I was just admiring this beautiful intake in Flip's post and didn't want to hijack his thread. This thought popped into my head and now I'm wondering if anyone can explain, in words of two syllables or less, why the runners are generally soooooo looooong in Throttle Body intakes but typically so short in Carb manifolds. Both are mixing air with gas.
W/ any Port Fuel Injection just easy to make longer runners. Those runners Do Not carry fuel just air.
Carb & TBI can run long runners but fuel plumbing etc limit this in most engine setups. That's Even ignoring getting flow from the carb to all cylinders evenly is hard. For many Carbs, A quick boost of "runner" length is spacers or just very thick gasket between it & intake manifold that does same thing. Depending on how, also can insulate the carb from getting engine heat but may cause other problems.
------------------ Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should. (Jurassic Park)
Carbs and TBI are runner length limited by wall wetting and transient fueling. Basically under some conditions the fuel falls out of the air flow, wets the walls of the runners. Then under other conditions, this wet fuel on the runner wall is vaporized and mixes with the current air/fuel charge. This wall wetting causes large swings in air/fuel ratio and impacts drivability during transients (throttle changes), not to mention increased emissions with this fluctuating air fuel mixture. The longer the distance between the fuel deliver and the intake valve, the larger the wall wetting issue is.
Port Injection (fuel injector close to intake valve) only adds the fuel a few inches before it goes in the cylinder (and against the back side of the hot intake valve), so while wall wetting still happens, it is much less significant, and typically only applies to the runner length within the cylinder head - so there is no longer any restriction on dry runner length. Without wall wetting restricting the length of the dry runners, the mfg can start tuning the length to maximize performance.
Every time the intake valve closes, the air in front of it must stop (runner is dead ended). When the intake valve opens, then the air has to start moving again. However, as the intake valve opens and closes faster (increasing RPMs), the velocity of the air in the runner significantly increases as does the air charge momentum, which makes is harder to stop. The last inch or so still has to stop, but the rest of the air can and does continue to flow which packs the runner. This runner packing, forces a greater air/fuel charge in the cylinder once the intake valve is opened.
This intake tuning will boost torque within a specific RPM range and make the engine feel more powerful for its given size.
Generally speaking, Small runner diameters increase low rpm torque, but at the expense of high rpm power. Longer runners lengths increase low rpm torque, but at the expense of high rpm power.
On an existing application, Shorten the intake runners, will reduce low rpm toque and increase high RPM hp. Enlarge the intake runners, will reduce low rpm toque and increase high RPM hp.
Thanks, gents. What you say makes a lot of sense. Carb runners would be longer, if they had a way to manage the condensation problem, but they don't.
So generally, returning to the TB world, if we had an intake with variable-length (or diameter?) runners, the system could optimize itself at every rpm. Am I right in thinking one would want to optimize for Torque at low rpms (to get a strong launch) and Horse Power at mid-to-high rpms??
Here is an alternative idea: what if we ditch intakes entirely and just direct-inject the air? Instead of a supercharger globally pressurizing an entire manifold like we do today, it would just pump straight to the air injectors. Then we could tailor WHEN the air mixes in, how long the injectors stay open, and the overlap with the direct fuel injectors. This would eliminate intake valves, leaving just an Exhaust valve to manage. Is a cam even necessary at that point?
So generally, returning to the TB world, if we had an intake with variable-length (or diameter?) runners, the system could optimize itself at every rpm.
Yup! The Mazda 787 Lemans car had individual throttle bodies with variable length intake runners for that exact reason! so they could optimize runner length at any engine speed
Is why most carbs/TBI have Heated Intake manifolds including some V engine have coolant running thru them. &/or Many V engines uses the manifold as Cover for the valley that gets more heat from engine & oil hitting the bottom.
Why? In cool to cold weather... fuel can try puddling right under the Carb/TBI w/o the coolant healing the "floor." Some intakes have small "heatsink" fins etc to heat that area better.
In some Race Apps like old school Drag cars where don't run in winter... you often see intakes going thru the hood, (or imaging where the hood would be on many race cars) before 2 or more carbs are attach to them.
Again, is mainly for space allowed causing Very short carb/tbi "runners." Even when some have a hood scoop etc because most states & maybe FMVSS limit "scoop" height because big ones block driver's view of road. Many OE "shaker hood" cars had same or close to same intake & carb setup w/ the "shaker" assembly is the Air Cleaner combine w/ cold air intake thru the hood. Some "Shakers" still had "Thermac" etc to heat the air to prevent carb icing. Even still very short Aftermarkets scoops often get stop by cops. & that's even when does nothing because owner only install them just to "look cool."
