92 Firebird with rebuilt 350 long block. Kept it tbi. All new sensors, etc. New ECM with newly burnt chip for it. New 350 injectors, etc.

QUESTION:
1. What should base timing be?
a. A 305 L03 base timing is 0
b. 350 tbi base timing should be????

Help. I would appreciate your help on this. I get a 50/50 reponse from here and some say 0 for my 350 and others say 6 BTDC.

THanks so much guys!!!

 

It depends on what 350 heads are being used on your 350 long block engine.

The 350 TBI engines were used in fullsize passenger cars and light C/K/G trucks and vans.... and they all got 0 deg base timing because they had fast(er) burn heads from the factory. The fast(er) burn heads are also known on TGO as swirl (intake) port heads. Those heads need less ignition advance to make optimum power because the mixture can burn faster due to the forced convection (flow) of the flame.

All swirl port head v6 (4.3) and v8 (5.0, 5.7) TBI engines got 0 deg base ignition timing.

OTOH if you are using non-swirl port heads, like L98 iron heads, then you want the extra 6 degs advance because the heads don't burn fuel as fast.

So check the casting number on the head (under the valve cover). If the number ends in 187 (for a 305) or 191 or 193 (both for 350), then you have swirl port heads and you want 0 deg base timing. If they end with anything else (like 081 or 083, as used in iron head TPI 305 and 350 engines), then you'll want 6 deg advance BTC. HTH.

 

you have to take the valve cover off?
My 350 tbi is also set at 0*TDC..

 

The base timing needs to be set to the same position that the initial timing value is in the chip.

RBob.

 

To check the casting number on the head, yes. Checking the head casting number allows you to verify that you have swirl intake port heads.

RBob is literally correct in that the base timing stored in the chip ought to match the engine -- my answer was directed more at physically what you ought to be using for timing and why.

 

here's the casting mark for the 193. I don't have the others but this might save you the trouble of pulling your valve cover.

 

I really appreciate all the info. Will check to see what heads i have. Again, thanks so much.

My idle is a little rough and it still surges. EVERYTHING on this car is brand new. LOL. Sensors, Modules, distributor, pickup coil, ignition module, injectors, etc etc etc. New harmonic balancer, etc.

When base timing is set with EST disconnect i have it set to 0 for now assuming that is correct casue that is what the chip is burned to, but when i clear the computer and connect the EST back up the timing mark jumps 4-6 degrees. Any ideas?

THanks

 

My idle is a little rough and it still surges. . . Sounds like it needs a little more tuning.

When base timing is set with EST disconnect . . . What you are seeing is correct. Once the EST is reconnected and the ignition cycled off for at least 10 seconds, upon restart the ECM will control the timing. Although, I would wonder why it is only advancing (it is advancing, correct?) 4-6 degrees. That isn't much timing for idle conditions.

RBob.

 

It is advance 20 degrees but the timing mark fluctuates or moves back and forth 4-6 degress with the EST connected. It acts spastic, lol.

Thanks!

 

Still using the stock '8746 ECM? If so then the timing change is normal. There is a routine that adjusts the spark timing to smooth out the idle. OTOH, if the idle has issues this functionality can make it a little worse.

RBob.