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.

[lapuwali]

Nice theory. Wrong, but nice-sounding.

You can the below (which is very imperfect, and diagrams are really called for), or you can just look at any motorcycle made in the past 20 years. Bikes make power (lots of it: 150-160hp per liter is common) at high revs, and very little power at low revs. They also have very short intake tracts. The typicla car engine makes power much lower (like half the revs or less, and most don't break 100hp/liter, many are below 80hp/liter), and have long intake runners. Short = high revs, long = low revs.

Now, the treatise:

Intake length is tied to valve timing and engine speed, and only really makes a big difference with cams that produce lots of overlap.

You open the exhaust valve as the piston is about 80% of the way to BDC on the compression stroke, as you've extracted all of the useful energy from the pressure rise after combustion. This sets up a positive pressure wave out the exhaust and down the pipe. This wave keeps going until it hits a major change in section (like in the collector), where it reflects back up the pipe to the valve as a negative pressure wave. This helps to suck the burnt charge out through the open valve. The wave reflects again, and keeps travelling back and forth, diminishing in strength, until the next time the valve opens. A similar thing happens on the intake side, except the "sign" is reversed. A negative pressure wave travels up the tract until it hits open air (or the airbox), when it gets reflected back as a positive pressure wave.

In an engine with lots of valve overlap, the wave coming back in through the exhaust can travel across the cylinder and try to push the new charge (and some residual exhaust gases) back through the open intake valve. With ideal timing, the reflected intake wave will arrive at the valve just in time to prevent the exhaust wave from pushing any charge out the intake valve. With bad timing, the charge will be blown all the way back up the tract, only to be reflected back. With really bad timing, the intake valve will close before it can re-enter.

If you look at the torque curve on an engine with lots of overlap, you'll see a series of hills and valleys. The hills are where the timing works out, the valleys are where it doesn't. Typically, each hill is higher as the revs rise, and each valley is deeper, until the torque peak is reached, which generally happens about when the speed of sound is reached by the charge attempting to flow through the intake valve.

Less overlap smooths out the hills and valleys, but at the mean between the peaks and troughs, not at the tops of the hills. Changing the length of the intake and/or exhaust tracts also shifts the height and position of the hills and valleys. With variable length intake tracts, variable valve timing, and variable exhausts, you could conceivably smooth out the bumps with the torque peak up near the peaks, rather than at the mean. This is the sort of thing current F1 engines are using, which is how they're managing to get any kind of drivability with 20K rpm 3.0 engines. Most motorcycles do not have these kinds of toys, but they have the severe cam timing, so you end up with peaky engines that make very little power below (say) 5K rpm, and huge power above 9K rpm. Motorcycles make lots of power at high revs, and have very short intake tracts. Short = high revs. Long = low revs.