i have a midly modified 305 TPI (edelbrock intake and runners, ported plenum, ported MAF, SLP cold air induction, relocated MAT sensor, GM high performance distributor, accel coil, flowmaster cat back exhaust, hypertech thermomaster chip with low temp thermostat and fan switch, stock 19lb injectors, throttle body bypass, airfoil, adjustable regulator, edelbrock waterpump and headers, shift kit, stock heads blah blah blah) now i started fooling with fuel pressure and timing. found that 46psi for fuel pressure was the best setting but for timing i kept advancing until i felt a drop off in performance. well i never got a drop off. i have the timing at 20 degrees advanced right now and the 305 is like a rocket right now. i advanced it as far as it woul go to 26 degrees but didn't notice anymore gains so went back to 20 were it felt the strongest before a drop off under this setting. 20 degrees sounds like a lot of advanve for a midly modified 305 with over 150,000 miles on it. anyone have this much advance on a 305? engine temp stays cool as ever so detonation doesn't seem to be taking place. any input would be appreciated.

 

SD,

The EST and ECM will retard the timing whenever detonation is detected. Because of this, your advance may not actually be where you think it is, and you may never hear a knock or ping. The danger in this is that the EST/ECM can only retard the spark a given number of degrees. High base timing settings could have disasterous consequences if you start to get detonation and the timing cannot be retarded enough to prevent it.

The other side of the coin is that with higher mileage, there is probably lower dynamic compression, so some additional advance can usually be tolerated before combustion pressures get high enough to cause preignition or detonation damage.

Above all of this, I'm presuming the EST bypass connector is getting disconnected for base timing settings, then reconnected for normal operation. I'm also guessing that you've verified that the harmonic balancer outer ring has not slipped, so that your indicated timing readings are correct.

Later,
Vader

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"Some Body...Someone"

 

replaced the timing chain and balancer when i did the intake swap. chiltons manual says to short the A and B terminals and if no advance is seen then you don't have to disconnect the EST. i didn't get any advance when i reved the engine with the A and B terminals shorted so figured i was getting an accurate reading for my base timing. the distributor is almost maxed out for advance so i'm assuming i'm getting a correct reading of 20 degrees. what do you think?

 

Chiltons suck. YOu need to disconnect the bypass connector by the pass. side strut tower area. Once you disconnect it, your timing will probably drop back to about 10-12*. That is your base timing. The ECM advances timing when you connect the bypass wire. Don't forget to clear the code 42 when done.

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"I used to have a handle on life, but then I used it as a plunger and broke it" -The Wave

ASE Master Tech + L1
Savannah, GA
'87 Trans Am-K&N,Cold air induction, SSM SFC, Flowmaster, 16" GTA rims.
'97 Bonneville SSE
About Me

 

P.S. by singling out people like Vader and I, you decrease your chances for a response. Some people won't reply even if they can help because you are addressing an individual. There are a plethera of people other than Vader and I that is capable of helping. Good luck.

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"I used to have a handle on life, but then I used it as a plunger and broke it" -The Wave

ASE Master Tech + L1
Savannah, GA
'87 Trans Am-K&N,Cold air induction, SSM SFC, Flowmaster, 16" GTA rims.
'97 Bonneville SSE
About Me

 

i must be getting a false reading because after diconnecting the bypass i can't even get a reading on my timing gun. i would have to retard the gun probably 5 degrees before zero to get a zero reading on my timing marks so i'm guessing the timing is more like 15 degrees advanced than 20. the TA tears it up so i'll leave it where it is for now. if the engine self destructs then i know it was to far advanced and won't have anymore excuses why i haven't upgraded to a 350 yet. i swear this 305 will give a 350 a run for it's money anyday. it seems to get faster with age so maybe i'll just stroke it and blow it if it grenades. thanks for the help. if you want the best answer you have to ask the best person, hence the calling out of the Vader/Tech duo.

 

scotty d,

GMT is correct. Asking for advice from a specific person can exclude some excellent resources. This one is fairly simply, and you could have had the correct response much faster from any number of people who have done this before.

Thanks for the vote of confidence, whether I deserve it or not. I can hold my own when it comes to the "make-it-run-right", chassis, and "flow/power enhancement" questions, but I suck exhaust when it comes to axial superchagers, NOS, and transmissions. My total experience with transmissions has been manuals, some valve body work, and one complete PowerGlide overhaul. As far as the newer axial blowers and nitrous, all I know is that they beat me when I line up next to them.

Nobody knows it all, and that's why the board is here. We have thousands of members with a broad range of experience. You may have just made several very skilled people more reluctant to help out next time. There are a lot of excellent technicians like RB, Mac, the Doctor, etc. (I apologize for all those I left out, but you know who you are, and I don't have all day.)

Maybe next time you could start your thread with something like "GM Tech, or anyone, ..."

BTW - GMT is correct about Chiltons as well. You're better off with a Helms shop manual and/or several "cheapie" manuals to compare notes. Also, you MUST disconnect the EST bypass connector to set base timing on your ECM.

If your engine has high mileage and some wear, the 15° may not be that far out of line, but I'll bet the ECM is already doing some spark retarding under heavy loading. The only way to check this accurately is with a data scanner.

Later,
Vader

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"Some Body...Someone"