Vacuum And tuning Help

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805moparkid

805moparkid

Slant and AFX Guy
Joined
Oct 29, 2008
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Anthem AZ
i have limited tools and am trying to get my slant run better. the best i have been able to get is about 10-11 inches @ 1000rpms on the vacuum gauge.

the setup is an .080 bore, heavilly ported head w/ oversized valves, 9.2:1 comp, .510lift 246 dur @ .050 and a 108 LSA. holley 600dp and clifford manifold w/ hooker super comps.

im running 67 primary jets and 72 on the secondaries. no vacuum leaks that i know of but i will smoke it when i get to work. i have the stock points dizzy with the petronix ignitor II setup(for now) @15* intial.

any ideas on tunning? i trying to buy a wideband and gauge but thats 300bucks...
 
Your carburetor is too big. Put a more appropriately-sized one on and you'll be miles ahead even before your first round of tweaks.
 
lets see 600 cfm, 300cfm per each 2 barrels...
And slant dan says no....


you could f around and close the secondaries completely along with the idle screws and see if ya pick up any vacuum.

imo you're doing good with 10 or so inches with that 246* @.050 cam.
 
1wild&crazyguy said:
lets see 600 cfm, 300cfm per each 2 barrels...
And slanted dan says no....


you could f around and close the secondaries completely along with the idle screws and see if ya pick up any vacuum.

imo you're doing good with 10 or so inches with that 246* @.050 cam.
Click to expand...

i was at 12" with a valve lash of .024. doug dutra told me to tighten up the lash till it just got quiet. so now im at .019" and i want to tune it. i have no complaints off idle, just just need to fix idle.

and it dies if i just close the idle screws if u were being serious...
 
What you NEED to do IS tailor the spark curve.

For now, remove the heavy spring in the points distributor and only run the one lighter spring, this will get maybe 18-20* initial at idle being that the running of only one spring will have it partially advanced, this better for vacuum/setting-or ability to lower the blades more throttle response etc...

or if you have a welder, weld the advance slots, set it to the highest initial timing it will start with 'fully warmed up' and what ever total/full advance it likes limit by welding the slots.
with a light spring kit=$4 from mr gasket, it should be full advance by like 22-2400rpm w/2 light springs or 1200rpm with only one light spring.

it should be lumpy with 245*@.050 + slant 6.jmo

you should be around 3/4 turn out on each idle screw.
 
1wild&crazyguy said:
What you NEED to do IS tailor the spark curve.

For now, remove the heavy spring in the points distributor and only run the one lighter spring, this will get maybe 18-20* initial at idle being that the running of only one spring will have it partially advanced, this better for vacuum/setting-or ability to lower the blades more throttle response etc...

or if you have a welder, weld the advance slots, set it to the highest initial timing it will start with 'fully warmed up' and what ever total/full advance it likes limit by welding the slots.
with a light spring kit=$4 from mr gasket, it should be full advance by like 22-2400rpm w/2 light springs or 1200rpm with only one light spring.

it should be lumpy with 245*@.050 + slant 6.jmo

you should be around 3/4 turn out on each idle screw.
Click to expand...

well its lumpy! do u have any pics or directions on how to do the adv trick? never done that before! sounds fun tho! here is a vid of when it was happy!

http://www.youtube.com/watch?v=pviXVh2Uaq0
 
When I built my \6 I found I needed 17 degrees of initial timing and that was with a smaller cam. You are going to need that much or more. Keep advance in small increments while maintaining a constant rpm until you find the peak manifold vacuum, that's where you initial wants to be.

With that kind of initial you are going to need to restrict the mechanical advance, as 1wild&crazyguy stated, or you will have way too much on the top end. Take the point/pickup plate out of the distributor and look at the advance mechanism. The slots in the plate attached to the top shaftis where you need to weld or epoxy to restrict the advance. I use a protractor glued to an old rotor with a piece of coat hanger taped to the distributor housing to measure my progress.

I would shoot for 36-38 degrees total so subtract your initial from that to find the amount you need in the distributor. The protractor will read 1/2 of that (timing is expressed in crank degrees and is twice cam degrees).

One of the springs from the two spring MP kit replacing the heavy spring will get you close on the advance rate.

Don't expect wonders on manifold vacuum, a [email protected] cam in a \6 is huge and with only 9.2:1 compression you are just not going to have a strong signal.

