I was looking in my bin last night and discoverd that I only have 29.7deg of total timing. I have initial set a 6deg as per the program. The bin reads 23.7deg at 70kpa and above after 3,500 rpm if I am remembering correctly. The Auto X-ray reads 29.7deg at WOT. I also get 42deg at cruise and the highest advance in the bin is 36deg. All of this leads me to believe that the Auto X-ray is reading total advance to be 29.7degat WOT. Isn't normal timing on a small block chevy around 36-38deg total all in by 2,800rpm. My question is this. How much HP do you think I am loosing due to this timing problem? I would guess 50-60hp.

 

I would guess 50-60hp.

Why guess?

You can easily go out and advance the timing 2 degrees and see if performance improves. If it helps try more if it does not help try less. Tune for the least spark with best performance. You will have to burn a new PROM to get this right.

You should also understand that you AFR heads should have a much faster burn rate due to chamber shape and improved squish and swirl. I would not be surprised if you see no gain but the only way you will know is to test.

HTH

John

 

John: If I turn up my initial instead of going into the Prom will there there be any adverse effects? Can I turn up the initial as a short time fix, until I have time to fix the Prom?

 

If I turn up my initial instead of going into the Prom will there there be any adverse effects? Can I turn up the initial as a short time fix, until I have time to fix the Prom?

The only thing I can think of is that the spark gap (from the rotor to the cap) will change. This may make an effect of its own that will not be there when you change the PROM. It is best if you can change one thing at a time.

John

 

John, the only time that is true is in an ignition system that is triggered by a crank sensor (like in an '83 -'85 lincoln w/302, the distributor tower is nothing but a rotor and the cap sitting on top). The ignition on a sbc is triggered by a pickup in the distributor, when the distributor is rotated this either advances or retards the ignition timing. The ecm has no way of determining how much initial timing is dialed into it, it is basing it's calculations on the initial factory settings.
(correct me if I'm wrong here) If you have an initial setting of 6 degrees BTDC and you're reading 29 degrees (that's 29 degrees of advance being initiated by the esc) on your X-ray that would be a total advance of 35 degrees.
One sure way to check the advance is with a good old fashioned timing light, preferably one that has the advance dial built into it.

 

John, the only time that is true is in an ignition system that is triggered by a crank sensor (like in an '83 -'85 lincoln w/302, the distributor tower is nothing but a rotor and the cap sitting on top). The ignition on a sbc is triggered by a pickup in the distributor, when the distributor is rotated this either advances or retards the ignition timing. The ecm has no way of determining how much initial timing is dialed into it, it is basing it's calculations on the initial factory settings.

Please allow me to clarify. Everything above is true. I was lazy and assumed since he asked a question about rotating the distributor that I need not restsate the above. I am sorry for any confusion.

The comment about the spark gap is in addition to also either advancing or retarding the timing. This is why you would be changing two things at once.

(correct me if I'm wrong here) If you have an initial setting of 6 degrees BTDC and you're reading 29 degrees (that's

The total shown should be the actual advance from the dist plus the intial on the PROM. 29 should be 29 and not 35

One sure way to check the advance is with a good old fashioned timing light, preferably one that has the advance

This is highly recommended.

John

 

All that is in the ECM is 23.7*. I have checked the timing with a dial back timing light. The problem is that if the dial is on 0 and the Set Timing Connector disconnected it reads 6*. If I roll the dial forword to 6 (to read 0* on the timing tape) nothing happens. I have to roll it all the way to 29* to get the timing tab to read O*. I will double check all of this later when I have a friend to help. The other problem is that there is no load on the car when you rev it up to check the timing, therefore it reads 42* the cruise timing. The MAP kpa isn't at 90-100 as in WOT. So you get a false reading. What does a standard timing curve look like. I have the AUJP bin and the vette bin, they have no upper timing in them.

 

Quickest way to ruin an engine is just tuning to numbers.
The days of 36d timing being the perfect number are long gone.

Tune for performance, and read the plugs.

Ain't no easy answers or short cuts.

WBs, EGTs, acclerometers, all neat and help you along, but you still gotta understand what the plugs are telling ya.

 

I'm puzzled as to why you are having to dial in 29* on the timing light before you see any result.
As far as setting the timing....you start by using the engine specs to determine a desired total spark advance. Then you have to test and tune from there.

Here is a basic guide to setting a basic curve....
The rule of thumb is that the higher the compression ratio, the less total timing it can handle before detonation, and also the higher octane rating it needs to control detonation. Low octane fuels ignite faster, thus require less timing advance. Conversely high octane fuel can handle slightly more advance. Dyno testing has shown that most small blocks with 9:1 to 9.5:1 compression make peak HP with 38-42 degrees total advance. Engines with 9.5:1 - 10.5:1 run best with 35-38 degrees total, and above 11:1, should not go higher than 35 deg. total. When using power adders such as nitrous, super or turbo chargers, the timing should be advanced accordingly.

Ideally you should keep the initial between 4 and 12 degrees, and the total in the ranges listed above for your compression ratio. For performance driving, the best acceleration comes when total advance is achieved before 2500 rpm.

I know this still leaves a wide open area but it is one that only you can fill in the blanks.

 

Thanks for the advice Rebel. I have 11.2cr and run 92 octane. I will try to set the curve the way you put it and see what happens. I had to go with the compression to make up for the cams overlap. Those AFR heads are extreamly efficient also and fight detonation. I have turned initial up to 14* with no detonation but turned it down for GP. I am going to leave the initial at 6* and tune from there. I am puzzled as to why the timing light does that too. I may have been drunk when I was looking at it and became confused. I will check it again when I have time.