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SlantSixDan
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PostPosted: Thu May 31, 2012 2:32 pm 
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Have you verified that the timing mark on the outer ring of your vibration dampener is correct?
…and that you're in fact looking at the right timing mark, if you have one of the dampers with multiple marks? And that your damper and timing indicator are a matched set?

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bigslant6fan
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PostPosted: Thu May 31, 2012 2:43 pm 
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I agree with jbc, I've seen many engine combos develop a smoother idle and higher vacuum readings with idle setting in the 15-30 BTC range. However, I don't recomend more than 10-12 BTC for the following reasons. #1. The engine could kick back against the starter during cranking, damaging the starter or ring gear. #2. With a manaul trans, it could ping if the clutch is engaged at low speeds. I too, have a setting of 20 BTC on my turbo car,but it's an automatic trans with a high stall converter,and I have a MSD ign. system with a starting retard funtion. If it's working well for jbc, then so be it.

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Rex Baker
PostPosted: Thu May 31, 2012 3:01 pm 
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This is just an FYI of my experience and my results may be of interest to those interested in more power, improved drivability and dramatically better performance and milage.

I've been experimenting with my Super Six'd 225 with bigger valves, head cut .080, HEI conversion, 350 cfm Holley 2bbl, Doc's smaller Erson cam and his famous duals.

I've found my motor loves the initial timing set at 25 BTDC with the total set at 35 BTDC. The timing is all in around 2000 rpm. Of course, I had to weld up the slots in the mechanical advance to pull this off. I'm also running vacuum advance hooked to manifold vacuum, so the timing at idle etc is even more advanced.

The motor runs fantastic, starts right up hot or cold and doesn't ping or gas knock. I have WAY more torque and horsepower and improved milage. I have yet to find a downside.
Should work ok. :shrug: Distributor setup is similar to my 8.6 to 1, 440 I built years ago, still running actually, though I'm not sure how mine would crank with 25 degrees initial. I have 8 degree mech. in dist., 18-20 degrees initial and a stop-limited vacuum 8 degrees @ the distributor, and I use ported vacuum. Total mech. is between 34-36, all in around 1300 and 50-52 w/vacuum hooked up under no load. I set my old dual point up on my Sun 404. Cam is 218/223 @ .050 and about 470 lift, 112 lsa if I remember correctly, 2 degrees retarded from straight up. Runs fair, 391 hp at the wheels @ 4,900 rpm. Car & me are 4,160 and runs 13.1 @ 107 at 1,800 elevation on 87 octane.

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olafla
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PostPosted: Thu May 31, 2012 6:20 pm 
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You guys should really reflect on the processes going on here before rejecting the idea.
An accellerating engine receiving a richer mixture, usually benefits from a retarded timing, while a leaner mixture, as when high speed cruising, demand an advanced timing to ignite the mixture properly. In this case the total timing is kept on level with an ordinary distributor, but has the advantage of the retarded timing given by a reduction in the vacuum to the distributor, because it uses manifold vacuum that drop during accelleration.

What jbc528 has done, is simply to limit the timing range, thus taking advantage of the fact that the slant likes plenty static timing, while adding the benefit of a retarded timing during accelleration.

Just like Ford did...

Olaf.

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xjarhead
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PostPosted: Thu May 31, 2012 7:05 pm 
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Here is a write up on the subject of ported vs manifold vacuum by a gm engineer. http://performancecarbscience.com/weber ... onversion/

Dave

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SlantSixDan
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PostPosted: Thu May 31, 2012 7:25 pm 
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xjarhead
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PostPosted: Thu May 31, 2012 8:08 pm 
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Ok

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Reed
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PostPosted: Thu May 31, 2012 10:19 pm 
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Ported vs full manifold vacuum advance is also a function of distributor design. I used to own a Pinto that had a distributor with a vacuum advance mechanism that used ported and manifold vacuum. There was a nipple on either side of the vacuum advance diaphragm and the timing was advanced and retarded in a push-me-pull-me fashion.

I noticed the linked article was written by an anonymous ex-GM engineer. I am wary of anonymous "authority."

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SlantSixDan
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PostPosted: Thu May 31, 2012 11:10 pm 
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Ported vs full manifold vacuum advance is also a function of distributor design.
+1.
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I used to own a Pinto that had a distributor with a vacuum advance mechanism that used ported and manifold vacuum. There was a nipple on either side of the vacuum advance diaphragm and the timing was advanced and retarded in a push-me-pull-me fashion.
Yup! Lotta Fords like that. Some Volvos, too, I think, and some that had a vacuum-retard unit only.
Quote:
I noticed the linked article was written by an anonymous ex-GM engineer. I am wary of anonymous "authority."
+1.

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sandy in BC
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PostPosted: Fri Jun 01, 2012 8:31 am 
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I have exactly this advance setup on the race motor in my bogger.

It allows me to start with 15 degrees advance ....and get another 10 when the thing starts from the manifold vacuum advance. Easy to start and enough advance for idle and low speed.

As soon as Im off idle I get mechanical advance (22 degrees full in by 3000) and no vacuum advance.

The thing is its a 6500 rpm 11:1 ....3500 stall ....10.5 gears Chev on 35" tires....2400 lbs

My 9.25:1 5 speed 2800lb Valiant was far happier with a conventional curve.....

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xjarhead
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PostPosted: Sat Jun 02, 2012 5:40 am 
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Ok I found the article with the authors name here http://rockridgefarm.com/vettdoc/Timing_101.pdf
and here http://www.camaros.org/forum/index.php? ... 680.0;wap2
and some info on the author here http://www.youtube.com/watch?v=mfXNaGLUohk
He's no longer anonymous

Dave

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SlantSixDan
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PostPosted: Sat Jun 02, 2012 10:25 am 
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He's no longer anonymous
Real good. He's still demonstrably wrong on central basic facts underpinning his argument. Because his recommendation is based on incorrect "facts" it is not worth much—certainly not worth anything like what he thinks it's worth.

Here's a thread (with links) with results of a practical, real-world experiment with ported vs. manifold vacuum spark advance hookup on a real, actual slant-6 engine.

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xjarhead
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PostPosted: Sat Jun 02, 2012 4:56 pm 
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From your post "My guess is that the driveability improvement with manifold vacuum spark advance is specific to the underhood configuration. Mine differs substantially from the setups on which manifold spark advance was used; I have no EGR, my base timing is about 8°, etc. Still, I can't help wondering why I remember seeing manifold vacuum at the spark port of various more-or-less stock carburetors on my '62 Lancer back when it was my dad's car. I'm pretty sure the vacuum porting particulars depended in the pre-emissions era on manual/automatic transmission, but back then I think it was manual-trans cars that got porting closer to manifold-vacuum-at-idle."

I think you you hit the nail on the head there Dan.

Dave

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WagonsRcool
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PostPosted: Sat Jun 02, 2012 5:09 pm 
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You look up theory of operation for the ignition system on early 40's Packards- they used ported/ timed vac for the dist vac advance, like pretty much everything else I've worked on. (the big exception is most 70's era GM cars)

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SlantSixDan
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PostPosted: Sat Jun 02, 2012 7:13 pm 
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You look up theory of operation for the ignition system on early 40's Packards- they used ported/ timed vac for the dist vac advance, like pretty much everything else I've worked on. (the big exception is most 70's era GM cars)
I don't recall those '70s-era GM cars running very well. :lol:

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