Ford timing issue
My buddy has a 67 mustang with a rebuilt 302 in it. Not sure of the exact engine specs but he is pinging at light throttle while cruising and at WOT. I asked him to plot his advance curve, this is what he sent me:
Initial 8° (idle 750 rpm) - without vac adv
1000 RPM 8°
1250 RPM 8°
1500 RPM 8°
1750 RPM 10°
2000 RPM 13°
2250 RPM 15°
2500 RPM 17°
2750 RPM 18°
3000 RPM 19°
3250 RPM 20°
3500 RPM 22°
It seems his vac advance adds 14°, starts adding advance at 1200 RPM and is all in by 1500 RPM for a total timing of 36° - he has a Crane adj vac can.
I'm assuming a high-revver like a 302 wants it's timing in at a higher RPM than my 350 but I am looking for any input I can get to help my buddy out.
Here is what I know about the car/engine:
-mild 302 with factory replacement pistons, cam and heads; CR unknown
-100,000 miles on motor since rebuild
-avg 145-150psi compression per cyl
-Performer intake and 600cfm Holley vac sec, stock jetting
-C4 auto trans w stock converter
-2.76 rear gears w taller than stock rear tires
Vacuum advance is variable with manifold vacuum, more vacuum begets more advance. It can become additive to the mechanical though it shouldn't be set up like that. One would have to know which heads are on this engine and what pistons are in it. These two things make the combustion chamber shape, different shapes have large impacts on an engine's sensitivity to detonation.
Stock jetting is meaningless information, what matters is the fuel to air ratio. Extra fuel can be used to trick the engine into reacting like the octane rating of the fuel is higher than it really is. Which a Holley this can be manages with where the power valve turns on and what size power jets are in it. Or you can trade this complication for larger main jets, but that risks too much fuel being put in when it isn't needed which causes wear on the engine and you wallet.
100,000 miles on a rebuild is a lot, the screaming majority of rebuilds don't come close to factory quality and don't last as long. This is especially true of the timing set, which typically aren't that great to begin with regardless of whose they are. As the cam gets out of synch with the cam and the distributor is constantly advanced to unknowingly readjust the ignition timing, the engine becomes increasingly detonation sensitive.Toss in a little oil from the rings no longer up to par and a bit down the wearing out valve guides and you've got detonation trouble in River City.
Short term is the engine probably needs some freshening up or greater compromises in ignition timing.
Thanks for the thorough reply Bogie, no doubt the engine is soon due for attention as it is a bit long in the tooth for the generic rebuild that it was.
I agree that some key variables to truly tune the ignition are missing as you mention, but I am not experienced enough with different engines to know what they characteristically want/need for optimal ignition, my experience has been myopically focused on the 350ci chevy small block. Time to branch out for me.
I agree that the vacuum can appears very aggressive bringing it all in so soon, but the mechanical curve looks off as well, not advancing till over 1700rpm and only a total of 14 degrees total, seems progressive enough but a bit limited.
I realize saying he has a 600cfm carb with stock jetting is somewhat meaningless, but he doesn't have the ability to measure the A/F ratio so we are going to have to find a way to analyze and optimize that aspect of the equation - there is no budget for the dyno.
Taking the gearing/load aspect you mentioned into consideration, along with the unknowns of the current, tiring engine, I want to help my buddy optimize his current combo while he saves up for a 351W build. I think some good attention to what the current engine wants for ignition timing and fuel will go a long way and be a good experience for both of us.
When it does come time to swap the motor everything will be re-evaluated including transmission, gearing, tires, heads, quench pad, compression ratio, exhaust, etc - we will design and build a combo that will suit the car and his intended use of it rather than make all the problems a nail and hit it with bigger hammer.
What can you suggest to get us started?
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