Gas Flowing

[carrierdave]

Just a quick question for you all. When gas flowing a head is it better to leave on the small length of valve guide in the air path or should it be cut back to the same level as the airway?

Thanks

Dave

[john.p.clegg]

Dave
On mine I've shaped it to help the airflow but left all the inner,the valve needs all the length it can get...

John

[rgh0]

Exhaust valve guide is not critical in terms of flow and can be left. SHortening the inlet guide can help but I would not do it as the twin cam guides are really to short already and making them shorter just makes valve guide wear and oil leakage worse. The guide though should be tapered to minimise its cross section area in the stream flow.

regards
Rohan

[steveww]

Another tip: pay attention to where the valve seat meets the chamber, think flow at small lift. A regular head is a bit untidy here and gains are to be had here.

If you are going to fit large bore headers/manifold you can flare out the exhaust port to match, there is plenty of material here. QED do large bore exhaust gaskets to match.

[bill308]

In addition to the taper that Rohan suggests, the guide tip champher can also be rounded off a little to improve flow.

Bill

[carrierdave]

Hi Gents,
Thanks for all of your input. Unfortunately is was one of those "after the event questions" so I'm now not sure where to go.

You will see from the attached photo that the machine shop I used decided to remove that little "sticky out bit" which now appear to be quite critical!

One step forward - Three steps back comes to mind!!!!!!!!

Dave

[john.p.clegg]

Dave
If you can put up with rapid valve guide wear and the hit and miss sealing that goes with it you will benefit from improved gasflow....

P.S.
Get it back to the machine shop....

John

[carrierdave]

Thanks for the note John.
My annual mileage does not exceed 3000 miles at most; so I think I will see how it goes.

I think next time I will leave specific instructions!!

Dave

[msd1107]

RE: Gas flowing a TC head.

Although the question was directed at the valve guides, there is the general question of modifying the TC engine for optimum gas flow.

It is useful to recognize that conditions in the ports and valves are not nice to the poor gas molecules, with instantaneous velocities in the ports being quite high, approaching Mach .5 or so, and with bends, area changes, obstructions etc to impede their progress.

Basically, we are looking for what is called laminar flow, where the air molecules are not subjected to rapid changes in velocity, area, or face sharp edges. If this happens, the flow becomes turbulent, which causes an increase in local pressure, a decrease in velocity, a decrease in mass flow rate and you get less air in the engine or more losses on the way out.

So, let?s see what can be done to improve the life of the poor gas molecule in its trip through the engine.

Start at the carburetor. If you open the throttle, you will see the throttle plate presents a sharp, rectangular shape to the incoming air. Now, a flat plate has a Cd of 1.15 while an optimized streamline shape has a Cd of maybe .06, so you see the potential loss here. What you want to do is round the leading edge of the throttle plate (an ellipse is close to the ideal curve) and taper the trailing edge.

Then look at the throttle shaft. It is a bulky cylindrical shape, with screw heads and threads that may protrude. Gas flow hates these little things that stick up into the air flow (and, interestingly, depressions also, like screw slots). Smooth this down to the throttle shaft. And if you are brave, thin the throttle shaft so it presents less area to the input stream.

To put this into perspective, some of you may remember a carburetor called the Amal GP. This used a cylindrical throttle slide that was completely out of the way at WOT, and the metering needle was in a chamber at the side, so did not intrude on air flow. This gave a completely unimpeded flow at WOT and the most HP for any particular carburetor size. The Vanwall GP engine used modified Amal carburetors to help it achieve its outstanding power output for the period.

In the cylinder head, make sure the carburetors are matched to the intake port (they usually are on Webers), and there are no obvious casting irregularities. Smooth these off, but do not polish the intake port. It is known that a mirror finish is not optimum, what is not known is the optimum surface treatment to maximize flow. So smooth but not mirror is the word.

Now we get to the valve stem and valve guide. The gas molecule now has a substantial obstruction, and a substantial decrease in area, neither of which it likes. The solution is not to cut the valve guide off, since that leaves the valve stem in the way, and the cylindrical cross section is not very efficient. Remove material at the side of the valve guide (fortunately, with bucket tappets, you do not have to be concerned with large side loads, so you can go quite thin). Round the valve guide on the upstream side, taper it on the downstream side (although there is little you can do there). It is perhaps easiest to do this by installing new valve guides. Mark the old valve guides where they meet the intake port. Remove them. Transfer the markings to the new valve guides. Now you can more easily remove the material on the side and smooth them up. Insert the valve guides, making sure they are oriented properly.

