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Old 05-03-2013, 02:51 PM
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Initial Ignition Timing Newbie

I have an XE274H cam in a SBC350 with Vortec heads and 9.9:1 compression ratio. I called Comp Cams and they said to aim for about 14-16║ initial ignition timing. Agree? Disagree? I plan on starting this (my first) motor for the first time tomorrow for cam break in. I am excited and nervous!

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Old 05-03-2013, 03:43 PM
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Originally Posted by Silver Surfer View Post
I have an XE274H cam in a SBC350 with Vortec heads and 9.9:1 compression ratio. I called Comp Cams and they said to aim for about 14-16║ initial ignition timing. Agree? Disagree? I plan on starting this (my first) motor for the first time tomorrow for cam break in. I am excited and nervous!
Your not going to see initial during cam break in anyways. Have a light on it and just make sure at your 2000-2500rpm 20 min break in that your timing isn't over 34 Ish. Once the can break in is done and your doing your initial set up I would start at 16 initial and see how it feels, I know vortecs need less timing then alot of heads. But I suspect with that cam your still going to find it responds best to somewhere in the 24* initial range. But your going to need to limit your total to an 8-10 degree curve for that to work. No more then 32-34 total, and in by 3000rpm. Use the search function for limiting total advance if needed.
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Old 05-03-2013, 04:01 PM
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Thanks bygddy! How did you obtain these numbers? Are they just something you know after tinkering with engines or are there some rules of thumb out there I need to know about?
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Old 05-03-2013, 05:21 PM
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Thanks bygddy! How did you obtain these numbers? Are they just something you know after tinkering with engines or are there some rules of thumb out there I need to know about?
Trial an error, and spending way too much time on here lol. I ran an XE284 in a 350 with alum heads, rpm intake, 750dp and messed with timing a bunch as it had an Msd e-curve dist so it was real easy to do. Just little dip switches to change timing, rate, curve etc. It wound up best with it actually locked at 36*. In your case depending on Carb choice I would likely do the same but you would likely need a start retard or at least an interupt switch wired into the dist. So for simplicity a short curve and 24* initial should allow it to start when hot assuming a good battery and starter.
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Old 05-03-2013, 06:46 PM
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A short curve with 24 initial will run great. That engine should love timing. Will it start hot is another issue. 9.9 compression 24 degree will not want to crank when hot.

Initial timing help keep the combustion happening at the right time. The front needs enough lead to keep ahead of the piston in regard to rpm.

Getting the front to light at the correct lower time will make more acceleration. If you are accelerating through 1500-5500rpm. After 3-4000 on up, locked seemed best.

Here is a old small engine pic depicting the concept.
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Old 05-03-2013, 08:53 PM
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A short curve with 24 initial will run great. That engine should love timing. Will it start hot is another issue. 9.9 compression 24 degree will not want to crank when hot.

Initial timing help keep the combustion happening at the right time. The front needs enough lead to keep ahead of the piston in regard to rpm.

Getting the front to light at the correct lower time will make more acceleration. If you are accelerating through 1500-5500rpm. After 3-4000 on up, locked seemed best.

