84TransAm

Do the stock Quadrajets on the computer controlled F-bodies have a vacuum port that allows a vacuum advance distributor to hook into, even though the stock distributor was computer advanced?

------------------
--Steve S--
1984 Trans Am 305 LG4, 5 speed
Daily Driver, Flowmaster 80 Series

84TransAm

Also, what Holley carb would be a good replacement? Not going for emissions, just need a 4 bbl with vacuum secondaries to replace the quadrajet. should it be a double pumper? Should it be 600 cfm or 750? I will have a relatively stock 305 LG4 with headers, and Performer RPM intake.

------------------
--Steve S--
1984 Trans Am 305 LG4, 5 speed
Daily Driver, Flowmaster 80 Series

burnoutrpm

no its computer driven no reason for a vac advance can. the timing is computer driven. the computer advaces and retards the timing as needed to maintain a contant air/fuel mix!

------------------
1989 firebird formula

Mods: converted from T.B.I. to a carburator 305 to a 350. Flowmater exhaust,hedman shortie hedders,202 160 (882) heads,.447/.447 lift 222@.050 duration speed pro 327/350hp cam 350 .40 over (356) Edlebrock torker2 and a 600 edlebrock manual choke (1405)

Future mods performer rpm air gap intake (polished) comp roller cam, and way better headsa 400 defintely in the works!

Biochem

You can use mech secondaries on a manual tranny... that is the general rule of thumb. You wouldn't want to go much over 600-650cfm on a 305 though. They have some good Holley DP that would work well for your setup I think, take a look in Jegs or Summit for them.

------------------
1984 z28 w/ a 357 cu in. monster engine which is looking like the posterchild for Edelbrock with the exception of the Holley 750vac... all the suspension stuff... 9-bolt posi disk is in...

-=ICON Motorsports=-

five7kid

iTrader: (14)

Not sure why you're asking if you can hook a vacuum distributor to a CC q-jet. You don't want to use either a mechanical/vacuum distributor with a CC carb or a mechanical carb with a CC distributor. They both need each other.

Sounds like you're really looking at getting a different carb, so get the right distributor as well.

------------------
82 Berlinetta, orig V-6 car, now w/86 LG4/TH700R. 2.93 limited slip. Cat-back from '91 GTA, Accel HEI SuperCoil. AMSOIL syn lubes bumper-to-bumper. Daily driver, work-in-progress (LG4 w/LB9 block, ZZ3 cam and intake, WP 305 heads ported & polished, Hooker headers & y-pipe, hi-flow cat).
57 Bel Air, my 1st car. 0.030 over 396, Weiand Action+, Edelbrock 1901 Q-Jet, Jacobs Omnipack, 1-3/4" headers, TH400 w/TCI Sat Night Special conv & Trans-Scat shift kit, 3.08 10-bolt w/Powertrax, AMSOIL syn lubes bumper-to-bumper. Best 15.1 @ 5800' Bandimere. Daily driver while Camaro was being put together.

[This message has been edited by five7kid (edited February 11, 2001).]

84TransAm

five7kid: What has happened is my current Quadrajet is taking a dive, so i thought i would replace it with a non CC carb, and now it is apparent i need a non CC distributor with that. I already found a cheap ($92 new) non CC distributor that comes off of 350 Chevys from 1983-1993 from their Police cars that were carbureted. I was just wondering if i could get one and not the other, but apparently that isn't the case

------------------
--Steve S--
1984 Trans Am 305 LG4, 5 speed
Daily Driver, Flowmaster 80 Series

The ODB

Yes you can.
just use any available vacuum port from your intake manifold. If none available then put a T-port on one of the vacuum lines to add the port you need.
good luck
ODB

84FTA

You want to use a ported one. (I.E. one that supplies no vaccum at idle, not until you get into the transfer/primary circuits).

