VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1912-1928 1928 Capitol 1 Ton Truck - Head Intake Port

I'm rebuilding a 1928 Four and when I removed the intake manifold I noticed the honeycomb device inside each intake port. Does anyone know the purpose of these devices? Only thing I could think of is air heating... Thanks, Maxie.

You're close. They are air diffusers.

I knew what they were, but given what we know about engines today, won't the engine breathe better without them?

Restrictions in the air flow can be a limitation for combustion engines. They can cause reduced flow rates or increase the pressure drop which might result in lower horsepower. Another consideration is laminar flow versus turbulant flow. The size of the pipe, number and radius of bends, smoothness of passages all have an effect on turbulance. The less turbulance the less pressure drop. All of the above is really only significant with high air flow rates.

Another consideration is efficient mixing of gasoline vapor with air to produce an uniform mixture. Part of that assumes that the liquid gas is completely vaporized. Therefore some turbulance (to improve mixing) and heat (to aid in vaporizing the droplets of gasoline) are helpful in carburated engines. As above the lower the air flow rate the less turbulance.

Now for the air diffusers. What do they do? How about reduce turbulance? Good answer! Long relatively narrow passages (length vs width is the key) can quickly reduce turbulance and produce laminar flow. So even though the diffusers take up space in the intake they may be very beneficial in engine performance. At least the '28 Chevy engineers felt that way. Otherwise they would not have put them in. Particularly since they increased the cost and increased cost with no benefit was not a Chevy engineering policy.

Oh yes, with the normal operating range of a 1928 engine @ 200-2000 rpm the flow rates in the intake (and exhaust) are not high enough to reach the limitations of the passages. Now if you balance the engine, increase compression and raise the operational range to 1000 - 8000 rpm then 3 to 5 times as much air will need to pass through the same size passages. Restrictions then become significant.

HUH??
Mike

Maxie
What I think chipper was trying to say, Chev put the diffusers there for a reason but considering the size of the manifolds and air flow the diffusers are not going to provide any restrictions. Leave them there.
Tony

Hi Maxie,
The diffusers were used mainly(I stand to be corrected)to help break up the heavy fuel particles in the old fuels that were around when these cars were built.The fuels back then would probably been similar to kerosene,and they needed all the help to break the fuel into a finer mist/spray for more even combustion.
That's probably why the 4's needed the hot air feed to the carb,to help vaporise the heavier grade of old fuels.Whereas today,we face the opposite problem:fuels today are more volatile,and vaporise far easier,and excess hot air into the carb on a hot day/heavy climb,could lead to vaporisation.
So,as tonyw says,if in doubt,leave them in.