I've been playing with the timing on my '64 401 w/single 4 barrel hoping to replicate the factory dual quad ignition advance curve. One thing I found is that according to the factory repair manual the two mechanical advance curves are identical except for the initial advance. I did a little digging in the master parts book and found there's a "type 2" dual quad distributor for only A/T equipped cars. I assume this is the one that is desirable to recurve your distributor to. The bad thing is I cannot find this curve posted anywhere. The factory manual has only one curve for the A/T dual quad so I assume this is the "type 1" curve. I've bought an advance recurve kit to play with but went too agressive and have some pinging at WOT. The max advance of 28° is all in by 2400 rpm. I plan to try different springs to bring that down up to 3000 rpm. With max timing being 40° at 3000. (12° initial + 28°). I'd like to find the "type 2" curve to compare my results with.
Definitely not the resident expert here, but as I understand it the dual-quad cars got the same distributor with factory settings as used in the early 401 Dynaflow cars (so up to 63). Maybe this helps....
^^^ Yup, the Dynaflow distributor uses 10 deg initial advance too. It's a good unit to start with. Biggest difference from a 1x4 dist is the amount of mechanical advance. The 2x4 and Dyna has less mech advance so you can run more initial. I once welded to slot up on a 1x4 dist to make the slot shorter. Or you might be able to put an offset bushing on it to also limit mech advance. 40 degrees total is too much. 30-32 total is about right.... 10 initial + 20 mech. The 65 Chassis manual is online at 65gs.com. It might have the 2x4 curves shown in it.
A have a 401" 65 with single carb.. I have the lightest springs from a recurve kit, which is all in at 2200rpm as far as I recall and total timing is 32degree.. I have no pinging and smooth power.. Do you have opportunity to run higher octane gas?
The cylinder pressure curve from the actual cam timing, along with any mods like porting or other non-factory parts/conditions....negates and may drastically change the engine's timing curve needs (except for total timing). Combustion chamber shape has much to do with the total timing, but the need to change things based on the engine atomizing fuel differently through the induction system based on non-factory specs is something that would have me questioning using the factory timing curve as anything more than a starting point.
I am running 93 octane Shell gasoline from a busy and reputable station. I currently have 19° mechanical advance at 800 rpm and a max of 32° at 2100 rpm. Throw in the vacuum advance and I have 38° and 50° respectively. This is with 5° initial timing. The car runs OK except for WOT with pinging starting around 2200 rpm. (~50 mph cruise then WOT for passing). With 12° initial advance it is pretty much the same even though there's 7° more advance across the board. There's no performance difference that I can tell with the 5° initial plus Accel weights and springs. However, there's a huge difference when I change to 12° initial, at low rpm anyway. Yes, the 2x4 curve was to be my starting point. Engine is in good shape and has a 750 CFM Edlebrock. It is otherwise stock. It ran OK with the stock weights and springs. Looking to pick up some cheap HP. I'll check the '65 manual, thanks. I have more weights, springs, and limit bushings on the way.
Just going by your observation of noticing that 12* initial working better than 5*, I'd start my curve with 12* and limit to 32* max total...just as you are being advised for a safe max. Add your vacuum advance beyond that. Safer yet is to slow up the curve some, so you don't have your max timing at so low of an rpm. Once you are closer to that, I'd be reading new plugs for physical signs of detonation, rather than going by sound from behind the wheel. Being that you aren't noticing any performance difference by increasing your total timing, it might be safe to assume that from 32* to (39*?) ...you are not hearing the almost silent portion of destructive detonation. If audible det. is an amplitude of 10 on a 1-12 scale, near silent det. from 1-9 is still quite harmful. If you hear pinging at WOT from 2200 up, then you are hearing the sound of your engines demise. There is no more cheap hp with more total advance, only damage.
Yes, I'd start there. There's more of a load on the engine in high gear, so it's more likely to ping. A colder spark plug might help. A richer carb mixture might help. It all depends if something is 'off' from where it should be. Ping at light/ moderate throttle may be tuned out by limiting your vacuum advance, but that's not your problem. Here's a great article on reading your spark plugs: http://www.4secondsflat.com/Spark_plug_reading.html
Thanks for all the help. I've got spring and limit bushing kits on the way and also an adjustable vacuum advance limiter as well. I didn't think about reading the plugs. I've got a new set on the shelf. I will update with results ASAP.
