LS1 timing
Teching In
Joined: Apr 2006
Posts: 9
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my idle was swinging pretty bad after cam and i changed timming to 13-15 and played w base airflow which fixed it but id like to know why jhon d said itle make coolant run hotter, ill log with timming up high again and compare.btw do you not like the way it sounds at lower timming?
That's an old tuning rule from way, way back...
I think the reason is that the engine is running less efficiently and it takes more fuel to idle at the same rpm. Burning more fuel makes more heat, and less of the heat energy is converted to moving the piston since the spark is late.
The more efficiently the engine is running, the more intake vacuum it will make (lower kpa). The higher vacuum is because the throttle (or IAC) is closed more, but the engine is still turning the same rpms. (the throttle is a smaller intake leak when it's closed more...)
You can see in the example data a few posts back, that the lowest kpa was at the highest advance, and the middle range was close to that. As the timing advance moved lower, the kpa came up even more, indicating the IAC had to be held open more to keep the same idle speed.
The higher timing advance does use even less fuel, but typically the idle there is less stable, especially as you transition from unloaded to loaded (neutral to in-gear).
I think the reason is that the engine is running less efficiently and it takes more fuel to idle at the same rpm. Burning more fuel makes more heat, and less of the heat energy is converted to moving the piston since the spark is late.
The more efficiently the engine is running, the more intake vacuum it will make (lower kpa). The higher vacuum is because the throttle (or IAC) is closed more, but the engine is still turning the same rpms. (the throttle is a smaller intake leak when it's closed more...)
You can see in the example data a few posts back, that the lowest kpa was at the highest advance, and the middle range was close to that. As the timing advance moved lower, the kpa came up even more, indicating the IAC had to be held open more to keep the same idle speed.
The higher timing advance does use even less fuel, but typically the idle there is less stable, especially as you transition from unloaded to loaded (neutral to in-gear).
Last edited by John_D.; Mar 2, 2008 at 09:57 AM.
That's an old tuning rule from way, way back...
I think the reason is that the engine is running less efficiently and it takes more fuel to idle at the same rpm. Burning more fuel makes more heat, and less of the heat energy is converted to moving the piston since the spark is late.
The more efficiently the engine is running, the more intake vacuum it will make (lower kpa). The higher vacuum is because the throttle (or IAC) is closed more, but the engine is still turning the same rpms. (the throttle is a smaller intake leak when it's closed more...)
You can see in the example data a few posts back, that the lowest kpa was at the highest advance, and the middle range was close to that. As the timing advance moved lower, the kpa came up even more, indicating the IAC had to be held open more to keep the same idle speed.
The higher timing advance does use even less fuel, but typically the idle there is less stable, especially as you transition from unloaded to loaded (neutral to in-gear).
I think the reason is that the engine is running less efficiently and it takes more fuel to idle at the same rpm. Burning more fuel makes more heat, and less of the heat energy is converted to moving the piston since the spark is late.
The more efficiently the engine is running, the more intake vacuum it will make (lower kpa). The higher vacuum is because the throttle (or IAC) is closed more, but the engine is still turning the same rpms. (the throttle is a smaller intake leak when it's closed more...)
You can see in the example data a few posts back, that the lowest kpa was at the highest advance, and the middle range was close to that. As the timing advance moved lower, the kpa came up even more, indicating the IAC had to be held open more to keep the same idle speed.
The higher timing advance does use even less fuel, but typically the idle there is less stable, especially as you transition from unloaded to loaded (neutral to in-gear).
So if I am reading you correctly, should I take the spot where the kpa begins to move up out of the low 50's (that will be around ~25*-30*) and use that timing even though its not as stable as high or low spark?
And SSMIKE, yes I have never heard the idle sound that good because I have never gone that low before because most say the LS1 needs ~20*-30* of spark, but man I have never heard the motor sound sooo good, but guess should not be that low.
If it's not running hot when idling/staging, the lower advance won't hurt anything.
I'm surprised it likes so much advance. Usually the engine will want to surge when it's real high. Or the idle will want to fall off when putting it in gear.
Pretty much it comes down to what you like, as far as the stability of the idle and the sound too. Unless heat or gas mileage overrides those. (most folks with a cam don't have gas mileage at the top of the list though)
The transition into the running spark table may be a consideration too. If there's a big change in the timing as it moves from the idle table to the running table, there may be the feeling of a bog/stumble.
I'm surprised it likes so much advance. Usually the engine will want to surge when it's real high. Or the idle will want to fall off when putting it in gear.
Pretty much it comes down to what you like, as far as the stability of the idle and the sound too. Unless heat or gas mileage overrides those. (most folks with a cam don't have gas mileage at the top of the list though)
The transition into the running spark table may be a consideration too. If there's a big change in the timing as it moves from the idle table to the running table, there may be the feeling of a bog/stumble.
I just want it to be the best, stable idle I can have, it is not that bad and really doesnt surge, it is more like stumbling, expeciall when comming to a stop, and I would think that the most stable idle would be the most efficient and best for gas mileage as well....but mabee I am wrong about that.
And on every new file I smooth the idle and main spark tables to either be the same or have no more than 2* different so that this is not a issue...but thanks for the advice.
Thanks for your help!
And on every new file I smooth the idle and main spark tables to either be the same or have no more than 2* different so that this is not a issue...but thanks for the advice.
Thanks for your help!
