Timing questions

Discussion in 'Technical' started by Tumbler, Feb 7, 2019.

  1. Tumbler

    Tumbler Member

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    I want to get my engine timing dialed in. I discovered when I was having a fiasco with the ignition system, that if I disconnected the vacuum advance I got a dramatic boost in performance. I knew something was off, because I order to get maximum acceleration I had to give about 2/3rds throttle. If I gave it anymore, it would go slower. I checked the timing with it connected, and as soon as I nudged the throttle, it would jump to 40 degrees. I replaced the vacuum advance with an adjustable one and it no longer does this.

    Right now I get maximum acceleration when the throttle is about an inch from the floor. The initial advance is set to 12 and the total advance goes out to 48 degrees. I'm guessing this might be too much. The distributor is currently using the 18L reluctor arm. I was wondering what the total advance should be set to? I've only been able to find total timing suggestions for compression ratios from 9:1 to 11:1. The compression tester is giving me a reading of 120psi, so it should be around an 8:1 compression ratio if I got that right.

    I don't have any detailed specs on this engine. My dad bought a Ford truck 302 short block from an engine builder. The things I do know is it has a high torque flat tappet cam of some sort. The intake manifold is from a 83 Mustang, and the heads are from a 86 Mustang. It also has a Holley 570 Street Avenger carburetor.
     
  2. baddad457

    baddad457 Member

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    If those are truly 86 Mustang heads, then you're running WAY too much advance there. Look at the lower front corner of the passenger side head next to the valve cover, you should see a large "S" cast there if they are 86 Stang heads. These are the fast burn heads and do not need the total timing other SBF heads require. Total timing for SBF is often listed as 36*, so I would start there and work my way down till you get it dialed in.
     
  3. Krazy Comet

    Krazy Comet Tom

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    The '86 E6 fast burn heads use the ¾" reach spark plugs, all other OEM heads are ½".

    When comparing head performance, those don't even make first rung on ladder. They are not horrible for street but can't breathe past 5300 or so.

    Best performance is going to be with around 15-18* advance, and 10* in dist with very light spring(s). All timing advance should be in by 3000 RPM at latest, plug the vac adv and forget it's there. This is the abbreviated version, Mr Roberts will fill in the details.
     
  4. RMiller

    RMiller My name is Rick

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    Sounds like your vacuum advance is hooked to a manifold port instead of ported vacuum. If you pull off the hose do you have full vacuum at idle from the source? Of course if that was the case your idle would go to hell with it removed.
     
  5. groberts101

    groberts101 Member Supporting Member

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    Right. But I think he may be confused as to the advance pots ability to make any difference during abrupt heavy or WOT no matter what source it's hooked up to. Both need manifold vacuum above about 7-8 inches to even operate the vac pot diaphragm.. one port supplements at idle and cruise speeds while the other is "timed" to only add vac assist when the throttle is opened a little further during light/highway cruise.

    If you're having to drive the car at partial throttle for improved response and quicker acceleration then you potentially have too light a secondary spring matched to your current spark curve. One affects the other so you'll need to calibrate back and forth to really dial it all in. It's been said by quite a few old timers that 90% of all carb problems are timing related.

    And you guys are so conservative on timing numbers! Unless of course he has stock cam with 3.00 rear gears which puts you both spot on. lol

    Always keep in mind the part throttle spark requirements are entirely different than the WOT spark curve requirements. Slower piston speeds(weaker induction speed) and far less densely packed combustion chamber A/F volumes(NOT to be confused with A/F ratio) mandates more spark lead to light the mixture off and hit peak cylinder pressures at the optimum time. Vac advance works great for that because it is on "on demand" type device that reads engine load by way of existing manifold vacuum and only allows assist to be added when the engine(fuel octane) will actually tolerate it.

    A timing light and vacuum gauge is required to dial this stuff in or your just stabbing in the dark as to what change is doing what. Engines always run damned good when they're leaning out... right before they take out a piston crown or ring land.
     
  6. baddad457

    baddad457 Member

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    I'm not being conservative here. I'm going by experience with these heads. I've run them with one build they don't need that much total timing. And while they don't make the HP numbers, they do make more torque in the lower rpms which translated to quicker ET's in the 86 Stangs vs the later models with E7 heads.
     
  7. baddad457

    baddad457 Member

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    They use the same plugs the GT40 heads used. (not the P heads)
     
  8. Krazy Comet

    Krazy Comet Tom

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    Yeah I forgot about those and I owned a '93 Lightning.

    I bought it from a guy I know & he'd just repl plugs and wires, were still installed when I sold it.
     
  9. groberts101

    groberts101 Member Supporting Member

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    In hopes that the OP might learn something useful for his situation.. generally speaking more timing increases torque everywhere at and below the engines torque peak.. while usually extending it slightly past(bit wider peak). Less timing makes more peak horsepower which comes at the cost of less torque at and below peak.. resulting in less average horsepower from idle to redline. In essence, less timing will make the pedal/motor feel softer because it reduces combustion pressures.

