Carb Airbox Question

I am considering removing my airbox and using independant filters (KnN?) for each carb. Has anyone tried this and with what results? I'm concerned that the air supply on an "independant system" would be warmer because of the use of the ambient air around the engine. Would this be any kind of issue?

Thanks, Blaine

 

There are a few threads on this, debating pros and cons of higher airflow as an upside with warmer air as a downside. Have you considered gutting your airbox?

 

I don't know a thing about Ferrari's airbox design BUT a properly designed airbox can enhance engine performance. So don't be too sure that removing it will improve performance, it could easily do the opposite.

Airbox design is part of the intake tuning so it will have an effect if you remove it - whether the affect is positive or negative will depend on whether it was designed to augment the engine's performance or just thrown on there to filter air with no further thought.

FWIW, you can't thoroughly evaluate airbox/no airbox performance differences on a dyno; Airflow over/around the carburetor intake throats at speed can adversely affect cylinder filling at WOT depending on vehicle speed.

 

I don't know a thing about Ferrari's airbox design BUT a properly designed airbox can enhance engine performance. So don't be too sure that removing it will improve performance, it could easily do the opposite.

Airbox design is part of the intake tuning so it will have an effect if you remove it - whether the affect is positive or negative will depend on whether it was designed to augment the engine's performance or just thrown on there to filter air with no further thought.

FWIW, you can't thoroughly evaluate airbox/no airbox performance differences on a dyno; Airflow over/around the carburetor intake throats at speed can adversely affect cylinder filling at WOT depending on vehicle speed.

 

You will get more flow which will probably require raising the main jet size a bit but you will be sucking in a lot of hot air. Up to you really.

 

"You will get more flow "

Do you know this based on some actual performance data?

I have personally seen examples of noticeable power drops and/or "holes" in power curves caused by removing airboxes and going to individual filters/no filters. Again, it will be determined by the airbox design in the first place but most competition engines use airboxes nowadays whereas years ago they did not.

One quick example - we gained a .2 sec improvement in the 1/4 mile times by fabricating an airbox (with filter) for a SS/F 440 Dodge Challenger as opposed to a hood scoop sealed to an unfiltered carb. THe only change was the airbox - sized to enhance power at WOT. They can be sized to adjust power anywhere in the curve - just like headers and collectors.

 

You're more right than I am. My view is very subjective. I see the stock airbox with a very narrow opening at the side only vs the individual route which has easily 400% more filter area exposed to air. It very well could cause a flat spot or cause for serious need to noodle with the timing/fuel curves.

 

WIthout actual measurement of the flow or track testing, it's not possible to say if the oem setup is restrictive but the thing to consider is the airflow the engine requires at full power VS the amount of air that can be provided by the ducting/filter.

If an engine needs 450CFM of air to produce max power - and that's more than what a 3x8 would need at the oem HP/RPM ratings - then having more air "available" doesn't do anything at all.

OTOH, I have no idea what the Ferrari carb ducting/filter can provide airflow-wise at full power; perhaps it is restrictive but it doesn't take a very large diameter tube to provide that flow assuming the tubing is smooth, not excessively long, and has no abrupt changes in direction.

 

This issue has been thoroughly gone over in an old thread:

http://ferrarichat.com/forum/showthread.php?t=49407&highlight=Carb+air+box+modification

It definitely gets more air and makes more carb "noise".

I believe the factory did this to quiet it down.

I took the mesh and fiberglass out and can confirm the benefits. Glad i did.

chris

 

If the dyno shows a HP loss with the filter/box in place than that pretty much makes it clear that the filter system was "stuck on there" to filter and fit in the space available with no thought to actual performance. Disappointing! I would have expected better...

 

Hi Mike,

Many high end carr buyers are not quite as hard core as some of us on this thread and the factory obviously felt that the engine needed to be quieted down a bit to make it appeal to a broader public.

Once again, compromise reaises its ugly head, (not you mike, .

viva enzo,
chris

 

I have to admit to being periodically both IMpressed and UNimpressed by Ferrari "technology." Enzo is reputed to have said something to the effect that when you buy a Ferrari you're paying for an engine and he throws in the car for free.

Sometimes I think that it's the opposite, at least in the case of the 3x8 - you bought a beautiful Pininfarina body and Ferrari threw in the rest of the car's components for free. Some of them were pretty good; some of them weren't.

I guess part of the issue for a small company is simply a matter of R&D necessary to meet changing safety/emissions requirements. As noted, if the engine has to be quieter, it's easier to just get an air cleaner/box that will fit as opposed to doing extensive body/manifold redesign to accommodate a box/filter that will quiet the engine AND provide max power.

The air intake hose from the scoop to the box on the 89 328 is a good example - quieter (as required by US regs) but the hose is a less efficient design than the '88's tubing. Whether it's less efficient enough to adversely affect the amount of air required by the engine, I suspect probably not but I don't know.

That being said, they were no worse than any other company in that regard BUT since it is Ferrari, and Ferrari's were always expensive, I assumed that they did whatever was technically necessary to ensure there were no compromises performance-wise.

