Port volume vs flow vs velocity

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Re: Port volume vs flow vs velocity

Post by mag2555 »

Also keep in mind that the piston does not pull air in behind it as it descends down the bore, all it does is make a space for whatever atmospheric pressure you have at that place and time to fill that bore and good port velocity will start to compress that air and fuel mixture even before the piston starts it’s way up the bore.
This equates to higher VE numbers .
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Re: Port volume vs flow vs velocity

Post by GARY C »

BLSTIC wrote: Tue Jun 15, 2021 4:19 pm
GARY C wrote: Tue Jun 15, 2021 4:10 pm Whats your thoughts on the OEM like GM's LS engine's going to big ports and big valves for low rpm truck engines and being up 50 hp and tq over the traditional Vortec truck engines?
In my mental model, big valves and big ports allow you to run smaller cam timing to get a broader spread of torque while still allowing decent top end. The problem is that with massive ports you can never get decent inertia ramming.

95% VE everywhere vs 110% somewhere makes for a broader torque spread. The extra peak torque is probably down to non-port factors like manifolds, combustion, exhaust tech, and far superior knock sensing
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...
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Re: Port volume vs flow vs velocity

Post by mag2555 »

Gains power at a higher rpm.
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Re: Port volume vs flow vs velocity

Post by Steve.k »

GARY C wrote: Tue Jun 15, 2021 4:30 pm
BLSTIC wrote: Tue Jun 15, 2021 4:19 pm
GARY C wrote: Tue Jun 15, 2021 4:10 pm Whats your thoughts on the OEM like GM's LS engine's going to big ports and big valves for low rpm truck engines and being up 50 hp and tq over the traditional Vortec truck engines?
In my mental model, big valves and big ports allow you to run smaller cam timing to get a broader spread of torque while still allowing decent top end. The problem is that with massive ports you can never get decent inertia ramming.

95% VE everywhere vs 110% somewhere makes for a broader torque spread. The extra peak torque is probably down to non-port factors like manifolds, combustion, exhaust tech, and far superior knock sensing
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 thought most porting your trying to increase port velocity while also gaining cfm/ volume. You can get to a point where one or the other or both start to drop. I believe i watched a webinar where Morgan says if your fps gets to high the air cannot make the short turn and fuel drops out settling closer to floor. Does that sound correct?
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Re: Port volume vs flow vs velocity

Post by GARY C »

mag2555 wrote: Tue Jun 15, 2021 5:05 pm Gains power at a higher rpm.
This example from Larry with stock heads gains power through out the pull.
viewtopic.php?f=1&t=860
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Re: Port volume vs flow vs velocity

Post by GARY C »

Steve.k wrote: Tue Jun 15, 2021 5:12 pm
GARY C wrote: Tue Jun 15, 2021 4:30 pm
BLSTIC wrote: Tue Jun 15, 2021 4:19 pm

In my mental model, big valves and big ports allow you to run smaller cam timing to get a broader spread of torque while still allowing decent top end. The problem is that with massive ports you can never get decent inertia ramming.

95% VE everywhere vs 110% somewhere makes for a broader torque spread. The extra peak torque is probably down to non-port factors like manifolds, combustion, exhaust tech, and far superior knock sensing
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 thought most porting your trying to increase port velocity while also gaining cfm/ volume. You can get to a point where one or the other or both start to drop. I believe i watched a webinar where Morgan says if your fps gets to high the air cannot make the short turn and fuel drops out settling closer to floor. Does that sound correct?
If increasing port velocity was the goal then wouldn't we want to add material making the port smaller instead of removing material making areas of the port bigger? I think it's trying to equalize port velocity like Larry demonstrates in this thread how different heads with the same cc and cfm but different air speeds effect power. CFM has to be controlled by air speed like you referred to on Morgans webinar.
viewtopic.php?f=1&t=860
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Re: Port volume vs flow vs velocity

Post by BLSTIC »

GARY C wrote: Tue Jun 15, 2021 4:30 pm
BLSTIC wrote: Tue Jun 15, 2021 4:19 pm
GARY C wrote: Tue Jun 15, 2021 4:10 pm Whats your thoughts on the OEM like GM's LS engine's going to big ports and big valves for low rpm truck engines and being up 50 hp and tq over the traditional Vortec truck engines?
In my mental model, big valves and big ports allow you to run smaller cam timing to get a broader spread of torque while still allowing decent top end. The problem is that with massive ports you can never get decent inertia ramming.

95% VE everywhere vs 110% somewhere makes for a broader torque spread. The extra peak torque is probably down to non-port factors like manifolds, combustion, exhaust tech, and far superior knock sensing
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.
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Re: Port volume vs flow vs velocity

Post by GARY C »

BLSTIC wrote: Tue Jun 15, 2021 5:26 pm
GARY C wrote: Tue Jun 15, 2021 4:30 pm
BLSTIC wrote: Tue Jun 15, 2021 4:19 pm

In my mental model, big valves and big ports allow you to run smaller cam timing to get a broader spread of torque while still allowing decent top end. The problem is that with massive ports you can never get decent inertia ramming.

