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Ported Vaccuum?

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by RustBuket, Aug 30, 2003.

  1. RustBuket

    RustBuket 1/2 ton status

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    I am planning on removing all the vaccuum hoses from my engine except for the necessary ones. I would just like to know where I can get ported vaccuum from on a stock Q-Jet (81) carb. Which port is it? Also, how can I cap off all the other ports other than a peice of hoses with a screw and silicone? /forums/images/graemlins/1zhelp.gif
     
  2. desertrat67

    desertrat67 Hawk Driver

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    Parts stores should have small rubber caps. Same diameters as the hoses.
     
  3. RustBuket

    RustBuket 1/2 ton status

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    Thanks man, I'll look into it.
    Anyone know which port to use for ported vaccuum?
     
  4. Mastiff

    Mastiff 1/2 ton status Premium Member

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    [ QUOTE ]
    Thanks man, I'll look into it.
    Anyone know which port to use for ported vaccuum?

    [/ QUOTE ]

    What are you running with it? I'm running a PCV valve off the big port on the front, and the brakes off the threaded connection in the back, and that's it. My other connections (vac advance, tranny kickdown and vent actuators) run off of a little tower screwed into the manifold behind the carb. I think it's been this way from day one.

    I know there's been some discussion about what kind of source to use for vac advance. I'm pretty sure mine is straight from the manifold and is working fine for me.
     
  5. BorregoK5

    BorregoK5 1/2 ton status

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    You can get ported vacuum off the front of the carb on the drivers side just above the adjustment screw. I would suggest going this route, especially if you have an ajustable vacuum advance kit and wish to really tune for peak performance. If your running the vacuum advance very conservative, then you can run manifold vacuum with a high degree of success and a higher rpm at idle.
     
  6. Mastiff

    Mastiff 1/2 ton status Premium Member

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    [ QUOTE ]
    You can get ported vacuum off the front of the carb on the drivers side just above the adjustment screw. I would suggest going this route, especially if you have an ajustable vacuum advance kit and wish to really tune for peak performance. If your running the vacuum advance very conservative, then you can run manifold vacuum with a high degree of success and a higher rpm at idle.

    [/ QUOTE ]

    Can you explain why ported is preferable? To me it always made perfect sense to have a lot of advance whenever manifold vacuum was high, with the vac advance retarding under load to avoid ping. What's the rest of the story?

    Thanks.
     
  7. BorregoK5

    BorregoK5 1/2 ton status

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    I was always a fan of manifold vacuum on my 68 Camaro since it was mostly a straight line car and I wasn't concerned with anything other than raw horsepower, weight and traction. The blazer however is much heavier and has a wider range of needs to fill. Some of these would be increased low end torque, increased fuel economy, an even power band and of course every day driveability. There are two common conditions for detonation that you are targeting on a fresh engine; part throttle and full throttle ping. A full throttle ping is a sign of too much static advance either by mechanical advance weights or clocking the distriburer. Part throttle ping comes from too much vacuum advance too soon in the power band. Using a good (and relatively inexpensive) adjustable adjvance kit, you can set the maximum mechanical, maximum vacuum and initial vacuum to exactly follow your setup through the power band. With manifold vacuum, your advance is imediately engaged to full vacuum advance which raises your idle. Lowering the idle to compensate sets you into a rather unstable condition where a small loss of vacuum at low rpm has a drastic effect on timing, dropping it back to its static timing where the idle is now too low. Setting the vacuum to ported vacuum allows for a low stable idle at its static advance that can't drop any lower so you reduce the possibility of stall out. Now the mud drags and dune drag trucks dont really care since its hard pedal to the floor or no pedal at all, but if your daily driving, rock crawling or looking for the most predictability and consistency then ported vacuum is the better way to go.
     
  8. 88Silverado

    88Silverado 1/2 ton status

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    This has been the great debate on many sites.
    Up until 1970 the vacuum advance was on Manifold. Around 71, due to smog issues they changed it over to Ported. Ported pollutes less at idle is why they made the change. Emissions.

