How much total advance in timing is too much?
05-14-2012, 11:54 PM
#21
Race Director
BBC needs a little more sometimes. When Elle88's engine was dyno tuned we were at 19 or 20 initial and 38 or 39 on total for best power with modern aluminum heads. It has been a couple years and many runs so Im not quite sure but I remember thinking it was pretty high. I could see old iron heads wanting more if the fuel has enough octane for the additional advance.
Thats what im talking about...
But this works for my cast heads as well... modern? ..... ported, polished and port-matched by Valley heads, World Spotsman II's
Last edited by pauldana; 05-14-2012 at 11:57 PM.
05-14-2012, 11:59 PM
#22
Race Director
Cleaning up the combustion chamber can do a lot. The only real way to know for sure is to experiment on the dyno like Billa said.
05-15-2012, 05:34 AM
#23
Burning Brakes
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BBC needs a little more sometimes. When Elle88's engine was dyno tuned we were at 19 or 20 initial and 38 or 39 on total for best power with modern aluminum heads. It has been a couple years and many runs so Im not quite sure but I remember thinking it was pretty high. I could see old iron heads wanting more if the fuel has enough octane for the additional advance.
but checked the dyno sheets made at 35 deg and power was down about 5hp , so no big difference
05-19-2012, 07:33 PM
#24
Le Mans Master
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You still don't know what dist rotor phasing is! Google it!
I know what you're trying to say but you've got it a bit wrong. As you say, if you have 45* at the crank, the rotor "advance" is only half that, i.e. 22.5*, exactly as you said. However, this puts the rotor HALF WAY between the 45* spaced distributor cap terminals, not directly underneath the next one in the sequence. I agree with you about the tendency of electrons, I just hope that the OP can understand all of this so let's all try to help him, eh?
Regards from Down Under.
aussiejohn
Regards from Down Under.
aussiejohn
Regarding the timing advance, there's a pretty big disconnect in the logic.
You can advance beyond 45* of timing advance. Why couldn't you? The distributor curve and travel of the cylinder are related mechanically, yes, but the distributor fires whatever cylinder post the rotor is under. If it fires with 60* of crank advance it certainly will. It only fires when the rotor tip is pretty much under the corresponding post (unless you have some ignition problems.)
What that means is the piston is only a third of the way up on the compression stroke when the ignition event happens for that cylinder.
It's not firing on the wrong cylinder, although it could be happening at the wrong time.
(it's probably not a good idea to have that much advance... but there are definitely cases where cars have advance in the 50* range at high engine speed, low load, etc.)
You can advance beyond 45* of timing advance. Why couldn't you? The distributor curve and travel of the cylinder are related mechanically, yes, but the distributor fires whatever cylinder post the rotor is under. If it fires with 60* of crank advance it certainly will. It only fires when the rotor tip is pretty much under the corresponding post (unless you have some ignition problems.)
What that means is the piston is only a third of the way up on the compression stroke when the ignition event happens for that cylinder.
It's not firing on the wrong cylinder, although it could be happening at the wrong time.
(it's probably not a good idea to have that much advance... but there are definitely cases where cars have advance in the 50* range at high engine speed, low load, etc.)
Starting again
Ignition timing: 45* BTDC
means that the ignition event happens (rotor tip reaches plug tower post) while the piston is halfway up on the compression stroke.
There's no need to worry about the distributor math. None. not at all. Helps when it comes to stabbing the dizzy or just understanding the principle behind how a cam driven distributor works, but it doesn't matter in this conversation.
Having your timing at 90* BTDC does not mean that the rotor will be pointed at the previous cylinder, it means that the ignition event for that cylinder occurs as the compression stroke starts.
Ignition timing: 45* BTDC
means that the ignition event happens (rotor tip reaches plug tower post) while the piston is halfway up on the compression stroke.
There's no need to worry about the distributor math. None. not at all. Helps when it comes to stabbing the dizzy or just understanding the principle behind how a cam driven distributor works, but it doesn't matter in this conversation.
