Valve flow potential vs. port flow potential

[kenikh]

I was thinking the other day (dangerous) and was wondering if anyone had ever put together a table that separated air flow potential in a port (CFM) from air flow potential of a valve (CFM)for a given cam (let's say in a 2.2/2.4/2.7 head). For example, I was wondering what the delta is between the flow potential of a 36mm port and the flow potential of the intake valve on a variety of cams (GE80, GE60, GE40, etc).

The flow potential of a 36mm ports is static if measured with no valve in place. By measuring the delta of what a valve will flow at various lifts using cams as reference against the port, it would be pretty easy to match cams to port sizes in addition to using the anecdotal data I typically read here.

Has anybody seen a table like this or have any thoughts?

[jluetjen]

Kenik;
I know that I haven't looked at that problem the way that you described, but I do have a couple of thoughts... (note that this is a very multifacited problem, and I'm making a couple of fairly linear observations. So take this for a "grain of salt".)

1) It's not clear to me that a valve's flow at a certain lift is going to be affected by the cam. In general a valve will flow it's peak flow at about half the head diameter. Beyond that and the surface area around the paremeter will be greater then the surface area of the port diameter. There are exceptions, but it's a good rule of thumb to start with.

2) The flow requirements won't depend as much on the cam as on the cylinder's swept volume that you are trying to draw through the port and past the valve in a certain period of time.

2a) Ports: In general it seems like 911 motors have their peak torque engine speed when the ports are flowing less then 75 meters/second. After the peak torque engine speed, it's all down hill as far as an engine's combustion efficency is concerned. Generally the drop off is fairly gradual to start and so the RPM's will increase faster then the torque on the crank drops. This is why the HP continues to climb until the effiency starts to drop faster then the RPM's increase. I've discussed this more in the Selecting a Cam thread. So let's assume that based on your swept volume, and the diameter of the port you have calculated that they will be adequate for your camshaft with a desired peak torque engine speed of X RPM.

2b) Valves: Here's a post that I did a while back where I plotted the actual head flow numbers that I collected for a number of different heads, including an early 2.0 head with small valves, and a later 2.2 head with larger valves, but the same sized ports, and a later 2.4TK head with larger valves and smaller ports.


Note that the the bigger valves flow better at across the board, but the gap decreases some at higher lifts. This might suggest that at higher lifts small valves act like small ports, but at lower lifts the performance of small valves is significantly less then big valves.

3) Camshaft profile:
For kicks I tried poking around with this question a while ago. Just looking at the published seat-to-seat duration of some factory cams (T/E/S/906), I calculated out the amount of time that the valve is open at different rev's. When I looked at the peak torque engine speed, I noticed that it always occurred when the valve was open for about 0.00015 seconds (+/- .00001 seconds) irrespective of the cam. Hmmmm.... That's interesting.

My conclusions?
1) The cam will determine the engine's peak torque speed (and by extension the peak HP engine speed).

2) The ports can have affects on the equation:
a) The ports will determine the maximum flow available to the engine, and thus will put a limit on how fast you can spin the engine and still efficiently produce torque.
b) The ports can also limit the flow at high lifts, so if your cam has too much lift for the ports to flow, it will act like a cam with less lift (but you'll still have the frictional and inertial issues of the high lift cam to deal with).

3) Valve: The valve size especially affects the flow at low lifts. When is the flow at low lift important? During overlap (which affects how high the torque curve will reach). So I'm guessing that undersized valves are a bigger issue for engines which are running cams with significant overlap. If you're not running any overlap (as in a CIS or pollution controlled engine), a slightly undersized valve may not be such a big deal (although still not optimal), especially if it is not a high-rev engine.

Does this help any?

[kenikh]

This is good stuff. I have read the toher threads you pointed to in the past, which lead me to the thinking above. You rpost is a much easier to digest summary.

What I have been trying to figure out is when the ports become the restricting factor on air flow and what effect lift has on the tipping point (of valves vs. ports being the limiting factor). This formula:

In general a valve will flow it's peak flow at about half the head diameter

Is a piece that I think will be important in asnwering my question. Regardless, I think a 2 axis grid with valve lift (at a given valve size) and port size as the axes, showing the air flow differential would be a very cool reference tool.

[jluetjen]

Actually, it would need to be a 3 axis grid since you will need to be looking at that flow compared to lift, or time. So either

Flow resulting from Valve size, port size and time open

- or -

Flow resulting from valve size, port size and lift (incidentally, this is what I plotted in my valve flow graph above).

Time for lunch...

[kenikh]

Yes, yes indeed. Perhaps the axis would be best represented with all of that data being expressed in terms of a cam, which encapsualtes all of those variables.

