tdc

[fiat132t]

hey there guys...i have a question to ask
+20btdc is 20atdc
-20btdc is 20dtdc
do i have this correct or what

[lani16]

When you talk about btdc or atdc you talking about advance. That means +20 BTDC the spark is bein produced 20 crankshaft degrees before the piston reaches top dead center so the piston is on it's way upp when the fuel is ignited. This is nesesary because the fuel takes some time to fully ignite. If you have your time perfectly dialed in the fuel mixture is going to be fully ignited when the piston is at TDC.too much advance and the piston is still gonna be on it's way upp when the burning fuel starts to push it down and that's no good. On the other hand ATDC (wich you will never use) the fuel mixture is gonna be fully ignited when the piston is already on it's way down after TDC so that's a waste and most likely will give you overheating problems and even some nice glowing red headers because the fuel is still burning when it goes past the exhaust valve which is gonna be burned too if you leave it like that.

[fiat132t]

so if someone was to write
+ or - 20 btdc....
what would that mean?

[PSIG]

so if someone was to write
+ or - 20 btdc....
what would that mean?

Normally there is no + or - symbol. 20° BTDC is exactly that. It could also be said as 340° ATDC. Now, if there is another connotation being used in the sentence - that's different. However there would then normally not be any A/BTDC as it would mean (+) adding/advancing or (-) subtracting/retarding by 20° from the current setting.

A few shops use other symbols to indicate whether it is the compression or exhaust stroke such as "20° BTDCF" for "20 degrees Before Top Dead Center - Firing". Also, the D for Dead is often left out for simplicity as in "20BTC".

Hope that helps,
David