Timing on vintage 302 Z/28 A/S

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Vintrcr
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Timing on vintage 302 Z/28 A/S

Post by Vintrcr » Tue Aug 14, 2018 4:20 pm

Looking for advice on total timing for a vintage correct 68 Z/28. Stock bottom end. Standard bore, factory pistons. Jones solid flat 252/260 .544/.525 108L/C. Crower stainless 1.5. Smokey Ram , 830 Holley.
Heads are GM slant plug “492” #336746.
Should I expect best results with less total, say 34* with the slant plug heads?

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Re: Timing on vintage 302 Z/28 A/S

Post by GARY C » Tue Aug 14, 2018 5:43 pm

Vintrcr wrote:
Tue Aug 14, 2018 4:20 pm
Looking for advice on total timing for a vintage correct 68 Z/28. Stock bottom end. Standard bore, factory pistons. Jones solid flat 252/260 .544/.525 108L/C. Crower stainless 1.5. Smokey Ram , 830 Holley.
Heads are GM slant plug “492” #336746.
Should I expect best results with less total, say 34* with the slant plug heads?
Do those pistons have a dome?
I built one years back with a good size dome I think it was it the 11.5 compression range and it wanted 42 degrees to keep the headers from glowing, You will probably have to find a way to test it, track or dyno to find out the exact needed for your combo.
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Re: Timing on vintage 302 Z/28 A/S

Post by Vintrcr » Tue Aug 14, 2018 5:54 pm

Yes, they do have a dome, and right about 11.5:1.
I expected to need about 40*, but have been told the slant plug location requires less timing.
Only had the car on the track for its maiden run, and didn’t have an opportunity to try different timing.

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Re: Timing on vintage 302 Z/28 A/S

Post by F-BIRD'88 » Tue Aug 14, 2018 7:07 pm

What distributor? What is the mech timing curve?
Fuel used ?

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Re: Timing on vintage 302 Z/28 A/S

Post by F-BIRD'88 » Tue Aug 14, 2018 7:09 pm

What distributor? What is the mech timing curve?
Fuel used ? If you are road racing it rather than 1/4 m drag racing it will want a bit less total wot timing.

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Re: Timing on vintage 302 Z/28 A/S

Post by Vintrcr » Tue Aug 14, 2018 7:19 pm

Factory GM distributor with Hays Stinger conversion.
18* initial, 40* all in by 3000rpm
Road race usage with Leaded 110

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Re: Timing on vintage 302 Z/28 A/S

Post by fdicrasto » Tue Aug 14, 2018 7:27 pm

You didn't mention combustion chamber cc's. If you are running a "stock" 302 bottom end and the angle plug heads you shouldn't need more than 38-40* total. If you are road racing, 38* would be safer.

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Re: Timing on vintage 302 Z/28 A/S

Post by F-BIRD'88 » Tue Aug 14, 2018 11:12 pm

I would not expect it will need more than 37 deg. But it may well want more initial timing at idle. 26 ish.

Thus ID try a 10 ish deg advance curve and play with that a bit.

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Re: Timing on vintage 302 Z/28 A/S

Post by F-BIRD'88 » Tue Aug 14, 2018 11:14 pm

You can try a 110 octane unleaded race gas , (with a lead substitute built in. Will probably make more power yet want a bit less timing too.

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Re: Timing on vintage 302 Z/28 A/S

Post by GARY C » Wed Aug 15, 2018 4:15 am

The big dome can hurt flame travel, mine were the older speed pro pistons I think it was a .500 dome and I was running c12, 1/8th mile drag race.
I wouldn't know how to test timing on a road course, it's pretty easy on the drag strip.
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Re: Timing on vintage 302 Z/28 A/S

Post by wwmtlineman » Wed Aug 15, 2018 12:18 pm

Long time ago, early 80s, but we used to run 38 in our NHRA stocker 68 Z/28

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Re: Timing on vintage 302 Z/28 A/S

Post by Vintrcr » Wed Aug 15, 2018 2:54 pm

Thanks for all the replies.
I’ll give 37-38* a try next time out. The car hadn’t been run since the 70’s. The original owner/racer still claims his best times were run with 34* total. I’ve always had best luck in the 38-40* range. The slant plug heads are new to me, thought that might make a difference with flame travel and the timing required.

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Re: Timing on vintage 302 Z/28 A/S

Post by F-BIRD'88 » Wed Aug 15, 2018 4:11 pm

Start at 34 degrees then... Too much timing is bad.
Watch your plugs. All of them.

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Re: Timing on vintage 302 Z/28 A/S

Post by F-BIRD'88 » Wed Aug 15, 2018 4:14 pm

Here is a trick. Blend in some 92 octane Street pump gas in with the 110 leaded. It will tend to speed up combustion add a bit if power and not be so timing hungry too. All while still plenty of detonation resistance "headroom".
Start at 34 deg.
My SBC stuff has always wanted 34-36 deg.

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Re: Timing on vintage 302 Z/28 A/S

Post by Truckedup » Wed Aug 15, 2018 4:17 pm

F-BIRD'88 wrote:
Wed Aug 15, 2018 4:14 pm
Here is a trick. Blend in some 92 octane Street pump gas in with the 110 leaded. It will tend to speed up combustion add a bit if power and not be so timing hungry too. All while still plenty of detonation resistance "headroom".
Start at 34 deg.
My SBC stuff has always wanted 34-36 deg.
Are you saying the pump gas added to the race fuel makes the race fuel burn faster? If so, you have proof of that?
Motorcycle land speed racing... wearing animal hides and clinging to vibrating oily machines propelled by fire

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