ignition timing vs cam profile theory - MustangForums.com


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Old 07-16-2017, 08:11 PM   #1
Gun Jam
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Default ignition timing vs cam profile theory

alright so a discussion came up at a car show about how its helps to know your cam so that you can time the engine proper.

My first thought was this guy is off his ****...cam profile seems irreverent to ignition timing to me... if anything determines ignition timing it would be heat, compression, altitude, fuel, AFRs and any type of induction. All that could effect timing thats easy to see why...He quickly ramble on to other topics while trying to help a noob understand engine theory...he seemed pretty **** sure about what he was saying which made be suspect he was off his ****....But I didnt feel like getting involved.

Here is my issue.

No matter what cam is in that engine be it an RV cam or a 8500RPM beast from hell they all have one thing in common when that spark fires the valves are closed. They've been closed for a while and they will remain closed for a good while after that spark is long gone.

So how the hell could cam profile bother with timing at all (unless it changes compression or ARFs which is really a compression and ARF issue secondary to the cam)

thoughts on this would be cool

Thanks
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Old 07-17-2017, 12:36 AM   #2
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Cam profile definitely affects optimal ignition timing.

The simplified explanation:

The goal of ignition timing is to ignite the mixture at the proper time to reach peak pressure just after TDC. Too late and you're wasting energy, too early and you're fighting the piston as it comes up on the compression stroke, as well as risking detonation.

One of the biggest factors in mixture burn rate is cylinder pressure. More pressure results in a more unstable, faster burning mixture.

Cylinder pressure is determined mostly by the static compression ratio and the cam profile - when the valves open and close. With a cam with low valve overlap, the cylinder builds pressure lower in the rpm range, so the advance curve starts low and tops out at a relatively low rpm.

With a high-winding cam with a lot of valve overlap, cylinder pressures are relatively low at lower rpms, then build up at high rpms. Since the low pressure mixture burns slower, the advance curve can start out higher and peak at a higher rpm.

Unfortunately there are more factors in burn rate than just cylinder pressure, like chamber size and shape, head and block material, fuel atomization, etc., so there's no easy way to set the timing curve perfectly without a dyno. That said, you can get it close by following rough guidelines and erring on the site of safety (slightly retarded to avoid detonation).
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Old 07-17-2017, 12:55 AM   #3
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Both the OP and the response pose a number of quandaries, I think. Both of you guys are obviously well-bred and experienced in the area being considered, far more so than I. All the nitty-gritty factors mentioned are relevant, I believe. Now, are we talking about an advance curve whose characteristics are fixed (old style carbureted distributor-determined curve with modification built in for vacuum level and eng. speed), OR, a sophisticated EFI computer-driven advance system which utilizes many other input parameters beyond the old dinosaur? Today's technology of engine management makes the OP question much more difficult to address, or, am I wrong? imp
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Old 07-17-2017, 01:25 AM   #4
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So Starfury you might agree that I am close by saying we are really just setting timing base on compression, heat and fuel mix which would be effected by the cam. I still offer resistance to the "need to know cam profile to time proper" idea really shouldn't we be asking how much Octane are you going to run and whats your compression ratio and how hot that engine runs... I was unaware that a cam could change cylinder pressure as RPM changed...or at least I never really thought much about it but I could see how knowing at least that its a high revving cam could help you guess at a starting curve. So I guess his cam profile theory isnt totally wack

This discussion also occurred right after some guy told me that a holley 4150 style carb (again "helping" the nooob) timed vac port increase vac at WOT. He said that it was independent of manifold vac and would continue to increase InHg as revs came up at WOT. I said no its the same as manifold vac but doesn't create vac until the throttle plates open a few degrees. I was thinking "**** me dead have I not learned a damn thing in the last 17 years of messing with this stuff?" Then I said well if that was the case wouldn't anyone who hooked vac advance up to timed vac port encounter engine ping because they would be running 43deg advance at full load at 5000+ RPM... He was like nawh those are race engines...I said oh and in my head I was like "**** this im out" but I was scared because if that old bastard was right everything i learned would be wrong so i went and hooked up my vac gauge to timed and got what i expected...I just wrote it off as his explanation of timed vac being relevant to some other carb that was popular in the 40s maybe

and yes all this is related to cabred 60s style engines with no computer business.
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Old 07-17-2017, 09:38 AM   #5
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Lol...how would the timed vacuum port increase vacuum past manifold vacuum? You can hook up the vacuum advance to manifold vacuum, which can help in some situations, particularly if you have a really aggressive cam. But the engine only produces so much vacuum; the carb can't provide additional vacuum. Nor, as you said, would you want full vacuum advance under full load.

Octane rating has a little bit to do with timing, but only in that it prevents detonation. Despite common belief, octane rating (which is not the same as octane content) does not affect combustion energy or burn rate. So if your timing is being limited by detonation, you need to be running higher octane gas so you can run the correct timing advance.

Also, if you have a more aggressive cam, low-rpm cylinder pressures are relatively low, so you have less risk of detonation. This means you can run lower octane at a certain compression ratio than you would with a more torquey cam.

You don't need to know the cam profile to be able to time the engine, particularly if you have a dyno, but it definitely helps to get it in the ballpark. For example, I have a fairly aggressive cam and a manual transmission (low load at low rpms), so idle advance is set at ~14* BTDC, and I set my mechanical advance curve per Comp Cams' tech recommendation based on my cam profile, compression ratio, etc., which was something like 34* all in by 3000rpm. A more mild cam would see less initial timing and possibly peak at later in the rpm range to avoid detonation under load.

Concerning modern fuel/ignition systems, things definitely get more complicated. Chambers have been optimized for rapid and symmetric burns, cam timing can be altered by the computer, direct injection allows for fuel injection just prior to ignition, the computer can read exact manifold pressure, etc. The engine computer takes all of these things into account when adjusting the timing, and it will continuously adjust, as it's not confined to a single advance curve.
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Old 07-17-2017, 09:53 AM   #6
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He suggested that timed vac would increase with engine RPM independent of manifold vac because at WOT and high RPM air velocity running through the ventures would create its own vac and thats where the timed vac gets its signal.

The above statement is actually mostly true which caused me to second guess. The issue is there would have to be a system inside the ventures that would create or boost this signal to the timed port...very much like a booster venture can create vac with zero manifold vac....but that system does not exist because thats not how we want timed vac to work
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Old 07-17-2017, 01:59 PM   #7
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http://www.hotrod.com/articles/ccrp-...amshaft-power/

http://www.hotrod.com/articles/cam-l...ne-angle-tech/

http://www.compcams.com/Pages/413/ca...ion-angle.aspx

In a nutshell, yes, cam profile affects the sweet spot of your timing. Lift and duration all affects the intake, compression, power stroke and exhaust/scavenging stroke characteristics which means to take advantage of the cam design, the fuel must be ignited at the proper time. This may be sooner or later than what an OEM cam profile might need.
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