nate perron

my repair manual says to set the base timing to 6btdc how close can you get to tdc before you cause damage to your engine I have checked the valve to piston clearence with some modeling clay and they clear if I run it at 2btd or even tdc does anyone think they will hit eachother?

InfernalVortex

iTrader: (4)

Cam timing and distributor/ignition timing are completely different things. Which are you trying to set?

nate perron

I guess distributor timing what I am asking is can I set it so it fires when the piston is at the very top of the stroke instead of before or after and as the engine speed increases is there a risk of a piston hitting a valve as it goes up and down in the bore

86LG4Bird

iTrader: (1)

Piston to valve clearance is controlled by the mechanical link between the crankshaft and the camshaft, ie. the timing set (chain and gears).
Your distributor is driven by a gear at the back of the camshaft, and and is totally unrelated to the link between the camshaft and crankshaft. Therefore, when you turn your distributor housing to adjust ignition timimg, you cannot affect valve timing or piston-to-valve clearance.

The reason the manual tells you to set it to 6 * btdc is because it takes time for the air/fuel mixture to burn and generate cylinder pressure, so you need to start the burn there to have maximum cylinder pressure to generate torque on the crankshaft when the rod/crankthrow is near a 90 * angle. Just follow the manual and you'll be good

AlkyIROC

Ignition timing directly at TDC is bad. It takes time for the fuel to burn across the top of the piston. Base timing depends on the cam grind but it's only a base for idle only. As the engine rpms increase, the timing increases depending on rpm and engine load.

Theoretically, the best timing has the fuel burnt in the cylinder by the time the piston is about 20* ATDC. Since the fuel takes the same amount of time to burns, as engine speed increases, the timing moves more into the BTDC range to give the fuel time to burn.

Even with a base timing of 6* BTDC at idle, an engine under full load should reach total advance by 3000 rpm for best performance. Total advance depends on a lot of factors but could be anywhere from 28* to 40* BTDC. With an older distributor, that could easily be changed with weights and springs inside the distributor. Modern distributors are all computer controlled so to change the timing curve, you need to reprogram the ECM.

Ignition timing simply changes when the spark plugs fire in relation to the piston location in the cylinder. Valve timing has nothing to do with ignition timing and is controlled by the cam grind and if the camshaft is installed straight up, advance or retard. This determines exactly when the valves open and close in relation to where the piston is in the cylinder.

nate perron

okay I'll set it for six I just was wondering my static compression is only 8to1 and thought running it closer to (0) would help also I removed my ecm and using older vacuum advance dizzy and a carb with a spacer thanx for the advice

AlkyIROC

How much base timing an engine needs really depends on the cam grind.

If you have a stock cam then use the recommended setting.

Typically, it's recommend 10 to 12 degrees of initial timing when the duration of the camshaft is less than 220-degrees @ 0.050” of valve lift

14 to 16 degrees of initial timing with a camshaft duration of less than 240-degrees @ 0.050”

18 to 20 degrees of initial timing when the camshaft duration is less than 260-degrees @ 0.050” of valve lift.

Again, that's only the base or initial timing. All engines are different so knowing where an engine likes it's total timing is also important. Adjusting the base timing will change the total so when changing the base timing, the total also needs to be changed to bring it back to where an engine likes it.

When people have claimed that increasing their base timing made more power. What they really did was increase their total timing which is what the engine really liked. If you increase the base timing too much, an engine can be hard to start so you need to keep the base timing in that above range but play with the total timing to see where the engine makes it's most power. Changing things like jetting, valve lash etc won't change where an engine likes it's timing.