    Now in my experience, manifold is better. You get better mileage while cruzing (no or light load on engine) so you can run alot of timing without pinging and as load increases, throttle blades open more, vacuum drops, timing retards as load increases reducing pinging. I have tried both ways on different vehicles and full-time was always better. Lot less pinging when under load. Havnt had the experience with the Blazer crawling up hills tho.

    But...its suppose to be hooked up the way the manufacturer said /forums/images/graemlins/thumb.gif
     
  9. BorregoK5

    BorregoK5 1/2 ton status

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    It wasn't said above, but the only difference between ported vacuum and manifold vacuum is at idle, everything above idle (over 850 rpm) is directed to straight manifold vacuum and has all the benefits.
     
  10. Mastiff

    Mastiff 1/2 ton status Premium Member

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    Pretty confusing stuff. I can find as many people advocating manifold as ported when I surf around. I'm running manifold now, and everything seems to be working okay. I'm tempted to try ported and see if I can tell any difference.

    I don't really hot rod the truck much anymore anyway. As long as it starts and idles well and can accelerate on the freeway (which it can) then I'm happy. I'm too paranoid about blowing U-joints and stuff to really gun the thing much. /forums/images/graemlins/grin.gif
     
  11. BorregoK5

    BorregoK5 1/2 ton status

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    I've never really had that much trouble running either way on my 350ci and smaller engines since the static timing was well up between 6 and 8 degrees btdc. With my 415ci the static advance is set to 2 degrees btdc and little off idle vacuum drops when running manifold vacuum caused all kinds of off idle issues that ported vacuum did not have. It is an interesting debate. I have to go re-adjust all my advance timing again since I just changed out from 76cc to 75cc heads. Running cheap 87 Octane on 9.6:1 is tricky when your not running aluminum heads, but with the ceramic coated pistons and adjustable vacuum advance kit I can dial it in pingless with lots of power and 15mpg.
     
  12. 88Silverado

    88Silverado 1/2 ton status

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    [ QUOTE ]
    well up between 6 and 8 degrees btdc

    [/ QUOTE ]
    On my 71 350 street car, static is at 14* at 800, mech advance starts around 1000 and pulls 22* in by 3000 (total 36*) I have vac adv limited to 10* max, so idiling with vac advance im at 24ish. Im running flat-tops with camel hump 64cc heads running 87 octane at altitude of 2200'
    I cant run it on ported cuz if im slowly cruzing or start lugging in OD, and press the accel to downshift, ported causes it to ping really bad prior to downshift. I dont have that problem on Manifold. Some of my friends swear by ported. You really need to try it both ways and see which way feels and responds better and run it there.

    Theres a lot of different engine params that will effect timing and performance. Some will work better on manifold, others on ported.

    The 88 blazer I set at +2 and let the computer handle the rest /forums/images/graemlins/thumb.gif

    Since the ported vac ports are above the throttle blade, vacuum should start when the throttle opens and remain pulling vacuum as long as high velocity air is flowing in the upper area of the carb. Mech ports being below the blades will drop vacuum when the throttle is opened and climb as blades are closd. At some point both will pull about the same vacuum during crusing. Both pretty much drop enough vacuum during medium to heavy accelleration for the vac canister spring to pull timing out. I think its faster on manifold.
     
  13. BorregoK5

    BorregoK5 1/2 ton status

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    Since I had to retune my truck anyhow, I just ran a little test on the Q-Jet using my two vacuum guages, one to ported and one to manifold, routed up and stuffed under the windsheild wiper. It's something I've wanted to do for a while now but just never got around to it, this thread got me going on it again.

