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 New timing recurve kit
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gscherer78ta
Buffalo

USA
51 Posts

Posted - 01 Jan 2012 :  10:53:37 AM  Show Profile  Reply with Quote
All,

I got a new Accel timing recurve kit for Christmas and was messing around with it yesterday and ended up with more questions rather than more performance. So, a little background- I got the Pontiac L78 motor with TH350 and 3.23 gears and it runs well- starts right up every time and runs down the track or street as good as the day it was new. It's dyno'd at 200hp and 300ft-lb torque so that's as good as it got back then, oh-it's the original motor to the car too.

I have had my timing set to 18 degree base and that jumps to 32 degree with vac advance. Without vac advance at 3000 I'm all in with 32 degree of timing. I think the car would run stronger in the higher revs with more timing, thus the recurve kit. Well, the weights and the center post were very different than what was in the car oringinally- I pulled the original ones out and the center post had "346" on it and the weights had "045" on them. When I put the recurve parts in with the lighter weight spring I lost all my timing- I only had 12 degrees with vac advance and 2 degrees base. That wasn't in the direction I wanted to go, so the next iteration was to use the recurve kit weights with the original center post and that didn't have any difference either.

I don't even know what help I'm asking for here- maybe if someone can tell me what the "346" center post and "045" weights are and whether I can get to 36 degree all in and a little sooner than 3000 rpm.

Happy New Year!

Greg

Greg
1978 Trans Am Y88, L78

Admin
Ye Olde Webmaster

847 Posts

Posted - 01 Jan 2012 :  11:18:58 AM  Show Profile  Reply with Quote
You're not all in at 3000 rpm! The HEI will provide 20 degrees of mechanical advance. You'll see it run up to 4400 rpm with the stock springs in there. The curve on these HEIs is SLOW.

The object is to get this curve to come in quicker and peak at 3000 (or a little less). The trick is the combination of springs. One can't rely on the formula provided by the company it's too generic. Also, the tension has to be enough to bring the weights back to rest fully at idle. When one puts in weak springs, you can pretty much guarantee it will not pull the weights back far enough all the time. This will give you an inconsistent idle rpm.

In my experience I recommend using the original weights and center plate. The springs are the thing. Try using a medium and a stock spring and check it all with a dial-back timing light.

Hope this helps.

"Dedicated to keeping the classic Pontiac engine alive."

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gscherer78ta
Buffalo

USA
51 Posts

Posted - 02 Jan 2012 :  07:41:05 AM  Show Profile  Reply with Quote
Thanks for the response. I think I'm all in because I don't show any further advance after 2800...

In a technical article on this site by Jim Hand, he is using a #398 cetner post and #139 weights- where can I buy those parts? Can anybody tell me what the numbers indicate? Is it weight, for instance my 045 weights are 45 grams and Jim is using 139 gram weights?

Also, I noted that when I put things back together that the weights have some room to move around a bit before the springs pull them back in- should the weights always ride snuggly up against the center post?

Greg
1978 Trans Am Y88, L78
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Bill Boyle
Horse Feathers (Charter Member)

USA
4794 Posts

Posted - 02 Jan 2012 :  08:59:19 AM  Show Profile  Visit Bill Boyle's Homepage  Reply with Quote
The springs should pull the weights back against the center plate at rest.

I've never weighed the weights to discover their weight so I don't know if there is a correlation between the ID number and the weight.

You will not be able to purchase these parts from any retailer. Most have been reused from salvage yard distributors.

The importance, IMO, is that you work with the weights and center plate in the distributor.

Jim Hand's article describes how he modified the center plate to gain more advance than what the original design provided. He wanted more than 20 degrees mechanical advance. He dropped his idle advance lower than what was used in the post 74 HEI Pontiac engines. He tailored it more like the point's distributor that typically used manifold vacuum at idle to boost mechanical advance. [The two worked together.] HEI typically called for 16 degrees of initial because they ran off ported vacuum that provided no boost at idle.


"Dedicated to keeping the classic Pontiac engine alive."

----
400 bored +.030, forged TRW pistons, ported 62 heads, Hedman headers, 2.5 SS dual exhaust X Pypes, Comp 276AH10 cam, Scorpion 1.65 RR, 850 Q-jet, stock intake & tuned HEI; original owner.

Edited by - Bill Boyle on 02 Jan 2012 09:02:10 AM
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gscherer78ta
Buffalo

USA
51 Posts

Posted - 03 Jan 2012 :  4:46:10 PM  Show Profile  Reply with Quote
So, the springs do pull the weights back in. Only when the weights are pulled in they are pulled against a point contact and not a surface so, they can rotate on that point without pulling on the spring. I think that if the weights were slinging out because of this I would see it in the timing and I don't. The timing seems very stable to me, the timing mark is very steady at a constant rpm.

