Not a corvette (69 Firebird) but know some of you run this engine
GM crate
Spec sheet says time at 10 deg!@750 rpm...cant find where it does or doesnt say wether or not it uses vacumm advance.
Doesnt one of thier crates not use it? This or maybe the 383 crate
Says everything on the engine is as delivered but his vaccum adv can looks super old made me wonder.

Will pull the cap and see what kind of dist it is.
Some knucklheads at the body sho pit was in decided to get it running and screwed teh timing and carb adjustments up royally.
Got it running pretty good but think it could be even better. Will drive it something this next week. Sounds throaty for what it is.

 

Your Spec sheet should say max timing as well. Put a timing digital light on it and see if your engine falls in spec. if your running a manual trans I wouldnt be concerned with having a vacuum advance can.

 

I would have smacked the **** out of someone ******* with my timing......you might as well try to bang my wife. IDK why it is like being **** struck when you get a group of guys around a Hot Rod they want to **** with the timing......I have lost friends and customers over this.....Morons......I get so pissed because not only do you know squat about engines....but you make the owner of the damn thing 10x confused when he gets it back.
I met a guy in my neighborhood, friend of a friend, who has a 69' Super Bee 383......he drug me over there to talk to him as it doesn't run right. You would think I was speaking a foreign language and threatning his Mom or something.....**** these people.....they have no business being around ANY old car. If your grandfather set timing by ear then I never want to meet him. Ever.
That said.....the HO has Vortec heads and set the total timing for 32-33 degrees......let the initial fall where it falls....initial is not important to anyone except the fella at GM that printed the specs. You do not drive or operate at idle.....and initial is "set" at idle.
Always use vacuum advance on a mild engine like this......my 406 is way more radical and I have it hooked up....it only works during idle and cruise......
About 1 in 10 know how critical ignition timing is and how it works.....the other 9 twist the distributor until it "sounds" better.

Jebby

 

At first I was quiet after reading the post above.Then I got a good laugh as I needed one today. When I was a teen my father taught me how to set timing by ear, vacuum gauge and by light. LOL. I also remember him setting up a mag in a an alcohol rail with a box with a couple lights on it back then too. Wow how things have changed.

 

Didnt check total, used a dial back and set initial where its supposed to be.
We are sticking a mach. fan, shroud back on it so he can put some miles on the car.
Another one of the "mechanics" or "Me cans" stuck an electric fan on with one of those probes in the radiator;
Put one wire to ground and another straight to the battery. Called my friend the next day and said the car needed a battery.
Thats when he pulled his car out.
Ordered a kit with a thermostatic switch, relay etc and do it right as a secondary cooling source
Remember the first shop i worked in as a teen (80s era) the guys who insisted you dont need a timing light.
Especially with a TBI car! EFI was a mystery to all back then.

 

So use a dial back and set the total where it is supposed to be for power and efficiency....
Initial means nothing.
Total will be set at or above 2500 rpm....wherever the advance stops climbing.
I am telling you this makes all of the difference.

Jebby

 

But..... Initial timing is responsible for a good idle quality and agressive off idle drivability. We dont have knock sensors or ion sensing systems to control ignition timing on these old engines designs but the whole objective to the timing puzzle is to produce a spark as most advanced as possible without a damaging predetonation ruining a piston but not have a retarded lethargic revving engine that gets poor fuel economy and exhaust that sounds likes its under water.

