308 carb airbox restriction discovered

Russ~

On these Jalpa airboxes, are there seperate air cleaners over each set of carbs, or one air cleaner in the air box feeding them?

 

Russ, did you do any dyno runs with the top off the airbox? This is very interesting, and suddenly made me recall something I seen years ago. Back in the days when guys ran Hilborne type fuel injection at the drag strip, you would see some with unequal length velocity stacks. I remember talking to someone about it and they said something along this same line. Air sure does not act as simply as it seems that it should. In fact there are things about it that no matter how much I read I still cant comprehend.

Case in point: An airplane flying above the surface of the earth, is effected by the pressure of the air below it up to a distance of about one wingspan. In otherwords, the pressure of the air out to a distance of its wingspan is being effected before the airplane gets there. I can only assume that the air ahead would be reacting the same way out to the same distance.

Now imagine the air going into the side vent on the 308. Every convulution, even right down to the throats of the individual carbs themselves, is changing the air pressure out at the intake vent on the side of the car. Heck, just sticking your arm out the window when you drive probably screws up the air flow going into the oil cooler. I think you are onto something with your dual aircleaners. In the stock 308 air cleaner, air pressure would be alternating back and forth from front to rear as each cylinder fired in turn. Seperating it would prevent that, and slow it down between the ports on each particular bank.

I am seriously considering relocating my oil cooler to the front of the car.

 

Has anyone seriously considered running the plenum to the top of the roofline, its not far and the potential for positive pressure at speed could add many pony's?

 

I believe you will find that the offset is going to provide some gains in mixture stability. This occurs when the reversion wave travels upwards, then reverses itself to go back down the runner. With an equal spacing, the charge stands over the carb, and is pulled back in on the next cycle. The problem being, it gets a second "dose" of fuel as it passes by the venturi a second time. By offsetting, it will tend to roll as it leaves the carb on the upturn, and the carb will tend to pull air from all sides missing some of the fuel laden air.
In the older dragstrip injection (Hilborn, etc) the racers first left the tubes open to the air. They quickly discovered that when the motor hit its RPM range that reversion occured, the windshield was quickly covered in fuel mist making it hard to drive.
Watching a motor on an engine dyno is a good example, as you can easily see the standoff when it ocurs, as it appears to be a fog standing over the intakes.

 

Matt, very good, most people are unaware of this phenonmenom.

 

Air does strange things....there is a post in a violin that holds the top and bottom apart, but it's not centered. If they part it in the center the violin barely makes any sound. They need to offest it about an inch and then the sound will fills a concert hall. Little thinks can make a big difference.

 

If you look at the 308 air box there is a small chamber around all the carbs. But the air can't get to it b/c of the insulating material.

When I took my air cleaner off to remove the carbs, I noticed it's only dirty in one spot, where the air comes in as I'm sure has been pointed out in this thread.

 

Just a thought on carburated engines. You will likely get more fuel stand-off If you go to bigger cams. Carburated engines need at least 110 degrees lobe speration angle or more to insure a proper signal to the carb. Injected motors can stand significantly smaller seperation angles.

 

simple math:
1.5L = .053CF
.053cf x 8000rpm = 424cfm per bank

these are gross numbers but should give you an idea. accutual would be based on your VE%, always under 100.

 

IMO Russ, the bigger the better, providing you can physically fit it in the space (within reason). I believe that the larger element will offer less flow restriction, and therefore more power. One item to look at though is the size of the snorkle, what will fit it? The goal is to provider plenty of cool clean air if you want power. Much larger than the snout opening isn't going to gain much, as it then becomes the restriction.
Thinking of offsetting the carbs relative to the air cleaner?

 

I always like the biggest filter that will fit. They disturb the air like and almost alway make more power.

 

I know you guys have been working hard on this and at first I was skeptical. I was messing around and came up with this. The engine is getting lots more air but I am wondering if it is getting too hot with the extra coverage. Let me know what you think.

Rob
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Damn Im busted! I just dont have the same real skills as you guys so whats a guy to do!! Did anyone see the Ferrari at the Olympics!

Rob
"the trouble maker"


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I ran dyno yesterday with the previous set up from last year,
Gutted airbox
Short velocity stacks
135 main jets
190 air correction jets
55 idle jets, (I think).
The carbs are rebuilt this year also.

Last year I pulled 184 HP with 140 mains and 200 air correction jets. The air/fuel ratio was around 12.2. A little rich.
The HP only came up to 177@6,000 RPM this year versus 184@6500 RPM last year due to ignition breakup above 6,000 RPM but the main point of the session was to verify the final jetting change to 135 mains and 190 A/C jets.

The HP and torque curves were about the same as last year year up to 6,000 RPM. The air/fuel ratio averaged 13 to 1, hitting 13.1:1 at 4,600 then dipping momentarily to 12.6:1 at 5,500 RPM and then moving back up to 13.2:1 at 6200 RPM when the ignition crapped out.

Note that the RED curve is the pull in 4th gear and the pull that I am referring to. The BLUE curve was done in 3rd gear as an experiment to try to anylize the ignition miss. Oddly the miss doesn't appear in the RED curve although I saw it on the screen "live" while doing the pull

I was happy with relatively the flat curve throughout the pull so I am sticking with this set up.

Next step is to pull the plugs and take a read on those guys and see what this high speed miss is about.
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funny thing...read post 92....

 

I'm sure that is the seed that got things rolling on the Jalpa airbox mods. I'm sticking with the stock box, gutted. For what I am using the car for, it's enough.