to those sticking with the nikki: free mod.
07-01-08, 07:40 AM
#51
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I have not kept track of who said what in this post but here is my two cents: transition areas from one size to another need to be smooth and consistent. (Fluid Dynamics 101) Limited port matching in any circumstance is beneficial. My background and experience is 15 years as an R&D Mechanical Engineer in the automotive industry as well as an Aerospace degree with an Airframe and Powerplant mechanics license.
07-01-08, 10:47 AM
#52
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This whole post is one piece of misinformation.
Ignition vacuum advance is never hooked up to venturi vacuum. It's either hooked up to pure manifold vacuum or ported vacuum which is just above the throttle plate so it sees manifold vacuum as soon as the throttle is cracked open.
Your theory defies the laws of physics. As load increases the pressure inside the combustion chamber increases which helps the mixture burn quicker. This means you need less timing.
I can't find one source on the net that says to use venturi vacuum for ignition timing yet there are countless articles that support manifold or ported vacuum.
http://www.vetteclub.org/technicalti..._explained.pdf
Ignition vacuum advance is never hooked up to venturi vacuum. It's either hooked up to pure manifold vacuum or ported vacuum which is just above the throttle plate so it sees manifold vacuum as soon as the throttle is cracked open.
Your theory defies the laws of physics. As load increases the pressure inside the combustion chamber increases which helps the mixture burn quicker. This means you need less timing.
I can't find one source on the net that says to use venturi vacuum for ignition timing yet there are countless articles that support manifold or ported vacuum.
http://www.vetteclub.org/technicalti..._explained.pdf
So, if you think that entire post was "Misinformation", think again. As the engine is placed under higher loads, the timing should be advancing, as the higher pressures inside are less prone to detonation, up to a point. Once the pressure gets too high, it can cause detonation, even without the ignition system being a part of the equation.
As the amount of air/fuel inside a combustion chamber increases, it takes longer to acheive *complete* combustion, therefore, the timing needs to be advanced. This is the reason for both the vacuum advance, and mechanical advance features designed into nearly every distributor ever used in an engine.
07-01-08, 11:35 AM
#53
I hate drum brakes
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I may or may not stick with the Nikki for the long run, as its one thing after another getting the pieces together (getting ripped off by other forum members, junk yards scrapping 7s as soon as they get into the junk yard, and 7s rarely appearing in the junkyards) Now I'm thinking of going EFI with Jenvey IDA/DCOE throttle bodies. But if I come across a cheap stock manifold, I'll compare it with my ported manifold and share the results with everyone... But all this wiil have to wait until I finish the metal work, brakes, GSL rear end restoration, full suspension, and ReSpeed steering rack; which are all happening at the same time for convienience and dependence on each other.
07-02-08, 04:31 AM
#57
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Notice the part I highlighted. What you are calling "Ported Vacuum", is in reality venturi vacuum. Venturi vacuum occurs above the throttle plates, from the airflow thru the venturis inside the carb.
So, if you think that entire post was "Misinformation", think again. As the engine is placed under higher loads, the timing should be advancing, as the higher pressures inside are less prone to detonation, up to a point. Once the pressure gets too high, it can cause detonation, even without the ignition system being a part of the equation.
As the amount of air/fuel inside a combustion chamber increases, it takes longer to acheive *complete* combustion, therefore, the timing needs to be advanced. This is the reason for both the vacuum advance, and mechanical advance features designed into nearly every distributor ever used in an engine.
So, if you think that entire post was "Misinformation", think again. As the engine is placed under higher loads, the timing should be advancing, as the higher pressures inside are less prone to detonation, up to a point. Once the pressure gets too high, it can cause detonation, even without the ignition system being a part of the equation.
As the amount of air/fuel inside a combustion chamber increases, it takes longer to acheive *complete* combustion, therefore, the timing needs to be advanced. This is the reason for both the vacuum advance, and mechanical advance features designed into nearly every distributor ever used in an engine.
The flame front travels faster as cylinder pressure increases. This means less timing. Timing should be the most retarded when cylinder pressure reaches it's peak. This is basic stuff. Try playing with forced induction and you'll be in for a rude awakening.
You can argue all you like but you can't re-write the laws of physics.
07-02-08, 11:43 AM
#58
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Venturi vacuum occurs below the venturi, due to the higher velocity of the air entering the air below the venturi, which is larger than the venturi itself. As this high velocity air passes thru the larger area, it causes a vacuum effect to occur above the throttle plate.
I never said it occured below the throttle plate. Anything below the throttle plate is manifold vacuum, which decreases at the plate(s) is opened. I stated very clearly that venturi vacuum occurs above the throttle plate.
If the timing should be retarded at higher pressures and RPM, why did every manufacturer bother to add a centrifical advance to their dizzy? Most were also assisted by a vacuum advance, to start the advance occuring at lower RPM.
As for playing with forced induction, that doesn't even apply here, as the entire thread is pertaining to N/A mods in the intake.