The 600 cfm carb can be made to work but if the car is primarily a street car you will be happier with something in the 500cfm range. I tried a 600 cfm vacuum secondary and was not happy at all with it and went to a 390 cfm. BTW, don't be confused by the ratings on 2bbl's and 4bbl's A 4bbl is rated at 1/2 the pressure drop of a two barrel so a 2bbl and 4bbl of the same rating the 4bbl will flow almost twice as much.
 
66aCUDA said:
Duster Idiot over on .org is the guru. I would have Rob set you up a dizzy. He is cheap also.
JMO
Frank
Click to expand...

wow guru huh?


whats a guru to you?

someone that knows more than you?

or someone with a sun distributor machine?
of corse he could be the only guy on .org that knows how to set up a distributor, so he would be a guru there huh...
I here 'guru' thrown around quite a bit.
 
1wild&crazyguy said:
wow guru huh?
whats a guru to you?
someone that knows more than you?
Click to expand...

"Someone whose manners are better than to mock and deride the demonstrated skill, knowledge and ability of someone he's never met, just because he's never seen the demonstration or heard of the guy himself" is a good start at a definition.

I here 'guru' thrown around quite a bit.
Click to expand...

"Someone who knows the difference between 'here' and 'hear'" is another good start.

Guess you're not guru material at this point.
 
lol@u!

News flash! there are a lot of guys with 'skill' that 'know' AND that laugh at the term 'guru' and how it's tossed about, just like my other favorite...'expert' ha ha ha
Say? does your 'guru' happen to be richard simmons?
maybe carrot top? tit 4 tat bud

you are I 'neet guy' I'm sure but why the personal attack on me for only pointing out the loosness of the term 'guru' and how seemingly anyone can inherit the title of 'guru', I'll secondly point out how much easier it can be through the internet to be a 'guru'.


Say... does the site hand out the 'guru' title?
Or does one just type it himself into the 'member title'?


where does one take this 'guru 101' exam??

nice rainbow btw
 
dgc333 said:
When I built my \6 I found I needed 17 degrees of initial timing and that was with a smaller cam. You are going to need that much or more. Keep advance in small increments while maintaining a constant rpm until you find the peak manifold vacuum, that's where you initial wants to be.

With that kind of initial you are going to need to restrict the mechanical advance, as 1wild&crazyguy stated, or you will have way too much on the top end. Take the point/pickup plate out of the distributor and look at the advance mechanism. The slots in the plate attached to the top shaftis where you need to weld or epoxy to restrict the advance. I use a protractor glued to an old rotor with a piece of coat hanger taped to the distributor housing to measure my progress.

I would shoot for 36-38 degrees total so subtract your initial from that to find the amount you need in the distributor. The protractor will read 1/2 of that (timing is expressed in crank degrees and is twice cam degrees).

One of the springs from the two spring MP kit replacing the heavy spring will get you close on the advance rate.

Don't expect wonders on manifold vacuum, a [email protected] cam in a \6 is huge and with only 9.2:1 compression you are just not going to have a strong signal.

The 600 cfm carb can be made to work but if the car is primarily a street car you will be happier with something in the 500cfm range. I tried a 600 cfm vacuum secondary and was not happy at all with it and went to a 390 cfm. BTW, don't be confused by the ratings on 2bbl's and 4bbl's A 4bbl is rated at 1/2 the pressure drop of a two barrel so a 2bbl and 4bbl of the same rating the 4bbl will flow almost twice as much.
Click to expand...

this is the kinda info im looking for! well the next time the head come off i'll cc it and calculate what i need to take off for my correct comp.

next on the dizzy "rebuild" do u have any pics? like i said i have never got to pull one apart and would love to learn!

last is about the carb. every body says that a 600 is to big? so what is the difference between a 2 barrel 300cfm and a 600 opened up before the secondaries open? yes i would agree is all 4 barrels opening all the time but from work and back i never need more than .25 throttle... unless that honda starts after me! but anyways i think u get my point.

thanks again for the info!
 
1wild&crazyguy said:
lol@u!

News flash! there are a lot of guys with 'skill' that 'know' AND that laugh at the term 'guru' and how it's tossed about, just like my other favorite...'expert' ha ha ha
Say? does your 'guru' happen to be richard simmons?
maybe carrot top? tit 4 tat bud

you are I 'neet guy' I'm sure but why the personal attack on me for only pointing out the loosness of the term 'guru' and how seemingly anyone can inherit the title of 'guru', I'll secondly point out how much easier it can be through the internet to be a 'guru'.


