308 carb airbox restriction discovered

Short story: I've decided to go back to a single airbox from the two plenums I have now. Why? Long story later sometime, but it has to do with power bands.

So, I'm re looking at the stock box again. After the removal of the wadding and screen, the next two issues I see are the opening at the front and a smooth path for air to flow. I am improving the engine airflow with bigger valves and carbs, so this is even more important.

My back to back dynos with the old airbox without filter and no airbox showed a 10+ rwhp difference. The dyno tech at the time suggested it was time to get a hole saw.

I've been reading up on different ideas about intake track design, and one modern school of thought is that the intake velocity becomes a factor once the air intake enclosure begins. With the removal of the wadding, although the flow is greatly improved, the flow path is not efficient and turbulent. Combining this with a larger inlet path to feed the larger carbs, I smoothed out the snout and added a larger inlet.

Assuming that flow is proportional to cross sectional area, I wanted the cross section of the oval inlet duct to match the rectangular area of where the snout runs into the box plenum to keep smooth flow and same velocity. I think for a stock 308 the way to do this would be to cut off the deep floor left from the wadding and re-weld a smooth plate giving a smooth flow track. Since I was replacing the snout as well, I opted to also round ot the sides and work them into the inlet shape intead of having a square drop off.

I will be using a K&N filter at the end of the snout which has the enlarged starboard scoop duct blowing on it.

The racing Maranellos in the ALMS series use a narrow central airbox to maintain intake air velocity with good result, so it is tempting to follow their lead to narrow the airbox to the same width as the opening to promote the continued velocity to the air horns. Not sure about that part yet.

Here are a couple of photos with the enlarged intake and flow optimized snout before I send it off to be bead blasted and crinkle black powder coated.

Hope this is entertaining for a few and a natural progression for this thread.

OBTW, My dual plenum airboxes are for sale if anyone's interested in a racing or high rpm optimized system.
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Thanks for the post!
I've certainly read a lot of opinions about induction design, and the rear wheel dynos were best with an open airbox lid. The driving force as you say is to get max [clean, cool] air to the velocity stack bellmouth. It seems once the induction system becomes enclosed, it's those other factors at hand I'm looking at as well.
The volume of the plenum seems to be important balancing capacitance and filling qualities, with one guideline I had read being the plenum volume should be 1.5x the swept volume. Then there is the value of an airbox Helmholtz resonator, with the Alfa Montreal being a good example. On dynos, the Montreal makes more hp with the airbox on than off!
Filtration resistance/flow also comes into play here as well, as it's written that filter elements flow better with a more steady vacuum source over a pulsitile source - i.e. summed vacuum sources (cylinders) make for a steady vacuum pressure and better flow across a filter.
Certainly beat to death on this board is inlet air temperature. The constant struggle of the value of separate carb filter boxes vs a cold air ducting system has been bantered here.
As mentioned in the velocity stack thread, Webers are sensitive to the free air space above the bellmouth, requiring at least 3/4".
On a practical level, weather suitability has shown itself to be an issue in the past. I found the old system funnels a lot of precipitation into the airbox.

These are the things I'm learning to balance in the design.
Many thanks
rt

 

There is also pretty much zero room in the Mondial. And, I agree with you about the filtration set up, which is why Kermit developed the LRHF short stacks, and we are now moving toward the adoption of a semi-ducted mongo sized cone filter.
According to Mike Pierce and other 'conventional wisdom' sources, there needs to be a minimum of 3/4" above the bellmouth. The stock mushrooms are 1.5" tall and the height inside the airbox is about 2 7/8". I've also read the distance above the bellmouth should be at least the throat diameter - in our case 40mm, or about 1.5" which is just a little 1/8" less than what is there. As others, I've run the individual air filters for each carb with the mushrooms, and they also just fit under the engine bonnet with the same 1 3/8" clearance above the bellmouth inside the filter. You'd guess someone is trying to tell us something. If you look at other Ferrari carb induction systems, the space above the bellmouth is usually in this .75" - 1.5" range, at least what I have seen. I have certainly seen other carb set-ups with less.
Even the Maserati Ghibli stock airbox with DCNF carbs (See Spasso, you can't escape) looks to have only about a 1" or so clearance.
I am now reading two older textbooks by Philip H. Smith on exhaust and intake design, and there is a mathematic suggestion that the cars might profit from a slightly higher velocity stack. I agree with Bill that having an unlimited supply of atmospheric pressure at the bellmouth is optimal, there may also be something to this tuning of the length of the airbox volumes and length based on wave theory since running open air horns is not as good an option.
Thanks all
rt

 

Makes sense - the residual fuel fog was absorbed into the stuffing, then sucked back out by the moving air as the engine was running. If you look back at the pictures early in the thread, there is an amazing amount of fiber material in there.

 

yes-- they look relly good but at least in the 308 application they are too tall for bonnet clearance based on the measurements I saw.