advanced cam effect total timing

[fryedaddy]

i have my cam advanced 4 degrees,does that effect where i set my total timing?

[My427stang]

Not really IMHO, but the dyno guys may have different experiences with A-B testing

Timing requirements for an engine are really based on how the entire chamber acts does during combustion, moving the cam doesn't change that physical characteristic

That being said, I'd say it likely changes the rate of advance the engine wants, especially at lower rpm  The less cylinder fill you have the more it wants advance, so if you advance the cam, it's generally filling better part throttle so it doesn't need to be as quick of a curve, but we are talking in such generalities at this point and such minor differences I wouldn't try to give advice based on X amount of cam advance

[WerbyFord]

Here is an anecdotal "yes":
I once ran Ford's "X" cam in my 2-liter (that's 122cid in volume units) Pinto. Great from 4000-7500, it would stall if you floored it below 3000.
So just for fun I advanced the timing belt a WHOLE COG. That's like advancing the cam something like 20-30 degrees.
I re-set the ignition timing to where it had been before and took the car out.

Instant torque monster, compared to before, but it also spark knocked - which it never did normally.
Obvious looking back - the intake valve was closing a whole bunch sooner so the TCR (Trapped Compression Ratio) went way up.
So my conclusion there is, if you advance the cam, the engine will REQUIRE you to cut back the total advance, due to octane limits.
That is not the same as comparing power output but it's one hint.

More recently, I tried the 245-245-114 FE 427-8v cam, 4 degrees retarded, 118 ICL 110 ECL, and then 4 degrees advanced, 110 ICL 118 ECL.
I do remember it liked a lot more timing at idle retarded, just to get a decent idle.
And the pair of timing runs I made were with 37 total adv for the retarded cam, and 34 total adv for the advanced cam, which was over 0.1 sec faster in the 1/8 mile. But I may have just bumped up the whole curve to 37 to get a better idle out of the retarded cam. It doesn't mean that was the best power setting. Again that doesn't prove anything, but if I had to bet, I'd say an advanced cam will like "a little less" total spark timing. No conclusive data.

[My427stang]

Werby, I think you were seeing differences in the curve and initial, not total.  Of course as we both said, inconclusive but our experience.

The only way to prove it would be a dyno run at WOT, with a/f mixture corrected for each after the cam timing change, and probably a locked distributor or at least only sample above full centrifugal and see where the engine liked total for peak power

No doubt part throttle differences are significant, and effects of overlap could be significant too with different timing compared to intake and exhaust resonance, trying to wrap my head around how much though once you get upstairs.

BTW many dislike DCR as a term, but I refuse to use TCR LOL, the effect may not be dynamic, but it isn't trapped either, it all gets out sooner or later. Just like Taco Bell  (All tongue in cheek)

[fryedaddy]

OK, i have my int set at 15 and total at 33 and cam advanced 4 degree, which springs should i use,do i want it all in at 2000 or up to 3500 .im running about 10.7 - 11 to 1 with 4.11 gears and under 3000 pound car

[fryedaddy]

with this setup do i want all in early or late or somewhere in between.i thought about swapping out the springs in different combos and test driving it.any imput on this?

[My427stang]

Assuming you are happy with the way it starts and total timing is correct for your heads, which I think you are pretty close.  I would have it come in by 2600 rpm and then if you have vacuum advance, run it ported and limit it to 8-10 degrees more.  That's a relatively aggressive street curve, but not overly aggressive for your gear and aluminum heads

Remember though, this is only for street driving, drag racing would keep you above the "all in" rpm, so experiment.  Spring it to come in at 2600, if it pings, slow it down, if it doesn't, try 2400, see how it drives.  However, I really think 2600 is darn close to perfect with that combo.

Heads, compression, cam, gear, car weight, heat and fuel are the big ones that will affect pinging at low RPM when cruising around. 

Also, I have never had to use your extremes (2000 and 3500), generally the sweet spot is somewhere between 2500 and 3000, with the latter being a heavy vehicle

[fryedaddy]

thanks,the reason im asking is i think i might be getting a little pinging at lower speeds and i was wondering what to do about it.

[My427stang]

That should do it, best bet would be to see where it is now, and then try something different.  If its at 2400, go 2600.  Don't over correct though, slowing it down gives up a little mileage and part throttle performance