EightStack Tuning

It's gotta be perfect -or you'll have issues forever

I learned with the Weber setup the importance of good linkage. It’s important to get all four to work together, in sync and at the same position. With the EightStack systems, this is even more true. If the linkage is off even the slightest amount, you’ll have issues. Some tips I’ve learned, starting at the loud pedal and working towards the throttle bodies.

Fuel pedal position, stops and movement

With carbureted engines, we tend to take the pedal position for granted. If it opens full and closes, that’s good enough. With EightStack throttle bodies, the majority of their throttle response is in the first 40% of the blade movement. And the increases are not linear. The first 5-15% open generates the majority of the air flow and increases are incrementally smaller beyond that. (If you’re a math geek, calculate the area of an ellipse in a 48mm circle then rotate that ellipse 2 degrees and recalculate. The difference will be an eye-opener)  You need the throttles to open fully and return freely and to the same position, but you also need precise control of the position. Any slop or free-play creates problems. I installed a throttle stop on the loud pedal so I don’t put the linkage in a bind, unintentionally. The height of the pedal can be adjusted to the desired height by adjusting the rod lengths. It is also important that any levers move equal amounts on each side of vertical or you won’t get smooth movement.

Center Pedestal must be in the center!

The location of the center pedestal the bellcrank is mounted to must be centered between the four linkage points. A quick way to check or determine this is to place rubber bands from the right front inner rear stud to the left rear inner front stud. Then do the same for the other two bodies so you end up with an “X” shape in the bands.

Finding Center

Where all the bands cross, a diamond shape is formed. The center of the pedastal MUST be centered in it. Again, only perfect is acceptable. I had to move my first set about 1/4″. The second build was a little trickier, it only needed to move about 1/16″. I ended up boring a new hole since the old one was slightly off-center then  using a bushing to adapt the post to the new larger diameter.

Again, it is all-important that this be centered to get the four throttle bodies to open exactly the same. Since only minor amounts of movement make large changes in the air that is passed, it is CRITICAL. If the post is slightly closer to one or another body, it changes the linkage geometry dramatically. Take your time and do it right. You’ll be glad you did. And don’t ask me how I know this.

Center pivot location

Center Links - getting closer but not done yet

With the center pivot exactly positioned, it is equally important the center links be the exact same length. One long, one short will fit and hookup but it will cause the throttles to open unequally. I marked  one side of the hex material so I had a reference to any changes I made. And changes are normally in just one flat’s rotation – very minute!

Using a caliper to measure between the balls on the rod ends, make sure both are equal.

Were almost done but need to transition to some sensor tuning before we go any further.

Throttle Position sensor adjustment

ECU’s need to know the position of the throttle blades. This must be set (coarse adjustment) at this point. Follow the procedures of  your tuning software to enter the settings for both closed and full open positions. This will get a final fine tune a little later.

Throttle stops and springs

Although each throttle body has a stop screw, the one on the first body is the only one used. The others should be removed and saved in your spare parts. Which is the first body? It’s the one the linkage first connects to.

Another tip: You’ll notice compression springs under all of the adjustment screws. It makes for a more reliable setup if there is substantial pressure on the screw to keep it from vibrating or turning. I stretched the springs 100% then reinserted the screws. Make sure they won’t move freely.

Run Tweaking

At this point, proceed with your initial tuning and idle setup. I recommend using the SK style  airflow gauge to balance the throttles. It must be done in a sequence that follows the linkage. Start with the first point where the linkage connects to a throttle body. This point will also have a screw for the mechanical idle stop. IMPORTANT: make sure all the air bleed screws are closed. In my setup, it was cylinder #7. 

SK Meter

Set the idle stop screw on the first body so there is .003 clearance between the stop-screw and the lever.

With the engine warmed up and idling check the airflow. Whatever that reading is, adjust the screw on the same side inter-link(The little spring/screw lever system that connects the two throttle bodies. Adjust as necessary to get the same or close reading. Should be within one indicator mark.

Now move to the opposite bank and adjust the cross-links to get the same reading on the body that the cross links attach to. It is important to adjust the links in equal amounts so they stay the same length. (if you shorten one side by 1/2 turn, you must shorten the other side an equal amount. I found the easiest way to do this is to count the flats as they turn. 3 flats shorter on one side, 3 flats shorter on the other side. Then adjust the fourth body to the same settings using the interlink adjuster. j

While doing this coarse adjustment, you only need to check the inner four air horns 2&3  and 6&7. Once you get these four close, now we start on the fine adjustment. 

Fine Tuning - almost done

Starting back at the linkage connection point to the throttle bodies (#7?), take a reading with the airflow gauge then check the airhorn adjacent to it on the same throttle body. Open the bleed screws at the base to get the two cylinders to match. The cylinder with the lower reading needs more air so crack that bleed screw slightly. Move to the other throttle body on the same side and adjust those also. You’ll find you have to tweak the inter-body link to get them to match. 

Now move to the other side and adjust them accordingly. Remember to follow the connection path of the linkage. 

Now the trick to all this – you need to work this around about four or more times before you can get all eight airflows to match. Be patient, it can be done but is tedious.

Almost there

At this point, you’ve got them in balance, but only at an idle. Now the scary part, you need to adjust the crosslinks at a higher RPM and check them at a couple of different RPM points. I use a pedal depressor or toggle clamp to push the gas pedal to a given rpm and hold it there My first point was about 2500 RPM. The amount isn’t important but holding it steady is. Check the airflow on each bank. They should match within 1 gauge graduation. It is doesn’t match, adjust the cross-links slightly. Remember, you have to adjust both links equally and the adjustment is typically only one flat.

After the adjustment is made, try the airflow reading again. adjust and test until you get equal readings side to side and all the way around. Try two or three different RPM settings.  When all done, have an adult beverage and admire your handiwork.

It’s difficult to convey how critical the linkage is. The black mark is a reference point used during final tuning. Rotating that point 1/4 turn in either direction created a change in lambda of 10%. The final linkage was tuned in 1/6 turns (one hex flat). And it made a perceivable difference.

Each throttle body has a torsion spring on it to close the throttles, however, it is not wise to rely on these solely. Most sanctioning bodies will require a dual return spring on the throttle linkage. I fabricated a bracket to attach to one of the mounting screws of the rear throttle body and ran dual springs to the bellcrank. 


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