The Mech. secondaries is not really a rule of thumb. I'd like to meet the person who made that one up. It's basically just a bad idea to use DPs on autos (not so much the 700-R4 but the ones that use vaccum like the TH-350), but doesn't make it a good idea to use DPs on manuals. A square is a rectangle but a rectangle is not a square. Your 305 would function much better on a vac. secondary holley or even better yet a edlebrock or non-CC Q-jet. The holley adapaters are crap and create some perfomance problems along with hood clearence issues.

Double pumpers really only need to be used when the engine is such a massive breather that it can't supply enough vaccum to open secondaries. An LG4 with a stock cam and stock heads will actually do worse with a double pumper and he'll just be shelling out 2x as much money for the DP over the VS and for the gas mileage. (DPs get about 2/3 to 1/2 the mileage of the VSs).

Vaccum secondaries work very well until you get into roots blowers and engines that aren't streetable. They are decently tuneable and you can't beat the cost. What ever you do don't get the Holley Q-jet replacment, that is the ultimate piece of junk.

------------------
1984 WS6 Trans Am Hartop
Former L69 Car under restoration
1984 Trans Am T-tops
4-bolt main 350, performer intake, headers, Holley 650, T-5, hayes clutch, dual elec. fans and 3.23's.
Daily driver and restoration
13.98 @ 101

[This message has been edited by 84FTA (edited February 11, 2001).]

The ODB

84fta
Good call on the carburetor part. Other than that I would like you to tell me why you would not want vacuum advance at idle?

thanks,
ODB

84TransAm

The current carb i think will work is the Holley 80451 spreadbore design. I think with the vacuum advance distributor, this carb would be emissions legal. Even though it isn't stock and renders the computer useless.

84FTA

The holley spreadbores are crap. They are the only carb I would not wish on someone. The three I've seen didn't even have adjustable idle circuits!!!! You don't want vaccum advance at idle because you won't really reach full advance.

I.E.

If I have 30 degrees total advance and with no advance at idle then I set it to -10. If I have the vaccum advance on, then it automatically increases my timing by 2 degress, so now when I set my timing to -10 I only have 28 degrees left. It really doesn't affect much in daily applications but when you start advancing your timing to the point of ping or best perfomance (which ever comes first) it starts to matter.


Even though the adapters are crap and hood cleanernce becomes an issue (mine still fit with a 14x2 non-drop aircleaner and a 14x3 drop base aircleaner but both rubbed just a tad bit) they are still better than the spreadbore holleys and you can always change the intake later when you have the money.

------------------
1984 WS6 Trans Am Hartop
Former L69 Car under restoration
1984 Trans Am T-tops
4-bolt main 350, performer intake, headers, Holley 650, T-5, hayes clutch, dual elec. fans and 3.23's.
Daily driver and restoration
13.98 @ 101


[This message has been edited by 84FTA (edited February 11, 2001).]

The ODB

FYI

vacuum advance (the degrees it gives), has nothing to do with initial advance, centrifugal advance, or total advance.

Vacuum advance only adds timing during high manifold vacuum (low load) conditions.

When you go to full throttle the vacuum advance adds nothing and all you have is intial + centrifugal. A vacuum advance will never change your total advance at WOT (near zero vacuum conditions).

The only instance that I can think of where you would not want vacuum advance at idle would be on a very radical engine with an unstable idle & low fluctuating vacuum. In that case most people stick with mechanical advance only so vacuum never comes into play.

ODB

84FTA

Actually no....it takes very little psi to pull the vaccum canister arm at idle.

I used a hand pump and had a signifcant pull at 16 psi, my engine idles at 22 psi which means if I plugged it into a non-ported vaccum source my initial timing would advance some, probably 2 or 3 degrees.

The idea of vaccum advance is that it can 'sense' load. Engines aren't magical, there is a reason. The reason why the canister can sense load is because when you're giving it more fuel and air but going slower the vaccum the engine pulls drops because the throttle blades are open more and thus the vaccum pull on the canister is reduced and this reducing the timing advance.

