Hey guys, long time no see. Been too busy driving the cars...

I got a strange question, that I couldn't find the answer to by reading the archives.

I have a 69 coke bottle, with the 427 390 horse motor (w/ an auto). the engine was professionally rebuilt a few years ago, but since then, I removed the points and installed a pertronix. My question is - will either of those things change where the engine should be timed?

The book on it says timing should be 4 degrees BTDC. On the notches by the dampener, i can see the "0", and then below it says "R" and above it it says "A". If the timing is supposed to BTDC, shouldn't it be in the "R" range? I've been running it all over town, stopping, changing the timing, and recording my findings, but the engine seems to run better at 4 degrees in the "A" range? Is that BTDC? I'm confused. