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Ignition timing questions
Posted: Wed Oct 14, 2020 10:24 am
by GRTfast
Just throwing out some thoughts/questions for discussion.
I have a 9.5:1 big block chevy, aluminum heads. It has a Jones HR cam, 110 separation, .600 lift, 240-ish duration at 0.050
makes about 160 psi cranking pressure.
The engine runs magnificently, pulling hard from down low to well north of 6000 RPM.
The strange thing (to me) is that the optimal timing curve seems fairly advanced compared to almost all the recommendations I see for timing. I am using the Progression ignition programmable HEI system (which is awesome BTW). I am running 28 degrees at idle, 40 degrees all in by 2500, and I have about 10 degrees of vacuum advance that is active in the cruising RPM range.
All the recommendations I see out there for an engine like mine are in the range of 14-18 initial, 34-38 all in. If I use those settings the engine feels lazy everywhere and noticeably down on power. I have confirmed multiple times that the damper is timed correctly to the engine so I know the readings are correct. Just wondering why my engine seems to like so much timing and tolerate it on pump gas. I suspect it means that I could raise the compression a lot, but I am not certain it is that simple. It has brodix 312cc rectangular port heads (probably too much head for this street setup), maybe that has something to do with it?
Thoughts/comments appreciated.
Re: Ignition timing questions
Posted: Wed Oct 14, 2020 12:08 pm
by ClassAct
GRTfast wrote: ↑Wed Oct 14, 2020 10:24 am
Just throwing out some thoughts/questions for discussion.
I have a 9.5:1 big block chevy, aluminum heads. It has a Jones HR cam, 110 separation, .600 lift, 240-ish duration at 0.050
makes about 160 psi cranking pressure.
The engine runs magnificently, pulling hard from down low to well north of 6000 RPM.
The strange thing (to me) is that the optimal timing curve seems fairly advanced compared to almost all the recommendations I see for timing. I am using the Progression ignition programmable HEI system (which is awesome BTW). I am running 28 degrees at idle, 40 degrees all in by 2500, and I have about 10 degrees of vacuum advance that is active in the cruising RPM range.
All the recommendations I see out there for an engine like mine are in the range of 14-18 initial, 34-38 all in. If I use those settings the engine feels lazy everywhere and noticeably down on power. I have confirmed multiple times that the damper is timed correctly to the engine so I know the readings are correct. Just wondering why my engine seems to like so much timing and tolerate it on pump gas. I suspect it means that I could raise the compression a lot, but I am not certain it is that simple. It has brodix 312cc rectangular port heads (probably too much head for this street setup), maybe that has something to do with it?
Thoughts/comments appreciated.
Sounds right to me.
Re: Ignition timing questions
Posted: Wed Oct 14, 2020 1:06 pm
by FC-Pilot
Performance engines are spoiled brats. When you give them what you want they are pains in the neck. Give them what THEY want and they are great ( just like my kid). Compression, cam event timing, chamber shape, spark plugs used, measured squish, fuel and afr, intake design and many other variables all play into it.
Nothing wrong with your engine, just find it’s sweet spot and let it do its thing.
Paul
Re: Ignition timing questions
Posted: Wed Oct 14, 2020 3:16 pm
by Schurkey
GRTfast wrote: ↑Wed Oct 14, 2020 10:24 amThe strange thing (to me) is that the optimal timing curve seems fairly advanced compared to almost all the recommendations I see for timing... ...All the recommendations I see out there for an engine like mine are in the range of 14-18 initial, 34-38 all in. If I use those settings the engine feels lazy everywhere and noticeably down on power.
What IS the quench distance?
Re: Ignition timing questions
Posted: Wed Oct 14, 2020 5:08 pm
by panic
My guess: your overlap flow at idle is large enough to reduce charge quality.
Re: Ignition timing questions
Posted: Wed Oct 14, 2020 7:07 pm
by GRTfast
Schurkey wrote: ↑Wed Oct 14, 2020 3:16 pm
GRTfast wrote: ↑Wed Oct 14, 2020 10:24 amThe strange thing (to me) is that the optimal timing curve seems fairly advanced compared to almost all the recommendations I see for timing... ...All the recommendations I see out there for an engine like mine are in the range of 14-18 initial, 34-38 all in. If I use those settings the engine feels lazy everywhere and noticeably down on power.