The vaccum advance as nothing to do with the centrifugal weights, that's because it doesn't move the rotor, it moves the _____ (damn it! I always forget that word for those pointy things that trigger the ignition! in the distrib! Damn!!) and aslong as the vaccum overcomes the spring in the canister it is adding advance.

------------------
1984 WS6 Trans Am Hartop
Former L69 Car under restoration
1984 Trans Am T-tops
4-bolt main 350, performer intake, headers, Holley 650, T-5, hayes clutch, dual elec. fans and 3.23's.
Daily driver and restoration
13.98 @ 101

The ODB

did you read this in Hot Rod magazine?

Please get someone who understands distributors to explain this all to you again.


ODB

84FTA

lol. What you wrote was from HR. What I wrote was exactly what happens.

------------------
1984 WS6 Trans Am Hartop
Former L69 Car under restoration
1984 Trans Am T-tops
4-bolt main 350, performer intake, headers, Holley 650, T-5, hayes clutch, dual elec. fans and 3.23's.
Daily driver and restoration
13.98 @ 101

The ODB

GM cars came factory stock with vacuum advance at idle.
If you don't have that, then your car has a problem.

ODB

84FTA

Hrm....not that I know of. They're ported just like the Holley and Edlebrock carbs have vaccum ports designated for the canister higher in the carb (ported).

------------------
1984 WS6 Trans Am Hartop
Former L69 Car under restoration
1984 Trans Am T-tops
4-bolt main 350, performer intake, headers, Holley 650, T-5, hayes clutch, dual elec. fans and 3.23's.
Daily driver and restoration
13.98 @ 101

five7kid

iTrader: (14)

The original 283 in my '57 had manifold vacuum to the distributor. My dad's '65 GMC V6 pickup had manifold vacuum to the distributor. My brother's '66 Impala SS 327 and the 396 now in my '57 had ported vacuum to the distributor (the beginning of economy/emissions consciousness). Every GM mechanical carb/distributor I've seen since had ported vacuum. The curve of the vacuum advance was modified to account for this change - THAT's why you don't want vacuum at idle.

Not sure how to break this to you, but I believe you're the one with the vacuum problem.

84FTA

MeanYellowZ also mentioned this one to me which is actually more important than running out of timing up high

If you run it off manifold vaccum, when you punch it the vaccum will drop and you'll either lose timing or go to a flat curve (loss of vaccum timing is canceling out the gain you should get from weights).

------------------
1984 WS6 Trans Am Hartop
Former L69 Car under restoration
1984 Trans Am T-tops
4-bolt main 350, performer intake, headers, Holley 650, T-5, hayes clutch, dual elec. fans and 3.23's.
Daily driver and restoration
13.98 @ 101

The ODB

Once again,
the vacuum advance mechanism will not negate the effect of centrifugal advance (the weights).
Vacuum advance at idle is by far the best way to go. Anyone who understands distributors and engine tuning would agree. I seriously hope I do not have to take the time to explain it all.
ODB

84FTA

No they do not affect the position of the rotor as the centrifugal advance does, how ever it does affect when the point where the coil is triggered. This all adds into total advance. The coil is need to be triggered before the rotor can spark.

I would like to hear your version on how it works, since you see to be against the majority and can't even site specific inner parts of the distributor.

------------------
1984 WS6 Trans Am Hartop
Former L69 Car under restoration
1984 Trans Am T-tops
4-bolt main 350, performer intake, headers, Holley 650, T-5, hayes clutch, dual elec. fans and 3.23's.
Daily driver and restoration
13.98 @ 101

[This message has been edited by 84FTA (edited February 12, 2001).]

84TransAm

Hey! My first flame post! keep up the good work guys!

84FTA

I am not flaming. I am not directing my comments at him, I'd just like to know why he thinks as he does. If I wanted to flame him the insults would already be flying. I'm just trying to convience him or be convinced.