Say... does the site hand out the 'guru' title?
Or does one just type it himself into the 'member title'?


where does one take this 'guru 101' exam??

nice rainbow btw
Click to expand...
:love7: i think u are my hero!
 
Time how much advance is in the distributor, then based upon what you find, weld up a given amount of the slot, for fine tuning will require filing/grinding out of the welded slots. fwiw
 
1wild&crazyguy said:
Time how much advance is in the distributor, then based upon what you find, weld up a given amount of the slot, for fine tuning will require filing/grinding out of the welded slots. fwiw
Click to expand...

well i was thinking about it today and the slants have that second adjustment bolt. maybe i can mess with that and get my intial up to 17* to start then my mech adv of 15* would be 32* pretty close to ur numbers!

well if i ever get a day off work a 60hr week this week, but i will order the light springs (v8 ones work??) and get it set up.
 
ok so ordered the light springs. next day off, if i ever get one, i will pull the dizzy apart and install new springs
 
805moparkid said:
last is about the carb. every body says that a 600 is to big? so what is the difference between a 2 barrel 300cfm and a 600 opened up before the secondaries open? yes i would agree is all 4 barrels opening all the time but from work and back i never need more than .25 throttle... unless that honda starts after me! but anyways i think u get my point.

thanks again for the info!
Click to expand...

Because air can be compressed the flow rate "cfm" has to be in reference to a pressure drop. The higher the pressure drop the more air in cfm that will flow. In an engine that pressure drop is created by the piston moving down on the intake stroke, you have atmospheric pressure (14.7 psi) outside the engine and zero inside or a 14.7 psi pressure drop. If you have ever driven with a vacuum guage hooked up you will have noticed that at idle and part throttle cruise you will have a relatively high vacuum reading. This is caused by the restiction of the closed throttle blade. At WOT the vacuum will be near zero because the restriction has been removed by opening the throttle blades but the carb venturies and boosters will still present a small restiction so you will have the samll vacuum reading. From a max power standpoint you want as close to zero as possible at WOT to maximize the amount of air/fuel that is drawn into the engine.

Because 4bbls are used in performance applications where the goal is to have as close to zero intake manifold vacuum at WOT as possible they are rated at a pressure drop across the carb of approx. 1/2" Hg which is close to where they are used. 2bbls are typically not used in perfromance applications and some restriction is not as important and are rated at a pressure drop of approx. 3"Hg.

What this means is that a 2bbl rated at 300 cfm will require you to suck harder (or blow harder) to get the 300 cfm than you would with a 300cfm rated 4bbl. So that 300 cfm 2bbl is only flowing about 1/2 the air as the primaries on your 600 cfm 4bbl.
 
dgc333 said:
Because air can be compressed the flow rate "cfm" has to be in reference to a pressure drop. The higher the pressure drop the more air in cfm that will flow. In an engine that pressure drop is created by the piston moving down on the intake stroke, you have atmospheric pressure (14.7 psi) outside the engine and zero inside or a 14.7 psi pressure drop. If you have ever driven with a vacuum guage hooked up you will have noticed that at idle and part throttle cruise you will have a relatively high vacuum reading. This is caused by the restiction of the closed throttle blade. At WOT the vacuum will be near zero because the restriction has been removed by opening the throttle blades but the carb venturies and boosters will still present a small restiction so you will have the samll vacuum reading. From a max power standpoint you want as close to zero as possible at WOT to maximize the amount of air/fuel that is drawn into the engine.

Because 4bbls are used in performance applications where the goal is to have as close to zero intake manifold vacuum at WOT as possible they are rated at a pressure drop across the carb of approx. 1/2" Hg which is close to where they are used. 2bbls are typically not used in perfromance applications and some restriction is not as important and are rated at a pressure drop of approx. 3"Hg.

What this means is that a 2bbl rated at 300 cfm will require you to suck harder (or blow harder) to get the 300 cfm than you would with a 300cfm rated 4bbl. So that 300 cfm 2bbl is only flowing about 1/2 the air as the primaries on your 600 cfm 4bbl.
Click to expand...

ok that makes sense @ wot the 4 barrel is flowing twice as much. but when u are not @ wot, or even in the secondaries u are flowing a max of 300 correct? because the secondaries arn't open.
 
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