Yeah, I used to believe that politicians had my best interests at heart too...

 

cold air good, hot air not good...

 

Is the box not also a spark suppresor?

 

"Is the box not also a spark suppresor?"

No - that's not possible. Nothing involving the ignition/plug wires is inside that box.

 

Think he might be referring to backfiring through the carburetors.

 

I'd keep the airbox on Blaine. Although it has no insulative properties, it will convey cold outside air to the carbs (like you surmise).

 

I'm guessing he may be refering to the "flame arrestor" fitting from the crankcase emissions (oil/vapour separator). Also a good idea to retain the airbox to help these fumes to be consumed by the engine. I've noticed a decent amount of vapour coming out of this fitting while I've had the 'box off for tuning purposes. Unless you'd come up with a way much like the old "road draft tubes", you'd just fill the engine compartment with more stinky fumes if you left the 'box off.

 

Those of you who are pilots may be familiar with the Mooney which has a port that can be opened to push fresh air into the induction system at altitude, creating a "ram pressure" effect, raising the pressure of air that is available, and therefore power.

Is it possible that the big duct to the air cleaner is pressurized a bit at speeds we travel in the 308, pushing air into the engine? It seems like clean cold air from outside the car is a far superior choice than aftermarket filters. Also the original system looks right, adds protection from fire due to the occasional backfire. It sure is a pain to get out of there, though. Think of the 16 8mm nuts and washers holding the air rams in place! Those things are possessed and long to commit suicide by plunging down the intake.

 

Is the airbox and all ducting sealed from the scoop onward? If so, then there would be a pressurizing affect at high speed. Typically there is a vent of some sort somewhere in the ducting/scoop/airbox to dump the air so it is not pressurized. But I have no idea if that is the case on the carbed 308.

 

It has long been a question, "does the scoop pressurize the air above the carbs"? Someone needs to hook up a sensitive vacuum gauge to the interior of the air box, get a stationary reading without the engine on, with the engine at idle, cruise, and high speed 60-100 mph and full throttle.

Similarly, i bet if the same readings were taken with individual air cleaners, placing the pick up tube close to the air filters, one would notice less pressure around the individual air cleaners.

Looking at the air flow around the engine, (a wonderfull air heater), you notice that air is drawn into the engine compartment from under the car, (see all of the grit and crap on top of your engine?) and the air exits through all of those nice louvers on top of the engine lid, partially because heat rises, but mostly, because as the car moves through the air, a low pressure area is created over the engine lid, behind the glass.

So, this hot, low pressure, dirty air is being consumed by those pretty individual filters.

Mike, yes, the air box is sealed with a gasket to the top, clamped rubber gaskets between the carb air horn and carb body. The air filter housing has a nice clamped rubber hose connecting the assembly to a fiberblass pipe, (with a small rain/carwash drain hole in the bottom), which slip fits over the side metal duct.

I would sure like to see vacuum readings.

Somewhere in that 15 page thread there is mention of dyno readings with/without stock air cleaners and modified air cleaners. This is obviously is a stationary test, so does nothing to answer the burning question of scoop effectiveness,/pressurization.

chris

 

I've been running individual filters for years now and can attest that there is much more intake noise (music). As for the air temp, it is negligible. I did some test with a data logger mounted right on top of one of the filters and at speed there was maybe a degree or two higher temp. Sure, when stationary, the temps can be 10 to 20 higher (even 30 on a hot day). So there seems to be good circulation within the engine bay when moving (those louvers actually do something).

As for the the ram effect, I highly doubt that there is any. I have no objective data, but I would think that the only place you could get any actual pressure would be in the nose. I think I remember reading/seeing something a few years ago that most scoops (even on the hood only provided minimal, if any, pressure at all).

 

I have a '79 308 GTS, it had/has K & Ns. I am about to swap the stock box in as I was told by a reliable source (Jon Tirrell) that it will run better (cold air good) than the K & Ns.

John also told me that if you remove the sound baffle it will significantly improve performance. I have not decided if I am going to remove the baffle or not. The downside is the passenger side is a little nosier (but it must be quieter than the K & Ns).

One note, the main jets do not appear to have been changed from the stock (and small) ones. I have changed the main jets to 135s (220 air correctors, F35 emulation tubes), which should also make a difference so it may be hard to discern the end result of all the changes.

I hope to remember to post my subjective results in a couple of weeks.

 

During the process of having the carbs rebuilt, we also removed the sound baffle/air restriction equipment from inside the airbox. I can tell you that you will notice an improvement in performance. You are correct the sound coming from the carbs will be increased as well.

 

Confirmation on the pressure distribution too: The other day, I neglected to close the engine lid properly. At speeds of about 85 mph, the lid started to float up. So there is indeed an air flow coming from below the car and coming out of the louvres, plus the louvres are NOT large enough to accomodate the full flow.

This on a GT4, which obviously has separate lids for the engine and the luggage space.

While tuning my carbs, I've briefly run the car with no air filter assembly on it, just the trumpets. Boy, does that engine sound wicketly raw then! MUCH more so than with my air filter assembly (Euro, no sound dampening).

Nice!!!