Vacuum advance
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threewood
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I'm drinking water, just not out of your trough. I'm still waiting to see why one is better than the other.
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Ported vacuum advance vs manifold vacuum source for the distributor......too many "expert opinions" based on what????.....it's "my way is better than your way....because I said so".....or.......hundreds of engineering hours were spent on the development of the correct application for each engine application, albelt emission requirements or performance requirements.......let's see some real data not just unfounded speculation or the "shade tree mechanic" approach of guess and bygolly.......why not just recurve the mechanical advance and eliminate the distributor vacuum advance all together?????.........
BOB RENTON
gtxrt
Well-Known Member
i don't really think there is a right or wrong answer. if you have 8 degrees or more initial advance at idle you can run ported vacuum with stock or mild cams. big cams usually like direct manifold vacuum to keep from opening the carb idle screw too much and getting off the idle current. most engines don't really like too much timing at idle or even having your mechanical advance come in too fast for a street car.
threewood
Well-Known Member
I honestly believe it is directly related to application. Performance or mileage?
I think RJRENTON needs to do a study on this.
My timing is all in at 2800 rpm. Initial is 21. All in 38. No pinging on 88 non ethanol with 10.2:1 compression. Ported works great for my application. No reason to fix what isn't broke. I could probably unhook the vacuum advance and be fine.
I think RJRENTON needs to do a study on this.
My timing is all in at 2800 rpm. Initial is 21. All in 38. No pinging on 88 non ethanol with 10.2:1 compression. Ported works great for my application. No reason to fix what isn't broke. I could probably unhook the vacuum advance and be fine.
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I run a Progression Ignition distributor on the smogger 440.
It's designed to be connected to manifold vacuum for the electonic control of the timing. It utilizes a built in MAP sensor.
I mostly run 87 octane ethanol swill. Iron heads. Timing is set at 22 initial, 38 total @ 3000 and has 10 degrees of vacuum advance programmed in.
No pinging, runs strong, lots of fun. She don't sip like no Prius.
Smiles per gallon.
It's designed to be connected to manifold vacuum for the electonic control of the timing. It utilizes a built in MAP sensor.
I mostly run 87 octane ethanol swill. Iron heads. Timing is set at 22 initial, 38 total @ 3000 and has 10 degrees of vacuum advance programmed in.
No pinging, runs strong, lots of fun. She don't sip like no Prius.
Smiles per gallon.
Geoff 2
Well-Known Member
Bob [ Post #23 ].
How many direct comparisons of MVA v PVA, showing the benefits of MVA, are necessary for a Bob Renton 'expert opinion'.
Would four be enough? Some people are slow learners....
How many direct comparisons of MVA v PVA, showing the benefits of MVA, are necessary for a Bob Renton 'expert opinion'.
Would four be enough? Some people are slow learners....
threewood
Well-Known Member
So using MVA increases timing, provides better throttle response, increased engine vacuum, improved idle. The same effect of me setting base timing to max engine vacuum and adjusting curb idle down then fixing total timing without using MVA. It says in your article when running MVA that,"22 degrees of spark lead, idle quality had improved. I'm at 21 using PVA. Why do I need MVA again? I don't think you get it.
At part throttle, the advance from the canister will be the same whether the vacuum source is ported or manifold.
At WOT, vacuum will be at or near ZERO, whether it is hooked to a ported or manifold source.
So at idle , whether I have 21 degrees using MVA or PVA, where is the benefit?
At part throttle, the advance from the canister will be the same whether the vacuum source is ported or manifold.
At WOT, vacuum will be at or near ZERO, whether it is hooked to a ported or manifold source.
So at idle , whether I have 21 degrees using MVA or PVA, where is the benefit?
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Geoff 2
Well-Known Member
I ger it, you don't.
No Chrys engine ever came from the factory with 21* of initial timing. You have increased your init timing to 21*, which is duplicating what MVA would do. Engine doesn't care how it gets it's timing, as long as it gets it.......
So you have realised the benefits.....
The examples I provided [ above ] are based on factory initial timing [ typically 6-12* BTDC ] & the extra idle idle timing ADDED using MVA.