What IS the quench distance?
I think 0.035
My dad built the engine and I haven’t found his build notebook yet (he’s passed).
Re: Ignition timing questions
Posted: Wed Oct 14, 2020 7:13 pm
by GRTfast
FC-Pilot wrote: ↑Wed Oct 14, 2020 1:06 pm
Performance engines are spoiled brats. When you give them what you want they are pains in the neck. Give them what THEY want and they are great ( just like my kid). Compression, cam event timing, chamber shape, spark plugs used, measured squish, fuel and afr, intake design and many other variables all play into it.
Nothing wrong with your engine, just find it’s sweet spot and let it do its thing.
Paul
Yeah thanks for that, I am pretty much doing exactly as your final sentence suggests. I just find it interesting that it likes that much timing. Kinda bums me a little because I think it means the combustion event is lazy and unoptimized. <——-( engineer and racer coming out in me, lol ).
Re: Ignition timing questions
Posted: Wed Oct 14, 2020 7:40 pm
by Circlotron
Unusually small plug gap?
Re: Ignition timing questions
Posted: Wed Oct 14, 2020 7:53 pm
by Hedley Bean
I believe it's more about the fuel than the build. Lower compression stuff does generally like more idle timing, and of course the intake valve timing affects it too but man the gas we're getting out of the pump today is different than it was even a year ago, and can have excess water and all kinds of junk in it from tank to tank. I've noticed the same thing on my last couple of mopar big blocks out here. The last one I had, I set it up for 10 to 1 but made sure the quench was tight and it still didn't really start running like a hot rod until I had the timing at 24/38. Anything less and it idled poorly with any carb tune. I was sure it would fall in line at the specs most people talk about--that 16 or 18 initial and 34ish total kind of curve--but it wasn't having any of it. I'm sure there's other influencing factors as was stated above but pump fuel seems to be the big one to me.
Re: Ignition timing questions
Posted: Wed Oct 14, 2020 9:05 pm
by GRTfast
Circlotron wrote: ↑Wed Oct 14, 2020 7:40 pm
Unusually small plug gap?
It’s 0.035, plugs are bkr5e
Re: Ignition timing questions
Posted: Thu Oct 15, 2020 3:46 am
by Geoff2
All looks about right for 9.5:1 CR. Will like a lot of initial timing with that cam. You could try connecting the vac adv to a manifold vac source to give you 38* at idle & this will help tip-in response.
Re: Ignition timing questions
Posted: Thu Oct 15, 2020 5:09 am
by shoedoos
I don't know how often I have had "that look" when dropping a 40 degree tune into small blocks (yes I know you have a bbc).....the naysayers soon pipe up with all manner of opinions telling me I'm doing it wrong.....
Like others have said OP....give your engine what it wants......hope you're keeping an eye on the plugs at the same time though....
And PS: OP, when are we going to see your youtube video playing with that Progression ignition? Yes, I'd love to see an unbiased run through of all its ingredients....
Re: Ignition timing questions
Posted: Thu Oct 15, 2020 5:52 am
by BobbyB
GTR, What intake, carb & spacer are you using?
How does it run now compared to when you posted a year or so ago that it was surging at cruise before the new cam?
Re: Ignition timing questions
Posted: Thu Oct 15, 2020 6:28 am
by Camaromeo
What is your altitude? The higher, the more timing is needed.
Is your WOT-AFR maybe a little lean? Dont go leaner than 12.5. Could also contribute to timing hunger of the engine...
What pump gas exactly? 93? Burns slower so more timing can be necessary.
remember: measured from the perfect amount of timing degree, 3° less timing produce still 99% of the max power with way less detonation likelyness.
Re: Ignition timing questions
Posted: Thu Oct 15, 2020 6:50 am
by GRTfast
BobbyB wrote: ↑Thu Oct 15, 2020 5:52 am
GTR, What intake, carb & spacer are you using?
How does it run now compared to when you posted a year or so ago that it was surging at cruise before the new cam?
RPM air gap
QFT 1050
1 inch 4 hole phenolic spacer Runs best it ever has. Bucking/surging all but eliminated. Very happy with it.
The engine had always liked this much timing, I just never ran as much initial because it was hard on the starter. This programmable setup allows for start retard.