As a fellow vintage MUSTANG-ER, I wanted to share this conversation I had with a professional engine builder as I thought is was helpful for me and maybe would be for any of you out there. Feel free to chime in with your own ideas/comments or just take in the knowledge. Cheers! First my set-up description and my question following:
1968 Mustang Fastback
SBF 302 (1968 cast) Pistons:
KB Performance Series Cam:
Comp XE-268-H - 1600-5800 XE 224/230 Hyd Flat
High energy Roller Rocker kit to accompany Comp Cam. Manf:
Edel Perfomer RPM Cast heads:
professionally ported and polished Dist:
Pertronix D7130700 billet, has vacuum advance 14* standard I THINK?.. not sure if adjustable, never looked. Carb:
QuickFuel 450 cfm HotRod series 4bbl Dbl-P Idles:
850 rpm regardless of timing.. slightly rough idle very lopey characteristic of that cam. Vacuum:
10Hg 1-1/2 turns out on 4x corners: 13.5 - 14.0 AFR | Cruise: 14-16 AFR | WOT: 11.5-13.5 AFR
Premuium 91-octane Trans:
4-Speed wide ratio, with 3.40 posi rear 245-50R-16s QUESTION:
Timing: I've tried everything... but I'm not sure what is best for this set up... ???
What I've tried below: A.
initial 12 /mech 24 (rough idle, snappy throttle, but weaker low end until past 3K B.
12 / 20 / 14 (snappy throttle, rough idle) accel okay. (so far I think like best but haven't tried a couple more combos
15 / 20 / 14 (smoothest idle of all, best manifold vacuum, bland accel)
Putting initial up to 15* smoothed idle, and had to readjust my idle down to 850 from 900. Total was 31/32 even though I used 20* mech limiters.
Not sure how I'm losing a few degrees? I used a new timing light that has dial back and RPM. I'm wondering if my pertronix limiters are not limiting correctly and giving me 18* Do I go more on initial?
Is my cam capable of more initial & still benefit with a 224/230 duration?
Maybe I'm just asking too much for my set up... I'm just trying to create best performance with out messing up my engine. (I plan on trying 18 int. / 16 mech for a total of 34* and use my silver spings to be all in at at about 2600rpm. Based on Arron's suggestion after speaking with him. I haven't had time to do this yet, but will and will leave a comment on what the results are.)
Arron from BAD-ASS-CARS
helped answer my question and give me some ideas of what he would do to improve the performance.
Here is his suggestion I wanted to share with all of you.. Every engine is different but almost all of them on the street like 36 degrees of total timing. You can't look at your vacuum advance other than part throttle driving. It ain't working at WOT anyway, nor at an idle UNLESS it's hooked up to manifold vacuum, which in your case it should NOT be. Now, because you are running KB pistons, they are hypereutectics which are still cast and are a bit "brittle". KB says to not run more than 34 degrees of total timing to avoid piston damage. I've run hundreds of sets of them over the years. Looking at your numbers, I don't see what the problem you are trying to solve is. You have a cam that is not going to, nor is designed to, idle smoothly, especially in a little 302. The more cubes, the less the cam affects the idle, so with only 302 cubes, a 224/230 cam with probably a 110 LSA is going to idle rougher than it would if that same profile cam was in a 460. I would have gone with something that had more like 214 / 224 on a 112 LSA or something for a little 302 so it could "pull" through the RPM range, but I can't say that Ii haven't ran cams with WAY more than that, BUT in quite radical engines and cars that were completely set-up to by in sync with such a set-up. You said in one of your combos below that it idles rough but pulled OK, so what else are you looking for? If you want a smoother idle, change the cam to one with less overlap and a wider LSA... it'll idle smoother. It'll also pull harder at lower RPMs because it would make more torque. The cam you have right now is never going to make any decent bottom-end torque, especially if you don't have the gearing to accommodate it, which with that set-up should be at bare minimum 3.55s and more like in the 3.73, 3.80s area. If you have taller gears, it's going to pull like a slug below about 3,500 RPM with a little 302 like that. Like trying to take off from a dead stop on a10 speed when it's in 5th gear. I don't care if you're Hulk Hogan, you are not going to be doing any jack rabbit starts until or unless it's in 1st or 2nd gear first. 302s are not torque makers with their little 3" stroke, and even when you do have some good torque, you still need the gearing to go with it or you end up like Hulk Hogan on that 10 speed in the wrong gear. A 10 year old girl on a bike in 1st gear would wipe the butt with the road from a dead stop up until she ran out of leg RPM. Firstly, I'd stop using your vacuum advance. They were designed for stock engines to increase part throttle economy when cruising down the highway, not for performance use. Again, they don't work at WOT, nor at an idle anyway, which looking at what you've said, seems to be your concern. You can run as much initial as you want as long as you don't exceed 34 degrees total, (KB's recommended total), BUT once you get up to the 28 degrees and-on area of initial, your engine will surge when running down the road at part throttle until you step on the gas where it will smooth out again. Your cam has zero to do with ignition timing or how much it can handle. 34 degrees is 34 degrees regardless of what the cam profile is. I prefer to see about 20 - 24 on the crank and maybe 10 - 14 of mechanical in the distributor. That ensures you get the most out of your initial timing for a snappy engine and high idle vacuum signal, yet still enough of mechanical to allow it to "idle down" and not "surge" when cruising at part throttle down the highway. I have a great article on timing at this link
https://www.badasscars.com/index.cfm...prod/prd76.htm and a couple of videos you should watch at these links
https://www.badasscars.com/index.cfm...rod/prd447.htm https://www.badasscars.com/index.cfm...rod/prd451.htm Some of the info on re-curving a distributor and on how a vacuum advance works you will already know BUT there will be some additional info in there that you probably didn't know, and that is the info you could probably use. Just don't exceed 36 degrees of total timing (excluding your vacuum advance) and you won't hurt your engine. All you'll do is change how it idles a bit, it's throttle response, it's idle vacuum signal, and how hard it'll pull below about 3,500 RPM. None of those will "hurt" or cause any damage to your engine. Too much timing will (detonation), and too little timing can (too much heat) if you don't pay attention to it. I hope some of that helps. Thanks!! - Arron