Centrifugal Advance - when should it start? - Team Camaro Tech
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post #1 of 16 (permalink) Old Mar 23rd, 11, 09:21 AM Thread Starter
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Centrifugal Advance - when should it start?

Having some issues with a Pertronix dizzy, and trying to get to a somewhat reasonable advance curve. I am actually thinking that something might be wrong with the dizzy itself.

I was trying to obtain a curve that started at 1200 rpm, and was all in with 20* by 3000 rpm (using 20* limiters). Thought this seemed reasonable for a mildly cam'ed 350 with Vortec heads. Was going to set my base between 12 and 14*, so be around 32 to 34* when all in. On top of that was going to be 10* VA.

Well, the mechanical does not start to come in till about 1800 rpm, and is all in by 3500rpm. Changed up one spring (to a lighter one) and the curve starts out at 1600rpm and is all in by 2500. One seems to quick, the other too slow. Both seemed too high up in initial rpm.

Am I right in thinking she should start at 1200rpm or so?

My complaint with driveability just seems to be a lack of torque coming off idle and up to about 1500 rpm or so. I am using full manifold vac on the VA. There is no hesitation or chuggles so I think I got all my carb issues resolved. This is a street driven car, manual with 2.73 rear (I know... there goes the torque!).

I am also debating dropping the Pertronix for an MSD E-curve. Any opinions on that? My Pertronix seems to have its own mind on whether it goes out to 24* total or 20* total without limiters installed!

Thanks
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post #2 of 16 (permalink) Old Mar 23rd, 11, 10:01 AM
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Re: Centrifugal Advance - when should it start?

Just a thought. Have you contacted the techies at Pertronics? They were very responsive to my questions.

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post #3 of 16 (permalink) Old Mar 23rd, 11, 10:41 AM Thread Starter
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Re: Centrifugal Advance - when should it start?

Hi,

I have been on the horn with them several times. Nice folks and they steered me in the basic direction (which was the same as the manual), but did agree something was fishy. They questioned my tach and timing light, so I borrowed a buddies to confirm the dizzy was the issue.

Unfotunately, the dizzy is way out of warranty (it was bought 2 years ago, but did not get installed into the engine until last summer)
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post #4 of 16 (permalink) Old Mar 23rd, 11, 02:15 PM
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Re: Centrifugal Advance - when should it start?

Why limit yourself to 34 total? You need some dragstrip tuning
You need to add advance until you level off, lose mph or start to knock when using the proper fuel.
Run it at 2 lower than where it started to level off or the lowest best mph.
You might be suprized it might like 44 advance, it might prefer 35 you don't know what you are leaving on the table without trying.

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post #5 of 16 (permalink) Old Mar 23rd, 11, 02:34 PM Thread Starter
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Re: Centrifugal Advance - when should it start?

Got it - guess I was just looking for a logical starting point, and definitely caught up in the mystique of the vortec heads and timing.

This may sound stupid, but I am actually afraid I will not hear her knocking over the sound of the exhaust!
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post #6 of 16 (permalink) Old Mar 23rd, 11, 05:00 PM
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Re: Centrifugal Advance - when should it start?

For a street driven car you want the cent advance to start about 200-300rpm above your idle. This way you don't get an idle that hunts up and down as the cent adv kicks in and out... Having about 20 degs mechanical is ideal both street and strip, if you want 34 total you set the idle at 14 degs, you want 36 total you set the idle at 16... It's not set in stone but is a good starting point, if you have a cam and carb that require 18-20 at idle you'll need to adjust the mechanical to accomidate both idle and total timing...

...Dennis

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post #7 of 16 (permalink) Old Mar 23rd, 11, 09:23 PM
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Re: Centrifugal Advance - when should it start?

dennis is right on--but you gotta make the cent weights show that but great starting point
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post #8 of 16 (permalink) Old Mar 24th, 11, 12:58 AM
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Re: Centrifugal Advance - when should it start?

Along the lines of Dennis.

It sounds as if u have a late model (72/74 on) dizzy curve for ported vac and trying to run it on a manifold vac car....doesnt work.

VA is only in at med lows comes in about 7 to 10" all in 12 to 14" vac, so doesnt work accelerating ot taking off..thats all in the cent.
I assume 60 mph is about 2000 2200 rpms with about 16 to 18" vac.
From you engine description and final ratio/car weight etc you are looking at a cent starting around 12 to 14 degs around 1000/1100 rpms and all in around 3000 3200 rpms/ around 34/36 degs.

The idle vac no (VA attached) should be the same as or higherthan the All in Vac of the VA.
With 34/36 deg in the cent u will need about 10 to 15 degs in the VA..NOTE the cent is not all the way in at 60 mph..the higher VA makes up for that at cruise/light loads...
But the Engine vac drops when gets up to 80/90 mph plus cruise, and the VA drops out so you dont over advance.

Do not mix Full throttle cent with cruise cent +VA.

