Reply to "Engeneering challange ... Progressive Throttle Linkage"

Dago,

I should have asked displacement and rpm too but at 2 3/16” it’s less of an issue for the power range you’re likely in. Not sure of your power level but <= 650HP can easily be supported by ~ 2.125” diameter. Many Kinsler systems can be 2.43” and even larger [up to 3”!!). Of course the bore area increases with the square of the diameter and just a little throttle plate rotation can change the open area, and thus air flow, ……a lot. If you want to model the response of your linkage it is better to do so by comparing to the open area in the throttle bore instead of just the angle of the shaft. Of course, the engine behavior you describe could also be induced by cam characteristics too.

I put a lot of effort into the linkage on the systems I built to achieve better drivability. The link earlier in this thread for my “for sale” post provides progressive linkage for better off idle and low speed transition as most of the folks that were buying these systems from me were putting them on street cars. The individual bores were either 50mm or 55mm depending upon desired power level but seem about right for 500-650HP range. These TBs flow significantly more than a 48mm IDA carb because of the absence of the venturi in the center of the throat. Properly sized bores combined with progressive linkage can achieve much finer throttle resolution. Larger than necessary bores are an aggravation to this but often pick up a few peak HP.

The cable and wheel provides a constant amount of rotation for every inch/increment of pedal travel. Cable & Wheel also allows for pulling from any direction so it can be side or forward (or even rearward for front engine cars). The initial position of both the arms on the wheel and the levers on the throttle shafts being driven are initially almost “over center” initially taking much more pedal travel per degree of throttle plate rotation….thus the progression. These systems also had counter-rotating shafts to promote bank to bank symmetry as it mattered whether the high or low side of the throttle plate was inboard or outboard.

There are many ways to mechanically achieve progressive linkage. You would likely have better results with the “jackshaft” approach shown at the bottom of the Kinsler catalogue page, but if you have very large bores you would likely need a two stage linkage scheme such as suggested in Gary's post or perhaps a scheme such as I used with another cam profile on the cable between pedal and where the cable attaches to the rest of the induction linkage.

Kinsler has been making such systems for a very long time and definitely are very knowledgeable. The initial position recommended by Kinsler for under/over arm and link rod style linkage that mounts on the end of their IR systems is design to minimize the error between the rotation of each bank of cylinders….but it only minimizes error and does not produce the same position of the respective banks throughout their motion to wide open throttle whereas there jackshaft system does.

I always placed the wheel and linkage between the cylinders because it also enabled the use of the Kinsler (or similar) spring screw clamps with arms to connect the throttle shafts. These can provide some individual rotational adjustment for each pair of throttle plates, allow for angular misalignment and thermal growth without binding, and fewer parts overall. Binding is why Kinsler recommends a toe strap. Having one stick WOT when you don’t expect it can wreck or kill you. If not toe-strap, consider a steering wheel mounted kill switch.

Irrespective of where you place and how you configure the linkage, I would strongly recommend you incorporate a throttle stop (wide open) on the accelerator cable or at a location that doesn’t stress the entire linkage system through all throttle shafts. It’s much harder to keep the linkage in tune when it is subjected to a heavy foot and pedal pressure.

Take care,
Kelly