Here is a old small engine pic depicting the concept.
Normally your right, but currently I run a stock, big GM starter, its brand new, but stone stock, with a brand new battery, good clean grounds etc, and I'm at 24 initial and it fires up like it doesn't know the difference lol. Just lucky I guess.
That being said, spinn is right, and you will likely run into hot start issues, but its an easy fix, search "ignition interrupt" on here and there's tons of info, and its really as simple as running a momentary switch off the power wire to your HEI. Roll the motor over, and while doing so flip the switch and it will fire.
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Old 05-03-2013, 11:41 PM
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Great info guys thanks! Also fwiw, I am using a 750 Q-jet. Just curious why you brought up the carb? How does that impact timing?
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Old 05-04-2013, 06:04 AM
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Great info guys thanks! Also fwiw, I am using a 750 Q-jet. Just curious why you brought up the carb? How does that impact timing?
A "big" Carb, or what alot of people will say too much Carb or wrong application, will want alot of advance to idle clean. In my case I use a 750DP, if I run say 16* initial, with no vacuum advance, without major tuning, it runs pig rich on the idle circuits, and has very little effect using the mixture screws. But with 24* I can adjust idle using a vacuum guage, it responds well, and is clean at idle. When you hear people complaining about Carb issues at idle being rich, 9 times out of 10 its a timing issue. I know very little about Qjets nut have always been told they don't work well with big duration cam's. Your 230/236 @.050 is going to require major tunimg to perform well. I would personally just default to a Holley 750 because I know they are easy to tune. But do your research if you want to stick with the Qjet.
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Old 05-04-2013, 06:19 AM
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Silver Surfer,
Others may disagree, but I don't think initial firing/cam break-in period is the time to be experimenting with max timing for performance. The cam isn't that big, and I would stay closer to Comps recommendation of 14-16(maybe even less like 12*) to lessen the risk of over advanced timing during break-in. Like someone said earlier, its easier and safer to just limit the total when its running at 2000-2500rpm and let the initial fall to where it may until the cam is broke in. Disconnect the vacuum advance until you verify the mechanical advance is around 24-26 degrees @2k, then re-connect the vacuum advance. also if your running an MSD ignition, don't use a dial back timing light as the strobes will be all over the place, and you'll probably wipe the cam before you get the timing figured out.
Wait until cam break-in to verify max total mechanical advance is about 34* at 3k, then see where the initial is at after you get the carb idle mixture/speed adjusted for most vacuum. Then drive it easy while listening for detonation.
Personally I would even wait until after I had 500 miles on it so that the rings would have a better chance of sealing before I tried to get the most advanced ignition curve possible for performance, including initial timing, and carb adjustment. Less chance of oil getting in the chamber that can cause problems.
Of course opinions will vary, and that's just mine. I'm no pro. Good luck!
FWIW
ssmonty

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Old 05-04-2013, 06:38 AM
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Originally Posted by ssmonty View Post
Silver Surfer,
Others may disagree, but I don't think initial firing/cam break-in period is the time to be experimenting with max timing for performance. The cam isn't that big, and I would stay closer to Comps recommendation of 14-16(maybe even less like 12*) to lessen the risk of over advanced timing during break-in. Like someone said earlier, its easier and safer to just limit the total when its running at 2000-2500rpm and let the initial fall to where it may until the cam is broke in. Disconnect the vacuum advance until you verify the mechanical advance is around 24-26 degrees @2k, then re-connect the vacuum advance.
Wait until cam break-in to verify max total mechanical advance is about 34* at 3k, then see where the initial is at after you get the carb idle mixture/speed adjusted for most vacuum. Then drive it easy while listening for detonation.
Personally I would even wait until after I had 500 miles on it so that the rings would have a better chance of sealing before I tried to get the most advanced ignition curve possible for performance, including initial timing, and carb adjustment. Less chance of oil getting in the chamber that can cause problems.
Of course opinions will vary, and that's just mine. I'm no pro. Good luck!
FWIW
ssmonty
Best advice in this thread so far. Put enough timing in to make sure the engine starts and will run for the twenty minutes needed to properly break the cam in. Break the cam in, then worry about fine tuning the advance curve.
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Old 05-04-2013, 06:47 AM
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When you first fire the engine for the break in you will not really have the change to adjust timing. But keep the distributor loose enough too move by hand but tight enough to stay put.

20 minutes of run time at 2000 to 2500 rpm seems like a week. If the headers/manifolds start to turn red then add a little more advance.

You may get to check timing as it is running but the timing should be in the high 20's to low 30 range at 2500 rpms and you will need a timing light with an adjustment knob on the back or timing tape on the balancer to see that.

Make sure to use an oil additive for the break-in! Comp xtreme cams are extremely hard on the lifters and cam lobes. And vary the rpms between 2000 and 2500, don't just stay at one rpm. Also add an extra quart of oil to help splash more oil onto the cam.

good luck.
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