Q&A

Mailbag: Timing Adjustment Recommendations for a Small Block Chevy V8

set-timing-at-12-degrees-advance

(Image/Hot Rod Network)

You’ve got questions, we’ve got answers. We work with the Summit Racing tech department to help you tackle your auto-related conundrums. In this week’s Mailbag, we’re helping a reader time his engine.

Q: I have a 1956 Chevy with a 300-horsepower 350. It’s hooked up to, and sends power to, a 12-bolt rear axle with 4.11 gears. My question is, how do I time the engine? It has an HEI distributor, and everybody around here has a different way of doing it.

Do I time it at 2,500 rpm with the vacuum advance hooked up, or do I disconnect the vacuum advance and plug it? Would an HEI advance curve kit help any? Do I go to 38 degrees total timing, or should I go farther? I have a good advance timing light, so tools are not the problem.

A: We would shoot for 34 to 36 degrees total timing at 2,800-3,000 rpm. Set the timing with the vacuum advance hooked up so you can do vacuum and mechanical advances at the same time. To find out what your mechanical advance is, remove the vacuum line from the distributor, plug it, rev the engine, and get a reading off the timing marks on the harmonic damper with the timing light.

As for the advance kit, this would help you optimize the HEI for your particular combination.

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9 Comments

  1. FRANK A ZYSKOWSKI says:

    I have a new Chevy 260/350 crate engine with a four barrel carb and cast iron manifolds. This is what I am using and it runs pretty good: 10*@idle/600, 16*@1000, 22*@1500, 26*@ 2000, 33*@ 2500, and 35*@3000. My vacuum canister is set to pull in 15* of advance at cruise. This is my rough starting point and with some fine tuning I think it will run well. I check the mechanical without the vacuum canister connected.

  2. I think that the answer may be confusing as it infers that total timing should be 34 to 36 degrees with the vacuum connected. To the reader’s question, I believe that you should set the initial timing with the advance disconnected to the 34 to 36 degrees number. This is pure mechanical advance. Then you can connect the vacuum and check the additional ignition advance resulting from the vacuum advance adding to the mechanical advance.

    • Absolutely Correct Gary…. any other method proves the writer has no experience in performance tuning. A good Vacuum Gauge along with a Quality Non adjustable Timing Light is essential.

      • forgot to mention the first thing to check is T.D.C #1 exactly matches 0 degs on Harmonic Balancer…. closely followed by Rotor/Cap Phasing… and to a lesser extent, but still important if your chasing the best possible ignition performance is to check the Air Gap between the Rotor Tip and the Cap Terminals. There appears to be a huge variation in dimensions
        of these 2 components… if you unlucky enough to have a Rotor with a ‘Short Tip’ length measured between the centre of the Rotor and the end of the Tip…. and a Cap with a Large Terminal P.C.D then its very likely you could end up with up to 6mm of Air Gap between your Rotor Tip and the Cap Terminals. This much of an Air Gap can seriously effect the Spark Performance especially if your running high compression and a big cam on the street with lots of low rpm use. Its quite an easy operation to perform these x2 checks on both the Rotor and the Cap by simply drilling a hole in the Cap as close as possible to any one of the Hi Tension Terminals which would allow you to clearly see the Phasing and Air Gap. The Air Gap is easier to check by removing all the Plugs and turning engine over by hand until the Rotor Tip comes into view at which point you should be able to see ( with a good torch) both the Tip and the Cap Terminal and using a twist drill be able to measure the Air Gap between the 2. Anything up to 3mm Air Gap is acceptable as long as the Phasing is correct. Another thing which affects this Air Gap is not only the Radial Clearance but also Ive seen quite a difference on the vertical alignment between the different brands….which also has a big influence on the Air Gap. Once you have finished checking you can simply fill the hole in the Cap with silicon RTV taking care not to let any excess Silicon inside the cap.In my experience this Air Gap problem is relatively new (within the last 10-15 years) and wasn’t a common problem with most O.E.M Caps and Rotors, but appears more common with a lot of replacement parts now
        being sourced offshore ;-( its really let the buyer beware… if you aware there could be problem you can simply check it by the above method. Another tip with a hi performance ignition system is when purchasing replacement Caps/Rotors is to go with a set from a known “Good Brand’ which uses Brass Cap Terminals, not cheap Aluminium terminals. cheers

  3. SOUTHSIDE Performance says:

    I agree with Gary. Also you did not tell the Customer there is (2) Two types of vacuum coming off the Carb. Manifold (sucking at Idle and cruise) – Ventura (sucking when the throttle plates begin to open). Check this by removing the Vac. hose off the Dist. put your thumb over end of hose, check your Idle speed, then install it on the Dist. If the Idle speeds up it is manifold vacuum. We set the mechanical advance in a Dist. Machine and use manifold Vac. on use Vacuum Advance for better gas mileage. Some people run Ventura Vac. cause they say it runs better. Good Luck !

  4. What is a dizzy? I have been a tradesman for 30 plus years and l have never heard of that term.

  5. James machuta says:

    Small block 400 tinming marks and gears and chain are in line But it is not running correctly and back firing.
    Could it be a vavle problem or the rubber slide on the balancer.

    It starts but will not run smoothly

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