engine displacement vs intake/exhaust port diameter

Call it curiousity, but i'm trying to understand why this is the case. On a cb175 the intake port diameter is 21mm, and the exhaust port diameter is 35mm (actually that's the OD of the header). On a ducati monza jr 160, those numbers are the same, although the displacement is almost double on a single cylinder. So here's my question, what's up with that? Would putting a larger carb even make a difference id the ID going into the motor is 21mm without opening it up? I know the 175 is meant to rev to gain power, and the 160 is intended to give low rpm torque, but I feel like I'm missing something.

Small intake= higher charge velocity=better fuel atomization=low rpm torque. Or something like that

datadavid said:

Small intake= higher charge velocity=better fuel atomization=low rpm torque. Or something like that

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That part I get, but what i'm having a hard time following is, literally the same size for close to double the displacement? If that's the case, with a 4 speed trans I should have a 2 speed transfer case to keep the revs low all the time. Lol.

jag767 said:

That part I get, but what i'm having a hard time following is, literally the same size for close to double the displacement? If that's the case, with a 4 speed trans I should have a 2 speed transfer case to keep the revs low all the time. Lol.

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Im just a pipe fitter welder, my understanding of combustion technology is rather limited
But it has to do with flow characteristics affecting engine performance at various rpms.
I think all the old grumpy fuckers who actually understood this has been banned by now for inappropriate conduct

The induction side (as is the exhaust, if we're really honest) is a MIGHTY complicated thing, you can break it down to a few common variables that affect the fill grade of a cylinder, one of which you've already pointed out:

1) port size - bigger ports obviously flow more fuel
2) port length - just imagine a wave (tsunami) being sucked into said port and bouncing off the valve and bouncing back from the back of the airbox, the carb slide and other obstructions. Now if the length is *just* right and the wave is moving towards the inlet valve, whilst air-petrol-mix is sucked in, you can actually get more fuel in than what you'd suck in regularly
3) valve size to port size - there's quite a bit of literature and depending on what you use the bike for the valve size will be somewhere from 50-60% (stationary engines like generators) to 85-90% (high-rpm-only race engines) compared to the port size
4) cam-characteristics: Not only does the size of the valve and point matter (they limit the max flow at a given time) but also how far (cam-lift) and how long (cam-duration) they are open. Add overlap (when both inlet and exhaust valve are held open), which will promote scavenging and improve the fill grade of the cylinder, eventually at the price of worsened fuel economy.

I won't go into cam specs as that would completely go beyond the scale of the question, suffice to say, you can't infinitely prolong the duration (prolonging the duration will at a point for example adversely affect the idle and low-end characteristics) or lift (the valves will either hit each other or the piston or both).

That being said, it is quite reasonable to assume, that a Ducati 160 sports a cam with a lot more lift and duration (lift affects the fill grade by the 2nd power, as it's the surface of the gap opening up), so with a bit more lift you can have tremendous amounts of improved flow, if the port is large enough. Assuming that both engines will (roughly) rev to the same RPM, the Ducati will also sport a bit more duration on the cam.

With those informations, you can quite easily see, why for example base aftermarket cams for US-V8s usually sport a bit more lift and a touch more duration (if any at all) and still you can often get quite impressive gains, without any accompanying mods.

Hope this helps
Greg

So essentially even with such a large difference in displacent, it's entirely possible depending on the cam and valve specs to get the required flow in and out of the motor. Interesting.

Everything I read thus far made me compare the characteristics of them to be like a low revving large diaplacement v8, vs a smaller displacement high revving v8. One will make gobs more torque on the low end, while the other will see a lower torque figure, but way more hp on the top end. I feel like this leads into the eternal discussion of what's better, more hp or more torque haha.

The basics of an engine is that you need fuel spark, air, and how fast you can do it again so i would think, the more fuel and air, the faster you need to do it agian


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jag767 said:

So essentially even with such a large difference in displacent, it's entirely possible depending on the cam and valve specs to get the required flow in and out of the motor. Interesting.

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Pretty much, yes. There are physical limits and limits of usability, but other than that you're totally right.

Another prime example would be the 50cc class GP bikes of the late 70ies and 80ies. They rev'ed up to around 18k and usually sported a 700 to 1000rpm powerband. They made stupid horsepower there (I don't have exact figure at hand, but I think it's somewhere around the 20hp mark), yet still they had so little horsepower down low, that the driver had to paddle with their feet to get them moving from a standstill.

In simple terms the inlet passes air at ambient temperature and at normal pressure. Exhaust on the other hand is carrying a much larger volume of hot gas. Because it has a much higher pressure differential, it moves the larger volume of gas faster and typically exhaust vale areas are around 80 or so % of the size of the inlet.

But there are lots of other factors including valve lift and cam duration. A small valve lifted a small amount for a short amount of time will tend to allow less gas to pass that a larger valve lifted higher for longer - assuming there is sufficient pressure differential for it to flow effectively. That means that some motors with aggressive cams can get away with a smaller valve and port. That increases gas velocity and fuel atomization and helps prop up the bottom end power.

Small Honda twins used to have suitably sized valves but the exhaust ports were short, lumpy and a terrible shape and flowed very poorly. After that, the pipes which were already too large didn't make much difference. Michael Moore used tiny pipes on his CB160 based 216cc racer to match the exhaust flow better.

As with most things in motorcycle design, it's a matter of compromise in design. I would be tempted to get the head gas flowed to see what it flows and then do a cam lift curve and work out how much flow it needs by the appropriate flow point for that cam and compare that to the actual flow rates and see if it's actually restricted. Or port it and test the flow rate after and see how it runs.

An of course a decent megaphone will extract gas more effectively than almost any other pipe design but....... See how it goes...