327 Timing Question - Team Camaro Tech
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post #1 of 12 (permalink) Old Dec 2nd, 05, 09:53 AM Thread Starter
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Question 327 Timing Question

I have extensivly read all the posts I can find about timing here and decided to give it a shot with my car. Here's my situation that I need help with. I had a buddy with a non adjustable light read off the numbers on the timing tape as I increased the RPM. Granted the car was a little cold the numbers still seemed odd. It's set right now at 12btdc at idle no vacuum. With vacuum it was hovering @ 19-20. I disconnected the vacuum and plugged it. FYI, I have a stock GM HEI, nothing fancy yet. At 1000- 1500 I was already at 30 deg adv. It kept increasing up to 39ish but that was at around 4000. I recently purchased the crane adj vac kit, which I have yet to install. I was going to replace the weight springs but it seems to come in very fast already, too fast. Should I get heavier springs? Will the crane unit help me that much? The car ran well this summer, I just want it to run at it's best for next year.I think it has more to offer. thanks in advance, no pun intended!

1968 Camaro Z28 clone 327 .030 over, Muncie M20, DUI coil, Edelbrock Performer w/1406 , headers, 274 cam
1963 Chevy Biscayne-2 door, 283 2bbl, powerglide, Mallory Digital ignition
1985 Z-28 LG4, Edelbrock hdrs, Lakewood control arms
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post #2 of 12 (permalink) Old Dec 3rd, 05, 11:39 PM
 
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Re: 327 Timing Question

Heavier springs will help change when the advance kicks in quite a bit. 39 degrees at 4000 is pretty high... Unless you're running a lower compression motor with 91 octane. I played with my 383 a bit and got it set at 35 degrees total before it started pinging. I run 91 octane, and my compression ratio is about 10:1. You always want to check the timing with the engine warm though.
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post #3 of 12 (permalink) Old Dec 4th, 05, 02:25 AM
 
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Re: 327 Timing Question

Just a thought I have found many old HEI dist to have alot of wear where the weights pivot from. You might want to inspect it if you havent yet this can cause irratic readings.
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post #4 of 12 (permalink) Old Dec 4th, 05, 11:04 AM
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Re: 327 Timing Question

I agree with TexasPP, check for wear in the mechanical advance. Replace the springs at a minimum, and make sure the weights and bushings are in tip-top shape. Get those parts super-clean and put a tiny bit of lube on them. Mechanical advance parts get rusty and stick, causing erratic timing.

Most HEI's will mechanical advance about 24 to 26 degrees, thus your 12 degree initial setting, plus 26 advance will give about 38 total. If it's pinging, then back it down, or use heavier advance springs so that the maximum timing comes in later. The timing needs to advance at higher rpm to compensate for the air/fuel burn rate remaining relatively constant.

Once you get the mechanical advance all set (no pinging), then you can play with the vacuum advance. An adjustable vacuum advance is a wonderful thing. I typically let the vacuum advance add as much as possible, again watching for detonation. Engines like a lot of advance for light cruising conditions. I like to use a carb port with vacuum at idle and part-throttle. You don't want any vacuum on the line at full throttle. That way, when you mash the go-pedal, you are working solely on the mechanical advance and will be limited to about 36 to 38 degrees max.

Dave F. in Rhode Island
'68 Camaro SS Accel DFI 454 (SOLD - I'm a boring Corvette owner now)
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post #5 of 12 (permalink) Old Dec 5th, 05, 07:01 AM Thread Starter
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Re: 327 Timing Question

Thanks for the responses. What's the best way to limit my mechanical advance? I understand that I will need to change springs to alter the rate at which it comes in, but I don't know how to stop it from getting up to 39 deg.? Do I limit how the weights travel? Should I reduce my initial back down to 10? The motor really liked the 12 deg though.

1968 Camaro Z28 clone 327 .030 over, Muncie M20, DUI coil, Edelbrock Performer w/1406 , headers, 274 cam
1963 Chevy Biscayne-2 door, 283 2bbl, powerglide, Mallory Digital ignition
1985 Z-28 LG4, Edelbrock hdrs, Lakewood control arms
rusty Z is offline  
post #6 of 12 (permalink) Old Dec 5th, 05, 07:06 AM
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Re: 327 Timing Question

I would suggest purchasing a "recurve kit" they generally include recontoured weights that change the advance rate and limit the centrifical advance, some include bushings to limit advance as well. Most of these distributors were from the '70's smog era. The trick at the time was to minimize emissions with near zero initial timing, bring in a lot of mech advance quick, then slow the rest of the advance curve. They did this by the shape of the wieghts.





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post #7 of 12 (permalink) Old Dec 5th, 05, 07:20 AM Thread Starter
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Re: 327 Timing Question

Thanks, yeah this distributor is a 70's era unit. I'll have to look into that kit. Would Accel or MSD sell a recurve kit for a stock HEI?