    Example: At idle, ported vacuum is 0 Hg and manifold vacuum is 17 Hg. Crack the throttle and the signals immediately match each other and remain for the most part matched through light throttle and little to no load, even the speed of their changes are the same. As the load increases I found some interesting results, the manifold vacuum maintained less vacuum from medium to heavy load than the ported vacuum did. The manifold vacuum hit zero before wide open throttle where the ported vacuum hit zero at wide open throttle. This being the case, you would have to advance your static or mechanical timing when running manifold vacuum in order to take full advantage of the same advance ported vacuum gives you up to wide open throttle. Now in either case, you set your static timing then your mechanical advance to achieve a maximum advance curve under full load through the rpms without detonation. This is a fixed curve so advancing it to compensate for manifold vacuum is not an option. Since an adjustable vacuum advance can take away vacuum advance, ported vacuum can be reduced to match manifold vacuum. What it can't do is simulate vacuum when none is present so theres nothing you can do to manifold vacuum to expand and tune the advance curve all the way to wide open throttle.

    Given the test above, I understand why many of the track guys would argue that manifold vacuum will never give you as much advance at wide open throttle as ported vacuum but until now i never really had proof of the difference. I'll bet that variances in different carburators implementation of ported vacuum can dramaticly change those results so that neither achieve advance all the way to wide open throttle so its a moot point, but on my 79 Q-Jet, it made a big difference.
     
  14. 88Silverado

    88Silverado 1/2 ton status

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    [ QUOTE ]
    As the load increases I found some interesting results, the manifold vacuum maintained less vacuum from medium to heavy load than the ported vacuum did. The manifold vacuum hit zero before wide open throttle where the ported vacuum hit zero at wide open throttle.

    [/ QUOTE ]

    Right, the manifold will drop vacuum faster as load is increased. Let the mechanical advance take care of the running engine, the vacuum adv is for low RPM takeoff and max advance while crusing. When the engine gets under load you want to retard the timing slightly because you want to delay ignition to prevent pinging, thus manifold vacuum tends to drop off under load. As I said, it can work both ways. At WOT both have no vacuum.

    [ QUOTE ]
    Now in either case, you set your static timing then your mechanical advance to achieve a maximum advance curve under full load through the rpms without detonation.

    [/ QUOTE ]

    Most people I know basically curve the distributor first so all mechanical advance is in by 3000. In most cases HEI will give you 20-24 degs of mechanical advance. Since most SBC like around 34-36* advance for best performance you set your timing for 34-36 at 3000 then let the static fall where it may...around 12-14. Plug the canister into manifold vac and the RPMs go up cuz it likes more timing, therefore you can back off the idle screw leaning out the air/fuel mixture.

    Most vac adv have 20* advance. The adjustable ones (where you crank the screw inside the canister) only adjust the tension of the spring and how fast the advance comes in. It does not limit the total advance the canister brings in. That has to be set by mechanical stop. Most people I know like the canister to have no more than 10* advance. I have mine set there.


    [ QUOTE ]
    Since an adjustable vacuum advance can take away vacuum advance, ported vacuum can be reduced to match manifold vacuum. What it can't do is simulate vacuum when none is present so theres nothing you can do to manifold vacuum to expand and tune the advance curve all the way to wide open throttle

    [/ QUOTE ]

    True, If you have an adjustable canister you can make these adjustments. Normal fixed type cannot. All GM cars, up to 71, came setup on manifold, not ported. Thats why you hear the "Unplug the vac adv before setting timing". t doesnt matter on a ported setup. On ported your allowing the canister to have more effect on the timing than the mechanical curve.

    What I have found is you can make ported work well to a point. If you have maximum timing dialed in, lets say, on your blazer, then you hook up a big heavy trailer, chances are because the ported will pull in more timing or doesnt drop off as quick as manifold, under load your going to start pinging because of too much advance. On manifold, the vacuum drops out and advance is left up to mechanical curve.
    I know many who dont run vac advance at all and let the mech curve handle it. Thats the most constant.


