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C2/C4 Airbox Modification / Upgrade
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Pelican Technical Article:

C2/C4 Airbox Modification / Upgrade

Tom Sharpes


1 hour1 hr






Electric drill, 2-inch hole saw, drilling table, flathead screwdriver, metric socket and metric wrench set, safety glasses

Applicable Models:

Porsche 964 Carrera 2 (1990-94)
Porsche 964 Carrera 4 (1989-94)
Porsche 964 RS America (1993-94)
Porsche 964 Turbo (1991-94)

Parts Required:

Air filter

Performance Gain:

Better airflow into your engine

Complementary Modification:

Replace your air box with a conical air filter unit
thumbnail This is a very easy and effective mod to perform on the C2/4 engine. The stock airbox does not allow enough airflow to get into the intake system.  Drilling a set of holes in the airbox housing will open it up and allow more air to breathe into the engine.  
thumbnail First step is to remove the airbox cover located on the right side of the engine compartment.
thumbnail When you do this you'll see the air inlet to the engine. These are more expensive way to accomplish the increased airflow to the engine such as cone filters, but in this case this is an easy and inexpensive way to increase the airflow and save you money for other mods.
thumbnail Place the cover of the airbox on a good working surface . I used a 2" hole saw to drill a pattern of holes that would provide a significant increase in the airflow to the intake.
thumbnail As you can see, the increased open area is dramatic compared to the stock cover. This would be a good time to change out your old filter for a stock one or a K&N type.
thumbnail Reinstall the cover and your done. I have removed the curved "horn" of the airbox in this picture to allow a more direct shot into the filter area.
You may also want to drill some small holes in the low point of the part of the airbox  bottom to insure that any water that might find its way in will be able to drain out.
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Comments and Suggestions:
Tech Dude Comments: Nick, I didn't say you were disagreeing with me nor did I believe that to be the case. My point was that there is enough information presented in this discussion for people to decide for themselves what works for them.

As far as air flow data, I do not work at the facility where I conducted the air box testing many years ago. Even so it was customer paid testing so I could not disseminate someone else's data without their permission.

Intake induction systems are measured as losses to air flow thru the intake valve at full lift. In other words you flow the intake port with a radius entry at max valve lift. Then you add the intake manifold and test again at full valve lift. Then you add the air box and test again. Then you add the air filter and test again. In a well designed system the air filter will be the only minute drop in air flow at maximum lift.

In the C2/4 964 engine, adding the air box to the system showed no drop at all in air flow. Thus it is not a restriction and cutting holes in it does not improve performance or air flow into the engine. In fact as noted previously the increased heat drawn in lowers power as does the loss of the tuning acoustics.
August 17, 2015
  Followup from the Pelican Staff: Thanks for the follow up and sharing your experience
- Nick at Pelican Parts
Tech Dude Comments: I'm not telling anyone what to do with their own car. There is enough information presented that people should be able to decide for themselve's what works for them. I pointed out the technical reasons why you lose power with this mod in addition to the water liability. If someone desires more noise with reduced power, great go for it. Since the air box is not a restriction in the system based on actual air flow measurements by Porsche and myself, concluding that modding the air box will increase performance is a false belief.
August 12, 2015
  Followup from the Pelican Staff: I am not disagreeing with you. But please share you air flow data. Would love to look it over verse the normal data. - Nick at Pelican Parts  
Tech Dude Comments: Porsche does NOT do the mod shown on the 993 air box. In addition the water that can get in the air box can hydro-lock the engine and cause total engine destruction. Third the snorkel on the air box is located at the rear of the engine compartment to draw in cooler outside air from the grille opening, not the hot engine compartment air. The snorkel acoustically improves low speed torque. Fourth when you cut holes in the air box as shown you also lose all acoustical tuning benefit from the air box design. Highway speeds force more airflow through the engine compartment mitigating some of the heat issues but slower speeds most definitely result in higher IATs and power loss. What Porsche does on race cars is not necessarily what is optimal for street driven cars. And as originally noted the air box is not a power/air flow restriction that is reducing power. If you want to mod the air box for more noise, so be it. That increased noise is costing you power and raising the inlet temps while endangering your engine if you get caught in the rain or water gets in when washing the car.
August 6, 2015
  Followup from the Pelican Staff: I am with you. No argument here. I am not a huge fan of Mods in the sense of changing things beyond factory function. I have dealt with repairing issues related to items that don't belong too many times.

With that said, to each his own. I may not like diesel truck stacks for my vehicle, but have to appreciate that some can. As well with this mod, if someone wants to hole saw the airbox for sound or promised performance gains, I can't argue.

Maybe some folks that have performed this mod can chime in with their experience and add some insight. - Nick at Pelican Parts
Tech Dude Comments: This is a really DUMB mod that is guaranteed to LOWER the power output as hot air is drawn in along with water that can hydro-lock the engine and destroy it.

Don't believe for a minute that the OEM Porsche airbox is restricting the power output. I have the OEM dyno test data from Porsche for my C2 and the engine exceeds the stated power of 247 HP, so don't believe the foolishness perpetuated by the clueless.
July 25, 2015
  Followup from the Pelican Staff: I would say that this modification is for those who wish to have a deeper induction growl when mashing the throttle, I am sure that this modification is safe because Porsche does this in the 993 cars which have the engine sound package upgrade. I have actually monitored the intake temperature sensor on the DME while driving and did not notice ANY difference while cruising down the road with a modified box, or the stock box, but thanks for your estimation of what might go wrong here. - Casey at Pelican Parts  
Carrera6 Comments: Has anyone Dyno tested this mod and if so how much HP gain can be expected? Is there also a throatier sound like that of the MotorSound Option?

April 1, 2014
  Followup from the Pelican Staff: I am not sure.

I opened a post in our forums. A Pelican community member may be able to answer your question.
- Nick at Pelican Parts
Cooper Comments: Won't this increase the air temp going into the intake, pulling air straight off the hot motor? Hot air doesn't burn as well as cool air...
August 24, 2013
  Followup from the Pelican Staff: Being that the factory opening is in the engine compartment, it should only be an increase in flow, or course the air will be whatever the engine compartment temp is. Being air is still being drawn from the same space, I don't think the tem will change much. - Nick at Pelican Parts  

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