    I've had distributors out for quick recurves in garages, driveways, backroads, and even parking lots full of fellow "experts" watching over my shoulder. If they give me a regular timing curve to work with?.. I pretty much guarantee my time will be worth the cash spent. In the end I care far far less about hitting a particular total timing number than about making additional idle and cruise engine vacuum. That's what really points to how much fuel is being burned, resulting in higher cylinder pressure(what we calculate as torque and horsepower) and manifold vacuum, which all by its lonesome, denotes true combustion efficiency. Static tuning is easy but then the vehicle needs to be driven to lug the motor enough to see if the fuel octane can still handle those now higher cylinder pressures caused by the added spark lead. Unless the carb's air bleeds are worked it's usually a point of diminishing returns and most settle on a happy medium of "good enough for now and better than it was".

    Heavier and/or underpowered parts combos need to be able to push the gears to effectively avoid detonation. So, in the end.. no blanket statements can be made about "optimum timing" without taking all variables into consideration. Despite two engine combo's having same E6 heads, the total advance numbers can easily be 8-10 degrees different between a meager 200 horse 302 in an auto/3.00 rear geared combo.. vs.. a warmed over 300 horse 302 in an manual/3.73 geared deal. If you have those combo's even remotely close to one another for base, centrifugal, and total numbers?.. you ARE leaving power on the table.

    Other little tidbits to maybe help some folks formulate better understandings. Lower compression ratios(this one is miserably low so it fits) equate to weaker induction pulses and less densely packed mixtures.. typically requiring MORE spark lead. More open style combustion chambers having less/smaller squish pads on either sides of the chamber(which being far far away from state of the art, this head has) have lazier combustion speeds.. inevitably requiring more ignition advance to keep the combustion pressure spikes where they're most beneficial for the go-pedal and rear tires.

    So again, an engines timing requirements should ALWAYS be based on the engines ability to rev under load. Sure GOOD combustion chamber designs(which you'll never truly achieve with a 20 degree chamber designs layed back long side wall) can and does impact tuning latitude(shorter length peakier pressure spikes located ATDC also reduce losses/improve efficiency).. but nowhere even remotely near the impact that pressure over time has on heat saturation and ultimately reduces the combustion spaces octane tolerance.

    IOW, taller gears in a heavier ride(except bigger CID) won't allow an overly aggressive ignition advance. This is because the longer period WOT cylinder pressure spikes(slower revving/accelerating @WOT) pushes the combustion chambers heat saturation points well beyond the fuels octane tolerance.. eventually pinging or detonating. Anyone who's ever witnessed the differences between a 600 rpm/sec dyno pull and 300 rpm/sec can attest to the motor not being as octane or "tune tolerant". Those detonation shockwaves can be so bad(even higher than the normal WOT pressure spikes and combustion temps) that they can quickly crack cast pistons or close up the top ring land which prevents ring rotation and kills ring seal. Due to that inherent brittleness, detonating or "rattling" an unwarmed/cold cast piston'd motor is a major no-no.

    Just the opposite effect for your car if it had a 5 speed with 4.11's out back. Even with stock powered motor, gear/torque multiplication would be so high the motor would rev very quickly and not allow the WOT pressure spikes to linger long enough to heat saturate the chamber and cause detonation. Cooler combustion space and cooler fuel charge allows more spark advance to push peak cylinder pressures even higher without detonation creeping in too quickly. Of course there are limits and need to be aware of high speed detonation and how extremely quickly it can kill the shortblock. Everything's usually a compromise for street cars and it's entirely combination dependent.


    Seems odd to try and effectively help someone who's not been very forthcoming with engine/gear data. What trans/rear gear is in the car? What camshaft?


    No offense intended here in the slightest.. maybe OP might consider an 8-10 hour online crash course in ignition advance and how they affect combustion dynamics combined with properly tuning Holley fuel curves. Bring the fuel/spark base knowledge up a little to better understand and communicate what's going on when tuning. I have several good links if you want to really wrap your head around the combustion dynamics involved here. Tough to ever know "too much" about any of this kinda stuff.

    PS. since the vac pot's not operating at higher throttle angles it almost sounds like your "more power at part throttle" scenario is possibly pointing towards vac sec tuning(needs stiffer secondary spring so the engine quits laying over). There's also an old saying amongst engine tuners. 90% of all carb problems are ignition related. Can't dial in one without the other and it's definitely a game of back and forth type of scenario.

    Whew.. that one was building up through the day today! Hopefully it helps anyone who can or actually cared to make it all the way to the end. :p
     
  10. lcrabtree

    lcrabtree Member

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    Lots of really good info there.
     