    [ QUOTE ]
    I understand why many of the track guys would argue that manifold vacuum will never give you as much advance at wide open throttle as ported vacuum but until now i never really had proof of the difference.

    [/ QUOTE ]

    If your running WOT there is no vacuum for the canister so how can they be pulling in more advance on ported at WOT??

    Both ways work and which ever way feels better to you is how you should run it. Thanks to taking the measurements. Those are the same results I have seen on the Chevelle board I belong to. There are many variables so different philosophys can and do work.

    Good Discussion /forums/images/graemlins/thumb.gif
     
  15. BorregoK5

    BorregoK5 1/2 ton status

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    Agreed! Discussions like these are the whole reason I belong to this board. /forums/images/graemlins/thumb.gif
     
  16. bigblock44k5

    bigblock44k5 1/2 ton status

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    Not trying to be mean, but it seems like you guys running straight manifold vacuum have things a little backwards. First of all the more gas your give an engine the lower manifold vacuum goes, until it is at zero. Ported vacuum climbs the more gas you give it. The whole reason behind advance is the higher rpms you go, the sooner the ignition needs to spark to get the spark to fire at the plug at the right time. Kind of like if cars are going down a highway and your on the side throwing eggs at it. If a car is going slow you throw when it gets close to you(kind of like a piston). If a car is going a 100mph you need to throw a second before it gets to you. I can understand having the highest vacuum poosible without pinging at or close to idle, this gives you more power, but when you hook the advance up like that, you are taking away your advance when the motor really needs it, at high rpms. If you want to make the most power you dont run vacuum advance at all. You want to rely on mechanical advance since it directly corresponds to rpm. The whole reason most of you ping in the first place is because you give it gas which puts the motor under load, but the main reason is because your not only putting it under load, you are flogging the motor with timing.

    Here is what you do, disconnect your vacuum advance. Get or borrow an advance timing light. Set your total mechanical advance to 40 degrees BTDC by whatching the light and revving the engine until it stop advancing, then adjust the distributor. Go out and test drive while leaving the distributor just loose enough to turn with your hand. Put your truck through a little abuse by flooring it from a light and also lugging around in low rpms. If it pings pull over and retard the distributor a little and try again. This will put you at full recommended total timing for max power out of a chevy 8 which is what you should always set to(this is why you are supposed to plug the vacuum advance port when you adjust timing). Then let it idle and check timing, you will probably be at around 20 BTDC. Which will be fine since it will only move with rpms, not with throttle. This way your motor will ALWAYS be running at peak efficiency, not just when you floor it.
     
  17. 88Silverado

    88Silverado 1/2 ton status

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    [ QUOTE ]
    Ported vacuum climbs the more gas you give it.

    [/ QUOTE ] Not exactly, only for a moment until pressures above and below the throttle plate equalize.

    [ QUOTE ]
    The whole reason behind advance is the higher rpms you go, the sooner the ignition needs to spark to get the spark to fire at the plug at the right time

    [/ QUOTE ]
    Not quite...

    [ QUOTE ]
    but when you hook the advance up like that, you are taking away your advance when the motor really needs it, at high rpms.

    [/ QUOTE ]
    Only the mechanical advance, maximized at whatever it maxes out to accelerates the engine to redline without giving it any additional advance.

    [ QUOTE ]
    The whole reason most of you ping in the first place is because you give it gas which puts the motor under load

    [/ QUOTE ]
    Exactly, thats why you need to reduce advance. The ported tends to pull slightly in the wrong direction at first but drops along with manifold.

    OK...Let me try this a different way /forums/images/graemlins/thinking.gif

    Under light load, part throttle conditions the manifold pressure is lower so volumetric efficiency is lower so the cylinder pressure is lower so the fuel mixture burns more slowly. This means you need to light the mixture sooner so you reach peak cylinder pressure at the ideal time. This is the purpose of the vacuum advance. The lower the load is the more it will advance the timing.

