Shark Attack

By mistake I took my 79 out for a 6 mile drive with out the airbox / filter installed. I was driving along and the thing was just runnin great.. for sure a seat of the pants increase. I was wondering what was going on then i remembered i had the airbox off, DOH!!!

the car has less than 2000 miles on the air filter, less than 100 miles on the plugs wires cap and rotor. FYI...

mark kibort

yes, anderson did test 7hp gain without a filter on the filter box

i did a test with a removal of the filter box and a bell mouthed inlet 4" cone filter . way up high, no way the hood would close, but we did a dyno. guess what.........? thats right, lost 5hp. put the air box back on with the KN replacement, and 5hp came back. the stock air boxes and filters are pretty darn good! now, if you want to run without a filter, the stock air box bottom is pretty optimal! bell mouthed inlets flow much better than just a MAF in the air.

Mk

heinrich

well, this topic has been studied over and over and over, and the very scientific conclusions are that the stock airbox is usually best. In high-displacement cars, holes in the rear of the airbox are said to add power but I have worked with Louis Ott in years past on airbox mods and there are some that will gain power, but most will lose it. And, the gains are very small compared to supercharging etc...

mark kibort

Ive done a lot of testing with the stock air box. its very efficient, due to its toliet bowl design. (funny, but opimal) however, when sealed, which means you need stronger clamps and sealed boarders for the filter, it produces .5"Hg vacuum under WOT, even at 100mph. so, there is about a 1-2 % gain for most, if you vent the air box properly.

mk

heinrich

I think you're quite right Mark

danglerb

Airbox also reduces intake noise, take it off and you may 'think" its faster when its just noisier.

Not to mention issues related to disrupting the flow through the MAF making it work inaccurately.

scott863

Back in 1991 I ran my '66 912 for a while without proper filter setup for a while (I was 16) and then the engine started running miserably. I can't remember what I did with that car, or who actually bought the thing, but, I do know the improper filter setup or lack thereof, had done some $amage.

Whenever I read about finding horsepower with some crude fix involving holes being but randomly (also popular in 944 circles), parts being thrown away, etc. it reminds me of the precision with which Nascar mechanics used to adjust spoilers during a pit stop, namely they'd give it a couple of wacks with a ten pound sledge hammer.

C.F.

FYI. I can remember seeing pics. on a UK 928 list of a '88 CS (euro only) aircleaner box compared to a stock ('90 I think) air box & it was different. Far more of a radius on all of the bends, tubes to box & box to maf. So may be they did improve the design , but then why not put on all S4's ?.

John Veninger

I gained 5-10HP on my hybrid EuroS motor with the rear of the airbox cut opened. Not a mod for street cars since a firewall section had to be cut.

Mike Frye

John,

Good morning!

Why did you have to cut the firewall? I think Mark said he did that but vented it up through the hood to address a 'low pressure area' I think? Still maybe not practical for street, but I was just wondering.

John Veninger

I extended the back of the airbox top with sheet metal to the base of the windshield. I closed off the stock tube inputs.
Needed the space for the large opening.

sublimate

Here's a tested airbox design which has proven to make about 10+ extra hp:

Fancy airbox setup












OK, maybe that should be 100+ extra hp.

danglerb

Boost works, but lacks elegance.

mark kibort

i actually cut the firewall too. only down to the fuel line pipes, but it gives the minimum clearance to vent to the base of the windshield. i used a couple of the air inlets pointed in reverse and mounted to the top cover of the air box. it lost the .5" Hg vacuum present under WOT conditions on a sealed air box. (stock air box will only make .25"Hg, because the other air is leaking in from around the sides of the filter!)

with all the "stuff" out of the base of the windshield area, this is easy, on a stock car, a vent could be made, but its tricky to cut the firewall and seal things up. certainly a good mod, even for a stock set up. you just have to get creative on how you vent and make the little bridge duct.

mk

SharkSkin

You mean with the entire airbox removed so you could see the air plate, or just the top of the airbox & filter removed?