On a side note. I was out checking things over on the car and decided to a do a little expierement.....I got out the timing light and tested the engine.

Ported Vaccum: Base, -10, maxxed out at -27 (hrm. I think I who ever put the lighter weights in this one must of be pinging on the stock curve).

Manifold vaccum. (I used the port normally reserved for the heater controls). Base -13 (Hmm....) total -27. If you had set this set-up to -10 you'd only have -24 advance total. Then I also tested MYZ's theory, I didn't have a retard, maybe a flat but I don't have a tach under the hood, in timing but I believe he was running a DP which would easily make up for it. Anyone else can try this at home with timing tape/timing light with advance options. Results will very on your vaccum canister year and condition (vaccum leaks,spring force, and linkage length). Have fun and time responsibly.

Edit: Base turned to -11.5 not -13...I meant -12. Proofread before you post....
------------------
1984 WS6 Trans Am Hartop
Former L69 Car under restoration
1984 Trans Am T-tops
4-bolt main 350, performer intake, headers, Holley 650, T-5, hayes clutch, dual elec. fans and 3.23's.
Daily driver and restoration
13.98 @ 101



[This message has been edited by 84FTA (edited February 12, 2001).]

Racerwannabe

I know I'm going to regret jumping in here but what the hell... Might as well put my knowledge to the test.

IMO two things are going to determine where to hook your vacuum advance; EGR and cam "overlap". If you have the stock cam and a functioning EGR system keep it where GM had it, probably ported if it's an emissions engine.

With the EGR system, when the EGR valve is open the engine can tolerate much more advance before detonation. Vacuum advance systems that are designed to work with EGR provide much more advance for this reason. However, since the EGR valve is closed at idle, if you run full time vacuum you will have a very rough idle. Also, If you disable the EGR system without reducing the vacuum advance throw, you will have detonation at part throttle. If you simply take out some base timing you will stop the detonation but you will also lose a lot of power at WOT.

Cam overlap is what produces the rough idle on bigger cams. The problem is that these cams require a relatively large throttle opening at idle to stay running. This uncovers the idle transfer slot prematurely and your carb will run very rich at idle because a large percentage of fuel will be coming from the main delivery system. In this case the idle mixture screws will have very little effect on the mixture. Additional advance at idle can help solve this problem by allowing less throttle opening which allows the idle circuit to function as intended.

I'm running a Comp Cams 280 magnum and no EGR. My engine idles much quieter and cleaner, and has much better off idle throttle response, with full time vacuum. I also used the Crane adjustable advance kit to limit total vacuum advance to 10 degrees.


FWIW
RW

84TransAm

thanks for the help guys, no i'm all confused!
So if the EGR is disabled, performance suffers? Wouldn't that be synonomous with a vacuum leak?

The ODB

If you want everything perfectly stock for emissions and do not care about performance then follow the Chiltons manual for vacuum lines.

If you want performance and insist on running a vacuum advance distributer, then hook it to manifold vacuum. Also get the adjustable advance kit so that you can match what your engine wants as far as vacuum advance goes.

Vacuum advance (when working properly) should add no advance at full throttle and therefore have no effect on your total timing or WOT performance... period.

Your total timing is the sum of initial timing (base) plus centrifugal timing (weights)... period.

If you choose not to have vacuum advance at idle, then you will have to increase your initial timing in order to get smooth idle and good throttle response. The downside is that excess initial advance makes the engine much harder to start, requiring up to 3-times the amperage and often killing or breaking starters. The racers solution is to run an ignition-kill switch so you can spin the engine without spark, and then flip it on.

You use a dial-back type timing light in order to see how much centrifugal advance you get from the weights. This can be changed by elongating or filling in the slots as necessary. Your inital plus centrifugal advance should be anywhere from 34 to 38 degrees total (on a well tuned car) and be at full advance by about 3000 RPM.

The vacuum advance is completely seperate from the initial and centrifugal and has no effect whatsoever on total advance.

