308 carb airbox restriction discovered

I didn't remove it as it was already gone. As it is now, it's just airbox, airfilter and accordion like ribber hose connecting to the right side duct. The most noticeable increase in performance I felt was in low to midrange rpm - more torque in addition to the SOUND. I swear if you put your ear next to the duct at idle you can hear the intake valves working. On the road at WOT it's nothing less than fantastic.

Last week I replaced the air filter. Examining the old one I could see debris caught in the filter all the way around. Before the mod, debris was isolated to just one area.

 

Birdman you need a huge, orange, REMOVE BEFORE FLIGHT banner!

 

You would think that the big while towel hanging out of the intake like a giant molotov cocktail would be sufficient, but when you are in hurry....well, I forgot once!

"Why won't this thing rev? Oh crap! I left the towel in the intake!"

You should see how far it sucked it in!

 

I don't know about "legendary" but hey, if I'm to have a legend, why not!?

The stumble is nowhere near as bad as it once was. You know what helped a TON? Replacing the needle valves (float valves) in the carbs. I noticed that every once in a while when I turned hard, I would get hesitation and some intake backfiring. I suspected that one of the needle valves was sticking because the fuel sloshed and jammed the float up, pushing the needle valve hard and sticking it. Then the bowl would run dry on fuel and two cylinders would go way lean. Then the valve would finally unstick and the carb would work again. So just for the heck of it I pulled the carb tops, checked my float heights and replaced the float valves. Problem gone. So I would check your float heights and replace the float valves.

Birdman

 

Good to know, thanks much!

 

That looks really good. Any idea or guesstimate as to how many more cfms that mod will add to the intake?

 

my 308 gtb (fibreglass) originally had 3 sound insulated perferated tubes, of diameters of about 50mm. so i thought i've got 8 chokes trying to suck through 3 pipes, and therefore a big restriction. in my enthusiasm i milled the whole lot out. the "modified" air cleaner box was refitted. what a mistake the whole thing turned out to be. the noise was horrendous! then something occurred to me... sure i had 8 breathing through 3, but those 8 chokes dont suck 100% of the time...it's a 4 stroke engine, right? so given that, logic says, i'm getting 8 chokes sucking for about 25% of the engines cycle, the orginal 3 tubes should provide more than enough breathing. any way i managed to get a replacement, so all's well again. this is my expeirence on the matter. any comments gratefully recieved.
alan.

 

Welcome aboard, Alan!

Yep, if you pull all the material out as the thread says it WILL be louder!

Birdman, my scoops have the mesh screens in them so the towel would not go far! I have to check them for road debris and low flying birds......

 

Alan, is your car a Euro dry sump or North American wet sump???

There are different specs for the two......

 

thanks for the reply. my 308 gtb is a dry sump right hand drive english car. unfortuneatly, the only replacement air box i could source was one similar to yours. the proof of the pudding is in the eating, and if you are getting better performance and better throttle responce, then the restiction must be there. but i found the noise gave me a earaick. (bearing in mind the intake is right behind my right ear!
regards.
alan.

 

I have to say that the noise gives me goosebumps, but not an ear ache!! However that being said, I do not like Tubis in general because they are too loud and draw unneccesary attention. I also don't want to listen to that drone all the time. I find that the airbox intake mod only makes the intake loud under accelleration. Cruising on the highway, it's not really that much louder than it was before.

Birdman

 

Here are some pics of the airbox from my 1979 Euro 308 GT4. The air filter had 3 black circles the same size and in the same place as the openings.

Needless to say I now have a small pile of metal and glass fibres in my dustbin! The car now sounds much better, is more free revving and no longer sounds strangled at high rpm.
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I know, it's quite surprising really. The 208 GT4 I had before was completely straight through.

 

wow thats like a restriuctor plate!!!

 

ZOINKS Russ!!!!!

Those look amazing.

 

I like many have just HAD to do this mod.
I have recently purchased a 1976 Glass 308GTB from the UK and imported into Australia.
After waiting months for the container to arrive and to get the car home, one of the first things I had to suss out was the airbox.
To make my waiting even longer, slight corrosion and mould crept its way into the car during the containorisation even though it was all sealed up.
So when i attempted to open up the airbox 3 of the 4 airbox screws were very reluctant to loosen. Even though I used spray lubricant, one of the threads sheared off.
Anyway I was quite surprised to find that the airbox was massively restricted even moreso than your boxes from the states.
The car had been used for track work and had a half cage and harnesses etc, but clearly wasn't breathing sufficiently.
Even though I was surprised at this, I was quite pleased because that meant I had to tackle the mod.
Purchased the Dremel and went for it.
Mine had the ridiculous 3 tubes of perforated metal and wading, and yes three nice dirty marks on the airfilter to line up with the tubes like the GT4 above.
Certainly this "Freeing up" was going to be successful.
As you can see from the before shots the box needed a tidyup anyway.
I stripped/sandbalsted the whole box down and then attempted my first "Crinkle Finish" paint finish.
The last photo shows you the end result of which I am very pleased with.
The car has a "Bell" brand of stainless exhaust from the UK of which I am not familiar. Nevertheless it sounds fabulous and is quite loud.
Keep in mind being a RHD car, the drivers position is just inside from where the carbs are sucking, and when you wind it up she sounds great.
Revs freely to redline and beyond!
Being a new car I am going over everything with a fine tooth comb. I have rebuilt the carbs, and now with the open airbox will have to check the AF ratio to make sure all is AOK.
Thanks Russ for starting all of this.
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Yep I agree, water gets in and with the poor airflow with the original, doesn't get much of a chance to evaporate the water out.
My air duct has the typical hole in the bottom of the air duct before the bellows. Driving rain would definately overcome this though.
Hopefully with this increased airflow any water will have a better chance of evaporating, be it that it would be ingested by the engine.

 

Steve,
Did you take a picture of the inside after the padding was removed?
Cheers
Nick