Having your timing at 90* BTDC does not mean that the rotor will be pointed at the previous cylinder, it means that the ignition event for that cylinder occurs as the compression stroke starts.
Your first misconception is the as the advance plate moves the trigger (points, pole piece,etc.) the rotor still points at correct plug terminal when it fires. It doesn't! It points at the advanced position at the time of trigger.
Second misconception is vacuum advance is directly added to mechanical advance. Vacuum advance is a moving target that is usually zero at WOT - so total timing at WOT is only mechanical (initial + centrifugal).
Not to steal the thread here LS6 but the danger of too much timing needs to be realized here (and this thread is mostly run its course already for you). The real danger here is the common practice to regurgitate the same buzz statements about total timing should be >50*. Where most fail to use their brain is after setting up mechanical timing near 35* @ 3000rpm and then chose a vacuum can to add adv under light load and low rpm.
If some would set their mechanical timing >50* and please post the results i'm sure we would all enjoy to read about it.
cardo0
05-19-2012, 08:01 PM
#25
Race Director
cardo0 - I believe your post about my "misconceptions" is a misconception.
While I certainly understand why you bring rotor phasing into the question, what you're talking about assumes an initial advance of ZERO.
Now, I wouldn't recommend doing this but you can certainly achieve advance exceeding 45*. Imagine if you started by adjusting the initial.
I think we're talking about two different things here.
Also, regarding the "total advance" "misconception", yes, at WOT vacuum approaches zero and you lose all vacuum advance, but what happens when you're cruising at 2500 RPM? Especially if your mechanical has been set up "all in" at 2400. (I'm thinking of this from a small block perspective). You will end up with more advance than you would at WOT. I would call that "total advance".
While I certainly understand why you bring rotor phasing into the question, what you're talking about assumes an initial advance of ZERO.
Now, I wouldn't recommend doing this but you can certainly achieve advance exceeding 45*. Imagine if you started by adjusting the initial.
I think we're talking about two different things here.
Also, regarding the "total advance" "misconception", yes, at WOT vacuum approaches zero and you lose all vacuum advance, but what happens when you're cruising at 2500 RPM? Especially if your mechanical has been set up "all in" at 2400. (I'm thinking of this from a small block perspective). You will end up with more advance than you would at WOT. I would call that "total advance".
05-19-2012, 11:16 PM
#26
Le Mans Master
Member Since: Oct 2002
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MSD even has a Y-tube video:http://www.youtube.com/watch?v=aWMlNwGW0tM
I couldn't explain it better,
cardo0
I couldn't explain it better,
cardo0
05-19-2012, 11:41 PM
#27
Race Director
OK. What does rotor phasing have to do with timing advance? Seems more like a maintenance issue than anything.
Last edited by Shark Racer; 05-19-2012 at 11:44 PM.
05-20-2012, 12:03 AM
#29
Race Director
cardo0 - I believe your post about my "misconceptions" is a misconception.
While I certainly understand why you bring rotor phasing into the question, what you're talking about assumes an initial advance of ZERO.
Now, I wouldn't recommend doing this but you can certainly achieve advance exceeding 45*. Imagine if you started by adjusting the initial.
I think we're talking about two different things here.
Also, regarding the "total advance" "misconception", yes, at WOT vacuum approaches zero and you lose all vacuum advance, but what happens when you're cruising at 2500 RPM? Especially if your mechanical has been set up "all in" at 2400. (I'm thinking of this from a small block perspective). You will end up with more advance than you would at WOT. I would call that "total advance".
While I certainly understand why you bring rotor phasing into the question, what you're talking about assumes an initial advance of ZERO.
Now, I wouldn't recommend doing this but you can certainly achieve advance exceeding 45*. Imagine if you started by adjusting the initial.
I think we're talking about two different things here.
Also, regarding the "total advance" "misconception", yes, at WOT vacuum approaches zero and you lose all vacuum advance, but what happens when you're cruising at 2500 RPM? Especially if your mechanical has been set up "all in" at 2400. (I'm thinking of this from a small block perspective). You will end up with more advance than you would at WOT. I would call that "total advance".