Vacuum advance is there to ensure a more complete burning of fuel under light loads or no-load. There is so little load on the engine at idle that the fuel burns at a much slower rate and will go out the exhaust to finish burning. This will give the idle a rough lopey sound and make your sparkplugs sooty. Adding spark advance at idle with a vacuum advance begins the burn much earlier and smooths the idle considerably along with cleaner exhaust. The same holds true when you are coasting or decellerating.

If your vacuum advance is having an effect on total timing at WOT then you have a problem... period. Do I have to say this part again? period.

Running the vacuum advance to manifold vacuum will allow you to run less initial timing and more centrifugal to add up to that same 34-38 degrees total, but with much easier starting. You will also have the benefit of smooth idle & cleaner plugs thanks to the added advance from the vacuum.

To sum it up.
If you want to stay stock, then fine.
If you want to go fast, then do it.

ODB

The ODB

If you are really trying to go fast then EGR will not be an issue because it will simply not be there.

ODB

The ODB

84fta

Also, why would you check base timing with the vacuum line hooked up? that is not the way to do it.

Guest

Performance suffers with a disconnected EGR... uhhhh no. Just requires different tuning.

As for the rest of the mess, this is how it works:

Base timing is base timing. Set it and forget it.
Vacuum advance: Many of the replacement and newer cans are adjustable. I have yet to see one add advance below 5psi vacuum. The amount of advance also varies depending on what can you use. You are supposed to set base timing with the can disconnected, and then connect it, which then adds more advance, hence the name vacuum advance.
Centrifugal advance: Another advance, using weights and springs. As the RPM's increase, the weights want to fly away from the distributor. The springs and the mass of the weights control how fast this happens. The design of the weights controls how far it goes.

All three of these are independant of one another. Adjusting one doesnt change the amount of advance of the others.

At WOT, you are seeing the base timing+centrifugal.
At part throttle, you are seeing all 3 more than likely.
At idle, you are seeing base+vacuum.

If you have a computer, you dont need to worry about all this crap, its in a table and you set it wherever you want. No springs, vacuum cans, or weights to mess with. Its a disadvantage only when you dont have the equipment to change where its set. Just about anyone has a screwdriver and pliers. Not many have a computer and programmer.

The ODB

madmax,
Thank you, and you are completely correct,
cept that you mean 5" inches of vacuum,
but I know what you mean.

When some vacuum advances are very old they may allow some advance below 5" but that means they need to be replaced and is not the normal.

ODB

Guest

5", 5psi, 5hp... ahhhhh all the same.

84FTA

Yes, the are independant but still all contribute to the total.

Let's put this into a table, most numbers are fictional just to save me the time of going and retrieveing the actual numbers

Vaccum timing advance, Centrifugal timing advance and Total timing advance respectively

RPM VACCUM VT+ CT+ TT+
750 22 3 2 15 idle
1250 17 0 4 14 WOT
2500 30 5 10 25 Part throttle

Now, as you can see, if I punch it at idle to 1250, the vaccum advance that I had at idle will disappear and the CT will have to pick up the exchange, if the net of the two do not equal more than the previous total then you have lost some total timing.

The base timing doesn't have to be set to a number, when setting for perfomance, you don't read the number you put it where it runs best. So if mine runs best at -10 then I add the vaccum canister on them my timing will be at -13 and will not be where I found my best base at. This is assuming you find the best base without hooking up the vaccum canister inbetween.

I think this takes more theory than anything and some people just can't see it.

------------------
1984 WS6 Trans Am Hartop
Former L69 Car under restoration
1984 Trans Am T-tops
4-bolt main 350, performer intake, headers, Holley 650, T-5, hayes clutch, dual elec. fans and 3.23's.
Daily driver and restoration
13.98 @ 101

[This message has been edited by 84FTA (edited February 12, 2001).]

The ODB

LOL

yep....

84TransAm

This is why there is a chatroom,