Carb hats and lean back cylinders
 

Ok so I have noticed that my rear cylinders are running a little leaner than the front and from what I have been hearing is that the carb hat needs to have a big spacer under the hat so the air has time to straighten out before entering the carb / throttle body , but the hood clearance is not really available. Has anybody tried make a deflector inside the hat ??

Carb hat
 

Here is a pic of what the Buick grand national guys use .. when you look at it it looks like it would be counter productive as it looks like it’s choking off air flow , but guys are running in the 9 second ranges with them and it solved the rear lean cylinder condition. maybe something could be made for a carb hat , just thinking outside the box ....

You run a FiTech correct? What intake manifold do you run and how much psi are you pumping through it?

Yes fitech 1200 right now 12 pounds on E-85

With a ported tomahawk intake

Okay, that rules out distribution issues caused by a dual plane manifold. You could look into an enclosure which is supposed to help, but I'm not certain how or if that would even work with a FiTech.

Another thing that may help is larger intake piping. Increasing the tube diameter size slows down the air velocity and aids in keeping it from slamming into the back of the carb hat. To my understanding that's what typically causes the lean condition. However that's typically due to airflow signal through the venturies on a carb. The FiTech doesn't really care about that. Have you looked into a turtle for the plenum in your tomahawk?

Another pic with different angle ...I already had to modify the hats inlet and it barely rubs the hood scoop now

Thanks Jason I have not looked at the turtle , I talked to Steve Morris and he likes to have a 2” or bigger spacer under the hat but I do not have the room .

Sorry about the rotated pics

How much leaner are the rear cylinders than the front cylinders? Can you tune around this by fattening up the entire mixture to deferentially make the rear's richer without going too rich on the front cylinders?

Hard to tell but the plugs are lighter on the rear , I like that idea of running richer across the board plus it will be a safer tune as well.. thanks

That's probably the simplest way around it.

When I was running turbo Buicks we had the same problem. A guy came up with a spacer plate with shapes cut into it to even the flow. Funny looking thing but ... no more blown rear headgaskets. Here are some examples. They would have to be pretty specifically designed, or else they would just move your lean issue to another cylinder.

https://gbodyparts.com/product_info.php?products_id=561

Dual inlets facing each other might resolve this. "Deflectors" made with 1/4" mesh screen should break up swirl and keep things more turbulent and help as well.

I did a bunch of testing with wire screens Bruce and never found that they actually straightened the air. You needed "FINS" (like a wind Tunnel has in the corners of the tunnel loop) to turn the air in the direction you want it to go.

A basic House fan with 1/2" squares 3/8" thickness on the fan frame just keeps kids fingers out of the fan blades. No real effect, wire screen even less effect. The wire screens on the old mass air meters were there to keep "rocks" from damaging the Meter.

That being said, your dual air feed system across from each other works every time.

Tom V.

Are you talking new plugs that are looked at right after a pass and not driven back to the pits?

no I just took them out to change and noticed that the back were leaner looking than the front , did a little research and found that a lot of hats that fit under hoods have this issue .,

Yeah I would make a pass, immediately shut the engine down and pull a couple plugs to verify. Under load is where this is going to be the biggest issue. Verify it's present in that situation vs. idle/cruise.