But we still have an area reduction caused by the valve stem and guide. So you need to remove metal on the port so that the flow area does not get reduced quickly. There is not much wall thickness here, so be careful. And remember to radius the port into the removed area, so that the air flow is not presented with a sudden direction change.

Now is time for the valves. You should probably start with new valves. Mark each one to its corresponding cylinder. Very gently lap in one, you only want to see where the valve is in relation to the valve seat. Look at the lap marks. You want the valve to seat as close to the outside as possible, since this maximizes the flow area. A race engine can get quite close to the edge since it will be rebuilt more often, a road engine probably should be a little more conservative. If the lap marks are not close to the outside of the valve, cut the seats sufficiently larger so that the valve fits optimally. Lap the valve in again. Now comes the ugly part. Look at the cylinder head and see the lap marks in the valve seat. You only want enough area to seal the valve; a competition engine can get by with a thinner area than a road engine. At this point, there is probably material that needs to be removed from the intake port as it approaches the valve seat. But you cannot use a cutter to do this, since the inner port needs to be gently curved into the valve seat. (If you look at airflow streamlines of a straight sided port, you will see airflow separation and turbulent flow on the inside of the port as it approaches the valve. This is exactly the wrong spot to lose airflow.) So be careful as you go up in the intake port taking material out. You do not want to screw up that valve seat, but you want to remove metal to increase the port size slightly and transition it into the valve seat. Now look at the combustion chamber side. Radius the combustion chamber into the outside of the valve seat lap marks. Remove any roughness in this area. Remember, the air is coming in at a 45 deg angle at very high velocities, so any sharp or rough edges creates a lot of unneeded turbulence.

Now move to the valve. On the upstream side, remove material to blend the shape of the valve head into the lapped in area, doing so until you get the lapped width as narrow as you want. Again, competition engines can be narrower than road engines. On the outside, smooth off the sharp edge that remains, remember, gas flow does not like sharp edges. Also, on the combustion chamber side, round off the sharp edge, and, if possible, continue that up to where the valve seats against the valve seat. What you are doing here is creating a slight nozzle that helps the gas flow transition from the area between the valve and valve seat and the combustion chamber. This is a large area transition, so the air flow is always going to go turbulent, but this will delay the point of flow separation and reduce the turbulence losses.

This has been a lot of work. It is useful to review the results of research work into this area. Researchers have measured what is called the Coefficient of Discharge (Cd) of the port and valve under different lift conditions. At high lift, the Cd can approach and exceed .9. But at low lift, the Cd can decrease to .5 or even less. All this work about carefully lapping in the valve, transitioning the intake port into the valve seat, smoothing the combustion chamber shape away from the valve seat, and carefully radiusing the valve minimizes the air flow losses at this critical juncture and increases the Cd and thus the gas flow.

Now we can proceed to the exhaust side. Bolt up the exhaust manifold. Verify the exhaust port is matched to the exhaust pipe, with no steps or ridges. Correct as necessary. If you are installing a large bore exhaust system, bolt it up, and mark where it meets the cylinder head. This guides you in properly porting the exhaust side.

The conditions are different on the exhaust side. The valve and port area are smaller, so the velocities are higher. However, the temperatures are much higher. So, whereas the air flow in the intake port may be as high as Mach .5, the exhaust flow, while higher in absolute speed, is at a lower Mach value of maybe .3 or less. This partly explains why we can get away with such smaller exhaust valves and constricted porting. But improvements can still be made!

Because of the temperatures, the valve guide cannot be thinned as much, and the boss supporting the guide cannot have as much removed. But you can still remove material on the outside of the port and properly radius it on both sides. Because the exhaust port is so restricted in stock form, these lesser metal removals have a proportionally greater effect.

Do the valve seats like the intake side, leaving a wider seating area to take into account the higher temperatures and less hospitable conditions in the exhaust.

Now one cylinder is done. Do the rest.

Obviously, this is extremely labor intensive, so is probably a DIY proposition. But does it work, is it all newfangled theory?

Well, I did this back in the late 50s and early 60s. I was a starving student so didn?t have access to dynamometers, flow benches, etc. But the engines seemed go substantially better than stock, performing on a par with engines 20% larger in capacity. I think the TC starts off with more efficient porting, so the improvements you get will be substantially less. But these changes are seen through the rev range, just like more displacement, so are better than a hotter cam, larger carburetors, etc that are only a benefit at high rpm.

Are there anymore improvements to be made? Well, I know of three area of theoretical interest that have not yet been pursued. But these take substantial time and equipment to pursue.