More Timing Questions - Team Camaro Tech
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post #1 of 3 (permalink) Old Apr 26th, 02, 02:08 AM Thread Starter
 
Join Date: Aug 2001
Location: Lawrenceville, PA, USA
Posts: 13
Sure are a lot of timeing questions.

First, does anyone know of a book on tuning the GM HEI?

Second, what determines total mechanical timing in the distributor? Is it the weights or the springs or a combination of both?

The reason I ask:
I bought a new HEI from a race shop. The initial setting had light springs. Advance started at 900 rpm and gave a total of 18* by 2300rpm. I thought this was a little too quick plus I couldn't get the timing to go back to static at idle. It was sticking at 20* @1200rpms.

So I took it back to the shop. They put it on a machine and free'd up the mechanism. It was sticking a little. They also put in another set of springs.

Now the advance starts at 1200rpm and ends by 3000rpm BUT I only have 12* of advance!!

I took the dist. apart and put in a set of MR Gasket Silver springs. The advance started at 1500rpm and ended at 3300rpm. BUT still only 12* of advance.

What would be limiting the * of advance? Should I change weights? The weights from the race shop and the Mr. Gasket weights look identical.

-Tom


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'79 Z28, 4spd, 3.73 10 bolt, 350 4-bolt .030 over with stud kit, 10:1 Compression, Keith Black Flat Tops, Comp Cams Magnum 294 Solid Cam .525 lift, 63cc Heads, Stainless valves, hardened exhaust seats, guideplates, Summit Needle Bearing Roller Rockers 1.5 ratio, Offy Single Plane Intake, Holley 650 Vac. Secondaries, Stock HEI (for now), Headers 1 7/8" primaries, 3" Collectors into 2 1/2" exhaust with "H" pipe and Turbo Mufflers.
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post #2 of 3 (permalink) Old Apr 26th, 02, 01:58 PM
Senior Tech
 
Join Date: Oct 2001
Location: Mill Creek, Wa
Posts: 107
total advance is the initial setting you give the motor at idle (say 12*) plus the mechanical advance built into the distributor which if it is adjustable, is a stop or a small bolt that stops the rotation of the shaft that the rotor is attached to. the weights and springs determine how quickly it can reach that total advance stop bolt by their combined effect against centrifical force as the rpms increase. vaccum advance only works at part throttle and mainly helps with gas mileage and engine drivability when you're not wide-open. you may want to try putting one lighter spring in to help get back to the total advance you had the first time or it may be that you do need heavier weights.try for total advance around 35-38* but listen for any ping under load.
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post #3 of 3 (permalink) Old Apr 26th, 02, 02:11 PM
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Matt Jones
 
Join Date: Jan 2002
Location: Renton, WA USA
Posts: 2,765
Actually, here's what you want to do:
1. Use stock weights and center piece.
2. Use MSD's middle-tension springs (the blue ones)
3. Grind the very tip of the center piece, the side that touches the small end of the weights. This will give you more advance.

I just did this today on my distributor machine. I wanted 24 degrees in the distributor (Total-initial). However, the stock setting only had 18 degrees, so I had to grind that area. Email me if you have any questions.

It is not adjustable (as far as a screw or bolt). Do not use aftermarket weights and cams (the center deal). Do not use any of the lighter springs. You will have a difficult time with them sticking, and they could start the advance as low as 500RPM.

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Vortec 355, Perf. RPM, Demon Carb., TH-400
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