great question on spark tables

[qbngolf]

ok so my question is pretty basic. when i increase the number in the spark table, does this advance timing or retard it. when ilower the number in the table, the gauge reads that the number reads closer to btdc. so i am actually advancing timing by lowering the number right?


also....... is btdc on the downward stroke of the piston or on the upward stroke? does anybody know?

[qbngolf]

come on, somebody has got to know.

[Careless]

also....... is btdc on the downward stroke of the piston or on the upward stroke? does anybody know?

i would imagine the b in btdc stands for "before" top dead center, and a piston only reaches top dead center on the upwards stroke, and starts from tdc on the downwards stroke.

so to be at btdc, youd have to be approaching tdc. or else it would be after top dead center, once leaving that position, correct?

i dunno, but that's what i think

[rs2000]

Going from memory in the spark VE table a positive number is advanced, and a negative number is retarded. When you adjust the values in megatune in the tuning window it will tell you if you are advancing or retarding the timing, so you should be able to confirm what i have said yourself (or its probably in the manual)
BTDC means before top dead centre, so the piston is rising. TDC is top dead centre, so the piston is stationary and ATDC is after top dead centre, so piston moving down. TDC is specified as being on the firing stroke of a 4 stroke motor.
Having the timing set at x degrees BTDC means it is advanced, having the timing at x degrees ATDC is retarded.
Generally the timing needs to be advanced to account for the speed at which the mixture burns under differnt conditions. This gets the actual bang at the right place, the 'right place' depends on boost, fuel, engine type and tune etc. A dyno tune is the best way as you can see where the power starts to fall off.

andrew

[redjetta]

so i am actually advancing timing by lowering the number right?

yes

the number you enter is the number of degrees before top dead centre that the coil fires

because the velocity of the explosion is fairly constant, and the velocity of the piston changes with RPM, the spark has to fire earlier at high RPM than at low RPM, because the piston will be at the top sooner.

edit: i've only worked with natural aspiration so this may not apply to turbos

[rs2000]

so i am actually advancing timing by lowering the number right?

yes

the number you enter is the number of degrees before top dead centre that the coil fires

isn't it the other way around, higher number in megatune = advanced? Lower (or negative) number = retarded

[qbngolf]

so why would you want the explosion while the piston is rising? wouldnt that tend to want to break things? its like you have a piston which is coming up and trying to force it back down but it cant go down because the crank isnt at a position to let that happen yeet

[qbngolf]

[/quote]

isn't it the other way around, higher number in megatune = advanced? Lower (or negative) number = retarded[/quote]

thats what i would think, but the lower the number, the closer it gets to btdc

[FoundSoul]

The number in the table is the degrees of advance BTDC. So higher positive numbers=more ignition advance=firing sooner in the cycle.

[rs2000]

so why would you want the explosion while the piston is rising?

I'm no pro, but heres what i've gathered from what i've read. You want the actual explotion right after TDC. The problem is the burn you get is not instant. There is a delay after the spark before the mixture ignites fully and explodes. The piston is travelling extremely fast so in order to compensate for the 'slow' ignition and burn, we make the spark happen BTDC, when in actual fact the 'explotion' of the fuel, and therfore the pushing force on the piston is happening ATDC..

The exact point at which you want the spark is found on a dyno by slowly increacing the spark advance until either the engine knocks or pings, or the torque output stops increacing. Excess advance can cause detonation knock pinging etc and after a certain point extra advance will just increace heat.

[FoundSoul]

so why would you want the explosion while the piston is rising?

I'm no pro, but heres what i've gathered from what i've read. You want the actual explotion right after TDC. The problem is the burn you get is not instant. There is a delay after the spark before the mixture ignites fully and explodes. The piston is travelling extremely fast so in order to compensate for the 'slow' ignition and burn, we make the spark happen BTDC, when in actual fact the 'explotion' of the fuel, and therfore the pushing force on the piston is happening ATDC..

The exact point at which you want the spark is found on a dyno by slowly increacing the spark advance until either the engine knocks or pings, or the torque output stops increacing. Excess advance can cause detonation knock pinging etc and after a certain point extra advance will just increace heat.

That's pretty much right on-- and since different octane fuels burn at different speeds you tune differently for higher octane fuel. Higher octane burns slower... which means you need more ignition advance to get the BOOM exactly when you want it. Lower octane fuel burns faster, so you need to retard the ignition some if you're running it or you'll get detonation as the 'boom' happens too soon and tries to send the rising piston back down before it's time....

Without advance though, if you tried to light it off right at TDC, the explosion would just be chasing the piston down the cylinder, rather than pushing it. It's all about........ timing. ;)

[qbngolf]

good info

[redjetta]

isn't it the other way around, higher number in megatune = advanced? Lower (or negative) number = retarded

yes, this is true. my bad

yes, the explosion has to occur before tdc, because the piston is moving so fast. if the explosion happened at tdc, the piston would be far away by the time the explosion got to it, and there would be no power.

if the explosion happens too soon, however, the piston will be before tdc when the explosion reaches the piston dish, and the piston will be pushing up on the explosion. this pressure is transferred to the cylinder head and creates knock, pinging, whatever you want to call it. higher octane fuel burns slower than lower octane fuel, allowing the computer to put in more fuel before there is any knock, producing more power.

edit: lol the same thing was said two posts ago

[krisr]

Howstuffworks (http://auto.howstuffworks.com/ignition-system.htm) has a brilliant write up explaining ignition advance which i'm suprised noone referred.

[qbngolf]

now to this question.

why is it when tuning the spark table in megatune in the realtime editor, when you hit shift then up, in parenthises, it says retarded when you are increasing the number? and advanced when you decrease the number

[jsmcortina]

now to this question.

why is it when tuning the spark table in megatune in the realtime editor, when you hit shift then up, in parenthises, it says retarded when you are increasing the number? and advanced when you decrease the number

What code version is this with? - this is a very simple error in the .ini file that needs fixing.

James

[redjetta]

I see... If you press up, it says ADVANCED, if you press down, it says RETARDED. it's telling you the last move you made. 2.25 final

[qbngolf]

i have a ms1 processor with a v3 board running msns-e i believe the code is 029s

[redjetta]

which version of MegaTune?

[qbngolf]

version 3 with a ms1 processor