Originally posted by DanM
I want to make sure I have a firm grasp on all this timing stuff before I order my new cams and cam gears. First, could someone tell me whether the following statement is correct:
Advancing ignition timing is moving the spark further into the compression stroke. Retarding ignition timing is moving the spark farther back in time, sparking earlier in the compression stroke. Do I have that right?
no. timing is based on BTDC, but actual ignition and fuel burn happens AFTER TDC otherwise you have the pistons being forced down on the upstroke. advancing the ignition timing sets the ignition to spark when the piston is closer to TDC, immediately after TDC. retarding sets the ignition to spark later than stock, when the piston has passed TDC and is further on it's downstroke than stock. basically can be summed up as "dynamic compression", as the static compression is the same, but by retarding the ignition, the compression is less than at TDC where static compression is measured as the piston has gone further beyond TDC before ignition compared to stock. advancing slightly raises compression as the piston is closer to TDC at ignition, thusresulting in more squish at ignition... but of course no matter how much advance you have, you can never exceed static compression.
My first question is with respect to cam timing. A cam gear allows you to change the cam timing .. but when you change the cam timing, you're leaving the ignition timing the same relative to the compression cycles, right? If your ignition was at 14deg, say, it will still be at 14deg, you're just adjusting when the valves are opened with respect to that?
that is correct to a point. you're more or less changing valve timing in comparison to the crank's angle/piston position during it's revolution. only valve timing is changed, to either open your valves earlier (advance) or later (retard). think about this in your head. one piston goes up and down...
step 1. ignition occurred, piston begins downward draw.
step 2. after BDC, the piston moves upward, and the cam opens the exhaust valve while the piston pushes exhaust gas out.
step 3. upon TDC, the intake valves begin to open, the exhaust valves are almost closed. this is called OVERLAP. as the piston hits TDC, having the intake valves open up assists in blowing exhaust gas out, giving you a less residual exhaust gas trapped into the fuel mix. adjusting for advance on the cam gear will obviously open the intake valves slightly sooner. the longer the intake valve is open, the more draw you get when the piston starts it's downward stroke.
step4. piston hits BDC as the intake valves close.
step5. piston goes on upward stroke as all valves are now closed, and combustion happens near TDC and repeats at step 1.
When you adjust cam timing, what are the benefits to advancing/retarding? I assume one is better for top-end and one is better for bottom-end?
advancing the cam will open all valves sooner on an SOHC motor. if timed properly, you'll get the most efficient draw of intake air combined with optimum timing for exhaust valves opening to push all of the burnt gases out.
refer to
www.howstuffworks.com if you cannot visualize all of this.
What is the stock ignition & cam timing on a D16Y8?
dont know offhand
When I add a more aggressive aftermarket cam, is it best to advance or retard spark timing, or just leave it?
leave it until you can tune it on a dyno. only a dynamometer can tell you if what you're doing is producing any useable power, or if you're even losing power.
If I were to step up to a higher octane gas, would that allow me to run slightly more advanced ignition timing, and make a bit more power?
the gas alone wont really make more power, but if the degree of advance combined results in significantly increased compression, it could result in detonation. this is where higher octane gas will burn more controlled, preventing detonation, and allowing you to safely run with the timing advanced, which is bringing you more power.