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Topic: Suggested 71 454 TIming

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Suggested 71 454 TIming (1/10)
2/10/15 1:39am

CI45471

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Vette(s):
71 Red Convertible - Big Block 454

Joined: 8/31/2004
Posts: 7

Need some help on the correct ignition timing for my 1971 454. The table on the cowl says to set the ignition timing at 6 degrees BTDC with the vacuum line plugged at 600 rpms. I did this, then hook up the vacuum line. I then increased the rpm to about 3000 and the full advance is close to 50 degrees, I immediately shut it off.

what is the suggest full advance with the vacuum line hooked up at 3000 rpms?

thx

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Re: Suggested 71 454 TIming (2/10)
2/10/15 2:19pm

Adams' Apple

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Duncanville, TX - USA

Vette(s):
#1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/ Am-Fm/map light National/Regional/Chapter NCRS "Top Flight" #2-1985 Bright Red/Carmine Cp.L-98/auto Member: NCRS, NCRS Texas, Corvette Legends of Texas

Joined: 11/8/2003
Posts: 20129

CI45471 said:...increased the rpm to about 3000 and the full advance is close to 50 degrees, I immediately shut it off.

How close to 50* is it? And is that a steady reading, or does the mark jump around?
I don't know for sure without looking it up, but somewhere around 43-46 should be right, for full advance. Gotta consider that the timing light you're using might be a bit off, the timing chain could be worn, the dist/cam gears worn, etc.
Are you running ported(above the throttle blades), or full manifold vacuum to the dist?

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Joel Adams
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Re: Suggested 71 454 TIming (3/10)
2/10/15 4:58pm

Vman73
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Gig Harbor, WA - USA

Vette(s):
1973 L82 4 Speed Coupe - Very original, well documented driver/survivor. 2016 Bend Regional NCRS Top Flight.

Joined: 10/2/2014
Posts: 405

+1 on Joel's response. I've never seen a full timing spec that included the vacuum advance. I look forward to his response if he can find one.

For what it's worth, I just completed a distributor/timing tune on my '73 L82 SB. I took a lot of information and inspiration from articles written by John Hinckley and Lars Grimsrud posted on Vettetech, see attached link:

http://lbfun.com/Corvette/Tech/vettetech.html

Scroll down and go to the section on Timing and Vacuum Advance, you'll see their articles on mapping your distributor advance and total timing.

My car is now set at 14 degrees BTDC initial timing @ 900 and 36 degrees BTDC total including full mechanical advance @ 2500, both with the vacuum advance disconnected. The factory timing spec for my car was 8 degrees BTDC.

I am using ported vacuum and my stock GM vacuum advance is a 355-15, which indicates 15 degrees maximum advance at full manifold vacuum. I did not take any readings with my vacuum advance hooked up but 36 total plus 15 max vacuum could potentially give me 51 ??

Once my total timing was set and I was happy with the mechanical advance curve, I hooked up my vacuum advance and checked for drivability per Lars' article. No pinging under load and/or no jerking at cruising speed to indicate too much vacuum advance. I also checked for any "hot start" timing issues.

I am currently running Premium unleaded and extremely happy with how the car performs. All of the timing work plus a Quadrajet rebuild according to another one of Lars' articles on the Vettetech site and I'll bet this car is running better than the day it was built.

I realize close to 50 degrees BTDC is somewhat unnerving when we're all used to numbers like 6 and 8 BDTC, but you may not be that far off. It does seem strange that you can get close to 50 with only 6 degrees initial. You may need some adjustment to your mechanical advance springs or a lesser vacuum advance (assuming you're running a stock distributor).

Look for further information from Joel, but these articles may give you some good input, I know they gave me a different perspective on timing.

Check out the rest of the articles on that website, it's too bad that some of the links are blown, I've saved every bit of information I could to my hard drive. Most of that stuff is "priceless".

|UPDATED|2/10/2015 1:58:54 PM (AZT)|/UPDATED|

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John Sigmund

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Re: Suggested 71 454 TIming (4/10)
2/10/15 11:37pm

CI45471

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Vette(s):
71 Red Convertible - Big Block 454

Joined: 8/31/2004
Posts: 7

Thanks for the information. I will look it over and work on my baby this weekend.

It is a stock distributor and I replaced the button cover on the advance pin before I installed the distributor. It is a fresh rebuild with new cam and timing chain.

Thanks again for the info.

I am planning on going to BG and Corvettes of Carlisle, hope to see some members there.

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Re: Suggested 71 454 TIming (5/10)
2/11/15 1:52pm

Adams' Apple

Direct from GM, here are the factory specs for the distributor/timing. According to this, your TOTAL ign timing(for the 425hp engine) would be all in at 43deg @ 6000rpm for the 4 speed trans, and 40deg @ 6000rpm for the automatic trans . Total with the 365 hp 454 would be 42deg for either trans. If you're showing 50 deg @ 3000rpm, you should be getting some pinging when hot, it would seem. Wonder if the timing mark on the balancer is correct....

I have a PDF file with EVERYTHING you could possibly want to know on the '71 I can email you. It has the above info also(on page 126). This is similar to the info you would get if you ordered the "Restoration" package from the NCM. Thumbs Up

|UPDATED|2/11/2015 10:52:01 AM (AZT)|/UPDATED|

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Re: Suggested 71 454 TIming (6/10)
2/11/15 3:24pm

Hey Joel if you have the same specs for a '73 could you post it or send it to my e-mail noted below. I have a '73 Service Manual that comes close to those specs. Unfortunately it does note list the maximum advance at RPM and it only lists the vacuum advance specs for an L82 AT, mine is a 4 speed. Thanks in advance if you have them, John.

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John Sigmund

valkman57@sbcglobal.net

NCRS Member 61302

NW NCRS Chapter Member

Duct tape is the new Black !!

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Re: Suggested 71 454 TIming (7/10)
2/11/15 11:16pm

Adams' Apple

I'll dig and see what I can find, John. Smile

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Re: Suggested 71 454 TIming (8/10)
2/12/15 12:38am

CI45471

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Vette(s):
71 Red Convertible - Big Block 454

Joined: 8/31/2004
Posts: 7

Great info, I have two timing lights, one static and one with an advance capabilities, I will check both.

This is a great place to get information.

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Re: Suggested 71 454 TIming (9/10)
2/12/15 10:16am

Adams' Apple

Vman73 said: Hey Joel if you have the same specs for a '73 could you post it or send it to my e-mail noted below. I have a '73 Service Manual that comes close to those specs. Unfortunately it does note list the maximum advance at RPM and it only lists the vacuum advance specs for an L82 AT, mine is a 4 speed. Thanks in advance if you have them, John.

E-mail sent...check page 55 for ignition timing specs... Thumbs Up

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Re: Suggested 71 454 TIming (10/10)
2/13/15 2:40pm

Thanks Joel !!

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John Sigmund

valkman57@sbcglobal.net

NCRS Member 61302

NW NCRS Chapter Member

Duct tape is the new Black !!

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