Intake Port Volume vs Displacement vs RPM

[Vee Dub Nut]

Hoping some of you more experienced cylinder head guys can provide some insight to the subject topic.

Understanding that there is a relationship between intake port volume vs engine displacement and desired/intended rpm range, are there any calculations or rules of thumb that help figure out the "ideal" scenario?

For example:
-What intake port volume to shoot for on a particular combo, given the displacement and intended rpm range?
- Or coming from the other side, given a displacement and intake port volume, what sort of working RPM range could be expected?
(of course realizing that camshaft selection plays into these as well)

To throw in another wildcard, how does this relationship change with forced induction engines (or does it)?

I've dug through quite a few old threads and gained a lot of insight from folks like Brian_e, Alstrup, et. al, but was hoping to have a more detailed/specific discussion on this particular relationship of factors. Most of the threads I found had a comment or two in relation to a specific engine combo that was being discussed, but I didn't find anything that really dove into the subject more generally and how to apply it to other combos.


And for some case study:
There is quite a bit of interesting insight to be gained from several of the combos posted on CB's website (see link below).

https://www.cbperformance.com/category-s/352.htm

There are a lot of combos on that page using the 044 Panchito heads, which are relatively "small" heads that have a pretty low intake port volume compared to most all of their other CNC heads. CB/Pat managed (IMO) some impressive numbers from these small heads on relatively big CC engines (like 2276+). This was on both NA and turbo combos. However looking more closely, all of those big CC/Panchito combos used quite a bit more camshaft than the "normal" recommendations people tend to toss around for cam sizing. In short, a lot of the cams used are generally more considered drag racing type cams versus typical street cams, but the resulting power at RPM of these combos yielded more street oriented manners (i.e. peaks below 6k rpm). This presumably is directly related to the small port volumes of the 044 Panchito's, and needing more camshaft to adequately feed these bigger displacement engines, versus a more traditional combo.

If the above is true, how does that practically change if you swapped from the Panchitos to something like the wedgeport or miniwedge (with 10cc+ more port volume)? Presumably the cam could be pulled back some while still maintaining same/similar performance. But what else changes? I understand the concept that lower port volume = higher air speed = more torque (at least generally speaking). But what are the limits? Is there a particular advantage or disadvantage of either particular approach?

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There is a lot in this line of questioning I know, but I'm hoping to stimulate some good discussion here, and hopefully continue to expand my deeper understanding of engine building. Thanks in advance!
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[Alexander_Monday]

Black magic, but from what I have gathered it is more to do with the minimal cross-sectional area in the port compared to valve size and where the minimal cross-sectional area is along with port shape creating turbulence and how close to the wall the flow is (size of the boundary layer).
All that affects flow velocity which you do not want to go sonic but also don't want it too slow.
I would love to have an explanation that us lay people can follow.
I try to take in what I can from Darrin Morgan, but he mainly talks about bleeding edge race engines.
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Danth’s or Parker’s Law:
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Alexander_Monday->What were the rings gapped at?
bedlamite->Almost enough.

[Brian_e]

Fantastic and very difficult topic.

There are formula to figure out most everything in an engine for sizing. The problem is lots of it is based on an ideal perfectly round, and perfectly straight piece of pipe for a port. We all know that is never the case.

As for figuring out the ideal port volume, here is how it "should" be done in an ideal world.

Use upper RPM and displacement to figure out the size of the minimal cross sectional area in the port.

An "ideal" port taper is around 7deg. according to different books, the Superflow manual, etc. Given the length of the port, the new MCSA at one end, and a 7deg taper, you can then figure out the volume of the port. This will get you the smallest "correct" port, with a minimal volume to keep the air in the port accelerating, but not choke.

You can also use the new MCSA to determine the ideal valve size. Lots of factors, but I prefer a 88% Valve to seat diameter ratio. This keeps the seat angles good, and makes for a really good venturi effect at the seat. Some heads can use a 90-92%, but they are usually very high RPM, and they need 50+ deg seats. Supposedly the 85-88% is the range for street engines, and it makes sense.

I have found the best thing to do is get a copy of the Pipemax software. It will do most all of the calcs for you, and if you start playing with it a bunch you can start to see what its doing. It will also calculate "theoretical" airspeed in the port which is also important to look at.

The Wallace calculators are also free, and they can offer a bit of insight.

As for the Panchito's on big engines, I have found they will do pretty much what Pipemax says they will for upper rpm limits, given the typical cams I am using. I am pretty skeptical of CB's dyno and flow bench numbers, but I do think using an otherwise BIG cam for the combo will help overcome the small port volume. Hanging the valve way open with a bunch of lift, and for a long duration is a sure fire way to help pack more air into the cylinders. As long as they don't hit sonic choke, it might just work.