[ChrisBennet]

I think anyone who is considering porting should take a look at these articles. Among other things the author points out that a flow bench measures the least important aspect of intake cycle efficiency

The 8 phase motor

Intake Porting Secrets

The index of all the Mototune/Power News articles is here

-Chris

[shbop]

Chris, That was an exciting read. I'm just curious, have you tried the less is more theory on port size? John, what's your opinion on this? Sorry, I didn't mean to divert the thread, but some of you guys are so advanced on the theory aspects, it gets really interesting.

[ChrisBennet]

Originally posted by shbop
Chris, That was an exciting read. I'm just curious, have you tried the less is more theory on port size? John, what's your opinion on this? Sorry, I didn't mean to divert the thread, but some of you guys are so advanced on the theory aspects, it gets really interesting.

I've never played with ports. I haven't built a motor that would have benefited much from head work (all street motors). Porsche did a very good job with the heads.
Still, it would be fun to experiment. I found info on making your own flow bench and JB Weld is cheap..
-Chris

[kenikh]

The article is interesting because it gets after what I was getting after: Making ports as small as possible, yet still letting them flow to the potential of the valves at a given lift.

[camgrinder]

Check out this software.
http://www.maxracesoftware.com/pipemax_win.htm

Not expensive and does valve curtain area and other stuff.

[kenikh]

Pretty cool; wish I'd bought before the 1st of the year.

[jluetjen]

Originally posted by ChrisBennet
Porsche did a very good job with the heads.

Chris and I are in agreement. Porsche did a really good job and there are most likely a lot more ways to mess up the head porting then "improve" it in most cases.

I tend to agree (in part) with the "MotoMan" article. In midst of the congratulatory self promotion, he makes the very good point that too much flow is not a good thing. But he's kind of silent on the flip side which is too little flow is also not a good thing. Engines seem to be like Goldylocks -- they need conditions to be just right! If the flow velocity is too slow, the engine doesn't run well, fuel drops out of suspension and general all around nastyness results. If the flow velocity is too high, you will lose HP at the top of the rev range which isn't always good either. This is the benefit of variable valve lift and timing systems like Porsche and Honda have developed. At low revs the valves have less lift and duration which increases the velocity which improves turbulance and mix, and thus combustion efficiency. At high revs the lift and duration increases which allows more flow and keeps the peak velocity in check at higher rev's.

[kenikh]

OK, a total random thought here, but the thought of reed valves for 4 strokes has always intrigued me. You want to stop reversion of the intake charge? Block it with carbon petals...any thoughts?

[jluetjen]

How many cycles would they survive for?

[kenikh]

I'm not sure, but considering the abuse they take on 2 stroke racing motorcycles, I'd expect they wouldn't need to be repalced often. I'd like Tadd to chime in here; he used to race an RZ two stroke. Considering 2 strokes take an intake charge with every rotation, they ahve have to be pretty robust. They would only get a quarter of the cycles in a 4 stroke.

[camgrinder]

Would you have a gain once the engine is in the powerband? Once the engine is "on the cam", the exhaust gas velocity out the pipe will help pull the intake charge in. If there is a something blocking the port you will defeat this advantage.

[tadd]

The RZ is my street bike .

Every once in a while a petal will let go, but it usually get spit out the exhaust with almost no trauma. Aluminum and steel vs a thin piece of fiberglass... Used to happen a lot more years ago. I have had a set of TDR reeds on my RZ for almost 15 years now. It gets rung out to 11.5k on most shifts. Dynoed at 70 HP many, many moons ago.

Would be an intersting idea.

tadd

[kenikh]

Originally posted by camgrinder
Would you have a gain once the engine is in the powerband? Once the engine is "on the cam", the exhaust gas velocity out the pipe will help pull the intake charge in. If there is a something blocking the port you will defeat this advantage.

I was thinking the advantage would be at low revs, especially at idle, like on a 2 stroke. At high revs, I imagine that the intake velocity and frequency would be such that the petals would essentailly remain open 100% of the time, mitigating the blocking problem. Thoughts?

[id911T]

The crabpot effect of the reeds would certainly improve the fuel reversion issue, but one would have to be careful not to cause problems at high rpms, because the reed cage itself can restrict the flow if not well designed.

[kenikh]

So this begs the question: is there a funadmental reason why reeds, when well designed, wouldn't add similar advantage to a 4 stroke? My thoughts are: if I can cut reversion by a factor of n, could I run a 906 (for arguments sake) cam on my 2 liter on the street without bucking and coughing and stil retain its top end?