If dont have a dyno....set the cent high (overadvanced) say 36 degs...then slowly drop back till a noticable drop in power..thisd happens within a couple degrees...then up a couple degrees.
Tuning the other way...advancing and engine gives a better note, the drop off of power is very marginal, but the change in note is like an opitical illustion and gives the impression of quicker
Tuning this way destories a lot of engines by slow death.

My Spelling is not incorrect...it is creative

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Last edited by Steptoe; Mar 24th, 11 at 01:29 AM.
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post #9 of 16 (permalink) Old Mar 24th, 11, 08:48 AM Thread Starter
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Re: Centrifugal Advance - when should it start?

Dennis , Darrell and Steps - thanks. This is just the pointers I was looking for.
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post #10 of 16 (permalink) Old Mar 24th, 11, 08:54 AM
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Re: Centrifugal Advance - when should it start?

Quote:
Originally Posted by stouchton View Post
There is no hesitation or chuggles so I think I got all my carb issues resolved. This is a street driven car, manual with 2.73 rear (I know... there goes the torque!).
Just thought I'd mention that you kind of diagnosed the real problem ?

2.73 gears and a manual trans are going to make your car very sluggish at low rpm.
Change those gears to 3.55's and it will wake up that engine.
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post #11 of 16 (permalink) Old Mar 24th, 11, 02:48 PM
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Re: Centrifugal Advance - when should it start?

I run turbo 350 and 1200 stall with 255 rear and 60 mph = 2050 rpm
Torque...I have problems with rear end chriping and a damp road....dont even think about pulling into a med space in traffic, just sits and spins.
I run 27 mpg equivelent around towm... 1.8 mile stop start in med traffic cold running, intersections and undulating landscape
Open road 34.1 mpg... not granny footing but with slight agression, and hard down passing huge logging and milk tanker trucks (bigger than u have state side)
Again undulating landscape , gravel roads, and winding backroads thru hills.

My Spelling is not incorrect...it is creative

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post #12 of 16 (permalink) Old Mar 24th, 11, 05:28 PM
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Re: Centrifugal Advance - when should it start?

I don't believe his distributor is the problem.

2.73 gears and a muncie 4 speed is a lousy combo unless you want to run 90 mph on the interstate all day.

Last edited by Nashville Beth; Mar 24th, 11 at 05:49 PM.
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post #13 of 16 (permalink) Old Mar 25th, 11, 09:27 AM Thread Starter
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Re: Centrifugal Advance - when should it start?

Quote:
Originally Posted by Nashville Beth View Post
I don't believe his distributor is the problem.

2.73 gears and a muncie 4 speed is a lousy combo unless you want to run 90 mph on the interstate all day.
Beth - you are correct, the 0-40mph is nothing spectacular but kinda fun - she will break free if you push her. 60 to 90 is wonderful. No problem taking the speedo for all its worth. Worst of all, it is a 3 speed Saginaw not a Muncie. With the 2.73 I am reducing my axle torque almost 25% vs the 3.55. And of course, this puts a magnifiying glass on minor carb/timing issues.

The thing is, I don't want to change the rear yet. You see, my dad bought the car new in 67 - it was the cheapest domestic he could buy ($50 less than the Firebird). He didn't have much money, and had 3 kids. So the car was a simple coupe, 230 inline 6, no options except an AM radio (which might have been standard). Of course I got the seat belt in the center rear, which was later subject to a recall (car was only designed as a 4 seater, even though dealers added the 5th belt).

So, I wanted to restore the car just the way he bought it. I made some changes along the way. Like putting a RS front end on it in high school cause I just liked the look, and some lady took the front end off the car for me. Engine, wheels/tires, power discs, and exhaust are the changes that took place during restoration. Colors are all original (deep water blue and light blue) except for ghosted deck stripes. Since it was a late 67 car, and subject to the changes for 68 some of the interior pieces are medium blue (like the dashpad to reduce reflection). Folks look at it and think its wrong, but I know that was the way she was delivered.

Later, I'll head towards at least a 4 speed, and gears in the neighborhood of 3.23 or 3.55. I have a 3.42 in my truck with an OD tranny and really like the combo.

Anyway, I am just trying to get the most of what she has to give at the moment.
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post #14 of 16 (permalink) Old Mar 25th, 11, 09:51 AM
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Re: Centrifugal Advance - when should it start?

Sounds like an awesome car. Any first gen that has survived is something to be treasured, especially if it was your Dad's car !

I hope to pass mine on to my son.
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post #15 of 16 (permalink) Old Mar 25th, 11, 02:37 PM Thread Starter
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Re: Centrifugal Advance - when should it start?

Thanks. I wouldn't say "awesome", but it is nice. Hope to hand it down to my son also - problem is he is 4, loves John Deere and wants to paint my car yellow and green

Hope to have some fun recurving this weekend and see how she runs.

Last edited by stouchton; Feb 15th, 13 at 09:33 AM.
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