 

And that lethargic revving sound was every GM car built from 1955 into the 80’s.....
There is a lot to understand how the mechanisms work on any distributor.....and you can control the time via curve.....but initial is a result of total timing.
You do not set timing via the initial....the initial number was a ballpark number that GM’s lawyers and warranty counter came up with.
You have a lot to learn about curves and timing....you should ask member Lars for his timing papers.....
Setting timing with a vacuum gauge is wrong
Setting by “ear” is wrong
Basically.....if you are not getting optimal total timing at elevated RPM’s via the curve....the engine with not run as intended....
I can curve a distributor that will match a computerized map.....no problem. GM, Ford, Chrysler....no one did this....
The first thing a “speed shop” would do in the 60’s was to curve the distributor and set total timing to optimum....the optimum number was determined by a lot of dyno testing by builders and racers and still is.
I know exactly what timing does.....for example, a 75’ 350 L-48 Vette will pickup about 40 horsepower at the tire by advancing the timing to 36 degrees total and changing the advance curve weights....no other changes. The factory initial spec was 5 degrees I believe....which put the total at about 24-25 degrees! To top it off....the vacuum hose was put to ported vacuum....which did not work at idle.....so advance timing to 36 ends up with about 15-16 initial and then another 12 or so by using full manifold vacuum.....
The car will be transformed.....and ALL cars back then were this way. This is fact.
Proper timing and curve is critical to performance....and initial timing is set by the total timing and not the other way around. There is no initial timing “spec”.....it is a misnomer, and a bad one because trying to convince people for 30+ years about this is exhausting.....
I have set timing on about 500 cars or more in my life....I would say 50 had a correct optimized timing curve and proper setting.
Thing is that your Dad, my Dad....the corner mechanic, your buddy, his buddy did it wrong for 65 years because not only did they not know what they were doing, but because the auto manufactures told them to do it that way.

Jebby

 

I hear what your saying and you have a lot of info for sure. I dont know how many engines i've tuned though. Never counted. I once had a dyno session over 200 runs though. There is tuning as in tune up specs recommended by manufactures and there is tuning for discovery of what works best for a given build. I get it.

 

I agree with 32 total, Vortec head.

 

Do yourself a favor and read Lars Grimruds info on how to set timing, you learn details and do it perfect right from the start:

http://corvette-restoration.com/wp-c...ine_Timing.pdf

According to him or Aftermarket specialists like comp cams and crane or holley it melts down to following:

- Static initial timing ranges for mild/stock to wild engines from 10° to 20°.

- Dynamic initial timing (set via Vacuum advance) - in case you use it - should add so much that you end up around 25° idle timing (maybe +-2°)
-> Use for example Crane adjustable vacuum advance kit attached to manifold vacuum.
So if you set 14 intitial static, adjust the vacuum cane in such manner, that it adds 10-12° at idle vacuum level.
So when you cruise you have for example 32° total plus 10-12° vacuum, which increases MPG.

- Total timing (set via bushing - MSD range is from 18° to 29°) should be 32-34°, with hotter engines and premium fuel maybe 36-38.
So for example with the black MSD 18° bushing and 14° initial you end up with 32°.

- All-in-rpm (set via springs - MSD range is from 1500 rpm to 5500, depending on combo) should be around 3000rpm (+- 500rpm)
Can use 2500 rpm (lighter cars, higher gears, premium fuel) or 3500 (heavier cars, lower gears, low quality fuel).
So for example with the two light silver springs and the black bushing and 14° intial timing you would have 32° all in at 3000 rpm.

Always aproach timing from the save side: rather too low/late than too high/early (Detonation takes one revolution and the engine might be gone. Happened to me)
AFR is equvalent: rather too rich, than too lean.
better too save, than too sorry.
better drink in smaller steps and enjoy the party till the morning than to pass out before midnight

PS:
Hot Big blocks go 24° sometimes - makes all the difference between bogging and thrusting, between undrivable and a pleasure.
Those often dont use vacuum advance or if they do, they used ported, which is ok in this case, since static timing is already high enough for proper idle.
So vacuum only comes into play when crusing high-rpm/low-load on highways for better mileage.
Some even go locked out with 30°+ timing, mostly in combination with automatic and loose converters, high gears and good fuel and/or higher altitude areas.
In those cases - due to the super long cam durations - cylinder pressure bleed off is so high in low rpm area and dyn. compr. ratio therfore so low, that just those mixtures accept high WOT timing (20°+) in low rpm area below 3000rpm WITHOUT detonation.
Stock cams / engines would detonate IMMEDIATELY down there due to high dyn. compr. ratio, caused by the short cam duration.
With Cam-Duration you directly alter the pressure situation and therefore timing capability / detonation-willingness of the cylinder charge.

