bot0611
1/2 ton status
Thats how i did it, i think its the best way even with a timing gun you dont really know what timing to get your engine on if it has aftermarket components.
Went to ported vacuum, but i will try both and see wich is better.
CK5
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Damn valve rattle wont leave my engine
- Thread starter bot0611
- Start date
Thats how i did it, i think its the best way even with a timing gun you dont really know what timing to get your engine on if it has aftermarket components.
Went to ported vacuum, but i will try both and see wich is better.
Every GM vehicle i've worked on uses ported vacuum from the factory.
I think ported vacuum is used from the factory with the assist of a manifold vacuum signal, and a delay valve in the system. If all the factory crap is removed, and the vacuum advance is connected straight to a vacuum source, then the only reliable vacuum source, which indicates how much throttle you have your foot into, can be manifold. If my theory is correct. I am open to different theorys though.
What are we talking, a couple degrees difference?
How can vacuum inside the engine, below the throttle plates, be appreciably different than anywhere else? If it were a velocity issue, ported vacuum would be through the roof at high RPM.
Anyone disagree that more advance produces a smoother idle?
Since you don't believe me or what i'm saying why not just go do the test i told you and find out for yourself. 
A four stroke gas engine is a vacuum pump. Camshaft selection will determine how much manifold vacuum the engine will make and air velocity through the carb well determine how much ported vacuum (maximum of manifold vacuum) will be made at any given time related to throttle postion.
A four stroke gas engine is a vacuum pump. Camshaft selection will determine how much manifold vacuum the engine will make and air velocity through the carb well determine how much ported vacuum (maximum of manifold vacuum) will be made at any given time related to throttle postion.
If I had two gauges, I'd test it out. I have no doubt that there are enough variables that results can be different.
I haven't said I don't believe what your gauges have shown. What I've said is I'd like it explained why or how they can (or are) different.
How about we clarify your results: under what conditions were the gauge readings taken?
I haven't said I don't believe what your gauges have shown. What I've said is I'd like it explained why or how they can (or are) different.
How about we clarify your results: under what conditions were the gauge readings taken?
Kenny78
1/2 ton status
I dont mean to step into the middle of the debate, but if you only had the throttle plates cracked the ported side wouldnt see full manifold.
I've already explained it a couple times now. Ported vacuum is created by air velocity past the transfer slot in the carb bore when the throttle plates are opened and will never be higher than maximum engine vacuum that can be achieved due to cam selection.
I have connected a vacuum gauge to a ported source and a manifold source. The ported source goes to full vacuum with only a small crack of throttle, and manifold vacuum source rises more slowly. With most vehicles they should reach maximum ignition advance somewhere around (give or take) 2000 RPM, which is approximately the same RPM a manifold source reaches maximum vacuum reading. With a ported source, full ignition advance would be reached at only about 700 to 900 RPM.
It is correct though that, even though ported vacuum rises faster than manifold it will never rise above manifold, or (i.e.) the engines maximum ability to create vacuum, which is determined by the camshaft.
It is correct though that, even though ported vacuum rises faster than manifold it will never rise above manifold, or (i.e.) the engines maximum ability to create vacuum, which is determined by the camshaft.
4xcrazy
3/4 ton status
i like pie
All measurements taken along the same flat stretch of pavement, same direction. All measurements in inches.
Idle vacuum (manifold of course) 19
Port; Manifold
1st 10MPH: 15; 19
2nd 10 MPH: 5; 17
1st 20MPH: 23; 22
2nd 20MPH: 19; 19
2nd 40MPH: 23; 22
The above readings are within my eyeball error range, of course with the exception of the obvious 1st/2nd gear 10MPH readings, when the slot probably wasn't uncovered completely.
If vacuum at the ported slot is based on inlet velocity, it sure isn't backed up above. Ported would ALWAYS be higher if it was (except under very low throttle position when the slot isn't uncovered), because the throttle bore always sees more velocity than the plenum, under load or not. I assume somewhere there is a carb large enough to flow more than all runners combined, but not here.
We're dealing with a 2 barrel (essentially) here, no second set of throttle plates to discuss. 1st @ 20MPH is 2400RPM FWIW.
What good would spark advance be if it only operated under WOT?
I'll have peach.
bot0611
1/2 ton status
This sure is an interesting subject, but still some preffer peach some apple, no i mean ported and full, would it depend on the carb? or simply preference or lazyness on trying it out, still every engine is diferent.
Seriously guys thanks for the help, my truck rides a lot better now.
Only thing i need i think are gears.
Seriously guys thanks for the help, my truck rides a lot better now.
Only thing i need i think are gears.
It depends on the engine. I don't have any engines with a really lumpy idle, apparently additional timing at idle can really help to smooth them out. But even on my motor with a VERY conservative cam, additional idle timing helped.
If you don't run manifold vacuum for your advance, additional base timing can be added, up until the point the engine gets hard to start. While that may be well and good, backing the mechanical timing off (say, 0* initial vs 8*), and using vac advance (manifold vacuum!) to get idle to 10*+, would make for an easy starting engine because timing is lower until the engine starts to run, that has good idle characteristics. Case in point, TBI trucks run 20+* at idle, with virtually the same engine that earlier was run with 8* at idle.
Discovery takes tuning, and perhaps some lack of experimentation comes from laziness, or because the vehicle runs ok the way it is. If someone really cares, they'll play with it until they figure out how the vehicle runs best...but we're talking carb tuning, distributor curve changing, etc., not just slapping parts on and assuming it's running the best it can be.
If you don't run manifold vacuum for your advance, additional base timing can be added, up until the point the engine gets hard to start. While that may be well and good, backing the mechanical timing off (say, 0* initial vs 8*), and using vac advance (manifold vacuum!) to get idle to 10*+, would make for an easy starting engine because timing is lower until the engine starts to run, that has good idle characteristics. Case in point, TBI trucks run 20+* at idle, with virtually the same engine that earlier was run with 8* at idle.
Discovery takes tuning, and perhaps some lack of experimentation comes from laziness, or because the vehicle runs ok the way it is. If someone really cares, they'll play with it until they figure out how the vehicle runs best...but we're talking carb tuning, distributor curve changing, etc., not just slapping parts on and assuming it's running the best it can be.
bot0611
1/2 ton status
Ive gotten a hard start 2 times since i fixed the problem, and im using ported vacuum, i thought timimgn was the deal.
It starded fine before the fix
It's easy to test. Retard your base timing a few degrees, and see if it starts easier. My experience has been if too advanced, the engine will not turn over nearly as easily as it will with timing retarded. That's turning over, not starting. It's obvious how hard the starter has to work when it happens.
