Small Block 350 Timing

Discussion in '40- & 55-Series Tech' started by nat, Oct 28, 2005.

  1. nat

    nat

    Messages:
    1,988
    Media:
    1
    Likes Received:
    130
    Joined:
    Sep 20, 2005
    Location:
    Los Gatos, California
    I want to tune up my sbc 350 as I am sure the timing is off. I know it is supposed to be 10 degreees BTDC at 700 rpm. Is that with the vacuum advance connected or with it disconnected and plugged? The timing mark is on the harmonic balancer not on the pulley on the front of it?

    Sorry for the questions, but I am used to playing with F and 2F motors.
     
  2. camcruiser13

    camcruiser13

    Messages:
    6,978
    Media:
    3
    Albums:
    1
    Likes Received:
    28
    Joined:
    Jun 22, 2004
    Location:
    sittin on the couch in UT
    yes it is on the harmonic balancer, and i have all my stuff hooked up. i run closer to 11 but im in the mountains.
     
  3. nat

    nat

    Messages:
    1,988
    Media:
    1
    Likes Received:
    130
    Joined:
    Sep 20, 2005
    Location:
    Los Gatos, California
    Cool, but is that with the vacuum advance hooked up or disconnected and plugged?
     
  4. ranger

    ranger

    Messages:
    1,169
    Likes Received:
    7
    Joined:
    Jan 18, 2003
    Location:
    Mountain Hole, Idaho
    Disconnected and plugged
     
  5. nat

    nat

    Messages:
    1,988
    Media:
    1
    Likes Received:
    130
    Joined:
    Sep 20, 2005
    Location:
    Los Gatos, California
    Thats what I always thought was the case, but wanted to make sure. Thanks!!!
     
  6. missipboy

    missipboy

    Messages:
    1,773
    Likes Received:
    2
    Joined:
    Oct 11, 2004
    Location:
    tunica, ms
    i thought the vacuum advance didn't matter since it's only at 700rpms??
     
  7. nat

    nat

    Messages:
    1,988
    Media:
    1
    Likes Received:
    130
    Joined:
    Sep 20, 2005
    Location:
    Los Gatos, California
    My understanding was there is more vacuum at idle, its the reason why my vacuum windshield wiper motor in my 1953 GMC works at idle, but not at speed.
     
  8. HI^C

    HI^C

    Messages:
    1,234
    Likes Received:
    2
    Joined:
    May 22, 2003
    LOL, depends on how the vac advanced is hooked up. but should be checked without
    vacum, and plugged.

    STOCK 6-8 btdc on most sbc.

    electric wipers are nice, convert it, 54 GMC :)
     
  9. fussy_cruiser

    fussy_cruiser

    Messages:
    276
    Likes Received:
    1
    Joined:
    Mar 13, 2005
    Location:
    Colorado
    Be careful, 10 BTDC is a good rule of thumb.

    There are some numbers under the pass side head, under the alternator if it is in the stock location, or on the block on the drivers side just over the oil filter. Punch those numbers into google or yahoo and that should tell you what it came out of and what kind of cam is installed.

    I went to the library and copied the pages out of the '67-'79 chevy pickup and blazer manual for tune up specs. Mine is set to 0 BTDC BTW '71 - '72 350. Runs better than at 10 degrees and I get an extra 1-2 MPG on average.
    Good luck.
     
  10. miker

    miker

    Messages:
    2,610
    Likes Received:
    47
    Joined:
    Jan 29, 2003
    Location:
    Nil Sine Numine
    Not all SBC alternators are mounted on the passenger side. GM had a wide range of alternator mounting locations. There are no numbers stamped on a SBC block that tell you for sure what cam is currently installed. Chances are, the SBC in your cruiser has been rebuilt at least once or had the cam swapped out at one time or another. Unless you installed the cam yourself, you just don't know. The SBC casting numbers will give you a general idea of the block year and type of car or truck GM used it in but they are generally not exact.

