[olddog]

I’ll make a few comments on fuel falling out of suspension. I recall having some SB engines that would load up at idle from fuel droplets forming on the walls. They had factory stock heads and intakes, but radical cams. So in those cases, it wasn’t from the ports being too large.

So let’s examine why a radical cam might be the root cause. Liquids have a hard time existing in a vacuum, especially gasoline. I think it’s called vapor pressure. A high valve overlap cam may idle at 10”Hg of vacuum, where a mild cam may idle at 19”Hg. I think a perfect vacuum is about 27”Hg, if memory serves. The higher the vacuum, the less likely fuel is going to drop out of suspension and form droplets on the walls.

With long intake duration, the intake valve may not close until the piston is well on its way up on the compression stroke. Often the intake valve does not close until 50, 60, or more deg after bottom dead center. At an idle this can cause the air to blow back up into the intake – reversion. So, at some point the velocity goes to zero before it reverses. Now if you want to talk about low port velocity, it doesn’t get any lower than none. You’re not going to avoid this no matter how small the ports are. Radical cams bring the port velocity to a complete halt at idle, regardless the port size.

Now the intake valve spends almost ¾ of it’s time closed. Then why isn’t a big puddle of fuel forming in the port and intake runner at idle during this time, if low velocity is the cause of the problem?

Now let’s talk about port velocity, when an engine is at an idle. A 350 cid engine at 100% efficiency is only pulling in 600 CFM of air at 6000 rpm. At 600 rpm that is only 60 cfm, with the throttle wide open. Close the throttle to let it idle, and where is the flow? Maybe there is 5 to 10 cfm at an idle, on a typical engine. Port velocity is practically nonexistent at an idle anyway.

I do not buy that large ports will cause fuel to drop out of suspension at idle. I think radical cams cause it, and big ports get blamed for it. After all, who puts large port heads on an engine without sticking a big bump stick into it, now days?

The big block engines in the 50’s and 60’s typically had large ports and mild cams. They lugged around big luxury cars, and had stump pulling torque from off idle to ~3500 rpm. They purred like a kitty cat at idle. Not all of them were large displacement. The FE started out at 331 cid.

However, I do think too large of ports will cause a bog or a lag in torque coming off idle, similar to turbo lag. No doubt it is hard to make low and mid range torque, when the ports are too large, but the cam choice can make it much worse.