I wanted to get some input on the intake. I thought about doing the log-style-through-the-hood type of air intake/filter:
Through the top:
or through the side:
Upsides? It's relatively cheap and guaranteed to get cold air. A couple downsides for me: The biggest one is noise. The car is already very loud and I would prefer a manifold to keep the noise down. Two, I think a properly designed manifold will result in more power and better throttle response. Three, I mocked up what it would look like and since my engine is very tall, the filter assembly would stick pretty far out of the top of the hood and it ruins the lines of the car. Finally, I'm not crazy about cutting a huge hole in the body-work.
So I'm leaning toward a manifold like this:
or this:
or this:
Since my engine is very close to my hood:
it works out rather serendipitously that a 90deg 2" CLR tube both re-routes my runners and extends their length to a total of 13" (from plenum to intake valve) which will give me a 4th wave harmonic at 7750 RPM (my peak torque) and a 3rd wave at 10500 RPM (my peak power). Given the plenum design features I want: a decreasing cross section to maintain air velocity and velocity stacks sitting above the floor and not too near to the sides so they don't sit in the boundary layer, I end up with about a 3 liter plenum volume. Besides the general rule of thumb that less volume = better throttle response at the expense of top end power I've had a hard time nailing down a theory that dictates quantitatively how large the plenum should be. FWIW the Internet says anywhere from 0.5 to 1 times the displacement of the engine for naturally aspirated. Seeing as it's 1.2l, that's not very practical. Also FWIW, I've read a few FSAE papers that put the volume between 2 and 3 times engine displacement. They run restrictors, so I'm not sure that's a good datapoint either. Anyway it flushes out, I'm at 3l which is 2.5 times my engine displacement.
What I've been winding up to the whole post, whew, is I'm mostly wondering where I should get the air from. As I see it there are basically two options: one, run a pipe (tube, duct or whatever) forward and over the top of the radiator to grab cold air as it comes into the nosecone (as I've seen a fellow MNRer do),or two, run a pipe back toward the scuttle and grab air from a duct in the side or top of the hood (as caterham does).
Option 1:
Option 2:
Under the hood:
In action:
A way to do it from the top:
If I were to do Option2 I would most likely do a side-version rather than a top version as I said before, I'm not crazy about ruining the lines of the car. The interesting thing about Option2 is that when travelling at speed I get to take advantage of the fact the air is running by the car. That should help drive air into the intake. On the other hand, the air basically has to do a 180 in order to get to the engine. I figure that the momentum would be enough to make the turn and still be better than grabbing what is most likely stagnant air from the nosecone. Finally, with Option2 I get slightly more room as the engine bay tapers narrow-to-wide as you travel rearward. This allows me to extend the runner length to 18" which gives me a 3rd wave at 7750 RPM and a 2nd wave at 11500 RPM (my redline). These are one order lower than the 13" design which means the harmonic effect will be stronger.
On the other hand, if I'm already running a pipe up and over the radiator, why not continue all the way to the grill and have fresh air driven right into it? Maybe that's the best option.
Thoughts?