after changes the MAP at WOT went down, not good?
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Joined: Feb 2004
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Originally Posted by P Mack
The reason timing affects your vacuum at idle is because your iac changes to keep your rpm the same. If you lower your timing, your rpm goes down, your iac opens more, your map increases. And vice versa. I don't buy the idea that a small change in timing at wot would have any measureable effect on manifold pressure.
Interesting, but I don't think it's a direct comparison because rpm probably changed too. I think i will have to give this w.o.t. thing a try and see if my map changes with reduced timing.
It is like JimmyBlue said - you are seeing lower MAP values when the engine is operating more efficiently (at best torque/power levels) - it is drawing vacuum on the intake system. Basically, it is drawing more air into the cylinders and can't get enough through the TB, so there is a vacuum in the intake where the MAP sensor is located. What this is indicating is that you would benifit from an increase in airflow - ported TB, LS6 intake if you don't have it, or even a 90/90 setup. As far as the timing, it just shows that the engine wants more air at WOT with one timing value vs. another. If you did this on the dyno, you would likely see some torque/power difference between the two. Make sense?
Thread Starter
Joined: Mar 2003
Posts: 8,319
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From: North Carolina
yeah it makes sense, the better timing values want more air>>means more power 
haha
yes i have ported TB and ls6 itnake, i wanna get a port matched lsx once i get money. just means ill see better gains from a larger intake. closer you are to ATM pressure more power seeing the intake is free flowing. now the timing tables are just set up to draw air in like it should. i would like to see if others see the same thing though.
hahayes i have ported TB and ls6 itnake, i wanna get a port matched lsx once i get money. just means ill see better gains from a larger intake. closer you are to ATM pressure more power seeing the intake is free flowing. now the timing tables are just set up to draw air in like it should. i would like to see if others see the same thing though.
Keep in mind you won't have atmospheric pressure in your intake unless the engine is stopped. Faster moving air will be at a lower pressure, so your map will probably never = baro, but close.
I tried this and I think i may have seen a 1 kpa difference going from 28 deg to 22 deg, but i'm not sure.
So is this what you guys think is happening? Less timing advance would mean the spark happens later, leaving the exhaust gasses at a higher pressure since they expended less energy pushing the piston down. With the exhaust gases at higher pressure at the beginning of the exhaust stroke, the exhaust will leave the combustion chamber with more momentum, and leave more of a vacuum in the chamber when the intake valve opens from scavenging. It will draw more air in on the intake stroke which will create more of a vacuum in the intake manifold. Disclaimer, I'm not saying i think this is true, just throwing out a scenario for you guys to agree or disagree with.
I tried this and I think i may have seen a 1 kpa difference going from 28 deg to 22 deg, but i'm not sure.
So is this what you guys think is happening? Less timing advance would mean the spark happens later, leaving the exhaust gasses at a higher pressure since they expended less energy pushing the piston down. With the exhaust gases at higher pressure at the beginning of the exhaust stroke, the exhaust will leave the combustion chamber with more momentum, and leave more of a vacuum in the chamber when the intake valve opens from scavenging. It will draw more air in on the intake stroke which will create more of a vacuum in the intake manifold. Disclaimer, I'm not saying i think this is true, just throwing out a scenario for you guys to agree or disagree with.
Thread Starter
Joined: Mar 2003
Posts: 8,319
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From: North Carolina
yeah i meant closer to ATM. try to go up on your timing, see if you notice any change, that is if you want to test it. it seemed more sensative for me to go to around 25-26 then to drop lower than that from 30ish.