I don't know if my description makes any sense.

Greg
1978 Trans Am Y88, L78
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Phil
The Great White Buffalo

USA
7216 Posts

Posted - 03 Jan 2012 :  7:49:55 PM  Show Profile  Reply with Quote
The weights and center plates that come in "recurve kits" are typically useless and suitable for only throwing in the trash. Try the lightest springs in the kit AS A BASELINE TEST. Note the changes/improvements and work back from there. I "redress" my weights and center plate as Bill mentioned and as outlined in Jim's book. But that should be done after you have experimented with the various springs. I usually end up with one light and one medium spring. Pay attention to the number of coils in the springs as well, there is a difference in the way they perform their function.

Bowties are for Pee-wee Herman. "Chevy": even the name sounds cheap, but not as cheap as your Pontiac will be with an LS transplant.
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455 Formula
Sitting Bull

USA
244 Posts

Posted - 04 Jan 2012 :  05:08:34 AM  Show Profile  Reply with Quote
quote:
Originally posted by gscherer78ta

So, the springs do pull the weights back in. Only when the weights are pulled in they are pulled against a point contact and not a surface so, they can rotate on that point without pulling on the spring. I think that if the weights were slinging out because of this I would see it in the timing and I don't. The timing seems very stable to me, the timing mark is very steady at a constant rpm.

I don't know if my description makes any sense.



Greg,

Check your e-mail....
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gscherer78ta
Buffalo

USA
51 Posts

Posted - 04 Jan 2012 :  08:03:11 AM  Show Profile  Reply with Quote
I got your Email Robert.

I put in a lighter spring from the kit and used my original weights and center post and now at 2200 I have 2 more degrees of mechanical advance. I had to re-do my base timing and re-set the amount of vacuum advnace and- I haven't driven it on the street yet. It's pretty close to where it was running and I was happy with that level of performance so this ought to be a little better.

Greg

Greg
1978 Trans Am Y88, L78
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455 Formula
Sitting Bull

USA
244 Posts

Posted - 04 Jan 2012 :  11:23:11 AM  Show Profile  Reply with Quote
Greg,

With the heavy, full-length, small-pin weights, like 060, 053, 105, 139, etc., I had to use a 'heavy-heavy' spring combination to get the timing 100% stable at idle.

I switched to the smaller/shorter unmarked weights with the plastic bushings and went back to a 'heavy-medium' spring combination.

Be sure to get the MSD #8412 Low Resistance Carbon Button to improve secondary output and reduce coil/module operating temps.

Lower compression Pontiac applications need more initial and slightly more total. Try bumping your initial up to 20 degrees.

On my all original 1979 T/A W72 I found it ran best with 20 degrees initial, a '449' center plate (numbers down) and '139' weights (for 24 degrees of centrifigul advance) and 8 degrees of ported vacuum advance, set to come in at 10 in/Hg of vacuum.

In contrast, my 8.75:1 SCR 413 likes 20 initial, with a '472' center plate & '139' weights which yields 20 degrees centrifigul and 8 degrees of ported vacuum advance, set to come in at a low 5 in/Hg.

Both being 4 speed cars, neither ran better with manifold vacuum advance...Robert
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455 Formula
Sitting Bull

USA
244 Posts

Posted - 04 Jan 2012 :  11:23:43 AM  Show Profile  Reply with Quote
quote:
Originally posted by Bill Boyle

You will not be able to purchase these parts from any retailer. Most have been reused from salvage yard distributors...


A few NAPA part numbers for HEIs:

DP109 HEI weight pins
DP112 HEI weight plastic bushings
DP114 HEI distributor weight "stamped 106"
DP115 HEI distributor weight "stamped 139"
DP126 HEI distributor weight "stamped 105"

Edited by - 455 Formula on 04 Jan 2012 11:24:40 AM
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Total Jackass
Tribal Scout

0 Posts

Posted - 14 Jan 2014 :  04:14:52 AM  Show Profile  Reply with Quote
Q. Why did the computer squeak.
http://www.bandgae.com/

I'm a first class jerk and don't you forget it!
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Phil
The Great White Buffalo

USA
7216 Posts

Posted - 14 Jan 2014 :  6:31:09 PM  Show Profile  Reply with Quote
TTT

Bowties are for Pee-wee Herman. "Chevy": even the name sounds cheap, but not as cheap as your Pontiac will be with an LS transplant.
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