Brian
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[Brian_e]

[Alexander_Monday]

[Brian_e]

[Wreck]

David Vizard "How to port and flow test cylinder heads" is worth buying , he mainly has examples of V8 stuff but it is relevant . He also has a YT channel but tends to waffle on a little too much for my patience level !

I use the above book and Pipemax . Also this online calculator .https://hotwater.gregraven.online/calculators/valve-sizing

[Brian_e]

These two books are also very good.
https://www.amazon.com/Practical-Engine-Airflow-Pe...amp;sr=1-1

https://www.amazon.com/Engine-Airflow-HP1537-Pract...amp;sr=1-2

Along with a copy of the Superflow flowbench owners manual if you can find one.

Brian
_________________
So more or less the lazier and stupider you want to be, the nicer quality parts you need to buy.
-Modok


www.type-emotorsports.com

Type E Engine Parts and Supplies
https://type-emotorsports.com/collections/engine-parts

[Vee Dub Nut]

[Dusty1]

[Zed999]

[Brian_e]

[Brian_e]

[chrisflstf]

The link Wreck provided is interesting as it shows higher rpm, more so than displacment , above 5k, mostly, is where larger valves become necessary. But from what ive learned, the port needs to be matched to the valve size for optimum performance, rather than just putting bigger valves in a small sized valve and port head

[Wreck]

[Alexander_Monday]

[modok]

That supercharger really throws a wrench into this question
best for you
just make the whole path from central plenum to the "intake runner length" large, the whole way.

You could just have a connection between your pairs of tubular runners on either side, the connection located at the correct distance from the head to make the runner the right length 16 or 20" or whatever you think, and then from that "connection" to the head is the actual runner length, and step up the size a bit if practical from the main plenum to this connection, so the actual path from the plenum to the runners is 2.5 or 3x larger area.
Why still have two tubes? well........cause it looks kewl?
Won't work any better than having huge 3 inch or 4 inch tubes leading to your runners on either side, but IMo would be easier and fit better and look more elegant than one huge tube or trying to make an oval tube.

Sorry to jump the the conclusion but we have to get that solved before we can even get into the quesion in the first post.......becase the intake harmonic chosen actually changes the ideal port and runner size.
A longer harmonic length, we should all agree, takes longer to accelerate and decelerate, therefore with a longer runner length the bulk of flow has to happen in a smaller window of time, meaning the longer the runner the larger the port and runner needs to be. Usually people design engines from the inside out so they figure out the right harmonic length for the head used.......... trying to figure out the right head for an overly long runner length.......your going to find out it's larger than would make any sense for sure.
ALTHOUGH, the rules of runner taper are a lot wilder than any angle.
It can work to have the last 1/4 or 1/3 (can't remember which) quite a lot smaller and that can work well, provided that small section is not jammed into a bend. and that could work for you, since it's port injected, it would mean your runner size would be larger than usual really. Most of the ram action happens in the lower third of the intake length, so runners can be straight, or tapered, or stepped, with similar overall result, the best being whichever fits best.

That's one subject where the lightbulb only lit up recently.....after watching
Darren Morgan and looking at a sbc again for the millionth time I realized that it kinda.............just, its all true, like, the pinch point. Some hate it, some love it, but the main thing is that it just fits, where it is -works-. It's gotta be in a straight line downstream and in the lower 1/3 of the intake length, and bend before the pinch point, not during or after, then it's a feature not a bug. But if you make it a problem, then it's a problem

[Alexander_Monday]

I designed the dual fan housings about 20 years ago now in the old Fluent CFD and I must say that all 4 CHT under the plugs and all 4 EGT are excellent, but working the intakes around them makes for a packaging problem.
I have done a lot of analyzing of the pressures in the intake system.
The harmonics are strong enough at low rpm that they create a problem for idle with a negative zone that makes it very hard to get idle much below 1400 with the FK10/87 cam I had in it.
I had to go to fully sequential injection and get the injection timing just right.
Here is a composite gif of the pressure differential between #3 runner just above the head and the plenum in red and the 36-1 crank pickup in blue swept from 1400 to 7200 to 1400 rpm with the vertical center artificially held at compression TDC.
As you can see, at high rpm the pressure differential is positive where it needs to be and drops off after the intake valve opens.
But at low rpm that nasty negative pressure differential is present just before the intake stroke.
Remember this is differential, not absolute pressure.

_________________
Danth’s or Parker’s Law:
“If you have to insist that you've won an internet argument, you've probably lost badly.”

Alexander_Monday->What were the rings gapped at?
bedlamite->Almost enough.

[sidemarkers]

In the 90's a friend made an efi manifold for his turbo sand rail. The cylindrical plenum with throttle body hovered over the fan shroud and each of the 4 long runners spidered along the shroud and mounted to the heads with flanges.

Idled well and pulled like a freight train up to about 5000 rpm then it fell on it's face. Think it has to do with the length of the pressure waves and your desired rpm range.