Timing ?

Discussion in 'Technical' started by Falcenac, Apr 20, 2018.

  1. Falcenac

    Falcenac Member

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    Trying to figure out my best bet for timing not positive of my targets looking for some suggestions. Fresh 351w swap I used an f4 roller block, 30 over scat crank stock stroke, forged flat top pistons 10.46 to1 compression , roller cam 632 and 619 lift (forget other specs) afr 195 heads, rpm airgap manifold , quickfuel 850 q series carb, and 1 3/4 long tube headers. Runs on 94 octane.
    Presently have timing at 11 degrees initial 41 degrees all in at 2500 rpm. I think my all in might be getting high . Car is a manual trans and even lugging have not heard pinging , any suggestions?
     
  2. 71gold

    71gold Frank Cooper Supporting Member

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    bump it up 2 degrees at a time until it pings, then back down 2...:drive::chirp:
     
  3. groberts101

    groberts101 Member

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    Your timing for those cylinder heads and compression ratio does seem a tad high. Most with similar combo's I've tuned or seen tuned(or even read about for that matter) are usually around 36-38° total. Also good to keep in mind that high rpm detonation is VERY hard to hear especially if it is very light. But the cumulative affect can be very damaging over time to ring lands and creates hot spots which quite literally erode away the aluminum. Usually the edges of the piston crown and sharper edges of valve reliefs, or even head gaskets get hit first due to the hot spots created and carried on to the next power stroke. At higher rpm those hot spots can literally start glowing and turn into mini glow plugs the longer you stay into the throttle under load.

    I can help get you much closer than you are currently but will need considerably more info.

    Primarily full cam spec's(including @.050" lift spec's) and rear gear ratio and tire size.

    How YOU drive the car and the typical loads(amount of extra weight from passengers) you carry around have an impact on tuning too. Such as preferring to downshift and keep the engine in a fatter powered sweet spot.. or conservative of engine life and fuel consumption by holding a taller gear and potentially lugging the motor a little more than the average hotrodder.

    Does the distributor also have a vacuum advance pot and where is it currently connected?.. from a ported(timed) source.. or full manifold vacuum source?

    And getting down to the nitty gritty about how well you maintain the engines tune with annual maintenance as well as expected mileage per yer and typical ambient temps will paint a truer picture on the tuning allowance and margins that need to be considered for the final tune.

    Also should include your degree of mechanical aptitude relating to reading plugs and generalized tuning knowledge when it comes to fuel and ignition curves and how they relate to one another.

    But off the top.. even without knowing any of that.. I can tell you for fact that you are leaving bunches of manifold vacuum on the table with only 11° of initial timing on a big'ish cam. I can only speculate based on those lift numbers that it's probably at least somewhere around 230-240° @.050" spec's. Bigger cams lead to somewhat lazy flow(induction velocities) and rather lazy combustion dynamics which necessitates even more initial timing than a regular cam would require. This is the primary reason that compression ratio must be increased with bigger cams to counteract that poor combustion efficiency or they turn into peaky powered turds on the street.

    A simple and easy trick is to advance the distributor at idle until the rpm rises a few or even several hundred rpm's. If you have the distributor vac advance pot connected to full manifold vacuum?.. then it is always best to remove it for reduced variables cause by the increased manifold vacuum gained from increased timing advance pulling harder on it. Once the idle speed has increased when advancing the timing by hand then readjust the idle stop back down to previous idle rpm's and adjust mixture screws until achieving best and consistent needle readings on the vac gauge connected to full manifold vacuum. Rinse and repeat until the engine makes peak manifold readings and starts breaking up(leaning out) with erratic needle movement beyond what the idle mixture screws will compensate for. If you run out of screw adjustment range before reaching peak manifold vacuum readings then the internal idle feed restriction(IFR) and idle air bleeds need to be tuned but we won't get that advanced at this point since this is just a lesson into learning the trends towards what ignition lead a motor wants and prefers.

    Then take note of what that timing numbers is with a light. THAT is what the engine prefers for best manifold vacuum idle efficiency and throttle response. Problem there is that the total timing will have jumped so severely when the engine is revved that you cannot possibly rev the motor very high or even drive the car without detonation setting in and potentially damaging the motor. This quick and dirty test is completely reversible and is only intended to show you the trend for what base timing(initial #) and centrifugal sweep needs to be dialed into the timing curve. If you wanted to feel the effects it would be easy enough to do VERY QUICK throttle snaps in 1st gear to feel what that extra torque and throttle response has done to the "feel" of the engine. BUT.. DO NOT DRIVE OR ROAD TEST IT THAT WAY.

    I don't have much time to play the texting game and would much rather do this type of stuff over the phone so PM me if you're really wanting more help to dial that tune in much more quickly and effectively. Will be in the garage most of the day working on my own engine projects so will have enough time to chat and steer you in the needed direction.

    Sorry for the long post.. I just rattled it off before starting my day off. Good luck with it!
     
