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Thread: Need to pick your brains about headers
          
   
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  1. #1
    HiboyGal's Avatar
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    Need to pick your brains about headers

     



    Woke up at 3 AM this morning and it hit me:

    Can using a 283 header on a 350 engine alter or affect in any way the vaccum of the engine?

    Educate me por favor

  2. #2
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    No.


    Don

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    bobscogin is offline CHR Member Visit my Photo Gallery
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    >Can using a 283 header on a 350 engine alter or affect in any way the vaccum >of the engine?

    Probably not to any noticeable degree. Most off the shelf small block Chevy "street" headers are generic with regard to primary tube diameter. When used on a 350 instead of 283 (all else equal), exhaust gas velocity may increase by virtue of the increased displacment, but that ain't a bad thing.

    Bob

  4. #4
    HiboyGal's Avatar
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    The reason I ask is because the common complaints Eric and Lonnie (2 previous owners) have about my current 350 engine is that the vacuum is not 'quite' right. Hector (friend of Lonnie) measured it and said it did not match what a 350 vacuum should be. Said maybe there is a different cam on it (but I don't think so). Vaccum being off, the 3 deuce set-up has not been working. I am currently running on 1 carb.

    Now I also know that Eric swapped the previous 283 Chevy engine (which died on his way to the NSRA nationals in 2002) with a crated 350 chevy. He did the swap while at the show, in one day.

    The same headers that were on the 283 still are on my current 350 engine.



    I don't know enough about engines yet to know for sure, but by simple logic, it seemed to me that possibly if the headers were restrictive it could create some kind of "pressure"...??

    Or maybe I am completely losing it - since i have NO CLUE what I am saying LOL

    But just in case I may be right, I thought I'd ask. Is it AT ALL possible it might be affecting my vaccuum, throwing it off just enough to mess things up?

  5. #5
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    THose are whats called "rams horns" manifolds and they make a very distinct exaust note.
    Its gunna take longer than u thought and its gunna cost more too(plan ahead!)

  6. #6
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    The manifolds you have are very good for that engine. If you have a different cam your vacuum will be effected. Could be a intake leak??? Have you had anybody look at it other than the people you bought it from???
    Charlie
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  7. #7
    bobscogin is offline CHR Member Visit my Photo Gallery
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    >Hector (friend of Lonnie) measured it and said it did not match what a 350 >vacuum should be. Said maybe there is a different cam on it (but I don't >think >so). Vaccum being off, the 3 deuce set-up has not been working. I >am >currently running on 1 carb.

    Vacuum is affected primarily by cam timing and the induction/intake system. It can vary from what you'd see on a stock engine to less vacuum on engines with more radical cams. Those look like small base Rochester 2G's on your three duece set up. I'm assuming you're running some type of progressive mechanical linkage?

    >I don't know enough about engines yet to know for sure, but by simple logic, it >seemed to me that possibly if the headers were restrictive it could create >some >kind of "pressure"...??

    You're not completely off base with your reasoning regarding the exhaust/vacuum relationship. Extreme restriction in the exhaust system can prevent complete purging of exhaust gas from the cylinder on the exhaust stroke. Any residual exhaust gas left after the exhaust stroke will diminish the volume of intake charge on the intake stroke and thus affect vacuum.

    Bob

  8. #8
    rumrumm's Avatar
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    My guess is that you have a vacuum leak. Check all of your vacuum fittings to see if the are tight, and check the hoses to make sure none are cracked. Vacuum leaks can cause havoc with the way an engine runs. That is a nice looking engine BTW.


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  9. #9
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    Could be lotsa things

     



    Faith,
    If you have a stock 'mom and dad' car's engine, no matter what the make, the vacuum should be steady somewhere between 17 and 21 inches Hg (mercury) at idle speed (5 to 600 rpm). I prefer 19-21"Hg

    A camshaft, different than stock can lower it - how much depends on the grind.

    You have triple carbs - I almost can guarantee that one of them leaks vacuum somewhere, unless totally blocked off between the carb base and manifold. Possible leaks can be found at the throttle shafts, at the base and intermediate castings, cracked castings, missing vacuum takeoff point plugs or split hoses just to name a few areas. If triple carbs didn't look so great, my suggestion would be to put on a single 4bbl and manifold, but.....

    The intake to head(s) gasket may also leak.

    The timing could be set incorrectly.

    The mixture screws in the middle carb may be set wrong.

    The mixture screws in the end carbs may not be seated - they should not be open at all on a 3x2 setup.