GM were smart. They used MVA until forced to use PVA because of emissions. My GTO came with a small cam 197* @ 050, 10.75:1 CR, but still left the showroom floor idling at 26* [ 6* init + 20* MVA ]. See below.
It is easy to see why bigger cams, low CRs, both need more idle timing, as much as 50*. The benefit of MVA v fixed [ locked ] timing is that is load sensitive & reduces timing with load at lower rpms.
No Chrys engine ever came from the factory with 21* of initial timing. You have increased your init timing to 21*, which is duplicating what MVA would do. Engine doesn't care how it gets it's timing, as long as it gets it.......
So you have realised the benefits.....
The examples I provided [ above ] are based on factory initial timing [ typically 6-12* BTDC ] & the extra idle idle timing ADDED using MVA.
GM were smart. They used MVA until forced to use PVA because of emissions. My GTO came with a small cam 197* @ 050, 10.75:1 CR, but still left the showroom floor idling at 26* [ 6* init + 20* MVA ]. See below.
It is easy to see why bigger cams, low CRs, both need more idle timing, as much as 50*. The benefit of MVA v fixed [ locked ] timing is that is load sensitive & reduces timing with load at lower rpms.
gtxrt
Well-Known Member
you don't need MVA you have plenty of timing at idle I am surprised it does not turn over slow when hot trying to start it.
gtxrt
Well-Known Member
run MVA for 45 years but the 5 years PVA. running a mild cam and recurved to 12 initial with PVA idle is smoother and not smelling unburned gas. think about it to much advance anywhere idle or wot the spark is pushing the piston back down while it is trying to get to TDC. for years I ran 36-38 total timing till Hughes engines said try 33-34 total yes it pulled better at the top end and better times in the 1/4 mile. every engine is different try both PVA and MVA also try less total timing.What do you have to lose.
68-500
Well-Known Member
I say initial timing, no more than it takes to get best idle. Total timing, no more than the engine wants ( keep backing it done until engine performance decreases). Part throttle, just under pinging. MVA or PVA, pick you poison.
threewood
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I think we are arguing the same point. That article on MVA is directed at stock or near stock engines with with little initial advance. The benefit of it being that they don't need to recurve their distributor and can still get more advance at idle. For those that build our engines and recurve our distributors, MVA is not needed.
Painting with a broad brush saying MVA is better than PVA is just confusing and incorrect.
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Exerpted references from unknown sources mean nothing.....show us where your references came from.......looks like a hodge-page of cherry picked notes and clippings from several magazine articles. Were the references from SAE sources or otherwise manufactured sources to reiterate your opinion.......inquiring minds want to know......
BOB RENTON
Geoff 2
Well-Known Member
Bob,
The quoted articles are not my opinion. They are the opinion of others, most of whom are much smarter than you are. The first quote in post #29 is from the two inch thick 1967 Pontiac workshop manual. Does it have to be signed by D. Trump for your approval?
With your constant harping negative criticism, haven't you got more important things to do...such as taking out the trash for collection.
The quoted articles are not my opinion. They are the opinion of others, most of whom are much smarter than you are. The first quote in post #29 is from the two inch thick 1967 Pontiac workshop manual. Does it have to be signed by D. Trump for your approval?
With your constant harping negative criticism, haven't you got more important things to do...such as taking out the trash for collection.
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mvent
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I’m with threewood, if you have a recurved advance unit which enables high initial timing to get desired idle with correct primary blade position and desired total then I don’t see the advantage. Stock everything, then yes manifold would be an advantage.
Geoff 2
Well-Known Member
mvent,
You don't see the advantge of MVA, post #38???
Some engines will detonate if they have 25* [ or more ] fixed timing at idle. They will also make life hard for the starter. Using MVA gives the same timing at idle, but the timing drops off with load & prevents detonation. GM knew what they were doing & the engineers were not as stupid as some people think....
You don't see the advantge of MVA, post #38???
Some engines will detonate if they have 25* [ or more ] fixed timing at idle. They will also make life hard for the starter. Using MVA gives the same timing at idle, but the timing drops off with load & prevents detonation. GM knew what they were doing & the engineers were not as stupid as some people think....
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