Engine timing

[Whiteshadow89158]

Hey guys, I read somewhere that the L81 motors were deadvanced at the factory for emission purposes and advancing the motor by 10 degrees would help restore some lost power. is this true? Also will I fail emissions if I do this?

 

[6880 Mike]

...I read somewhere that the L81 motors were deadvanced at the factory for emission purposes...

There was no actual engine tuning or detuning at the factory. At initial fire up on gasoline, there may have been tweaking, but not genuine tuning.

...Also will I fail emissions if I do this?...

Why take the chance? Tune it to factory specs; have it tested. After passing the test, tune it where you want it.

 

[Wajulia]

There are a lot of threads on this forum that says running an 81 at 12 deg BTDC brings our some hidden horsepower. The factory setting is 6. I have tried it and been unimpressed. I will soon get a little more scientific, setting at 6 -- 0-85 according to my GPS then set to 12 and run 0-85 again -- I will let you know if there is any difference.

 

[Whiteshadow89158]

O ok cool, ya definitely let us know what the outcome is.

 

[Sensei]

Yes, you get better performance with the timing advanced. Will you suddenly be turning 10s at the strip? No! But there will be improvement. 10-14 is the range you want to be in, the more the better so long as you do NOT experience detonation.

God bless, Sensei

 

[Peer81]

Some time ago I found a very nice read about engine timing on another forum:
Theory of the magical 36 degrees of timing

On older engines the magical number for the total timing is 36 degrees many think but the thing is. You don't want to know when the sparkplug needs to fire, you want to know at how many degrees after top dead center the cylinderpressure from the flamefront needs to be max. Logical would be you don't want max pressure when the piston is at the top in line with the connecting rod because little power will go into moving the piston from his point. There is a optimum point when pressure is still high (piston high) and the piston - connecting rod angle is good enough to transfer the power. Say this point is 6 degrees after TDC. We can't see this point so we need to guess when to start the spark(plug). This is also dependend on what the heads shape and size is, piston shape etc etc. I think the best way to figure this out is on a dyno but I think you're very close when turning till she starts detonating and then backoff 2 degrees.

Greetings Peter

 

[lars]

Due to variations in fuel octane ratings and compression ratio, the "back off 2 degrees from detonation" technique will typcally result in an over-advanced condition which is not conducive to best power. Having done dyno testing on dozens of engines (both Ford and GM), I can state with certainty that best power will occur with 32-38 degrees of total timing (usually rounded off to 36 degrees) on standard heads, and with 28-32 degrees on "fast burn" style combustion chambers. If you back off 2 degrees from detonation, you will often get about 40 degrees total timing, and some low-compression engines will never detonate no matter how far the timing is advanced. For the guy on the street, get a good timning light and set the total to 36 - don't do the "timing by ear and detonation" thing - it doesn't work. If you have access to a dyno and can afford it, establish total timing from the dyno numbers - it will always be in the range noted.

For complete instructions on how to correctly set the timing for peak performance, drop me an e-mail request for my "How to Set Timing" tech paper.

Lars
V8FastCars@msn.com
Denver

 

[Peer81]

But now the problem. Total timing is set without vacuum advance right? Our '81's like C4's don't have vacuum advance etc etc but a ECM that is changing the advance. So we can only set the base timing and "hope for the best" so to say. So what is the best way to do this if detonation and some degrees back isn't the way to go?

Greetings Peter

 

[herminator]

Due to variations in fuel octane ratings and compression ratio, the "back off 2 degrees from detonation" technique will typcally result in an over-advanced condition which is not conducive to best power. Having done dyno testing on dozens of engines (both Ford and GM), I can state with certainty that best power will occur with 32-38 degrees of total timing (usually rounded off to 36 degrees) on standard heads, and with 28-32 degrees on "fast burn" style combustion chambers. If you back off 2 degrees from detonation, you will often get about 40 degrees total timing, and some low-compression engines will never detonate no matter how far the timing is advanced. For the guy on the street, get a good timning light and set the total to 36 - don't do the "timing by ear and detonation" thing - it doesn't work. If you have access to a dyno and can afford it, establish total timing from the dyno numbers - it will always be in the range noted.

For complete instructions on how to correctly set the timing for peak performance, drop me an e-mail request for my "How to Set Timing" tech paper.

Lars
V8FastCars@msn.com
Denver

Lars,

I obtained your timing papers previously and they are very helpful, thanks. I don't remember if you address manifold vs. ported as far as best method. I have a '73 L-82 350 with Edelbrock RPM intake and heads, Holley 3310 and this cam: COMP Cams 12-600-8 - COMP Cams Thumpr Retrofit Hydraulic Roller Camshafts - Overview - SummitRacing.com and I have put the correct vac advance canister on for the low manifold vacume of this cam.


My comp ratio is 10.2:1, should I be taking my vacume from the manifold outlet at the bottom of the carb or the ported one at the top? I have heard differing opinions on this and would appreciate your expertise.

 

[lars]

Lars, I obtained your timing papers previously and they are very helpful, thanks. I don't remember if you address manifold vs. ported as far as best method. I have a '73 L-82 350 and I have put the correct vac advance canister on for the low manifold vacume of this cam.My comp ratio is 10.2:1, should I be taking my vacume from the manifold outlet at the bottom of the carb or the ported one at the top? I have heard differing opinions on this and would appreciate your expertise.

Your question is addressed in detail in my Vacuum Advance Tech Paper. E-mail me for a copy if you don't have a current version.

Lars
V8FastCars@msn.com

 

[lars]

But now the problem. Total timing is set without vacuum advance right? Our '81's like C4's don't have vacuum advance etc etc but a ECM that is changing the advance. So we can only set the base timing and "hope for the best" so to say. So what is the best way to do this if detonation and some degrees back isn't the way to go?
Greetings Peter

On ECM-controlled engines, the ignition curve must be altered by re-programming the PROM. You can obtain more total by advancing initial, but you're just shifting the rpm-scale of the curve. This can have a good effect, but it's not producing an optimum performance curve.

Lars