As for purple82, I quit paying attention to his incorrect theories long ago.
Now, I am no longer paying attenion to your incorrect info, as well.
BTW, I have not been "Arguing", mearly politely stating the facts. As for re-writing the laws of physics, you 2 have been trying to re-write them with misinformation.
The flame does travel faster at higher pressures, however, as the RPM increases, there is less total time for combustion. In order to acheive complete combustion, it must begin sooner, otherwise most of the energy would be wasted as it exited the engine, instead of being used to create power. Hence, the reason for the centrifical advance.
I never said it occured below the throttle plate. Anything below the throttle plate is manifold vacuum, which decreases at the plate(s) is opened. I stated very clearly that venturi vacuum occurs above the throttle plate.
If the timing should be retarded at higher pressures and RPM, why did every manufacturer bother to add a centrifical advance to their dizzy? Most were also assisted by a vacuum advance, to start the advance occuring at lower RPM.
As for playing with forced induction, that doesn't even apply here, as the entire thread is pertaining to N/A mods in the intake.
As for purple82, I quit paying attention to his incorrect theories long ago.
Now, I am no longer paying attenion to your incorrect info, as well.
BTW, I have not been "Arguing", mearly politely stating the facts. As for re-writing the laws of physics, you 2 have been trying to re-write them with misinformation.
The flame does travel faster at higher pressures, however, as the RPM increases, there is less total time for combustion. In order to acheive complete combustion, it must begin sooner, otherwise most of the energy would be wasted as it exited the engine, instead of being used to create power. Hence, the reason for the centrifical advance.
07-02-08, 12:58 PM
#59
I hate drum brakes
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I remember testing the vacuum ports under the carb with my vacuum gauge. I used a T-fitting and ran both pots to one line, and went into the port that they're supposed to go into (look at the vacuum line diagram, I can't remember but I think it's the second one from the left) When I tested it, I noticed that it is a venturi vacuum, not a manifold vacuum, the pressure decreases as engine speed (air velocity) picks up. I tried (just for the hell of it) hooking to manifold vacuum, and the car ran like crap all the time.
Now I could be wrong, my car's been out of commission for over a year now (thank you Lexus driving hoe) and it's getting hard to remember, so break out your vacuum gauge and FSMs and use your own judgement when it comes to hooking up your vacuum advance.
Now I could be wrong, my car's been out of commission for over a year now (thank you Lexus driving hoe) and it's getting hard to remember, so break out your vacuum gauge and FSMs and use your own judgement when it comes to hooking up your vacuum advance.
07-02-08, 04:28 PM
#60
Wow, alright guys let's get back on topic. I'm pretty sure there was a thread a while back discussing where to plug your vac advance and there was extensive discussion about what the different "types" of vacuum were and how they worked. You guys should search it up, dig it up from the dead and continue over there.
As for the manifold port matching to the engine, I think we're all in agreement at this point that nobody has enough firsthand knowledge or empirical evidence (dyno runs, flow bench test results, track time comparisons etc...) to sway the debate in either direction. We could banter all day about it, and either side could be right.
I think that until somebody actually gets out and tests this, there isn't much point to the continuation of this thread. I personally believe that port matching is detrimental, but that belief is supported purely by the tests of Yaw and Carl, and the theories surrounding reversion and loss of velocity. I could easily be proven wrong (or even right) and can't wait to see that happen.
I'm willing to do the dyno runs if someone is willing to provide the manifolds. I'm going to be doing dyno runs before the end of summer -anyway- and one run was going to be a bone stock carb and intake so it's no extra effort for me to add in one with port matched runners and nothing else modded on it.
I'm going to archive this thread and close it. If anybody gets any further information about this, feel free to start another thread or let me know, I just don't want this useful thread degenerating any further.
My apologies to xlr8planet if I seemed a bit harsh earlier. I didn't intend for it to sound that way.
Jon
As for the manifold port matching to the engine, I think we're all in agreement at this point that nobody has enough firsthand knowledge or empirical evidence (dyno runs, flow bench test results, track time comparisons etc...) to sway the debate in either direction. We could banter all day about it, and either side could be right.
I think that until somebody actually gets out and tests this, there isn't much point to the continuation of this thread. I personally believe that port matching is detrimental, but that belief is supported purely by the tests of Yaw and Carl, and the theories surrounding reversion and loss of velocity. I could easily be proven wrong (or even right) and can't wait to see that happen.
I'm willing to do the dyno runs if someone is willing to provide the manifolds. I'm going to be doing dyno runs before the end of summer -anyway- and one run was going to be a bone stock carb and intake so it's no extra effort for me to add in one with port matched runners and nothing else modded on it.
I'm going to archive this thread and close it. If anybody gets any further information about this, feel free to start another thread or let me know, I just don't want this useful thread degenerating any further.
My apologies to xlr8planet if I seemed a bit harsh earlier. I didn't intend for it to sound that way.
Jon
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