Making an A65 punch above its weight.

[Mark Parker]

"Do you think the use of a blowtorch gives an idea of the behaviour of the inducted mixture as in the YouTube video?"
I cannot see how that has any value, it's not measurable. Air going through a port can be at 400 foot per second or more. It shows something but it isn't fast. It would be nice to see what it looks like but measuring quantity and speed gives you an indication to clearly predict how the engine will perform.

In a motor as the piston rises on compression the inlet is still open. So that charge needs speed to have the energy to push into the cylinder against the rising piston, then the valve shuts maybe 72degrees after BDC. So, if it's both fast and in quantity more goes in. If it's slow it can be pushed back out. So a 325cc cylinder can have less than 325cc of mixture to burn or it can have more. That's why speed and quantity at lower rpm can boost power just as it can at higher rpm. It's a fun thing to mess with. But you need measurements to see what your results are.

[DAVE BRADY]

Hi Mark,

Would it possible to have some dimensions on the images and a cross section of the port from the side and above.  I am going to try and make a basic vacuum measuring device and see what I do over the winter.  I will also try to see what affect any alteration to the single carb manifold makes.

Dave.

[Mark Parker]

I start by getting some 34mm id alloy tube and angle cut it to the std port, then putting it in a vice and squashing the end to make it oval so it's 30mm high. I helicoil the bolt holes to suit smaller cap screws and weld ears to bolt the manifolds on. As in that previous photo.

They need to aim so the port roof is straight before it curves over. It goes up either side of the guide and the bowl is deeper. leave a tail after the guide and you can see how deep you are going down, this is to turn the flow onto the valve. Std ports are 17-18mm wide, get something to measure with, it can go 37mm without breaking through usually though it varies, take the valve guide out and widen both sides work on the roof area to come into the bowl. It can go out and come back in. If you break through on the inside it needs a shaped alloy plate glued on with JBweld That plate is hard to shape so avoid it if you can. If it's just JBweld it will crack. Don't go up into the valve spring pocket, so aim the manifold with that in mind the carb doesn't need to aim down so much but aim where the port can go.

Mark the tail after the guide with a texta and go down leaving it to meet where you cut either side the guide. An old guide turned down to just slip in and out helps, I get readings before fitting the permanent guide and cutting the seat.



The port is only about 22mm high when I rough up the floor and add JBweld. It needs to end up straight then curving over, or curving over slightly if it goes up then over the air will ski jump and leave the floor. If that reduces flow it can be shape or it may need to be wider.

https://www.youtube.com/watch?v=r5xwnGQ9DiM

[Mark Parker]

That should read the port at its shortest distance between top and bottom after adding JBweld to lift the floor is around 22mm.

[DAVE BRADY]

Hi,

Is it worth considering reducing the valve guide a bit to ease flow past it as it is a major obstruction to flow?

Dave.

[DAVE BRADY]

Hi,

I have just done a rough measurement and calculation, and the guide and valve stem have a volume of 2cm cubed. How much does this affect the flow?  I have also noticed that whilst the guide is obviously concentric with the valve seat it is not concentric in the inlet tract with a slight constriction on the inboard side looking from inside the head.

Dave.

[DAVE BRADY]

Hi Mark,

Sorry for asking more questions.  How far in to the port does the oval shape go?  Also, I have found a vacuum cleaner that has a capacity of 48.5 l/sec which is 102.7 cfm.  Is this enough to give an idea of improved flow?

Dave.

[DAVE BRADY]

Hi,

A few photos of the head I am going to try to improve.  It has the number 71-2200 on it and there is a little J also near the number.  I have old guides that just slide in so that I can work around that part of the  port.  On one I have done a bit of easing so that there is fairly even space either side of the guide and blended the throat a bit so that there is less shielding of the valve.  Nothing has been removed from the roof or bowl.  I also noticed what look like machining marks in the port and the bowl has a very rough finish.  I suspect that the port surface may have been left to create a surface that helps the fuel to stay in suspension but the roughness in the bowl just look unfinished.

[DAVE BRADY]

Hi,

Two more images.

Dave.

[DAVE BRADY]

Hi,

A bit of an update on the port work.  I roughly calculated that the guide and stem has a volume of 2cc.  They offer a restriction to the flow.
So easing the port at the guide area was all I wanted to do at this time.  By filling both ports with water I compare the volumes.  The modified port has volume of a bout 2cc more than the std. one.  Theoretically I have removed the restrictive affect of the guide and stem.  There is now more space on either side of the guide.
I am not going to do any more until I have a way of measuring the vacuum and flow.

Dave.

[Mark Parker]

It's the complete shape. If it's 17mm wide at the guide it can be 39mm on a port done to 34mm, Lightning head, a thunderbolt would likely go through. It has to turn the air onto the back of the valve and the air has no choice but to follow the top of the port as it runs into it, if its deeper and a smooth turn it has to follow it. Straight ports, or straighter are only better if they aim at the back of the valve not across it, these low ports must turn. The floor can be lifted for the same purpose, but it restricts the port, so width is needed. Once the air turns and hits the valve from behind flow jumps because it uses the whole circumference, not just the top section.

[DAVE BRADY]

Hi Mark,

Thanks for the input.   I have an engine to put back together over the next few weeks but I am hoping to make a basic flow bench after Christmas.   This should help me to see what I am doing.  I will get a vacuum cleaner that has just over 100CFM capacity but will probably need two to get the best results.

Dave.