95% VE everywhere vs 110% somewhere makes for a broader torque spread. The extra peak torque is probably down to non-port factors like manifolds, combustion, exhaust tech, and far superior knock sensing
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
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Re: Port volume vs flow vs velocity

Post by PRH »

It’s seems pretty simple to me......... if bigger and slower was the universal answer....... the head companies would stop selling all those different sized heads.
Why didn’t GM put the rect port heads on dump truck engines?

The size of the port should match the application and it’s intended use/operating range.

Looking at what’s best for a high rpm NHRA SS motor isn’t going to be much help in picking a head for someone looking for top end parts for their warmed over small block DD with a 3.0 or taller rear gear, OD and a cruise rpm of less than 2k.
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Re: Port volume vs flow vs velocity

Post by Steve.k »

When you look at the monstrous looking ports of 4v Cleveland one would assume naturally in street engine they would need high rpm to be beneficial. However I’ve built a few that were pk hp at 5900and tq around 4000. These were 450-500 hp 9.5 compression street engines. While not instant tire shredders once the motor hit 2500 the hp ramped up hard. The little softer launch on street actually was beneficial to get rolling.
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Re: Port volume vs flow vs velocity

Post by BLSTIC »

Sorry to reply twice. I forgot to reply to this bit and thought it's better to have a second post than edit something and have it missed.
GARY C wrote: Tue Jun 15, 2021 4:30 pm
Most if not all port work reduces air speed and gains power...
Yes, and also no. Port work almost invariably reduces local airspeed, not arguing that. However if you're doing it in the right places you should see other parts of the port increase in velocity. The unattainable ideal port has even velocity all the way across its cross-section. Even velocity reduces turbulence and fights reversion.

If you measure at a fixed flow, yes any porting that increases any part of the port volume decreases velocity, but that's not how flow benches *or engines* operate.
A flow bench will increase flow to maintain the same pressure drop, which can go either way with regards to calculated velocity, depending on what you've done with your grinder.
An engine operates very different to a flow bench, and at low RPM when cylinder filling is dictated more by cam timing you'll see lower velocity if you've removed any material. As you increase the RPM to the point where the port is a restriction and the cylinder isn't full at BDC anymore the port is much more likely to have increased velocity (but again, not guaranteed because if you've just hogged out the port at any old location you'll have increased flow less than you increased port volume).

Honestly this whole making horsepower thing reminds me of that Dr Who quote that, paraphrased, reads "Time isn't a straight line between cause and effect, it's more a big ball of wibbly wobbly, timey wimey... stuff". It's a mess and you can't usually just do one thing to achieve a goal without it affecting something else.

Also the large ports probably *did* hurt torque and you could almost certainly increase torque by filling them in. But back to the big ball of powery wowery... stuff... you'd almost certainly do something unintended like raise the octane requirement or something
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Re: Port volume vs flow vs velocity

Post by PRH »

Having not done this particular test, I’m going out on a limb and am going to make the “assumption”, that if a typical 351w combo was put together to make somewhere around 1.0-1.2hp/ci(cam, intake, headers, carb) and I had one set of heads, and successively reworked them to these three levels.......
Stock D0OE head w/1.84” intake valve. 183cfm peak at .400” lift, 145cc port

Bowl hogged only D0OE with 1.94”, 193 cfm at .400, 320 fps at peak flow, 148cc port

Bowl hogged and blended/smoothed D0OE w/1.94”, 229 cfm@.500”, 152cc port, 415 cfm at peak
........ that there would be an increase in power at each progressive step.
Last edited by PRH on Tue Jun 15, 2021 6:17 pm, edited 1 time in total.
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Re: Port volume vs flow vs velocity

Post by PRH »

double post
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Re: Port volume vs flow vs velocity

Post by GARY C »

PRH wrote: Tue Jun 15, 2021 5:57 pm It’s seems pretty simple to me......... if bigger and slower was the universal answer....... the head companies would stop selling all those different sized heads.
Why didn’t GM put the rect port heads on dump truck engines?

The size of the port should match the application and it’s intended use/operating range.

Looking at what’s best for a high rpm NHRA SS motor isn’t going to be much help in picking a head for someone looking for top end parts for their warmed over small block DD with a 3.0 or taller rear gear, OD and a cruise rpm of less than 2k.
I agree with sizing the head for the application and is why I included Larry's links on a factory 165cc head... What size would you recommend on a small block DD?
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Re: Port volume vs flow vs velocity

Post by Walter R. Malik »

Maximum horsepower at Wide Open Throttle has little to do with street driving. Keep it all in perspective.
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