308 carb airbox restriction discovered

When I originally fabricated the new snout for the box I made it with an opening that was taller than stock to maximize the flow from the side-of-body opening. I started running into clearance problems with the cooling system outflow tube below and the buttress shelf above.

I sat down and rethought what I was working with and what I was trying to achieve

1. I am working with a little 3 liter engine that is choked up with a restrictive airbox and low profile emissions cams.

2. The engine is not modified and the carburation is essentially stock so what REALLY needs to be improved?.

3.Start with the aspect of the intake tract that cannot be modified and optimize the system from that, the side-of-body opening. Appoximate the total volume of air that could potentially pass through that opening and open everything up down stream that may hamper the flow.

4. Gutting the airbox was the first step, improve flow through the snout by opening it up and reconfiguring the shape. Install a smooth bellows.

Considering the requirements of the engine and the type driving I was going to do I felt that returning to a slightly larger oval intake on the snout was sufficient. A smooth bellows was in the plan but time restraints and lack of material nixed the idea. That can be worked down the road.

Except for the red airbox the system looks stock.

Ultimately, I believe a large single inlet is the way to go if you want to maintain the basic layout of the engine bay especially if you are running a "4 liter" engine.
. Not sure about pulling air from the wheelwell, a lot of junk flies around in there.

The side scoops do indeed work. When the temps go down in the winter I have to plug the inlet on the drivers side to get the oil up to a decent tempurature.

 

Spasso,
Still waiting patiently, stuck in the arctic North here, for your impressions of the new airbox design! WELL????

Birdman

 

I just put the car back on the floor this morning before going to work. I may fire it up tomorrow morning (Wednesday)just to make sure it will but won't be able to take it out until Thursday morning at the earliest.

 

Man, threads like this are what make F-chat worth its weight in gold!

I've been and still am all ears!

 

Has anyone used anything other than the OEM rubber bellows to connect the airbox to the side scoop?

 

I believe Kermit has according to a previous post in this thread. Used a section of inner tube?

Or maybe in this thread,
Low Profile V-stacks

 

Fired it up this morning (indoors) after sitting for almost 4 months for maintenance. When cold I am getting some popping through the carbs and some through the exhaust like it is running lean, more so than before the modifications. When it warmed up the popping went away mostly but still did it. I may have a couple of air bypass jets out of adjustment.

I brought the engine up to 4,000 RPM and held it for a while. It sounds like I am getting an intermittant high speed miss which I think may still be a fuel problem.

I revved the engine hard 4 or 5 of times and it sounds like all 8 are hitting under load, a good sign. It seems to rev willingly and quickly with just a little flat spot on tip in (no big). The intake sound is distinctive but I can't tell any real difference in volume until I get the car out and under load.

I will check spark distribution with my timing light tomorrow and pull a couple of plugs to see what kind of shape they are in. The car came with 20 new Bosch plugs so I may try to change those before shakedown on Saturday.

If it is dry out tomorrow I'll take the car out for a freeway run and see how it pulls in 4th gear.

Update tomorrow pm.

 

<Cross Posted>


Well, I changed from 125's to 135's when I did the box mod and I am running a stock style Baldwin filter element. The true test will be on the road then........................

No changes to idle jets, no changes to air bypass jets (basically).

Dyno a week from Saturday, Woo Hoo.............

 

Make sure you get a fan blowing some air on/into that side intake!

 

Maybe we can hang a compressed air line in there.

 

I just returned from an hour long drive. Starting with freeway and then some twisty back roads.

The induction noise is much louder, the loudest and best sound between 5 and 6 thousand RPM. The engine seems to rev more freely but I cannot feel a "seat-of -the-pants" increase in power. I seems that I have lost a little low end power but not enough to compromise the over all performance.

Judging from the color of the exhaust tips and the more pronounced intake popping when cold I am guessing that I am still running lean, even after changing from 125 Mains to 135's.

I may try to stick some 140's in after I pull a few plugs and see how they read. If the plugs are indicating a lean out condition then I think the 140's would do the trick.

I am out of time today but tomorrow morning (Friday) I will pull some plugs see what they say.
__________________

 

At cruise the induction noise is the same as stock. I hear more going on inside the box when I work the throttle, transition noises, pulsating at lower RPM's etc............

Being a '78 my cams are also "conservative", to a fault. Your typical emissions profile.

Throttle sensitivity at all ranges seemed the same but didn't pull quite as hard from the bottom.

I am in the process of securing some 140 mains, hopefully in the next few days so I can change the mains if required.

I will take pictures of the exhaust tips and spark plugs when I pull them out tomorrow.

 

These are the dyno numbers taken from a 79 308 GTB with a STOCK airbox and the Low Profile High Flow velocity stacks. It appears that they are good for at least 5 out of the 10 to 12 HP gains according to subsequent dyno numbers with the modified box and "short" stacks.
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Hi Russ
Which was better?
John

 

Here are the pictures of the exhaust tips and spark plugs from my drive yesterday. It's obvious by the condition of the plugs that the car has been running very lean for quite some time. Even with the jump in jet sizes it is still running really lean.
The plugs in the car are currently NGK BP6ES. Two boxes of brand new plugs came with the car when I bought it. They are Bosch W7DTC. Enclosed are pictures of those with the multi-tip.

I think I need to put some 140 main jets in and change the plugs. I would like to use the Bosch (I have 20).

Any suggestions for gap?
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Dana,
It does appear to be a bit lean yet to me. I prefer to be able to look down into the inner part of the insulator for a truer reading though. The base of the plug does not show any dark however, which is a good indication that the lighter color will be seen all the way down the center.
Interesting, in that it would appear that the more open air filter snout is getting more air flow the stock gutted unit, IMO.
FWIW: I'd say that the 140's would not hurt your application a bit. Others, like Russ may be able to call this one better, but in my experience, an air correction jeet change will not give you the mixture you need.
Kermit

 

Here is a shot of the inside of the tips. Not sooty at all. Dark and dry.
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I do indeed have F36 emulsion tubes. Don't know what my idles are but a pretty safe bet they are stock 55's?

I cleaned up the old plugs, regapped to .032" and reinstalled them. I will be going on a 100 mile+ drive tomorrow to do some road "research". I'll pull them again and see what they look like.

Those Bosch plugs have a triple tip on them and I am curious if the gap requirements are different than a standard plug.

What is the stock gap on a standard plug? I am at work right now and can't check.

Thanks,
DJ

 

So BP6 ES is hotter or colder than the BP8 ES?

I couldn't even find a listing for the Bosch W7DTC plug or an application for them in a recent Bosch book. The highest it went was W6DC. WTH?

I measured the factory gap on the Bosch plugs and they range from .026" to .028." They can't be cheap and judging from the condition of the plugs I have in there now it would be nice to replace them with some new ones before dyno day.

 

BP6ES is hotter than a BP8ES or BP7ES (smaller number is hotter). You should be using a 6 (most carb 308's are running NGK 6's), and depending on whether you get any fouling...possibly even a 5. But 6 would be a good starting point. With your ignition system (same as what I had on my 78) I would open up the spark gap to around .034. When I ran .026 I ended up with a high rpm miss. Opening up the plug gap smoothed it right out.

Regards,
Dave