for those that dont understand vacuum

[ari666]

and i am numbered amongst "those that dont understand vacuum"

can i get some help with 308 vacuum?

i am running a crane 286, and i know that i can expect less vacuum with a long duration blah blah blah. i am getting 10 inches running at 60kms an hour, 5 inches in 'drive' at idle and 0 vacuum at WOT. the only time i get over 10 inches is when i am backing off the throttle at higher speeds, 80-100kmph

my question is realting to a bunch of things;

1. i have a redback 'sports' 2 and 1/2 exhaust with primary and secondary mufflers and was wondering if i took off the secondary muffler if i should see an increase in vacuum at WOT.

2. i cant figure out vac advance. i am running vac advance on the same ported vacuum as the guage (holley 600DB off the idler block) so does that mean that vacuum advance only works when you are coming off throttle? so how the f.. does recurving the dizzy give you an extra 35HP if it is only working at deceleration and idle?

3. what would a 308 non EFI vacuum be with a stock cam? 60kmph, idle in gear and WOT.

i think thats it for now.

thanks

 

[ScoHar]

are you running the vacuum tank?

 

[delcowizzid]

you will allways have 0 vacuam at WOT if you see vacuam you have a flow restriction in the intake so its good its at 0.the vacuam advance advances timing when it has vacuam ie cruise and deceleration.it retards timing at WOT thats what its made to do thats what confuses most people they think it advances timing at WOT but it does the opposite.as for regraphing the dizzy you regraph using counterweights or springs on the centrifical advance normally you add stronger springs or lighter weights which means you can advance the base timing more and have more advance early but still end up with the same maximum advance

 

[greenfoam]

5 inches at idle , we can figure out your compression from there, is it the same not in drive?

 

[ari666]

5 inches at idle , we can figure out your compression from there, is it the same not in drive?

nah, it jumps up to around 7 inches in neutral. thats the highest i can get with the idler screws, soon as it goes in D it drops 100 rpm and dives down to 5 inches.

for vacuum advance, i have blown into the hose while engine was running (purely for scientific purposes) with the timing light on, and the timing didnt move but if i sucked it would move up a few degrees(im a smoker and dont have much puff though), which is why i have been baffled as to its purpose. so, if i stick my air gun on it, i should see the timing retard?

i have a feeling my dizzy needs to go away to regraph-land again...

 

[delcowizzid]

no if you put pressure on it it will do noting apart from pop the diaphram.it gets vacuam at low load and idle which is where it advances the spark under high load with no vacuam ie WOT it gets no vacuam and goes retarded to stop pinging.no vacuam is the same as the pipe not connected

 

[ari666]

no if you put pressure on it it will do noting apart from pop the diaphram.it gets vacuam at low load and idle which is where it advances the spark under high load with no vacuam ie WOT it gets no vacuam and goes retarded to stop pinging.no vacuam is the same as the pipe not connected

ok, but the dizzy never retards further back than where it is set, right? so mechanical advance is the only thing really that advances the timing at WOT. does vac advance hold back mechanical advance at all?

i dropped my car off at my exhaust guy this morning to make me a pipe that fits where the secondary muffler is. this is the conversation

"hi mate, can you make a pipe for the rear section of my car so i can ditch the rear muffler"

"no"

"really? why"

"cos it would be loud"


....

dumbass

even though it wont make a diff to WOT vacuume, i will post up if it makes any difference at idle and cruise

 

[darcy]

which is why i have been baffled as to its purpose.

delcowizz is correct.


The intent of Vaccume advance is actually "anti-vacumme retard"

The vacumme advance unit is sitting at min advance with the engine off.
When start up, you get vacumme and it sucks on the diaphragm of the unit and brings the advance in. (Light throttle, low load is highest vaccumme, meaing highest vac advance)
When you go large throttle openings, that creates low vaccume, and allows the unit to pull back and retard spark.

Stick your air gun in the hose and all you might do is rupture the diaphragm, as you have already allowed it to retun to min advance when you disconnected the hose from teh manifold vacumme source.

 

[ari666]

surprise!!!

no change in vacuum, maybe an inch or two at cruise but nothin to write home about...

 

[ari666]

i am so confused by this vacuum advance thing.

if i hold the timing light onto the timing mark and give a light throttle from idle to say 2k rpm, the light seems to drop back (retard) about 6 degrees then it advances. it only does this when i go from idle to revs. if the revs are 2k and i go higher, then it will just advances.

is it supposed to do that?

does the mechanical advance and vacuum advance have any relation to each other? i.e. does one retard and advance, and the other stop it?

since my car doesnt ping EVER i am pulling my hair out as to where the static timing should be set.

i read online that static timing should be set "5 degrees retarded from where the engine hits its highest rpm idling" so you advance the dizzy till it hits its highest rpm, then back it off 5 degrees. i did that but all it does is give me some crazy induction note and not really any more power.