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Port volume vs flow vs velocity

General engine tech -- Drag Racing to Circle Track

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digger
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Re: Port volume vs flow vs velocity

Post by digger »

GARY C wrote: Tue Jun 15, 2021 5:37 pm
BLSTIC wrote: Tue Jun 15, 2021 5:26 pm
GARY C wrote: Tue Jun 15, 2021 4:30 pm
If power is increased by reducing air speed does the term "inertia ramming" really matter?

Most if not all port work reduces air speed and gains power...
I didn't say that power was increased by reducing air speed, I was meaning the high flow was allowing the increased top end for the same or smaller camshaft. Perhaps I should have been clearer. I should have said "big valves and big ports that have high flow...".

If you have an infinite flowing port/valve (bear with me for a sec here) the cylinder will be full shortly after BDC at most any rpm, so you can close the valve much earlier and avoid pumping charge back into the intake at low rpm. This is pretty much how early 4-valve stuff operated, small cams, huge valve area, large ports, and they were stupidly nice to drive compared to 2v engines with similar power per displacement. It also appears to be how these LS-truck engines are operating.

And yes the inertia ramming thing really does matter, if you're chasing significantly above 100% VE. A manufacturer can usually increase engine size (or boost) to make more torque. We in the aftermarket/racing world have displacement limits for one reason or another and need to keep filling the cylinder after BDC, and the only way to do that is with port velocity/inertia.

However I don't have all the answers. It's pretty clear that the factory COULD have had a port that flows *almost* as well that was much smaller, especially at the port opening. I'm not sure why the 'large port volume' appears to have been a target in and of itself. There could be some inertia-related reason at the start of an intake stroke that works well with low overlap cams, or some other thing. The factory wasn't stupid, they just have different goals. I do wish I understood their reasoning on that.
Yes I don't understand the OEM big ports either but it didn't hurt tq the way I was taught it should. Lots to learn I guess... :D
OEM big ports are for a flat/wide predictable powerband. They don’t make big torque per cube
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Re: Port volume vs flow vs velocity

Post by Steve.k »

What would a example of that be?
CA4E9132-D682-4951-B27F-ADF8A8211605.png
This is a big port chevy. Tq seems thick to me.
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digger
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Re: Port volume vs flow vs velocity

Post by digger »

Steve.k wrote: Tue Jun 15, 2021 9:54 pm What would a example of that be?

This is a big port chevy. Tq seems thick to me.
Every post 2000 oem NA Chevy, just look at tq/cube it’s not in the impressive range at the stock power level.

The ports are too big for stock power level as it gives the best overall compromise for them. Not that it’s a bad thing for the aftermarket
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Re: Port volume vs flow vs velocity

Post by travis »

Walter R. Malik wrote: Tue Jun 15, 2021 6:31 pm Maximum horsepower at Wide Open Throttle has little to do with street driving. Keep it all in perspective.
And I think, yet again, that this is where my confusion comes from.

I’ve found a large number of posts on here that state that anything over 330-350fps is too high and you start getting more pumping losses, more possibility of the port going turbulent, etc.

Larry’s posts are highly informative, no doubt about that. But…I don’t build SS engines. I primarily build stuff that needs to perform well in a 4000+ pound daily driver with highway friendly gears, or needs to lug up a rough mountain trail, or needs to pull a trailer full of hay out of the back 40. Cam timing is a lot of it, but not all of the picture. So…if I am interpreting correctly, 350+ FPS port velocity in the stuff I build isn’t a bad thing, especially when part throttle performance is more important that WOT performance.

I’ve never really driven a 6.0 LS powered truck, but spent plenty of time with 5.3 powered GM trucks and the Coyote based 5.0 in ford trucks, and to me they just don’t seem to be a good truck engine. I think they are more tolerable because of lower rear gears and multi speed transmissions, and they certainly get better fuel economy than anything I’ll ever build with ancient technology, but IMO they just don’t have the low end and part throttle grunt that I like…especially in a truck. I guess that’s why diesels have become so popular
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Re: Port volume vs flow vs velocity

Post by travis »

Now here’s another question. With the higher port velocity of a small head, is it more beneficial to have a slightly later closing intake valve to take advantage of the higher intake inertia? If I am understanding correctly, this may help from the midrange and up, with less detriment to the low rpm torque than if you had a bigger, slower port?
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Re: Port volume vs flow vs velocity

Post by johnef »

Port Volume is a byproduct of desired flow and velocity.
I look at MCSA first and map out from there to the front and back.
Upstream of MSCA is 2° change included angle, obviously measured as area change, and 5° downstream of MCSA leading to back of seat. This is for a 4 valve head.