PS2:
Adjustable timing lights show errors on capacitive discharges ignition systems (Info from MSD technician).
Dont use the adjustable feature on CD Systems like MSD 6AL or similar.
On HEI it would work.

 

Hey Jebby,
No way would I even think about messing with your timing, but what is your wife's name? Sorry, I couldn't pass that up. Please don't come looking for me.

So after reading your passionate explanation on timing, here's a question for you based on some really vague, limited info. I have a 76 L-48 with what I believe to be a mild cam, dual exhaust, FiTech EFI and an upgraded distributor. The PO made most of the improvements with the exception of the cam, which he said was done before he took possession of the car some years ago. He said he liked to tinker, and do a few rudimentary things on the car. My question is this. Is it likely that my engine has had the timing adjusted by some clusterhead that does not understand the intricacies of this process the way you do? The car seems to run well enough, but I would certainly like to get all that I can out of it, in spite of its limitations. I know your are probably thinking, "How in the f*** am I supposed to answer that?" Just doesn't seem like your run-of-the-mill good ole boy mechanic would have set the timing the way you have described.
* I am mechanically challenged and have not messed with timing since my 55 Chev almost 50 years ago.

 

I always appreciate a good sense of humor LOL!
Anything is possible, but I urge you to check it yourself using the super easy to understand guidelines in the papers from Lars....it doesn’t get any better trying to describe it.....
If you can pick up a wrench, hook up a timing light....you can set timing.
There is no mystery....that is why I have problems understanding why millions of ********* told each other to do it exactly as wrong as you could possibly do it for over 60 years......amazing.
BTW....my wife’s name is Carmen.....as in Carmen for your ***

Jebby

 

Sh*t Jebby,
Now I'll be hiding in a safe house somewhere, looking over my shoulder for a Corvette guy on a mission to end me. Damn, I should have followed a personal credo: never pass up an opportunity to keep your mouth shut!

Thanks, I'm going to do some reading and research, borrow a timing light, follow Lars's notes and dive in. Comes a point you just gotta say WTF? Thanks for sharing your wealth of Knowledge on the forum.

 

Not me you have to worry about.....we own a lot of guns down here LOL!
Read the papers slow and have confidence.....follow instructions.....it really is simple.

Jebby

 

Jebby,
I have to say I wouldn't mind joining you down there. Visited Nacogdooches and Frisco a few years ago when our kid was playing football, and I really liked it. The folks there were great. The political climate here between Portland and Seattle is so f***** up. Just think, we could be neighbors, and you could work on my car for me, and we could drink beer together. And I would promise not to hit on your wife.

 

Hey Jebby,
I started thinking about a timing light, and stumbled upon another related thread, in which a fella was inquiring about a particular light and taking recommendations. OTC 3367 came up frequently, They run close to 3 bills, and I assume, are used by the pro's. I am as far away from a professional mech. as a guy can get. Can you recommend another less expensive timing light for a true novice that will suffice? I understand one gets what he pays for, but since I'm the weekend warrior guy, trying not to f*** it up, I don't think I need top of the line stuff. Thanks.

Tom

 

I actually have the Bosch Re-Box copy CP7529 which is $90 on Amazon

Jebby

 

Jebby,
Thanks a bunch; I may just go pull the trigger on that one and pretend I know what I'm doing. Brace yourself for a barrage of questions.

 

I have a GM 350HO in my 1970 coupe. +1 for Lars Grimsruds' timing paper. I had Lars rebuild my original distributor some years ago and I followed his timing procedure after I installed it in my 350HO. The engine runs clean and strong when set by his method.