I was wondering what the biggest cam i could run with stock pistons? Im going with gt40 heads for now. and this may be pure ignorance but can someone explain the whole 1.6 or 1.7 roller rocker thing to me? What is this a ratio of thanks
Peak lift has NOTHING to do with Piston to Valve Clearance issues.
Follow along:
There are 4 simple strokes to an engine: Power, Exhaust, Intake, and Compression.
First, there is the power stroke, which is created after the spark ignites the compressed air/fuel mixture the piston is pushed downwards and relates the power to the crankshaft.
Second, there is the exhaust stroke where the piston is now coming up and the exhaust valve opens to push the excess air out the exhaust port into the exhaust manifold.
Third, there is the intake stroke in which air is pushed down into cylinder as it travels downward.
Fourth, there is the compression stroke in which the piston moves upwards to compress the air/fuel mixture that entered the cylinder on the previous stroke.
One should notice that the intake opening typically happens before top dead center (BTDC) and the intake closing typically occurs after top dead center (ATDC). The exhaust opening typically happens before bottom dead center and the exhaust closing typically occurs after top dead center (ATDC).
I will discuss this better and try to combine the strokes with the valve timing of a Trick Flow Stage 1 camshaft.
Again, for simplicity I will start with the power stroke. The piston has just been "exploded" downwards to transmit all the power to the crankshaft in order to rotate it. Before the piston reaches the bottom, the exhaust valve begins to open (49* BBDC) in order to begin scavenging the exhaust, and after the power stroke passes bottom dead center, the exhaust stroke begins. The reason the exhaust valve will open before the piston reaches the bottom of its travel is because cylinder pressure is much higher, even at this point, than atmospheric pressure. This helps scavenge some of the exhaust out the exhaust port.
As the piston is coming back up to push out the extra gasses out the exhaust, the exhaust valve opens up fully and then begins to close as the piston approaches top dead center. Just before the piston gets to the top and the exhaust valve closes, the intake valve begins to open (3* BTDC). At this point, called overlap, both the intake and exhaust valve are open. This is the point where piston to valve contact occurs, during the period of overlap. The exhaust valve closes a little before (4* BTDC) or after top dead center, which is when the intake stroke begins.
The intake stroke is where the intake valve continues to open and air is pushed in from the atmospheric pressure. The intake valve continues to stay open until just after the piston reached the bottom of its travel, (ABDC). After top dead center and after the intake valve closes (38* ABDC), the compression stroke begins to compress all the air/fuel that was just entered into the cylinder. The ignition occurs a little before the piston gets back up to the top dead center position, to continue right into the power stroke. The cycle repeats over and over, from 600 RPM to 9,000 RPM.
Let’s take a .600” peak lift camshaft, 108* intake centerline, stock 4.6L 3.543" stroke, and the factory 5.933” rod.
At the intake centerline (ICL), that is where peak lift occurs.
The piston will be down the bore 2.561", while the valve is open .500"-.600".
So, how is the valve going to hit the piston top? Also keep in mind that a valve is not on the same level as the deck height. It is seated up into the combustion chamber of the cylinder head.
On another note, take the factory 4.6L stroke for a total of 3.543". Why would anyone want to open up the valve all the way (.500"-.600"), when the piston is at TDC? There is no volume to displace since the piston is at the top of the bore and the greater pressure on the cylinder head side would have a hard time overcoming a lesser pressure, when the piston just got through pushing upwards.
The pressure differential and volume are next to nothing.
A 302 with a stock 5.090” rod, 3.00” stroke, and a 107.5* intake centerline (TFS-1) would have the piston 2.156” down the bore.
A 347 with a 5.400” rod, 3.40” stroke, and a 108* intake centerline would have the piston 2.554” down the bore.
I hope one can see where this is going.
As rod length increases, the depth the piston is at during peak lift decreases.
As stroke increases, the depth the piston is at during peak lift increases.
As the intake centerline decreases, the depth the piston is at during peak lift increases.
Back to the TFS-1 camshaft for the 5.0L:
At peak lift, a 107.5 ICL (TFS-1 camshaft) is 2.156" down the bore, when the valve is open .499".
Watch the exponential change (non-linear) that occurs as the lobe lift increases to open the valve more with the roller rocker ratio change.
Now a stock 5.0L lift camshaft (.444"/.444”) with an early intake opening, a late exhaust closing, and a steep ramp rate would create contact. The speed the valve comes off the valve seat would be so quick that it would catch the piston. However, a camshaft with a lazier lobe, late intake opening point, early exhaust closing point would be fine at .700” lift.
If you degree a camshaft, the below information may help you out.
Advancing the timing opens the intake sooner and closes the exhaust sooner. That will gain you some clearance on the exhaust valve, but you will lose clearance on the intake valve. Retarding the cam causes the timing to open the intake valve later, and closes the exhaust later. You'll gain clearance on the intake, and lose clearance on the exhaust. So changing the cam timing won't result in increasing clearance on both valves.
Adjusting either way can shift the power band +/- 200 rpm that direction.
The general rule of thumb is .080" of clearance on the intake side, and .100" on the exhaust side.
The exhaust side needs a little more clearance due to a couple reasons:
- Piston chases the exhaust valve as it is closing, which causes the spring to have less control.
- Heat expands the valve material
- Chain stretch (Retards the camshaft)
Peak lift does effect coil bind though. Higher install heights help out coil bind, but then you run into spring surge/resonance problems at upper RPM's.
The rocker ratio takes the camshafts lobe lift and multiplies it. The camshaft help link in my signature may help a little better.
Wow...thanks alot i have been confused for a long time about that, ill have to read that about 4 more times to soak all the info in thanks. I guess what got me confused was that some of the ford racing camshafts say may require piston modification for piston to valve clearance. (x,z cam) or is this due to increased duration?
Aside from that excellent info up there...I don't know who told you an X had to have extra notching...i'm using one now it's safe to say that an X303 will clear unless you mill the heads...so you can use the X's specs as a sort of template when shopping around for a cam. There are better cams than the X, but it has a good amount of lift and it's working great for me. Of course just for the record this doesn't apply to the '86 blocks.