1968 Camaro Z28 clone 327 .030 over, Muncie M20, DUI coil, Edelbrock Performer w/1406 , headers, 274 cam
1963 Chevy Biscayne-2 door, 283 2bbl, powerglide, Mallory Digital ignition
1985 Z-28 LG4, Edelbrock hdrs, Lakewood control arms
rusty Z is offline  
post #8 of 12 (permalink) Old Dec 5th, 05, 07:58 AM
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Re: 327 Timing Question

Quote:
Originally Posted by rusty Z
Thanks for the responses. What's the best way to limit my mechanical advance? ......Should I reduce my initial back down to 10? The motor really liked the 12 deg though.
Well, the 12 deg only comes into play at idle and just off-idle. The advance curve is well into play by the time you are making any power.

When you say, "The motor really liked the 12 deg though", do you mean that it just idles better? Assuming that's what you mean, then I would just back the timing down to 8 or 10 degrees, for 36+/- total mechanical advance, and then use the vacuum advance to give you an additional 10 to 12 degrees of timing at idle and part throttle.

This way, you will have limited your total advance to your desired goal, and you will have the better idling quality from about 20+/- degrees advance at idle (10+/- mechanical plus 10+/- vacuum advance). Additionally, you can do it this way without having to buy any additonal parts.

I worked in the trade from 1970 to 1985 and tuned hundreds (if not thousands) of these types of cars. Typical factory setup is as I describe above, with base timing a few degrees advanced and then quite a bit of vacuum advance added in.

Dave F. in Rhode Island
'68 Camaro SS Accel DFI 454 (SOLD - I'm a boring Corvette owner now)
Bought my first big-block Chevy in 1970
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post #9 of 12 (permalink) Old Dec 5th, 05, 08:21 AM Thread Starter
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Re: 327 Timing Question

It idled well at 12 and then when I drove it, it ran the best at that setting. I dropped it to 9 ish and it still idled well but the car was a dog. I lost all mid-range power, so back up to 12 it went. Would 10 or 11 btdc make that much of a difference at idle? That 38 deg total just seemed high, maybe that car likes to run that way?? I'm running around 9.75:1 compression and I have not heard any pinging either which is also throwing me off. Why would my readings shoot to 30 deg at 1500 RPMS?? Shouldn't I be around 20? I wish I could drive it now to test these things, this snow is not helping!

1968 Camaro Z28 clone 327 .030 over, Muncie M20, DUI coil, Edelbrock Performer w/1406 , headers, 274 cam
1963 Chevy Biscayne-2 door, 283 2bbl, powerglide, Mallory Digital ignition
1985 Z-28 LG4, Edelbrock hdrs, Lakewood control arms
rusty Z is offline  
post #10 of 12 (permalink) Old Dec 5th, 05, 08:52 AM
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Re: 327 Timing Question

it really depends on how you want to run it.
The (extremely conservative) factory specs for these motors were 4-12 degrees initial (depending on model/year) vacaum on ported (so no vac adv at idle) and total around 32-34.
A "run it how it's happy" performance tune would be much more agressive, 12-18 degrees initial, vac on manifold (full vac advance at idle) and total between 32 and 38.
The more advance you run at idle, the happier it will be, up to the point where you get part throttle pinging. My own 327 loves 18 degrees initial, with another 18 vac on manifold. Idle timing is 38 degrees! But it's happy there. off idle and midrange torque is great, it never ever pings, even lugging down to idle rpm's in third gear. And you don't have to open the throttle much to get a good idle speed, so run-on and overheating isn't a problem. The important thing when you run it like this is to limit the mechanical advance to keep your total under control. detonation at WOT is very destructive, and also hard to hear / feel.





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post #11 of 12 (permalink) Old Dec 5th, 05, 09:02 AM Thread Starter
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Re: 327 Timing Question

That was my concern too. Without hearing the ping I can't tell if it's the right setting. It just runs so much better at 12. I too can run it in third or even fourth at part throttle with no ping. I am running full manifold vacuum not ported which I think helps it out. It never gets above 190 deg while running and it starts ok for me while hot. Once in a while the car will surge in first gear during light throttle, a really slow cruise speed. I attributed this to a high idle speed, maybe the advance is partially to blame.
I may pull the plugs anyway and see how they look this winter. They should be light tan from what I have read.

1968 Camaro Z28 clone 327 .030 over, Muncie M20, DUI coil, Edelbrock Performer w/1406 , headers, 274 cam
1963 Chevy Biscayne-2 door, 283 2bbl, powerglide, Mallory Digital ignition
1985 Z-28 LG4, Edelbrock hdrs, Lakewood control arms
rusty Z is offline  
post #12 of 12 (permalink) Old Dec 5th, 05, 10:00 AM
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Re: 327 Timing Question

Quote:
Originally Posted by JimM
.........The (extremely conservative) factory specs for these motors were 4-12 degrees initial (depending on model/year) vacaum on ported (so no vac adv at idle) ..............

I remember many GM vehicles using "manifold" vacuum for the vacuum advance mechanism. The timing would clearly jump significantly when you connected the vacuum line to the distributor. This would smooth out the idle quite a bit and also make the idle a little faster. "Ported" vacuum was the exception to the rule, but was still used quite a bit. The tuning instructions (mostly my Chilton Manuals) in the old days always said to disconnect the vacuum advance prior to setting timing, regardless of make or model.

Dave F. in Rhode Island
'68 Camaro SS Accel DFI 454 (SOLD - I'm a boring Corvette owner now)
Bought my first big-block Chevy in 1970
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