Project 68mm – Part 3

OK, it’s time to get the motor back on the dyno with a few new bits and see how it stacks up.
With a new modified head, again based on a 12G202 head, it has inlet valves of 1.3″ in diameter, with a nice radius seat, all blended into the throat area and modified around the divider. The inlet port has been opened up a little more than the last head, but still not as big as a 12G295 head, yet. The head is flowing around 113 CFM at 25″ on my flow bench.

Flow benches are essentially a vacuum cleaner that can tell you the air flow rate. They tell big lies, heads are just like a drain pipe, the bigger the hole the more they flow. Yes, we need flow to make Torque and Horsepower, but it is the way we achieve the flow that really matters. We need velocity to fill the cylinder, especially at higher speeds. Just think about it, the valves opens and you have to get air moving from being stopped and it does not have a lot of time get in and fill the cylinder before the valve closes again. Now it takes a lot longer to get air moving in a big hole as it does in a small one. But you can go too a small with a port and the air becomes supersonic and stalls. This is not really the problem with a siamese inlet ports. You can defiantly go too big with a mini port, especially with road cars where you need to chase torque.

Now I’m not against big valves in a road cars as long as you keep the choke area down. One thing you must keep in mind, is the air does not flow through the diameter of the valve, but around the circumference of the valve. So with the bigger valves you have more area to pull from, but with the smaller choke area you will get the higher air speed across the seat to help fill the cylinder.

I’ll give you an example of what I mean. In my dyno room, I have a fan for pushing air in the room, now if I open the door wide open you can not feel the air exiting the room, but if you open the door about a third, you can feel the air rushing out. Now have a think about what I just said. This leads us to the curtain area of the valve, which is the amount the valve has to lift to equal the area of the valve diameter, which is about 25% of the valve diameter, no matter what the size the valve is.

The theory is that you don’t need to go any higher in lift than this, but of course, sometimes the theory doesn’t always work out and that is why we lift the valve 30-36% of the valve diameter. This is why I like camshafts with a large nose radius as you open the valve quickly and hold it open for as long as possible to take advantage of the high speed air coming in.

Now with this motor, I have my new 266SS camshaft fitted, which has 0.270″ lift on the lobe, minus the tappet seating which gives us 31% of the valve diameter in lift with the 1.26″ diameter inlet valve and 30% with the 1.3″ inlet valve.

Lets take a look at the exhaust valve and port. The valve I am going to use is 1.070″ in diameter. With the std exhaust valve which is 1″ in diameter, you can actually get quite a lot of air through this, but I like to a little bigger so I can get a better shape seat. So the first step is to cut the new seat, to get a nice radius and a good lead into the 45 degree seat. With the seat cut, lets put it on the flow bench.

1st Setup                                                      2nd Setup
Std Exhaust port and Std Bowl                Modified bowl and port

Lift              Scale                 %                Lift                Scale             %

0.50             110                    50               0.50             110                 52
0.100                                     63               0.100                                  65
0.150                                     71               0.150                                   74.5
0.200           150                   58               0.200          150                  61
0.250                                     60.5            0.250                                  69
0.300                                     62.5            0.300                                  66
0.350                                     62.5            0.350                                   67
0.400                                     63               0.400                                  68.5
0.450                                      64.5           0.450                                   70

Next we tidy up the bowl and remove the guide boss.
After doing this test, I then decided to open up the exhaust port itself along with some reshaping, but not going too much bigger, I threw it back on the flow bench only to find out it makes no difference at all to the flow figures. The only thing I managed to achieve was to get the air flowing out of the port more smoothly, yet there was still no gain in flow. This means, a good clean up would suffice on STD head with this size valve, the exhaust port only needs to be around the same area as the valve. A big exhaust port is around 110% of the exhaust valve. We still need to keep high velocity in the port, so don’t go too big!

68mm big head68mm big head dyno

Well that was interesting, the motor did not like the bigger head, so when building a motor, you have to consider what you are going to use it for. If you are going to build a good road motor, don’t go overboard with cam and head modifications, as you can see by the dyno results, with the smaller cam you don’t need a big head to get good results.

What this head did do, was carry the power through to 8000RPM, which surprised me, as normally a cam of this duration would not carry that kind of horsepower through to 8000RPM. As you go up in cam duration, you are going to move the torque curve further up the rpm range. So think about what sort of RPM range you will be working in. This is why I am trying to design a new cam that can make strong torque and horsepower as low as possible in the rev range.

In the next article, I’ll be trying another cam(The new RE274) with this bigger head, to see if we can make it work.

Stay tuned for the next article.

Cuppa tea time now.