Intake length vs torque, Just anthoer carb question No boobs Options

[Joe Ricard]

So I have short intakes and 44IDF's with 38mm vents
How long of an intake should I go to with these carbs or should I go to 40 IDF's with 32 vents or 28 vents.

I have some longer runner intakes and the other carbs so it's just a matter of experimenting I guess.

2.0L Bus piston Web 86B portmatch intake and exhaust ports. 13lb flywheel Bursch SSI Car weighs 1965 lbs 205/50-15 Kumho V700 R tires.

Thought of playing with spacers under the carbs to get more length and maybe spacer under the common length velocity stack. Looking for more torque down lower to pull harder exiting the apex of tight AX turns.

Whose got the experience of really been there done that? great gains or don't bother.

[Dr. Roger]

Excellent info.

Quote:
But there's also a performance benefit because the airbox is a Helmholtz resonator. That is, a resonance effect occurs when you connect an enclosed volume of a suitable size and shape to an engine's intake stacks causing the air inside to resonate at a frequency that results in pressure peaks coincident with the cylinders' intake stroke frequency (at a particular RPM.) This can theoretically increase power by 10-15% within a particular RPM range by boosting airflow into the engine. Air boxes need to be well sealed and stiff in construction to maintain these resonance characteristics. A simple way to illustrate this is to blow across the mouth of an empty bottle. The sound you hear is the natural frequency of a Helmholtz resonator.

If you change the shape and free volume of the airbox, you change its resonance frequencies, and the engine RPM at which it responds to enhance filling the engine with air. For example, a larger Corse airbox is tuned for optimum filling a at higher RPM where race bikes normally operate, but street bikes usually do not.

An airbox also functions as a plenum, a space where the air velocity is reduced so as to eliminate turbulent air flow prior to being smoothed and accelerated down the velocity stacks. In fact, if you just place your finger anywhere near the edge of the top of the velocity stack you'll see dyno power drop off due to the disturbed air flow pattern.

The air in the plenum is also considered "free air." That is, its already passed through the air runners and filters so it can be supplied to the engine without any flow restriction. If you use the auto industry's standard calculation of air required for "nil" vacuum restriction within the air intake system, you should have at least 130% of engine capacity in available air volume between the throttle butterflies and the air filter element.

All of this tells us why Ducati places the Superbike air filters in the air runners. This location avoids lowering the frequency of the airbox (by not filling up a large portion of the airbox volume with a bulky foam filter) and prevents disturbing the airflow near the velocity stacks as well as improving throttle response by maintaining a large free air volume between the filter location and the velocity stacks.

This is also why Ducati didn't use over-the-velocity stack bellmouth style filter. This location doesn't meet that 1,300 cc. plenum volume needed to avoid degrading throttle response (which a dyno doesn't measure BTW.) Dyno tests say you get somewhat less peak horsepower with these filter types but on a stock bike they don't seem to make a lot of difference.

Another point to consider is that unless you have customized a FIM chip on a dyno in an attempt to match the flow and resonance characteristics of an over-the-bellmouth filter you'll have to use a chip that was developed using the stock filters.

The computerized engine management system uses a fixed fuel injection metering scheme controlled by the EPROM chip that was developed in combination with the stock intake/exhaust configuration. Unless you install a programmable FIM chip and sort out any changes on a dyno with a knowledgeable operator/programmer you won't get optimum (low-end?, midrange?, high-end?, power?, throttle response?) performance.