  4. Falcenac

    Falcenac Member

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    No vacuum advance using a duraspark dizzy to trigger a McD 6al box, cam specs duration @.050 intake 237 exhaust 247 lsa 112 degrees, rear end gears are 410 ratio running 28x11.50 ET street tires. Car is a street driven and goes to track not a daily driver, basically built to run hard.
    As you mentioned my concern was I may not hear high rpm pinging car is very loud ! I think this weekend will pull dizzy and close up advance slots bump up initial and try to get all in around 38 degrees?
     
  5. groberts101

    groberts101 Member

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    Nice solid combo. Based on that info here's what I would do.

    Solder or weld the slots to achieve a minimal centrifugal sweep of around 8-10 degrees MAX. Then set base timing for the remainder of what's needed to reach the 38 total number. That will give you 28-30 degrees initial and make the car MUCH more responsive due to increased manifold vacuum. Now, with such high ignition lead at idle and light cruise you may need to decrease idle bleed sizing a bit more to richen it back up. Otherwise it'll likely surge, pop, or even backfire through the induction if severely leaned out.

    You'll also likely need to add some accelerator pump shot on the primary side as due to similar leaning affect as well. I wouldn't be looking at anything under a 38 nozzle and make sure the cam supports a nice fat shot right off the idle stop. Have to try a few to get it right. And don't forget to make sure you have high flow pump nozzle screws too.

    The other thing you could think about is fully locking the distributor at full advance. Then run either a start retard box or use an ignition power(coil cut) switch until the engine is cranking over before flipping it to add fire to the plugs. Otherwise your starter will likely kick back under the too great cylinder pressure created from such heavy advance. Makes them last much shorter lives even if they do actually pop off the motor without those tricks.

    PS. To quickly test effects of low speed bleed(LSB) enrichment effects all you need is a small piece of wire wrapped around the vent tubes and pushed down into the LSB's. This displace the orifaces available hole sizing which effectively makes it smaller and enriches the circuit. Very thin strands of copper wire from multifilement electrical wires work well. Even speaker wire works well too.
     
    Last edited: Apr 20, 2018
  6. greasemonkey

    greasemonkey Burnin corn

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    Lock it out at around 36. You be better to start at a lower total and try it at the track. There's a lot goes in to total timing but those heads have a pretty good combustion chamber shape And I'd be surprised if it needs much if any over 36. Run it at the track and guage mph.
     
    Last edited: Apr 21, 2018
  7. groberts101

    groberts101 Member

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    I fully agree with this. Just keep in mind that finding the optimum locked timing number is always going to be a matter of robbing peter to pay paul. Most engines pay dividends under the torque peaks by adding timing but fall off quicker up top. The only way around that is to use a programmable timing box like the 6 or 7 series programmable deals or bigger $$$ EFI style controllers with mapping. And they work awesome!

    IOW's, what you can gain in peak power.. can just as easily be lost under the peak for canceled out net ET reduction as well. For that reason it's usually best to look at the bigger picture by taking the 330' and 660' times into account. All about average power output to help accelerate the engine quicker and ultimately the car by the end of the track.
     
  8. Falcenac

    Falcenac Member

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    Going to pull dizzy later today and close up slots will post results when I test, I don't have a track day till mid May.
     
  9. bmcdaniel

    bmcdaniel Senile Member

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    Once upon a time I used to know how many thousandths of an inch of slot equaled a degree change in timing, saved a lot of trial and error. Also instead of welding and filing the slots I just put pieces of vaccuum tubing over the posts, fine tuned with additional pieces of heat shrink tubing.
     
  10. Falcenac

    Falcenac Member

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    Pulled dizzy yesterday took apart there is two slots for advance , I previously had it in the larger slot moved around to smaller slot and reassembled.
    Set initial ar 13 or 14 all in now 36 took for a drive runs well pulls hard, should be safe from detonation now, have a test n tune in May before first race will fine tune then when I can see some results.
    Thanks for input.
     
  11. groberts101

    groberts101 Member

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    If you read back to my first response about heavy initial advance and bigger cams you can imagine you're still leaving tons of manifold vacuum on the table for a street driven car. For a race only car so long as it starts well enough, doesn't load up on the line, and makes max power in the rpm range being run down the track it's not a concern at all. But low speed driveability is and will be affected by reduced manifold vacuum. Bet you have another 2-3" of manifold vac being tossed away with only low-teens at idle and no disty vac pot. The carb's idle and transition slot signal won't be nearly a strong either.

    My best advice at this point is that if you don't want to run heavier initial advance then would be better to run VERY fast centrifugal curve(extremely light springs).. maybe all in around 2,200-2400 rpm. Dowsnside to that is spring bounce and timing flutter becomes an issue when quickly snapping or abruptly off the throttle and can usually see some symptoms of it on the light. Only gets worse under load.

    Congrats though if you're happy enough with it.
     
  12. groberts101

    groberts101 Member

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    I hear you there. IIRC, and likely not, it was something around .020" slot adjustment per crank degree. Just numbers stuck in my head.. could be way off though. lol
     
  13. Falcenac

    Falcenac Member

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    Previously installed light springs and timing all in by those numbers. Definitely will be trying different settings you know how it is an ever evolving project no such thing as done just what's next.
     

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