I always go smaller, and has yet to let me down. We have set multiple Allmotor records with much less CC's.
Next will be applying it to the exhaust, haven't had time.
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Re: Port volume vs flow vs velocity

Post by mag2555 »

Look at it from this point of view that a hope we all can agree on?
If for a street or street limited strip motor it’s better to be a tad under Camed, then in that same vein it’s better to have a tad too much port velocity, then not enough.

As has been written about many times, not enough port velocity will spoil a whole power curve, and those of you who might say that this is not carved in stone are flat out nuts, ok nuts!!
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Re: Port volume vs flow vs velocity

Post by steve cowan »

travis wrote: Wed Jun 16, 2021 4:16 am Now here’s another question. With the higher port velocity of a small head, is it more beneficial to have a slightly later closing intake valve to take advantage of the higher intake inertia? If I am understanding correctly, this may help from the midrange and up, with less detriment to the low rpm torque than if you had a bigger, slower port?
It a can of worms at best,
You will have reversion no matter what,if you know people who have tested cams on the dyno and found best INCL point for that application that will help you otherwise its a guessing game.
Pumping loses are one of the reasons a 4 stroke engine is so inefficient.
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Re: Port volume vs flow vs velocity

Post by SpeierRacingHeads »

If we are talking 23º SBC, the floor apex trumps all MIN CSA.. Throw that out..

Prime example is a Super Stock port. 172cc and velocity is crazy high, it's all about the apex.

And IMO, volume vs flow vs velocity is smart porting. Most guys the head doesn't flow what it should for the available area, they want more air, they make it bigger instead of finding the real problem.
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Re: Port volume vs flow vs velocity

Post by RW TECH »

digger wrote: Tue Jun 15, 2021 10:50 pm Every post 2000 oem NA Chevy, just look at tq/cube it’s not in the impressive range at the stock power level.
Compared to what?
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Re: Port volume vs flow vs velocity

Post by RW TECH »

travis wrote: Wed Jun 16, 2021 4:10 am And I think, yet again, that this is where my confusion comes from.

I’ve found a large number of posts on here that state that anything over 330-350fps is too high and you start getting more pumping losses, more possibility of the port going turbulent, etc.
The driving situations you described do not sound like the ones where my first priority would be air speed measurements on an airflow bench, unless you're talking about an extremely large engine with an extremely small cylinder head (i.e.-427 SBC with stock iron heads).

In reality, some of the pumping losses and secondary impacts (like increased ring flutter, etc) on road vehicles actually show you are better off with smaller CID and more throttle angle to achieve a given torque vs a big CID engine pulling against the back of a mostly closed throttle blade.
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Re: Port volume vs flow vs velocity

Post by 1980RS »

Well here would be a good test. My peanut port with 8:1 compression that I ran 360 castings ported went the best of 11.16@118.26 so far. I wonder what a my 990's with a good valve job and a bowl clean up would do on the same engine. Large port super flowing heads vs. a high velocity head, think on that one for a while.
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Re: Port volume vs flow vs velocity

Post by Steve.k »

Most head porters will tell you that you always pickup velocity with port work as well as volume. Most guys who have bench tell you they know their close when head makes a different pitch than previous tests. Every set of my big Cleveland heads picked up power. Our own personal car over 7/10s with heads and cam swap.
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Re: Port volume vs flow vs velocity

Post by mag2555 »

Lets put some needed details to that last post ok?

Port velocity goes up at max lift being applied only when the percentage of air flow gain exceeds that of the percentage of area gain in the port choke / minimum port area location if it was reworked.

Steve, so it looks like you picked up some 75 hp, that’s not unheard of and looks like that would be achieved with a 22 cfm gain per cylinder.

How many CCs where added to those ports to attain that flow level of flow gain?

How much bigger did the minimum port area get or was this power gain had by just making other areas more efficient?
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Re: Port volume vs flow vs velocity

Post by digger »

RW TECH wrote: Wed Jun 16, 2021 3:15 pm
digger wrote: Tue Jun 15, 2021 10:50 pm Every post 2000 oem NA Chevy, just look at tq/cube it’s not in the impressive range at the stock power level.
Compared to what?
to something with the size and flow that is closer to what what the engine actually requires/consumes.
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