timing question - Team Camaro Tech
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post #1 of 8 (permalink) Old Jul 19th, 09, 03:09 AM Thread Starter
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Jason
 
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timing question

how many degrees advanced is a good starting point. evidently my distributor hold down was loose and the timing is off. my engine builder locally died about 6months ago, when he built the motor he dyno it in and told me not to adjust the carbs or the timing that it was perfect, that being said I asked no questions it was not the first motor he built for me and therefore it worked until now. the car was overheating as I posted in another tread. the timing was advanced probably 24deg btc.

I brought it back down to about 10deg btc and the car still runs great. I am assuming that the cam would have been advanced in the gear set and the initial setting would be pretty close to stock within reason.

any body using the comp cam 12-250-3
284/296 @ .5 in

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post #2 of 8 (permalink) Old Jul 19th, 09, 07:52 AM
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Re: timing question

I would set the total at 3000 to 36 degrees(no vacuum advance) and see where your at with the initial, then hook up the vacuum advance. Go for a test drive to check for pinging under full throttle. If it pings take a couple degrees out and retest
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post #3 of 8 (permalink) Old Jul 19th, 09, 12:16 PM Thread Starter
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Re: timing question

i am there now, 10deg initial 36deg total
car runs good. other than time trials how do you know if that is the best setting

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post #4 of 8 (permalink) Old Jul 19th, 09, 02:23 PM
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Neil
 
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Re: timing question

I'm no expert and can only speak from my personal experience. What motor are you running,cr, cam,heads and etc? Seems most performance cams like around 16-18 initial and 34-36 total. My 402bb is set at 18 initial 36 total plus another 14 vacuum advance. In my case I had slow the rate of advance down with a spring kit. I just kept experimenting until I had it where it ran the best. Every motor is different though.
A chassis dyno would be best if the motor is in the car. Otherwise just enjoy the testing
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post #5 of 8 (permalink) Old Jul 20th, 09, 12:07 AM
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Ron
 
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Re: timing question

Unless you have a stock motor, no 2 motors are a like when it comes to initial timing as there are just too many variables.

To find the initial timing on any motor: 1/ Ensure the Carb's primary Plates are located on the Idle Circuit and the Secondary Plates Set per Carb Mfgr's Specs and idle Air Screws set 1 Turn Off Seat. 2/ Disconnect and Plug all Vacuum Sources - Distributer Vacuum Pod, Auto-Tranny SX, Power Brakes ... etc - Try and make the PCV Functional but not critical. Start the Engine by turning the Distributer. Find the position where the engine Starts, Idles smoothly between 800 to 1000rpm and responds quickly to the throttle with minimal adjustment to carb settings. Usually the Idle air screws will be at 3/4 Turn off Seat and the Primary Throttle Plate NEVER off the Idle Circuit. This is the Initial timing for that Engine - Put a Timing light and Vacuum Gauge on the engine and read the results. This is the Initial Timing and Vacuum Idle for that particular Engine - Write it down for future reference - say it's 18 degrees with a 10Hg Vacuum - BBC example.

Rev the engine up and see where the Mechanical Advance stops say it stops at 36 degrees by 3200 rpm - meaning you have 36-18 = 16 degrees of mechanical advance all in by 3200rpm. May be you want it all in by 3000 so you will have to tailor the mechanical advance or retard the timing and adjust to carb accordingly but never take it off the Idle Circuit at Idle and the Idle Air Screw must respond never exceeding 1/2 turn or over 11/2 turns off seat - If this don't work - Something is out to Lunch ...eg: Dirt, Vacuum Leak, Float Levels, Fuel Pressure, Jetting, Points, Plugs - whatever. The Idle Air Screws of a 4 Corner Carb might like 5/8 Off Seat on the Primary and 1/2 on the Secondaries.

Connect the Distributors Vacuum Pod (a Manifold source not a Ported Source). If you notice any fluctuation in Idle or say a significant increase in speed (Advance in timing) you may have the wrong Hg Pod which is over-riding the initial mechanical timing too early such that the pod is floating on the idle vacuum, 10Kg in this example, while the throttle is on the idle circuit. The vacuum advance should never enable and override the initial mechanical advance while the engine is idling and the Throttle is on the Idle Circuit. With a 10Kg Vacuum at idle you would want something like a 7.5 or 8 Kg Pod. Once the throttle opens the vacuum drops to say 6 Hg and the Vacuum advance cuts in by then your into the Secondary metering circuit. Note that the Carburetors Power Valve responds to the Engine Vacuum in a very similar manner. Here's where you may want a 6.5 Hg Power Valve.

A mild SBC, and here again depending on Compression, CAM, Octane and other variables .. eg: tranny gear ratio, may like somewhere around 13 degrees at idle + 16 degrees mechanical advance = 29 degrees all in by 3200 or perhaps 2600rpm - every engine and drive combination is different.

Suggest you do some Googling into "Reading Spark Plugs".

Hope this Helps and as Neil says "just enjoy the testing" believe me your always looking to make your ride Faster and Behave and that's what Rodding and Hi-Perf is all about ;o)

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Last edited by Z15CAM; Jul 20th, 09 at 01:37 AM.
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post #6 of 8 (permalink) Old Jul 20th, 09, 04:55 AM
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Steps
 
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Re: timing question

Quote:
how do you know if that is the best setting
Without a dyno it is track time, or a measured distance up a steep hill.
Start about 38 total, no VA about 3600 rpms
then retartd till you see a drop off in time...then up 2 or 3 degs.
Over advance causes detonation , even if u cant hear it, increased NOx
There is a piont when retarting the performance drops off within a few degrees.
Advancing above 'premium' total the performance drops off very slow, it also makes the engine sound a if it is producing more power, so dont be sucked in by that

So lets say premuim is 34 degs dropping to 32 or 30 you will see signifiacant drop in times....but if u increase to 36 or even 38, the engine sounds grunty, and stuff all loss in power, increase in NOx , and possibly getting close to detonation.

Getting the curve right
Now drop in heavier springs, to drop the rpms back, till again see a drop off, then add about 200 to 500 rpms]

My Spelling is not incorrect...it is creative

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post #7 of 8 (permalink) Old Jul 20th, 09, 01:49 PM
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Re: timing question

Steps: Excellent method for timing from WOT and about as good as it gets for the strip but not necessarily practicable for a daily driver running various octanes. One item that is handy is a Timing Retard Module. I mounted one in my Glove Box. With one one of these you can set the Initial Timing and Dial in a Retard or Advance on the Fly or simply shut it off and the Timing will return to it's original setting. You set the Initial Timing with the module wired in series with the Distributer and the Dial set at say 7.5 Degrees, within the 15 Degree Sweep, and end up with a 7.5 degree control of Retard or Advance on either side of the Initial Timing without leaving the Drivers Seat.



It works excellent along with my HyFire VI Module and can't see why it would not work with most Ignition SetUps.

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Last edited by Z15CAM; Jul 20th, 09 at 02:55 PM.
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post #8 of 8 (permalink) Old Jul 20th, 09, 03:43 PM
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Re: timing question

Quote:
timing from WOT and about as good as it gets for the strip but not necessarily practicable for a daily driver running various octanes.
Daily /variable octanes???
change the fuel octane changes the timing, same as changing compression and/or cam which changes the dynamic CR.
For daily, once you set the idle timing...again measuring AF and NOx to get it right, you have X amount in the dizzy, and spring that come off about 150 to 200 above idle you will be so close its not funny...then it is a matter of getting the VA right..which is explained in several older posts.

My Spelling is not incorrect...it is creative

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