The golden ratio is 3/4

[modok]

the answer is 42....no wait, 3/4
The answer is 3/4

What's the question?

A long time ago, it was discovered by trial and error, that the best proportion for the size of the exhaust compared to the intake is 3/4, 75%
This was before flow benches, just plain size, in a very general sense.
Keep in mind this would be with straight pattern cams, conservative compression ratios. carburetors, and the intake and exhaust runners also designed by dead reckoning and trial and error.

And over the years all kinds of different engines, with different ratios have been built, and sometimes it seems like the rule is NOt true, but IMO when you examine each case in more detail, it ends up being as true as it ever was, or they just goofed.

To design an engine based on this literally, you would choose the size of the intake and exhaust valves and ports to match the engine RPM range, and the cam matches the RPM range and valve sizes, and it would just all work, although half of that is another subject.

the intake runner size would be 86% of the intake valve size, and the exhaust valve would be 86% of the intake valve size, and the exhaust port would be 86% of the exhaust valve size, and even the venturi size in the carburetor would be 86% of the carburetor bore, and the exhaust port ID 86% of exhaust header tube ID. Runners 86% of the intake riser/carb bore.
Go with what's known to work works.... you can't lose.

Does it work? basically yes. When you get into the details it can seem more or less true, but it's basically true.
WHy? Why not? I don't know.
for one thing...... what part is the restriction?,
the valve or the port?
the intake or the exhaust?
the head or the manifold?
It all needs to match, somehow. I suppose you have to BELIEVE that there is no free lunch in making one part larger than needed, and that's usually true.
Especially WHEN you are utilizing the velocity in the intake manifold and exhaust header, the velocities in there need to match what you are doing, and they need to match each other.


The cam controls the valve, so, changing the size of the valve size leaving all else the same, really has the same kind of results as changing the cam.
Think of the cam and valve together as controlling the "curtain area" that is open at any given degrees, and it makes a lot of sense.
At lower valve lifts, it is pretty much ONLY the size of the valve and how far it's open which controls how much of what can flow.
only above 0.25 L/D ratio does the size of the throat and port start becoming a factor.



About the MOST efficient you can get a intake valve to flow on a flowbench, is only 75% of what a straight pipe of the same size can flow.
In order to get high velocity in the intake runner, to make STRONg torque, then you can guess the intake runner size (area) should be about .866 of the valve size (75% area)
That would be for a GREAT flowing valve, with ideal throat shape and chamber shrouding.
Of course for less well flowing valves in reality, then the ratio should be less if high torque is desired. It can be adjusted to be in proportion to what the head actually flows. BUT testing on a flowbench is testing efficiency. At HIGH pressures the actual flow potential becomes more directly related to simply the size of the hole...... You can make a LITTLE exhaust port that flows as well as a bigger one, but it doesn't work like that. The size areas are still AS true, or MORE true.

[modok]

So...what's the point? it's not a hard rule, but, keep it in mind and don't get lost.
I think MOST have a generally right idea about most aspects.
What kind of cam is needed, or what size heads and carburetors are needed to do what you want, but get lost in the overall ratios of things.

Lets take a STOCK DP vw head.
The intake port is 30.5mm, 35.5 valve, 86%
Exhaust valve 32mm ....hmm, more like 90%....but, if you look at the throat under the valve, and subtract the stem sizes, it's closer to 88%
the smallest point of the exhaust port is the valve seat, which is kinda odd, maybe not ideal but not so bad.

Makes sense, in fact typical for a LOW compression INDUSTRIAL engine.
When the exhaust is just a muffler, cam is short, and compression is LOW, 80% can be better than 75%

BUT, lets fix that.
Add an engle 110 cam, raise the compression, put a header on it, now IMO it wants 75%, and we need to get the intake velocity UP, so 36.5, or 37mm intake valve. move everything toward the classic ratios. Will that work? OH YES.

Then we have.......

I have an "1835, engle 110, not ported heads, 40x35.5 valves, kadrons"
TOTALLY different proportions. Seems like a good idea, thousands have been built, but the proportions are wonky.

