69 BB L68 timing on pump gas
What would be the correct timing for my L68 using todays 91 octane pump gas? The manual says 4 degrees Before TDC. Should I use that or is there some adjustment to that spec?
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Re: 69 BB L68 timing on pump gas
All depends on what you want out of your car. If you want performance, keep advancing the timing until it rattles under load. Then back if off. Your car will tell you what the optimal timing is. If you want to protect the 5/50 factory warranty, set it at 4 degrees BTDC initial.
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Re: 69 BB L68 timing on pump gas
I'm not an expert on this - but I think getting your 427 with 10.25:1 compression to run on 91 octane pump gas is going to be a difficult task.
You could accomplish this task if you were starting with 93 octane.
If your engine is stock with cast heads, I think 91 octane won't work...
Maybe there is a small possibility by rejetting the carburator and working on the recurving the advance on the distributor.
Hopefully I am wrong --Comment
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Re: 69 BB L68 timing on pump gas
Bill,
I think your right on.
Octane is what it takes to get them to run the way they were designed.
They will run OK if you do what was advised by Dennis but to bring them to life is good old octane.
I use a mixture of avgas and pump gas and when I am on the road and have to put pump gas in I do as Dennis recomends and I have to retard my timing because it pings with pump gas.
The percentage of power lost with pump gas is much more than was posted in previous threads.
DOMComment
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Re: 69 BB L68 timing on pump gas
Richard
The factory recommended timing setting of 4 degrees BTDC was extremely conservative for a big block and undoubtedly helped keep the warranty claims down. It was also intended to reduce emissions in conjunction with the A.I.R. system. Is your engine completely stock and original? Do you have the A.I.R. system installed and functional?
My engine has been rebuilt with .030 over forged pistons. I am running neither the A.I.R. system nor the stock L36 cam...but rather a mild Comp Cams 268H hydraulic flat tappet. My C.R. is no more than 10.25 to 1. All I can tell you is that I am running around 15 degrees initial advance (no vacuum) and 21 degrees centrifugal advance for a total of 36 degrees by 2800 rpm. Big blocks like a lot of timing and I can run 38-39 degrees total without detonation. I have connected my vacuum advance to full manifold vacuum (as opposed to ported vacuum) for better idling and cooler operating temperatures. I normally run on 93 pump gas with 10% ethanol. I have put in 91 octane no ethanol and added 10% of C12 leaded race fuel at 108 octane. No real difference in performance and still no pinging.
Maybe someone else can chirp in here about factory recommended timing settings...and what that 4 degrees initial (no vacuum) will get you in terms of total timing...assuming stock distributor curving? You need to get a timing tape on your flywheel damper or an advance timing light in order to determine what the total timing is that your engine feels comfortable with...given the fuel that you are feeding it. Have fun waking that bad boy up while keeping it together in one piece.
DennisComment
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Re: 69 BB L68 timing on pump gas
There is no one "correct" setting for every engine and every driving condition. There are too many variables.
As a general rule, start off with the OE recommended setting. If you detect detonation under worse case conditions, back it off in two degree increments until the detonation is quelled.
If it doesn't detonate, and you want to optimize, increase initial timing in two degree increments until you detect slight detonation, then back it off a couple of degrees.
Keep in mind that if you run these experiments at 70F ambient and then drive it later when it's 90, you might get some detonation.
High coolant and/or ambient temperature increase the tendency to detonate, so everyone has to come up with their own "tune" that works best for their engine and their driving condtions.
Where you live can be a big factor, too. The higher your altitude, the more initial you can run.
DukeComment
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Re: 69 BB L68 timing on pump gas
As far as I know it is stock. It was rebuilt back in 1983. The paperwork doesn't show any special work done on it so I'm assuming it was just rebuilt.
I'm in Fountain Valley, CA pretty much near sea level.
I just set it a 4 degrees BTDC and it is dieseling when I shut down. So I'm going to bringing it back 2 degrees and see if that helps.
Thanks to everyoneComment
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Re: 69 BB L68 timing on pump gas
No, no, no! "Dieseling" on shutdown is an indication of too little total idle timing, so retarding the initial is the absolute WORST thing you can do.
The first thing to check is the VAC. What's the number? What are the specs and does it meet spec? Is it connected to a full time manifold vacuum source? What is the idle speed/manifold vacuum in neutral assuming it's a manual transmission.
Don't assume anything? Most engines that were rebuilt in the last 30 years had been unthoughfully modified. That screwed up a lot of engines!
Chances are you engine is not really and L68 anymore.
DukeComment
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Re: 69 BB L68 timing on pump gas
What I meant was instead of 4 degrees BTDC I was going to go to 2 degrees BTDC and work may way back until it stops. At TDC it didn't diesel on shutdown.
I will check the vacuum tomorrow night when I get home. All the vacuum lines are connected. The idle speed was 800 RPM, I did disconnect and plug the vacuum line at the distributor.
The work done on the engine back in 83;
Valve job
Mill heads
Press Rods
Recondition Rods
Bore Engine
Mill block
Balance Engine
Based on the parts used I do not see any special components
Guides
Crank kit
Timing Comp
Valves
fuel pump
Rocker armsComment
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Re: 69 BB L68 timing on pump gas
So you are saying that it diesels at 4 BTC, but not TDC?
Frankly, that doesn't make any sense. Remember that your spark advance map is set up for emissions, so it's retarded from optimum for nearly every speed/load condition, including idle
I don't have spark advance map data for '69, so it would help if you would post it. It's in the CSM and AMA specs that you can download from the GM Heritage site.
Most likely, as originally built, it has ported vacuum advance and a short, lazy centrifugal curve.
Reworking the spark advance map to something close to the '66 L36 would be a good place to start, and you can probably improve on that with some experimentation.
The L36/68 cam should yield about 14" Hg manifold vacuum in the 600-700 idle speed range with about 25 deg. of total idle spark advance.Comment
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Re: 69 BB L68 timing on pump gas
OK, I'm not sure I know what I'm doing but I did what I think was a vacuum test mapping by connecting the vacuum gage at the distributer. Timing is set at 2 degrees BTDC.
Here is what I measured:
RPM Vacuum
800 0
1000 2
1500 9
2000 13
2500 18
3000 21
3500 21Comment
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Re: 69 BB L68 timing on pump gas
You have ported vacuum advance. That's one of the reasons it runs-on after shutdown - not enough total idle advance, which heats up the combustion chamber boundaries to causing auto-ignition. Convert it to full time and advance the initial timing. It's been discussed in the archives a zillion times.
You may also need a new VAC. What's the data stamped on the installed VAC?
DukeComment
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Re: 69 BB L68 timing on pump gas
The only number I see is the 360, so I assume it is the original OEM VAC. Where would I find the date code?Comment
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Re: 69 BB L68 timing on pump gas
My 67 390HP ran with no problem with 4 degree advance and 91 octane califronia gas. It did not run on after the key was turn off.
JeffComment
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Re: 69 BB L68 timing on pump gas
That may be original, but you need to tell us the AMA spec data to determine if it matches the "360". There is no date code on VACs.
The next question is does it actually work. Does it meet spec?
And while your looking at the '69 AMA specs tell us the centrifugal curve.
DukeComment
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