Timing & Vacuum Advance 101 - Page 6 - Team Camaro Tech
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post #76 of 80 (permalink) Old Apr 24th, 14, 07:57 AM
Senior Tech
Marc
 
Join Date: Jul 2001
Location: Belgium
Posts: 546
Re: Timing & Vacuum Advance 101

I did post recently about a knocking noise I experience with my camaro, after distributor and carburator setting changes.

Now as a return of experience I want to post for reference for similar engine configuration people may use....
Engine is 327 ci vortec heads comp cam 268HE straight propane impco 425 edelbrock perfomer intake and tri Y headers. with T56 and 4.11:1 posi rear end.

As tuned now the car is pretty fast, certainly 2-2500 rpm untill 5500 with very good throttle response, which correspond to the cam installed.
Under 2000 rpm I guess I could have better respons still maybe with more advance but as now with the 6° speed can have high load on engine and I didn't want to experience and play with more advance...

- AEM wideband lambda gauge is installed and working.
- I could then test drive the car and tune it.
So here it is :

Ignition Timing
11° initial
12° mechanical starts at 1400 full at 2500 (so total = 23°)
10° vaccum advance

AFR
lambda values
0.96 idle
1.10-15 cruising
1.02-05 acceleration under 2200 RPM
0.95 acceleration at WOT higher RPM

For the understanding of my research and coming to these numbers after having these knocking issues :
- CR 10:1 high compression --> less advance
- vortec L31 heads are fast burn heads with high quench chamber design --> less advance needed. On vortec heads we have numbers on gasoline as low as 28-29 total with very good performance
- propane rule compared to gasoline: +3-5° initial and -5° total, with curve coming in faster.

Regards,

Camaro 1968, propane 327ci, Big Brake, Hotchkis, T56
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post #77 of 80 (permalink) Old Apr 24th, 14, 08:03 AM
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Marc
 
Join Date: Jul 2001
Location: Belgium
Posts: 546
Re: Timing & Vacuum Advance 101

Sorry forgot to say : with this configuration, initial with manifold vaccum is 20~21° and I get 17 psi vacuum. PVC valve is also connected to the holey base plate.
Idle is very good, i had to reduce idle rpm.

Camaro 1968, propane 327ci, Big Brake, Hotchkis, T56
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post #78 of 80 (permalink) Old Apr 24th, 14, 02:25 PM
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Steps
 
Join Date: Dec 2005
Location: New Zealand
Posts: 13,316
Re: Timing & Vacuum Advance 101

Quote:
12° mechanical starts at 1400 full at 2500 (so total = 23°)
You will need a ball park total around 30 to 34 deg at around 3200.
Asuming a non egr engine
intial 8 to 12 deg
Around 22/23 in the cent
And idle (intial+VA) 18 to 20 because of the lift to make stable street idle at reasonable
rpms.

A propane engine needs a very fast curve, petrol far slower
Propane also requires a low total than petrol
LPG also by nature will idle stable very low.. an engine like yours will idle at 550 in gear with an auto.
LPG when one leans out or over advances doest get hot, it cools.. the other way around to petrol.. and doesnt detonate at off cruise light load unless there is too much in the VA...
Typical non egr propane curve is around 10 to 12 initial, total around 28 to 32 depending on dcr, and all in around 2500 to 2600, and around an all up intial+cent+VA 42....petrol engine when dialed in with a knock sensor can often go way above this in to high 50s (thu if not dialing in with knock sensor never go above 40/42 on a petrol engine.)
Do not try to apply a gasious fuel characteristics to a liquid fuel

My Spelling is not incorrect...it is creative

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post #79 of 80 (permalink) Old Mar 15th, 16, 12:17 PM
 
Join Date: Mar 2016
Posts: 1
Re: Timing & Vacuum Advance 101

Wow this is great post for me .....Yer right Steps... have to keep that in mind. I'm assuming I'll be leaning out the idle mixture too. Actually, does anyone have a step-by-step process they use?
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post #80 of 80 (permalink) Old Oct 21st, 18, 01:41 PM
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Marko
 
Join Date: Oct 2018
Location: Magdeburg, Germany
Posts: 1
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Thumbs up Re: Timing & Vacuum Advance 101

Thank you so much!! It is very interesting and enlightening!

Best regards
Marko

´69 Camaro, 307 cui
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