Info on plenum and intake runners.

Yeah make the plenum too big and you will slow the air too much. The idea of the plenum is to slow the air and make it denser. If its too big your throttle response will be a bugger and low end power will drop.

Yet another post with Chas' rubbery figures that defy the experts, but work anyway

1. If you are trying to use tuned intake runners to get the advantage of the impulse waves then size the intake pipe to maximum of 54.9m/s air velocity. Inside dia (mm) = (litres x VE x RPM)/(V x 2.4) where V = velocity

Depending where you want the maximum efficeincy of the plenum power advantage, size the plenum about :

2500 rpm = 100% engine displacement x VE
3500 rpm = 85% "
5000 rpm = 80% "
6000 rpm = 70% "
7000 rpm = 65% "
7500 rpm = 60% " :

on a the three pot. (knock off 10% for a four pot, split a six pot into two three pots, knock off 20% for an eight)

Then you need to have the intake runners :

Same diameter sizing equation as main intake pipe.
Intake runners need to be tuned by doing dyno tests but about 70 + (RPM x 0.043) mm to start with.

Throttle body 0.1305 l/s/mm^2 for efi and 0.1023 l/s/mm^2 for carby.

2. If you're just going to drop a plenum on, then go about 65% of engine capacity to start with and increase until the throttle response becomes a dog. You can use spacers on the intake pipe where it bolts onto the plenum to do this, just like they do on carbies wher they meet the manifold..

3. Make sure you have rolled intake runner connections to the plenum (like trumpets) to reduce skin effect restriction.

4. Don't forget to taper the intake runners down to the size of the head ports to stop air damming (opposite of exhaust runners).

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