Intake ports slightly wider than head ports

[mag2555]

A simple way to look at would be as such!
Anytime a moving mass of air comes to a change in port area it will either speed up if it's going into a lesser area, or slow down when heading into a greater area.
One place the slowing down is of a advantage is when air flow must turn from the head runner into the bowl , the greater this turn angle the more the expansion in port area will need to be if you are looking to max out flow, ,but there is a disadvantage to this also which is that the air mass should be reaccelorated back up to what it was before the expansion in port area.
The problem is to reaccelorate the air mass it takes energy , energy that could have better been used to keep pressure on the back side of the valve so to speak.
Having another area mismatch other then preveously mentioned ,like at the head to manifold interface eats up more port energy as it creates another port choke point / minimum port area and makes for another energy loss point which in turn narrows up on the motors power band Shifting that up to a higher rpm point.

[ClassKing]

Old post, I know, and I've been gone a while. But I hope the OP sees this. As a student of air flow you never....want any....step in the air path. It creates turbulence which is a loss of power. Do like the guy mentioned with JB Weld or SplashZone. If it were the opposite where the intake is smaller than the head, that's a good thing because you're dampening the reverse pulse. We used to port every intake on our ported Edelbrock heads and for three years we did that and dyno'd every engine. Being a data hound I knew I had to try an out of the box 7156 RPM intake. At the Westech dyno here in So. Cal. I had a killer streetable 455 on the dyno and I did the test. When I say streetable it had a hyd roller I profiled but was 11.0 Cr. So it ran race gas. With a 150 shot it made 823 hp. On goes the out of the box untouched RPM..........made 823 hp. We never ported another RPM again, saving our customers a lot of money. That RPM for our Pontiacs is THAT good already. It makes the most torque at the lowest RPM of ANY other intake manifold anywhere. I've been able to design my high water mark for a 92 octane, hyd roller, stock stroke stock rod length 455 with a 750 Holley from Sean Murphy Induction is at THREE Thousand RPM has been 626 lbs ft. All I care about is torque on street and street/strip engine. When you have a 4.21 stroke from the factory you can make a massive amount of power. Obviously I love this stuff.

[Tom68]

Mismatch either way can create turbulance.

Intake runners that start out big and taper down to the valve work better than the opposite scenario where you have a big valve fed by a small port.

[mag2555]

Nothing is ever completely cut in stone when different situations are in play.
If you have fuel falling out of suspension and running down a port surface somewhere then the turbulence made by a sharp edged small step will shred some or all of the dropped out fuel back into the air stream where you want it.

This is why having no back cut on a intake valve and running a full radius valve job on the intake also is very likely cutting into efficiency and Hp that you otherwise could have.

[gunt]

i have seen sharp edges be delibrate adn sort out issues

just for the guys that said they had they had to put back a missmatch manifold to gain back the times fuel ecom, did they notice the rpm powerband from a dyno ,
has anyone a view on this as crab v injection ,

[panic]

When a famous manufacturer makes a mistake, it becomes a "feature".

[BLSTIC]

I do wonder if the step down (or step up if you're a fuel dribble running down the floor of the intake port) is more about atomisation than airflow.

A narrowing step is going to create a few vortices and also pushes the comparatively slow moving fuel up into a much higher velocity section of the intake flow where it can be atomised again.

A widening step is going to create vortices and a separation point but won't be shoving fuel into a higher velocity point like the narrowing step.

Has anyone tested the mistmatch/smooth thing with port efi? Or replacing the mistmatch with a textured port?

And now for some reason I want to put vortex generators on the presumed liquid fuel path in an intake tract

[1980RS]

Years ago I tested 2 single plane intakes against each other, one port match and the other out of the box, you know what? that car ran the same. The ported one was .5 mph faster but that could have been weather related. Wished I had a picture of how far off my 2925 S/V is on my 062 Vortec heads, it's a bad mis-match but the car still ran like a raped ape down the track that day.