DI Engines: Air Velocity is Dead? Long Live Air Velocity!

[NewbVetteGuy]

I'm going to ask a fairly open-ended question, as I find some of the best discussions on here either start or tend to move that direction anyway:


I keep seeing what appears to be increasing port sizes / decreasing CSA and decreasing air velocity in DI engines vs. their port injected and carbureted "ancestors": (Yes, I'm mostly looking at the SBC-> LS-> LT engine family progression) Does air velocity just not matter as much with DI? (My best guess is that velocity is still critical.)

At the same time, I see some very well respected head designers / porters touting DI head ports that improve velocity vs. the OEM designs and consequently restore air velocity.... What's going on?!? Did the OEMs make mistakes making these ports too big?


I thought that dry flow through a manifold and runners meant you could actually get away with less volume because you don't have fuel and atomized / evaporated fuel displacing air -again, then why larger volumes?




Adam

[midnightbluS10]

In this article from July 2019, they state that inlet velocity is no longer crucial for maintaining fuel in suspension so there is little concern over port velocity anymore. Their main concern is airflow at the moment.

Makes you wonder if GM did testing on flow with more velocity vs the same flow with less velocity and if so, what kind of results they saw to decide port velocity was no longer a main concern. Especially if that's what the aftermarket is improving on and making more power as a result.

"Reyman started this discussion by commenting that the LT1’s intake port is very large – over three square-inches at its largest cross-section. He says GM’s cylinder head designers have moved away from concern over port velocity because with direct injection inlet velocity is no longer crucial for maintaining fuel in suspension. The bullseye now is airflow. With previous engines, intake port velocity was crucial because if the air moved too slowly, the fuel (being heavier) could fall out of suspension. Direct injection eliminates those concerns."

https://www.enginelabs.com/engine-tech/ ... -di-heads/

[NewbVetteGuy]

In this article from July 2019, they state that inlet velocity is no longer crucial for maintaining fuel in suspension so there is little concern over port velocity anymore. Their main concern is airflow at the moment.

Yep. That's what I wonder about.
I thought that more velocity means you can support a later intake valve close, which is critical to trapping the most air in the cylinder. (Velocity is a key to fighting reversion, too, right? -Although I keep hearing statements that reversion isn't such an issue with DI engines; not clear on why...)

LS3 has a 257cc intake port volume.
LT1 has 297cc of intake port volume and flow tops out @ 304 CFM @ .550" lift (2.120" intake valve)

CID offers a 2.5 MCSA intake LT1 head which comes out to only 277cc and does very well...(Obviously should for $3,300 a head!)

Gwatney 320cc
Frankenstein 325cc
Katech's custom LTK casting 327cc
(I'll go pull CFM ratings at lifts to calculate velocity, eventually...)


The LT2 intake port volume spec is mysteriously missing-in-action, essentially anywhere... It's really weird that GM has released the intake port volume specs on essentially every engine EXCEPT FOR the LT2... They talk about the larger plenum (14.1 liters vs. 11.1 liters), more even air distribution, longer runners, 4% improve flow, but never the intake port volume.

-I'm guessing they saw the error of their ways and that +4% in airflow is happening with < 297cc intake port volume; but if not, that's saying something, too...


Adam

[modok]

Velocity is still critical for the engine, just changing where it IS in the engine.
Probably high velocity at the valve, or an inch upstream of the valve.

Velocity of the intake runner IS a supercharger.

But IMO you don't need long runners with forced induction, if the blower or turbo efficiency is better than the runners are

[Mummert]

In the motorcycle world cylinder heads seem to change every couple of years, bigger port smaller ports back to big ports, bigger valves than the riders complain and they put smaller valves back in them, straight ports that can be mirrored in master cam, then the offset the ports trying to get movement in the cylinder, then they go back to straight ports, they do clover leaf style combustion chambers and then they go to square box looking chambers back to clover leaf style.
All I can say is whats right is right, when the runners are too small we open them up, when they are too big we epoxy them smaller, when the valves are too big we try cutting a bunch of duration out of the cams, if its not enough you put venturi seats and smaller valves back in them.
When you remove the fuel from the runner its already bigger, ports can lose 4-5% flow wet vs dry. They'll go that way until they go too far and then someone will shrink one down and come up with some magic and then they'll go the other way. If engineers aren't changing something they must not be doing their job, right .

[KnightEngines]

OEM's are using large ports to get decent volumetric efficiency with small cams over a wide rpm band.
They're not concerned with making better than 100% VE, they just want a wide usable power band & low emissions.

Which is great till you actually want to make 110%+ VE, then you gotta either up the cubes, turn it much harder or shrink the ports, or a combination of the above - coz the ports are just too damn big for any decent inertia ram.

[KnightEngines]

Double post, dammit.

[David Redszus]

OEM's are using large ports to get decent volumetric efficiency with small cams over a wide rpm band.
They're not concerned with making better than 100% VE, they just want a wide usable power band & low emissions.

Interesting that you say that.

Many years ago we were working with Yamaha and one of their engine techs said:
"Racers want more horsepower. We give them more mid-range torque and they win
races and no longer care about peak power."

The more things change, the more they stay the same.

[hoffman900]

Electronic throttle bodies also play a role in the strategy of the entire intake system.

Not worrying about fuel suspension also allows a designer to focus on air flow as well as wave dynamics. More so as compared to a carburetor engine where you’re worried about jet signal. Sans rules, I don’t know why people tie an arm around their back from the get go in that regard.

[NewbVetteGuy]

Velocity is still critical for the engine, just changing where it IS in the engine.
Probably high velocity at the valve, or an inch upstream of the valve.

That makes sense.

Throttle body / plenum / runner area or entry sizing -not as big of a deal / error on the side of upper RPM HP as you don't have to worry about fuel dropping out of suspension?

-These are the areas that matter less with DI? Or can safely be moved towards the upper end of the rpm range?


Adam