    You may have valve, lifter or rocker problems.

    You may have a leaky piston ring (or several).

    You could have a bad spark plug or wire

    You can pick up a vacuum gauge cheap at Auto Zone (or whatever you have there on the Left Coast) - but these are the kind of things that can lower vacuum readings - and on a pre-computer engine are the cheapest and best diagnostic tools available.

    But since I've said all this, how does it really run? Skip, idle too rough, poor gas mileage, shift lousy(if it's an auto - I don't recall). If none of those are a problem, you may be worrying over nits.
    Dave

  10. #10
    topless is offline CHR Member Visit my Photo Gallery
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    Exhaust note?

     



    Quote Originally Posted by shawnlee28
    THose are whats called "rams horns" manifolds and they make a very distinct exaust note.
    Can you explain what you mean by a "distinct exhaust note" ? How does this differ from the way a 350 SBC sounds with headers or other exhaust manifolds? Thanks, Topless...
    Buy a Buick, they got plenty power!

  11. #11
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    As the Irish guy said..........vacuum leak. If you are not using the other 2 carbs, it would be best to seal off those carbs. A simple sheet metal plate under the carb between 2 gaskets would do it.

  12. #12
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    C9x
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    Rams Horn exhaust manifolds are an excellent way to go.

    Imo, they flow as good or better than most 'convenience' block hugger headers which are more manifold than they are headers.

    If you have the time they can really look slick if you grind the casting down smooth, remove unecessary protrusions and bumps then coat them with anything from ceramic to Jet Hot.


    As others have said, the cam profile (how radical etc.) makes the difference in vacuum at idle.

    How much vacuum do you have and what RPM are you reading it at?

    Fwiw, my 462" Buick runs 18.5" - 19" at 600 rpm idle at a 350' altitude with a very mild cam.
    Very mild being a step or so above stock with timing figures of 260 & 266 degrees advertised duration.
    (Don't confuse this figure with timing figures taken at .050 valve lift. They will be different to a considerable degree. This cam is 204-210 degrees duration at .050 lift.)

    When the bigger cam was in, idle was 600-650 RPM and vacuum was 11" - 12" at the same 350' altitude.
    286 - 292 degrees advertised duration in this case.

    These lower vacuum figures due to camshaft profile and not vacuum leaks.

    Once the engine is spun up to a no-load RPM of 1000-1200, vacuum levels are equivalent to a stock engine.

    Keep in mind too that changing altitudes will change idle vacuum levels.
    To the tune of 1" lower for every 1000' altitude gained.

    The milder cam noted above idles at 15" - 15.5" at the cars present 3333' altitude.
    When the engine is spun up to 1000-1200 no-load RPM, vacuum levels are the same as they were at the lower altitude.

    Note that these lower vacuum levels - due to camshaft profile - require changes in carburetor tuning.
    Different metering rod springs for Carter/Edelbrocks and different power valves for Holleys.

    When these carbs are in standard - as received from the manufacturer - configuration they will be too rich on the bottom end (idle and low-mid RPM levels at light engine loads) due to the metering rods on the Carter/Edelbrocks will open dumping excess fuel in and the Holley power valve will open which does the same thing.

    Along with the vacuum reading, what kind of cam is in the engine?
    C9

  13. #13
    Dave Severson is offline CHR Member/Contributor Visit my Photo Gallery
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    you could switch exhaust manifolds (headers are individual tubes going to a common collector) and probably not notice 1" of difference in vacuum....
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  14. #14
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    Question My 10 cents worth

     



    I notice that you are runing and oil tube directly into the intake manifold and down into the valley between the bores. I am asking on the other side of the engine are you running a pvc valve out of the other rocker cover. Because sometimes the spring in the pvc valve goes soft and the engine will draw air and a little bit of oil mist into the intake. A sign of this would be low vacume and a little bit of blue smoke out the exhaust under load. Of corse the oil level in the sump would drop a little too.
    "aerodynamics are for people who cant build engines"

    Enzo Ferrari

  15. #15
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    Quote Originally Posted by rumrumm
    My guess is that you have a vacuum leak. Check all of your vacuum fittings to see if the are tight, and check the hoses to make sure none are cracked. Vacuum leaks can cause havoc with the way an engine runs. That is a nice looking engine BTW.
    I agree ..sounds like a leak some place. On another note I dont like ram horns. Have you ever considered a header that goes over the frame?
    Might not be the look or sound you are looking for but it is an idea

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