    As engine rpm increases, piston speed goes up. But the burning and expansion rate of the fuel stays the same.
    The vacuum advance system is responsible for increasing ignition advance based on engine load.
    The lighter the engine load, the greater the advance. The greater the engine load, the less the advance.
    When the engine is lightly loaded, such as at idle and when cruising, manifold vacuum is very high and ignition
    timing can be advanced without causing detonation. The high manifold vacuum pulls on the diaphragm. It also is pulling vacuum on the ported side at about the same level.
    This increase in ignition timing and substantially improves fuel economy.
    The vacuum advance system adjusts ignition timing based on engine load, the centrifugal advance system adjusts timing based on engine speed. The more ignition timing you can run without causing detonation, the better the performance and fuel economy.

    As Borrego noticed in his measurements, I have located a Vacuum Chart which shows what his measurements probably looked like,
    Ported AND Manifold are basically the same Except at Idle !
    On manifold port, manifold pressure drops and removes timing when throttle blades are opening.
    On ported port, timing increases for a short time while airflow comes in the top of carb and until it equalizes pressure with manifold then becomes the same and BOTH go to Zero at WOT (Heres where the mechanical advance all by its self accelerates the engine to redline without adding any additional timing as the RPM's increase. Between 2500 and 3000, depending how the weights and springs are setup, there is No more timing advance applied to the engine. Vacuum is too low (zero) on the canister no matter what port so I dont understand how someone thinks you get additional timing at WOT from the ported side and thats where the 2nd and 3rd quotes arent quite right)

    So which one do you want to hook it to? Its up to you. I prefer manifold vacuum. This pulls in more timing at idle which is good since there is virtually no load.
    Your motor will idle smoother and cooler with the extra timing. The purpose of ported vacuum is to raise the temperature at idle, to lower NOx emissions.

    Thats about the best I can explain it /forums/images/graemlins/tongue.gif
    Cool /forums/images/graemlins/thumb.gif
     
  18. 84_Chevy_K10

    84_Chevy_K10 Banned

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    I run mine on manifold vacuum and I prefer it that way. I have an adjustable vacuum can and a limiter plate on my vacuum advance. I prefer to use manifold vacuum for a better idle and hopefully better part throttle fuel economy. I agree with Mike in that if the throttle is cracked just a slight bit and quickly closed it might cause an eratic idle for a second, but hopefully the advance will kick in to keep the engine from dying.

    I like to be able to turn my idle screw down a little more and I believe more advance at idle will help an engine stay cooler if it's going to just sit and idle for any reason.

    Interesting discussion actually. I'm glad to see both sides of the arguement. To each his own, there are advantages to both setups it appears.
     
  19. BorregoK5

    BorregoK5 1/2 ton status

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    That’s a great chart and yes, it basically corresponds to the measurements I took. But when was the last time you saw a small block pulling 25Hg at idle??? That’s one heck of a towing cam!!! To be fair to the sake of argument, I'm going to run manifold vacuum this month and see if I notice any other changes. /forums/images/graemlins/thumb.gif
     
  20. 88Silverado

    88Silverado 1/2 ton status

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    I always ran ported on my 71 ElCamino until I put the 200R Overdrive/lockup trans. While cruzing slow (35-45ish) in OD and the converter locked up or slowing down to make a turn from one street to another, Foots off the gas, engine under no load and I usually have to give it some gas to get it to downshift. (Controled via TV cable) On ported it would ping real bad as I gently increased the throttle (now ported is pulling vacuum and advance in until it equalizes with the dropping manifold pressure) while im increasing load to get it to downshift. I was able to virtually eliminate it on manifold. To me, the faster drop in timing from manifold works better and with the hot summers in the desert, with the air running, the advanced timing keeps things a little cooler while in traffic. On the down side, the cam sounds better without the additional advance at idle and it sure smells better too /forums/images/graemlins/frown.gif

    Thanks for the discussion /forums/images/graemlins/thumb.gif
     

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