Anytime you bring the mechanical advance in that early, you'll need to reduce the amount of vacuum advance. 8-10* maximum, but some cars need less. Your WOT should be no more than 34*. Plug the vacuum advance. Set the total timing and then make some WOT runs from low and medium speeds. If you hear any detonation at all, reduce the total timing and try again. You can't adjust initial and total timing independently unless you have a way to adjust the amount of mechanical advance. Only way to do that is to use a bushing, or weld the slot up and custom file it to your needs. The factory brought in total timing as late as 4600 RPM. That is why you could get away with 14-18* of vacuum advance. That won't work with light springs. Once you have the total advance optimized, then add the reduced vacuum advance in.
Your going to find that in short order those crappy silver colored light advance springs from advance curve kits are going to loose tension and not only not pull the timing back, but it will slap / bounce around!
I agree - I've found in both my nailhead and 455 distributors in order to keep the wieghts seated and not coming in at 700 rpm I have to use springs that bring total in around 3000 RPM...
I've got the mechanical advance limited to 18° (crank). I was able to adapt the distributor to accept a bushing from a Summit Racing distributor. Going to try springs that are about 30% stiffer than was in the Accel kit (also Summit springs). The vacuum advance limiter I ordered wouldn't work. I read somewhere that Crane makes on that works in a Delco points distributor. I'll likely end up with that and an adjustable vacuum can as well. No time to test it this weekend but maybe tonight. Once I get the mechanical dialed in I'll work on the vacuum side. Goal is 12° initial, 18° mechanical (30° total) at 3000 rpm, and max of 38 to 40° rpm TBD. BTW, the '64 factory manual shows the dual quad set-up to have a max advance of 39° at 2700 rpm, about 1000 rpm sooner than the max mechanical of 32° (initial + mechanical) is reached. Does that sound about right?
Yeah, that would be OK. The way all stock distributors worked, only part of the mechanical advance was in at typical cruise RPM. The vacuum advance added to that getting the cruise timing around 40*. Nail it, and the rest of the mechanical timing came in, and the vacuum advance went away.
Did a little (very little) testing last night and confirmed my mechanical advance limit mod worked as intended. I had a max of 30° at the crank, including the initial of 12°. The curve (Summit light) is still too aggressive as the timing before 2200 rpm is greater than with the Accel kit curve (Accel 1) that started this mess (actually it is fun as long as no hardware is harmed in the process). So, I need springs with the same tension at 3000 rpm as the ones tested but with a different rate. More testing planned for Saturday
Too close this out I finally had a chance to test my latest timing set-up and am happy with the results, although they aren't exactly what I was targeting. The final curve is in red along with the single 4bbl and dual quad curves from the 1964 factory service manual. I ended up with 12°initial advance, 18° mechanical advance, for a total of 30° at 3500 rpm (I was targeting 30° at 3000 rpm). This required the slot in the advance plate be modified and a Summit advance limit bushing be used (more on that in another post), OE single 4bbl advance weights, and Mr. Gasket heavy springs from their Delco point distributor kit. Adding in the vacuum advance there's a total max advance of 45° from 2900 to 3500 rpm (advance above 3500 rpm is interpolated from the OE curve and my measured curve to keep from having to rev my unloaded engine any higher for long periods of time). I plan to install a limiter in the future on the vacuum canister to limit max advance to 40°. I also switched from manifold to ported vacuum to eliminate having 24-30° advance off idle in part throttle conditions. This seemed to make the car more responsive Driving the car finds it has more acceleration from a dead stop and still revs out well for a nailhead. I always thought the car was a little sluggish off the bottom for a car with 445 ft-lbs of advertised TQ. There's no detonation at any point. I am likely leaving a little on the table but need to wrap it up before winter. Next spring I'll look into adding the dual quad advance weights (they are repopped by Standard Motor Products) and the vacuum advance limiter.
