Why can you only see one exhaust valve?

[8541Hawk]

This was brought up in my exhaust thread and instead of giving a compete answer there that would just get missed, why not start a new thread to go off topic....... lol

Besides I'm still waiting for my shock and got the "still waiting on one part" e-mail today.....

At this rate I'm going to need Virt to give me some riding lessons by the time I get her back together.....

Well back on topic....and that didn't take long

You would think that the most efficient exhaust port would look something like the intake port where you can look in and see both valves.

So why the counter intuitive exhaust port design? I believe the answer is something that many people overlook ( That is the best term I could come up with as I really am not trying to be insulting or a smart ass) is combustion chamber swirl.

What this means is that once the intake charge enters the combustion chamber you want it to spin in the chamber.
There are a number of reasons for this but, for me, then main reasons are to keep the a\f mixture in suspension and to also speed the combustion process as it is driving the mixture into the flame front.
Also keep in mind this swirl continues throughout the entire combustion process.

How do they get this swirl as it looks like a straight shot down the intake?
First, a VTR has what is called an "Open Chamber" design. This means that the combustion space is basically an open space with little restriction and no large differences in pressure throughout the chamber.

On this type of engine they use what is called "Induction Swirl" what this means is that they cant the intake tract slightly to one side.
The VTR, if you are sitting on the bike, has the intake canted slightly to the left, this induces a counter or anti-clockwise swirl in the chamber.

Now picture the piston on the bottom of the power stroke..... I would say BDC starting the exhaust stroke but the exhaust valves open 50 BBDC, so I don't want to get called out for that one but picture it whichever way is easiest for you.

So you have a cylinder of pressurized gas spinning anti-clockwise and the two (2) exhaust valves pop open.

Again if you were sitting on the bike, the right valve (or the one with the long port) "sees" a clean flow as it is spinning right into it, so nice high speed flow through the port.

The left valve, when first opened, "sees" a bit of flow that can't make it out the first valve but will also have an affect on the gasses that have already passed it, these will try to stop and turn around due to gas flow laws and suddenly being exposed to low pressure. Though by doing this, they slow down and also reach the valve after the initial "High Pressure Pulse" that happens when the valves first open. Which will slow the flow even more.

What you end up with is a strong flow in the right port and a slower, weaker flow through the left port.

With that in mind, my take is that it is an intentional design and what is happening is the fast flowing gasses in the right port hit the exhaust at the same time as the slower moving gasses in the closer left port.
Also the port design funnels the the right port over so the gas flow can act as a venturi and actually help scavenge the weak left side flow.

That is my take because it looked odd to me though when I asked about it I was told by the tuners I trust to not mess with it, as there is nothing to be found, though as always YMMV.

Though I also could be dead wrong and full of shite......

[VTRDark]

Porting is a very mysterious thing and a lot of it comes down to trial and error unless one has a flow chamber to play in. But opening something up is not always good thing as it also slows the flow down, though you get more mass. So I guess it's down to whether you want more mass or more flow or try to balance it out somehow.

Maybe a venturi type design would better, big at both ends but narrow in the middle a bit like an egg timer. Or maybe this is where the dimple design comes into play with a surface like a Golf ball. I'm sure there are some improvements to be made, a bit like the welds in the exhaust, as the engine was built on a budget and aimed at the consumer market not the racetrack. But less is more sometimes and it would be easy to take things too far where it would have the opposite effect to what one is trying to achieve. Like I said at the beginning it's trial and error.

I wonder if Mori had a budget and stopped short of what they could have produced if they had more time.

(:-})

[gl_s_r]

If it come out the middle of the two valves the suspension would be in the way of the exhaust of the rear head.. wouldn't it?

[8541Hawk]

I wonder if Mori had a budget and stopped short of what they could have produced if they had more time.

(:-})

Well they had the budget to source one off crankshafts and magnesium bodied carbs so I would assume they had the resources and time to do a bit of porting especially if there were major problems with the design.

Now you could say "look at the SP1 or 2 exhaust ports" as that is where all the work by the 3 tuning firms lead too (Moriwaki,Erion,Revolution). Yes you can see both exhaust valves in that design.
Though you need to throw a "but" in there. What I mean is the SP heads are designed for top end HP with bottom end Torque being secondary in that application. Even then the port is offset slightly to favor one valve.

This is the opposite what we have or what you want for a "road" bike. The exhaust port follows the same "rules" as the rest of the exhaust system.
Small, long ports = Bottom end - Large,short ports = Top end

So IMHO yes you could rework the port to what "looks" right but the very best you would see is a powerband equivalent to a SP curve or added top end while giving up the bottom end (remember nothing is free )

Then again the "race" bikes were developing between 130-150 RWHP depending on the state of tune with no major port rework, so how much do you really need?

[8541Hawk]

If it come out the middle of the two valves the suspension would be in the way of the exhaust of the rear head.. wouldn't it?

That would have been easy to take care of during the design but unlike the intake valve, the exhaust valves flow at different rates from each other due to the gasses spinning in the cyliner as I stated in my first post.