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Sun Aug 17, 2008 12:38 am

Hi all,

Had a quick question. Searched the forums but didn't see anything.

Is there a formula to figure out what degree your engine should be timed at? What are the factors that decide the degree BTDC? I am aware of many charts that tell you what it is but I'd like to understand the factors.

My engine is supposed to be at 7.5 BTDC (or at least that's what I'm told hehe).

How do I derive 7.5 degrees from my engine specs?

1776 DP
110 cam
043 40 x 35.5 heads (1.25 rockers)
009 Distrib (no compufire... just points/condense)
Dual 34 ICT carbs

How do you figure out what it should be at 3k rpms?
Would the timing specs change if you replaced the points with a compufire drop-in? If so, what variables would change in the formula?

Thanks for any enlightenment :)

Veei

Sun Aug 17, 2008 12:54 am

Distributor model# should tell you what to time it to,sense it determans
how far the ignition advances.

Sun Aug 17, 2008 2:27 am

The simple answer is that there is no hard and fast rule for what the optimum timing should be for a given combo, because there are so many variables. People that build a lot of these engines would be able to give you a pretty accurate ball park figure (I would say 32* at 3000 RPM should get you close), but the next step after building an engine has to be tuning it. Without attention to tuning, no engine will run it's best, and this is the stage where you figure out what timing works best (usually with the help of an engine dyno).

So unless you have access to a dyno, you won't be able to tell what works best. You'll get close, but without the dyno it's an educated guess at best.

Most (myself included) start at 32* at 3000, and see if it pings or runs abnormally, or runs hot. Is so, back it off a few degrees, and try again. Trail and error is all we have on the street. :wink:

Sun Aug 17, 2008 11:45 am

Thanks for the info on where to start when timing at 3k rpms. What about the 7.5 idle? Is that number nothing but specs for that distrib?

Other than the distrib, what are the other factors that directly affect the idle timing and the 3k timing? I've read much about timing and I know the 009s are pretty much el cheapo junk and give bad max advance since they have no vacuum.

For 009 most articles say that max advance should stay between 28-32*. I am sure that's right but why? Where does that number range come from? Numbers typically aren't arbitrary.

Do the type of points in your distrib affect the advance? 009 with typical points measures the same as 009 with compufire or similar drop-in?

Is a dyno the only thing that can measure EXACTLY what is correct? If a machine is measuring your engine, then there must be a formula! :)

Thanks

Veei

Sun Aug 17, 2008 1:47 pm

The machine isnt working on a formula, with a dyno its a tool that you can have an engine running on under load and can try different timings and see what gets most performance.
Under high load (foot to the floor flat out acceleration) 28-32 firing of the spark is usually the safe region before detonation occurs.
Only in high vacuum situations of light load (part throttle cruise) will the engine then not detonate at something like 40+ degrees.

In the modern world we have a engine spark advance map, and a knock censor that can keep the spark firing near to optimum.

With a 009 all you have is one pre set curve that is followed according to engine rpm.

With a SDVA you have the distributor working from rpm and from vacuum so its has 2 overlapping curves giving a much broader spark advance.

Sun Aug 17, 2008 5:02 pm

It's empirical . . . not a real science.