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How much vacuum should an engine make?

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Blue85, Aug 15, 2002.

  1. Blue85

    Blue85 Troll Premium Member

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    Just got a vacuum gauge. With my new timing it pulls almost 22 at idle and up to 25 when decelerating. Is this good? At WOT it never goes to zero, only down to 2 or 3. Is that just normal carb restriction? This is all vacuum right from the tree on the manifold.
     
  2. Sandman

    Sandman 3/4 ton status Author

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    Sounds fine for a stock engine.
     
  3. thatK30guy

    thatK30guy 1 ton status Premium Member

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    Those numbers sound real good.

    I believe the average is 18 inches.
     
  4. dyeager535

    dyeager535 1 ton status Premium Member

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    sounds even a bit higher than what mine pulls at idle, but I've got no idea what components this thing was built with way back when. Everything else is about the same.

    I would assume the 2 or 3 would be indicative of the secondaries only opening enough to support demand, meaning there has to be some vacuum left in the manifold as they open, based on primary venturi size.
     
  5. wrathORC

    wrathORC 1/2 ton status

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    Anything above 18 at idle is good. I never payed attention when decelerating though. I usually try to get my vacuum up to what it is at idle in order to get good fuel economy though.
     
  6. Jay73K20

    Jay73K20 Registered Member

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    Good vacuum but 1 would rather have a cam that would make 16-18 at idle then 22 because of power. 2-3 at WOT is to high, you want no more then 1.
     
  7. Blue85

    Blue85 Troll Premium Member

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    Are you able to get up to 18+ while cruising? I haven't been on the highway yet, but it seems like it won't stay above 15 when cruising. It is a Goodwrench longblock.

    I will do some more WOT pulls, to confirm the 2-3 numbers. If I still have vacuum at WOT, should I check the throttle linkage to ensure that the secondary butterflys are opening fully? Should I also look at the springs on the top flaps of the secondaries? Never hurts to have more power and I agree that I do need more cam. I still have lots of things higher priority, though. Plus, I am holding off on the cam/intake stuff, since I am dreaming of EFI.
     
  8. Jay73K20

    Jay73K20 Registered Member

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    I get about 12 + cruising but I cruise at 70+ mph more like 80.
    If it's a q-jet then there is a tab on the secondary air butterfly shaft linkage right where the pull off linkage connects to. When the butterflies are fully open the tab hits the rear of the air body to restricted air flow. Grind the tab and you will have a 750 cfm carb. But I would grind it half way and recheck your WOT vacuum, if it is below 1 then I would leave it.
     
  9. dyeager535

    dyeager535 1 ton status Premium Member

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    You mess with idle vacuum, (lower or "lopey") you are going to have horrid down low performance.

    My dads 454 has a cam in it better suited to a street/strip car than a truck, and even with 4.10's and smaller tires than me (350/3.42's, 33's vs his 32's) he can't "walk away" in second gear. I can. (both same trannies, t-case, same clutches, he has a 1000lb weight penalty, MORE than offset with the gears and engine size) His idle vacuum is pretty poor. Not "need an auxiallary vacuum cannister" poor, but its not a strong idle.

    Obviously vacuum isn't really the end all in this situation, but I wouldn't be looking at vacuum for power reasons, I'd be looking at where the engine *made* power, and WHERE it *needs* to make power for your use. High vacuum is a benefit for things such as brakes, vac advance, and fuel efficiency, so economy is where high vac numbers are great...

    I'd say hold off on cam too...EFI all the way. Not entirely comparable to a "standard" 350, but look at some stock LT1 HP/TQ "curves", (Torque curve means nothing to those motors) that will leave you drooling.
     
  10. wrathORC

    wrathORC 1/2 ton status

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    Well, I've got 31" tires, a 700R4, and airplane gears (2.73s). That means I'm pulling around 1400 rpm at 60mph.

    At sustained wide open throttle you'll still have vacuum due to the fact that a carburetor is a pretty restrictive piece of equipment (unless you go with a demon single butterfly carburetor that flows twice what your motor needs).

    I'd just make sure your secondaries are working right. A 350 only needs around 540cfm at 6000rpm anyway (get out your calculator if you don't believe me). A bone stock quadrajet is good for 600-625. You can tweak them to get as much as 750 (I saw an old-timer once use his dremel tool and an airhorn to get 825). In order to do this you need to get one of the earlier butterflies which are more flat and modify the stops so they open about 90% of the way.