I don't know what part to change first, but....i figure you move towards the proven ratios it SHOULD work better.
IMO what would be interesting to study is when it doesn't,
That's when it's interesting is to see what happens when the ratios are tilted, or just one part is skewed one way and it produced some benefit or more likely TRADE off, is neat.
But too far out of whack is just asking for pain.

[Wreck]

I've wondered if VW / Porsche went to a higher percentage of exhaust to intake to promote scavenging to reduce head temps at the cost of a little power .
I've read that with water cooled 2 valve heads the intake valve is normally half the bore size and then what ever size exhaust valve that will fit .

[Slow 1200]

well, I've always wondered why 42mm and 44mm intake valves both usually have 37mm exhausts

[modok]

DARN IT
That's a very difficult question.
OR, more specifically I have been wondering for a decade exactly why, to be specific, industrial engines, air cooled engines, and old motorcycles tend to go that direction. Each has their own slightly different reasons.
Motorcycles tend to use very SHORT intake lengths, so that skews everything for them.
industrial engines are often built for a different power curve.
They might be designed to be powerful AND efficient at...3600 rpm ect, and have no reason to operate at any RPM above that, ever.

Most air cooled engines ARE industrial or motorcycles.
while VW is maybe designed to be somewhat like an industrial engine "efficient, while "cruising" at it's "maximum speed" ??


but concerning "scavenging"....that'd be a no.
I've seen in countless "case studies" of people trying to apply longer duration cams and tuned exhaust systems to such engines, and it just doesn't work well at all in the conventional sense.

[modok]

yeah, I think I got it.
When the intake and/OR exhaust velocity is maxed, there are pumping losses.

A sporting engine could be designed so they both max out at more or less the same rpm range broadly speaking. the velocities in the intake and exhaust are matched to the same RPM range, and that's good.
Because when I'm at the TOP of a gear, or going 115mph I don't really care if it gets good MPG at the high RPMs, the pumping LOSS is returned as SCAVENGING, which makes it run cooler and makes more power.....but MPG is in the toilet.
It TAKES power from the piston, to accelerate the gas in the tubes to a high velocity, but we USE this velocity to make....MORE power, same as a supercharger. True on BOTh ends, exhaust also

While an industrial engine it would be fine for the intake velocity to MAX out at it's rated speed, but they don't want the exhaust velocity maxed out, because that's pumping loss, which would reduce fuel efficiency.

[Emeraldlion]

So where does the golden mean or the divine proportion fit into this Modok. I love the direction you have here and want to see one more step pythagoras . I bet it is there somewhere

[FreeBug]

This is where that whole "Z" value makes sense. Keep everything at Z<0.6, and you're good.

IIRC, you put in seat diameter, seat angle, lift, rpm, stroke, bore, rod length and rpm.

I have a 30 mm intake tract, for a 40mm valve. When i see big ports on (relatively) small valves, i know it works super well, but why? As i see it, weight of a long colomn of air + high speed = good filling with late intake..?

[Howard 111]

Let's throw a turbo in the mix!
_________________
1973 Karmann Ghia
Turbocharged, Fuel Injected
http://www.thesamba.com/vw/forum/viewtopic.php?t=531270

[Splitdog]

[modok]

[modok]

[Howard 111]

I run an old CB Turbo/FI, and the computer runs it all. I don't know what it's really doing, all I know is it does it very well.
_________________
1973 Karmann Ghia
Turbocharged, Fuel Injected
http://www.thesamba.com/vw/forum/viewtopic.php?t=531270

[oprn]

A 1600 is only 97 cu in.

A turbo in my view would be the idea fix for a mismatched engine. It will end up making more HP and torque than it ever could even if it were a well matched combination. Turbos are a bit of an equalizer in the breathing department.

On the other hand if it was a well thought out, well matched engine combination plus a turbo it would be even better.

[Howard 111]

[modok]

[oprn]

[FreeBug]

[modok]

Lets say every bit as useful as a dunebuggy.

[oprn]

Excellent come back!

You got me good on that one!