Originally Posted by
beebone
Interesting thing was found. I did the soldiering test the head clearance is about 0.53, good figure . But the engine is lack of power, only the punchy feeling i can feel when exit.the corner, then no power at all. It was about 120C.
When I take away the shim under sleeve, and add 0.1 head shim to maintain the head clearance in 0.5. The overall power is back!!!! About 110C. But the punchy low end power is lower then with the under sleeve shim.
So, I know the under shim makes the port higher, but what does the higher port mean?
I wish to put the under.shim back to retrieve more low end power, but I don't know how to tune it to get back the mid and top end power. Any clue ?thx
About Port Timing
Of course you know that when you place the undershim you will get a higher exhaust port, which will yield more rpm’s, so more top end, and inversely lower exhaust will yield less rpm’s, so more low end, but take into account that with the undershim you also move the transfer ports timing higher.
First of all we know that difference between transfer and exhaust duration needs to be at least 40 degrees so the tuned pipe works properly, then second thing you have to take to consideration is fuel consumption, higher transfer = more fuel will go, and some of it will not burn, then fuel consumption will go down.
In my opinion from what I’ve learned on RC cars motors is that .12 size transfer needs to be around 116-122 degrees. Everything over is going to hurt run time, everything lower will work against performance.
Just remember whatever you do, keep difference between transfer and exhaust 40 degrees at least.
Induction (crank opening) timing works like this: earlier opening = more low end . Later closing = more top end.
Also keep in mind that the greatest boost in power is gained by increasing total induction duration, and increasing inner diameter of passage in crankshaft.
With respect to head clearance:
Small chamber = Advanced ignition = Colder plug = More torque
Bigger chamber = Retarded ignition = Hotter plug = More top end
Plugs alone, with same chamber will give you: Hot plug = more top end. Cold Plug = more low end
So there is no special formula, it’s just more up to trials. You see on our RC cars application specially on .12 motors we need some torque, some RPM, and fuel mileage.
Hope this helps
AFM