perfect adjustments. Make no mistake when the term perfect is used it is literally what you need to get the system set up just right...and perfect is REALLY hard to accomplish.

This need for perfect adjustments is a factor of the single cylinder supply and demand. There is no common plenum to help smooth out and equalize the pulses, vacuum, reversion, and other airflow characteristics within a running engine with an 8 stack system.
Each stack works with one single cylinder. With 8 separate cylinders it is very common to have variations in demand and output based on how efficient each cylinder works. This makes perfect running VERY difficult to achieve over the entire range of a street engine.

I suggest you do everything you can to sync the throttle bodies for best running.
Begin with each single throttle body. Next adjust the throttle bodies in pairs. Follow that by side to side bank adjustments and finish with adjusting all eight together.
Begin by mechanically adjusting them all the same using technical methods to attain equal adjustments.
I use a thin wire as my feeler gauge for each throttle body adjustment.
Adjusting this will take a long time because each adjustment will effect the previous adjustment so you will spend a LOT of time going back and fourth getting it correct.

After you get the throttle bodies set equally you move onto the throttle opening via the gas pedal. This too can effect each throttle body and requires still more fine adjustments. Heck the return springs will change the adjustments they are that touchy.
Once you have all that set move on with running the engine and using an air velocity gauge to make running adjustments.

Only after you have all 8 set properly should you move on to computer tune adjustments. Once you have a good basic tune you can move onto cylinder to cylinder fine tuning to adjust for each cylinders specific needs.

Sometimes you will find the perfect adjustments simply can not be made due to limitations of the system...not to mention perfect is very hard to achieve. At this point I find it best to tune for best results and forget about all the perfect mechanical measurements and settings even if the system wants those perfect adjustments.

This whole process is a real pain in the ass and can become needed again with any difference between the 8 throttle bodies that can occur with normal use.

The worst part of all this lies in the owners tinkering as if they are adjusting a carb. Lets turn this and see what happens. Lets move that and see what happens. Maybe they need to remove some portion of the system to repair or replace something else, like and alternator.
This requires a complete reset of the entire system because things were moved.

8 stack systems look awesome but I will never have one because they are so very finicky and hard to tune for the street.

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(Login jjz103)

Depending on what 8 stack you have

October 13 2017, 3:41 PM 

you might take a browse of Roush website I found some good instructions on their unit there

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