    How much initial timing you have set is important but it is equally important to check the TOTAL advance timing -> initial+mechanical+vacuum =TOTAL advance

    TOTAL advance should not exceed 36 degrees. If it does, make sure the engine is not pinging under heavy loads otherwise you might find yourself having to rebuild the engine. Trust me, I know what pre-detonation can do to pistons, rings, and rod bearings. Is not pretty
     
  11. dgangle

    dgangle total rice

    Messages:
    4,002
    Likes Received:
    17
    Joined:
    Nov 24, 2004
    Location:
    Heart of Dixie
    I second Mike. It's the total advance you should be concerned with. Leave everything hooked up and measure/adjust. I have always set at 38* total advance @2500 RPM. You can experiement with what works best for you. Less advance will run cooler but I was told never go below 32* or the performance falls off pretty bad.
     
  12. miker

    miker

    Messages:
    2,610
    Likes Received:
    47
    Joined:
    Jan 29, 2003
    Location:
    Nil Sine Numine
    It's always a good idea to check the total advance even if you know what you purchased:

    I bought a "blueprinted" Summit HEI vacuum advance distributor for my SBC. It came with a spec sheet indicating that it had 11* mechanical advance @ 2200rpm and 12* vacuum advance at 2500rpm. So, I set the initial timing to 12* and expected to measure a total of 35*. I was surprised to find that I had over 50* of total advance! Even with 0* initial timing I was still borderline too much advance. It turned out that the "blueprinted" distributor was not at all as the spec sheet indicated and it really had nearly 30* of vacuum advance at 2500rpm
     
  13. dgangle

    dgangle total rice

    Messages:
    4,002
    Likes Received:
    17
    Joined:
    Nov 24, 2004
    Location:
    Heart of Dixie
    Wow! How did it run and what became of this? I am sure Summit exchanged it, and if so was the replacement correct? Assembled/tested incorrectly?
     
  14. miker

    miker

    Messages:
    2,610
    Likes Received:
    47
    Joined:
    Jan 29, 2003
    Location:
    Nil Sine Numine
    I never bothered to contact Summit on this one. I discovered that the vacuum module was not adjustable so I just purchsed an adjustable vacuum module and adjusted it to what I needed.

    I'm not bashing Summit. I love Summit! I've bought a lot of stuff from them and have even returned items that did not work correctly. The've always glady and promptly taken items back and quickly refunded money or replaced a defective item. At the time I found this, I didn't want to wait for a replacement so I just spent a little money and bought the part locally and got it working correctly myself. From now on I will always make sure if I buy a vacuum advance distributor that it has an adjustable module.

    The engine seemed to run ok with 50* advance but I never had it under a load with that much advance. I did distroy another SBC which had too much advance. It ran good up to the point where it blew a hole in the piston. When you are just trying to go fast you kind of ignore the occasional pinging. I guess it was really trying to tell me something.
     
  15. nat

    nat

    Messages:
    1,988
    Media:
    1
    Likes Received:
    130
    Joined:
    Sep 20, 2005
    Location:
    Los Gatos, California
    Thanks for the responses everyone. What will be more accurate, the degree plate mounted above the harmonic balancer, or the degrees marked on the balancer itself. Seeing that timing mark is a pain, I used white paint, but it is impossible to see.
     
  16. miker

    miker

    Messages:
    2,610
    Likes Received:
    47
    Joined:
    Jan 29, 2003
    Location:
    Nil Sine Numine
    You have to use a minumum of 2 marks. A mark on the balancer and then you also need a stationary mark on the engine block somewhere as a reference. Beware that it's possible to have the timing marks on the block in the wrong location especially if someone has installed one of those aftermarket chrome timing tabs. If it looks like a factory timing tab and if its the correct tab for the size of balancer, its probably close enough.
     
  17. miker

    miker

    Messages:
    2,610
    Likes Received:
    47
    Joined:
    Jan 29, 2003
    Location:
    Nil Sine Numine
    Just re-read your message. I prefer to use an adjustable timing light. Then you only need TDC on the balancer and TDC on the tab.

    As for best accuracy between the two you mentioned, I don't know.
     
  18. nat

    nat

    Messages:
    1,988
    Media:
    1
    Likes Received:
    130
    Joined:
    Sep 20, 2005
    Location:
    Los Gatos, California
    I do have one of those chrome tabs with degree marks on it, but it came with the crate motor, so I am guessing it is ok. I used a whiteout pen and marked 8 degrees BTDC and then lined it up with the 0 on my chrome tab. It runs killer now. Thanks for everyones input.
     
  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.