    But anyway, flow isn't likely your problem. If you don't mind beating the piss out of your motor just go under there and stuff the throttle and see if the secondaries are opening all the way. I wouldn't do this more than you have to. "Eh, mon, we can use this block for a chisel plow now." "Dude, those are my expensive forged H-beam rods stickin' out of da block."

    While doing this watch how fast they open. You can drill a bigger hole in the vacuum motor so they open faster which helps get rid of the infamous "quadrabog". As far as the spring, I'd just make sure it's adjusted properly. I once tried loosening mine up and the butterflies flung open way too quick.


    What size rods and hanger are you running? The rods will have two letters stamped on the side of them and the hanger will have a letter on top of it. I'd start with a set of DR rods and a "K" hanger. I'm running DR rods and an "I" or "H" hanger right now. They aren't quite what I want but that's because it takes three seconds for them to open (due to slow vacuum motor) but that's alright because with my airplane gears I'm always at 2/3 throttle.


    Website that's far more helpful than me:
    http://personal.clt.bellsouth.net/clt/m/g/mgervin/garage/damon.htm
     
  11. Blue85

    Blue85 Troll Premium Member

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    Thanks for the suggestions.

    A carb will always be a restriction. That is how it works. The question is just how much. The cfm rating of a carb is just how much air it flows for a given amount of pressure drop. If there was no drop, the venturi would have no signal and you couldn't meter fuel.

    I calculated 608cfm for a 350 at 6000rpm. What did I do wrong? Of course, a stock small block really shouldn't go over 5k, which should be about 506cfm. Using a bigger carb (higher cfm rating) would give less restriction, but won't necessarily give more power and will almost definately meter less accurately.

    I'm not trying to get a lot more flow, just to make sure that everything is operating the best it can. </font><blockquote><font class="small">In reply to:</font><hr />
    If you don't mind beating the piss out of your motor

    [/ QUOTE ] But I do mind! All I can do is make sure that the secondary throttle blades are opening correctly and then make sure that I have a spring setup similar to what others know works. Besides, the top of the carb may never fully open without a load on the engine, even with the throttle open all the way, so I'm not sure that's a fair test.
     
  12. dyeager535

    dyeager535 1 ton status Premium Member

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    For setting the air valve, I loosen the spring up as far as it will go, while still reliably closing the air valves all the way, everytime, from even a partially open position.

    As to drilling the vacuum break, BE CAREFUL! If you mess it up (too big, and it doesn't take much) you can end up with the air valve having a bad transition, which will hurt acceleration just as much as a bad restriction.

    Wells P/N CP115 is a "faster" vacuum break, apps include Oldsmobile 75-75, 79-86
    Chevy trucks 75-77
    (pretty vague though)

    asking for the break for these carbs:
    17051261
    17056167
    17060808
    17060832
    17064917
    17067331
    17067333
    17067466
    17068345

    should net you that replacement as well.

    I have yet to try it, but I need to wire the secondaries shut just so I can say "you'll KNOW if the secondaries don't work"...then again, people lived with the 2 barrels as well, maybe under normal driving conditions it won't be noticeable.

    As to secondary rods/hangars, I just have a wide assortment gleaned from swap meet carbs, and I played around with them. Mainly on my car though, as it would actually drain the float bowl with too rich of rods, or too aggressive of a hangar. Had to play around until it "just" wouldn't drain the float bowl. The truck..well, it isn't real quick, and I set the primary side up for max economy, so I really haven't played around with the secondaries. Probably play around with the different breaks, needle/seat arrangement, and even more secondary rods/hangars when the car is back up and running, since the truck will be EFI : )
     
  13. wrathORC

    wrathORC 1/2 ton status

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    Yeah, it is 608 CFM.. if you don't include a volumetric efficiency of about 85% (hey, we can dream, can't we?).

    I'm even less cruel to stock motors.. they don't see anything greater than 4400rpm. Yeah I agree about the bigger the carburetor bore the worse it is at fuel metering since it relies on the velocity of the air to help meter.

    On my truck the metering butterfly just about fully opens but mine doesn't open much more than 75° (I've got a non-APT carburetor I'm going to stick on when I swap distributors). I don't let the motor stay wide open, I just let it accelerate. It's even easier to do this with the air conditioning on or lights and a blower due to horsepower-robbing resistance created by the alternator.

    Anyway, I'd start with rods and a hanger. You might have some crazy rod with a big-assed power tip on it and a P hanger. For reference, a 305HO (found in Monte Carlo SS's) came with DR rods and a K hanger (or at least my Mom's did).
     

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