duck1

I have a 1990 C2 which is stock except for a K&N filter and some springs and shocks. I have considered "drilling" the air box lid, but when I asked my mechanic about this he said that it allows the engine to ingest hot air which would defeat any performance increase. I see the small "horn" on the airbox which apparently draws cooler outside air into the engine, but does it make that much of a difference. Before I drill out my stock lid or buy one of the lids from a catalog, does anyone have any input on this issue.
Thanks,

Rick
90 C2 Coupe

Phil Raby

I think you'd be optimistic to expect a performance increase, but it will sound great!!

Phil

Bill Gregory

Rick,

First, some assorted information: The 964 airbox was designed to meet restrictive Swiss noise laws. On the cup cars, the factory said that a hp increase of up to 7 hp was available by removing the cover and exposing the air filter directly to air. That's been debated whether it's real or not.

So, why drill the cover (most cost effective) or cut it mostly away, or buy one of the $175 cut away covers? Sound. Maybe there is some slight hp advantage, but that's an extra bene if it's there. What is real is a much improved sound. Idling and light pedal pressure show limited change in sound (some have described it as close to an older American V8 idling), however, floor it and a wonderful set of sounds flow. I also have a primary muffler bypass in place, which may influence the tone of the sounds further.

horst

"...full of sound and fury...signifying nothing..."

Drew_K

Most people like the sound of their drilled out airboxes, but I've tried it and actually didn't like the way it sounded. To me, it made a droning sound which I liked at first but eventually found annoying. It took away the 'scream' of the engine at high rpms, but then again I've always preferred the sound of an F1 engine to a big block V-8.

It's subjective I realize, but I didn't like it. I would buy an extra airbox lid and try it out. I found one for $30 from a Porsche dismantler in L.A.

Also, I'm sure it was just my imagination but it seemed like the engine didn't pull as well above 6000 rpm.

Drew
92 C2 (probably the ONLY person who didn't like the drilled airbox)

Cupcar

When the 25 Carrera Cup series cars were being built at Andial each engine was dynoed on an engine dyno to guarantee the engine's horsepower. One of the modifications to the racecars was cutting away the top of the aircleaner so,I asked Andial what the air cleaner modification was worth and was told 2 HP. Sounds like 102 HP <img src="graemlins/jumper.gif" border="0" alt="[jumper]" />

Jeff Curtis

I think the modification is well worth it, great sound quality improvement, if there's a HP gain - then it's all the sweeter!

I found that I really like my drilled airbox while on track, it helps me figure out if I'm lugging a gear or not...and it's a good audible reference in areas where I don't want to look down at the tach.

The car was so quiet before, the airbox mod, I believe it sounds louder to the driver than it ever would to a passerby, or another vehicle...being that it's up so close to the rear wall of the interior, again, makes for a good audible reference.

duck1

Thanks for all the honest input!

Rick
90C2

Emanuele Ferry23

A simple calculation about the airbox.

Imagine your 964 is running at 4400 rpm in 5th, which means your doing 170 km/h. The engine sucks ( in the literal sense of the word...I would NEVER mean something else!) 132 litres/sec of fresh air thru the short curved pipe of the airbox. Assuming a section of passage of 7850 mm^2 ( diameter of airbox pipe about 100 mm ) we get an average speed of air thru the airbox pipe of 16.9 m/s or 61 km/h. This value of air speed is relatively low and so there are no significant pressure losses in the short curved pipe connected to the airbox. The only perceivable effect of drilling the airbox is different sound. Take into account that the curved pipe allows the engine to breathe fresh air. With drilled airbox you suck air coming from the worst part of the engine : the side of bay with hot areas ( oil lines etc. ) This may result in sensible density loss.
However air breathing system in the 964 seems to be designed to eat power ( mechanical flow meter, long pipes from airbox to cylinders, airbox in the worst possible position etc... )

Hope this was helpful

Greetings

Emanuele "Ferry23"
'90 Guards Red Carrera4

Arthur.

That's what I'd call a pretty scientific approach as to why the only benefit is sound. I like your reasoning, it seems to be ok <img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />

Robert Linton

MMC wrist pins, saving 30g to 50g per pin (x 6), would be more useful than all of the air box modifications.

Kevin

Emanuele;

I like your velocity reference. However the curved bell shaped airhorn, was designed for noise abatement. Can you imagine air horns placed on the old weber carb rain/filter housing to prevent any heat buildup.

jonfkaminsky

Rick, Check the archive (or search) for Performance Enhancments Study II. I dyno'd the effect of this mod before and after. There is NO significant difference in power. The sound increase, however, is significant. If you still want to drill your airbox, let me trade you mine. I'll pay shipping and add $25.00 for your trouble. My neighbor's bedroom window is right next to my parking place and the throaty sound is annoying to them as I keep late hours with my coming and going.

Jon

GeoC2cab

High,
So is it harmful to the engine to have a drilled airbox cover?

Flying Finn

[quote]Originally posted by GeoC2cab:
<strong>High,
So is it harmful to the engine to have a drilled airbox cover?</strong><hr></blockquote>

Nope!