  11. Tumbler

    Tumbler Member

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    Wow, thanks for all the info. Right now I'm still waiting on the advance springs and a vacuum gauge I ordered to arrive. I've spent the last few days tracking down the loose connections in the ignition wiring. I've been having the worst time with these electrical crimp connectors.

    The heads do have an S cast on them. The transmission is a C4 with a 3.00 rear gear. The details of the cam are unknown other than it's some sort of high torque cam. Right now the carburetor is running the second from the heaviest secondary spring (the brown one). When I disconnect the vacuum advance I don't see any effects on the idle.

    I figure I'll start by switching the reluctor arm to the 10L one, and I'll try the lighter springs.
     
  12. groberts101

    groberts101 Member Supporting Member

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    Sounds like a good start for the tuning process. You may already understand this but the c4/3.00 gear is going to kill some fattening of base timing and advance curve under 2,500 rpm or so. Just remember that the quantity of fuel actually being burned below the engine torque peak is directly related to ignition advance. So as you pull timing back the carb can be leaned out a little more. Basically, more spark advance prefers more fuel.. pumpshot and all.

    Here's how I would likely end up calibrating that motors spark curve without mod's to the advance slot.

    12-16 base timing(this is a compromise setting and will be dependent on peak power drop off relating to total timing number).. 20 degrees mechanical(10L reluctor) all in by about 3,000-3,200 rpm.. vac pot running off FULL TIME(not ported) manifold source and adjusted tighter(5/32? allen wrench inserted inside vac pots nipple) to give an additional 10-12 degrees of MAX assist. Under abrupt moderate to full throttle angles this will equate to 12-16 base timing climbing upwards of 32-36(depending on where base timing start things off) all in at around 3,000-3,200 rpm to redline(likely around 5,400 rpm in this case).

    Lighter throttle angles will allow the vac pots assist to function and add the extra 10-12 degrees to equal somewhere around 22-28 base timing.. and upwards of 42-46 degrees of advance. Most think that sounds like far too much advance but it helps to understand a closed throttle engine under light load has substantially different combustion characteristics than WOT would impose.

    If it were mine I'd be quick to adjust/shorten the advance slot length and end up with 22-24 base.. 10-12 mechanical all in by about 3,000-3,200, and the same vac pot settings as above(tightened up to provide another 10-12 degrees assist). Despite the numbers being similar, the reason this calibration is generally preferred over the previous is this. Base timing never changes when you stab the throttle from a dead standstill and the more you have available before relying on the mechanical to contribute will help prevent the motor from laboring to rpm from an idled standstill. Why start out from the bottom of the hole to be able to quickly climb out when you can start higher and end up on top quicker? Most beneficial in the idle to 1,200-1,500 rpm range during a dead stomp. The motor will feel slightly bigger more responsive and also allow more fuel to be used(helps burn a bigger pumpshot too) to ultimately make more power and accelerate more quickly into the cams sweet spot. Makes the car funner to drive.

    I take it back.. if it were mine I'd get rid of the mechanical and vac advance altogether. Also need a MAP sensor to mimic the effects of a vac adavance pot for light cruise torque and efficiency. Besides the price, there is no downside and is night and day compared to this 100 year old "robbing peter to pay paul" design. This will give you a bigger cake and more enjoyment.

    https://www.holley.com/products/ignition/ignition_boxes/street_and_strip/parts/6530

    PS. I'd probably go to the black spring and see if keeping the secondary from opening helps matters at all. Without the specific carb spec's(model # and current calibration) it's just a quick stab in the dark though.
     
    Last edited: Feb 12, 2019 at 5:20 PM
  13. baddad457

    baddad457 Member

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    Unlike you I don't give answers with TMI and post a 5 page essay answer when something short and to the point will suffice. He's got a Crown Vic motor which was designed for a 2000 pound heavier vehicle which by putting it in a lighter weight Comet/Mav will only improve the way it runs, despite the C4/3.00 drivetrain. I ran mine with a stock reman points distributor with a Pertronix I, II and later a Crane XR-1 electronic unit. There was no need to spend hours and days trying to fine tune the curve in it. These motors only made 175 HP on their best days.
     
  14. stumanchu

    stumanchu Stuart

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    My block ID #s tell me I have a 71 torino block. I got compression test #s from 150-165 when I first checked it. Two years later I checked it and the #s were 120-130. (used the same gauge.) This puzzled me, but I let it go. Then I saw a chart that showed how cranking RPMs affected these numbers, and it dawned on me that I had upgraded my old style starter for a new style. Do the new ones spin slower than the old when you have all the plugs out??? off the subject, but just wondering.
     
  15. groberts101

    groberts101 Member Supporting Member

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    Obviously my posts aren't aimed at folks who have it all figured out and accept run of the mill results. I tend to always maximize tuning towards that last 1-2% of improvement but not everyone cares to work that hard to find it. Some will and some won't and to each their own.
     

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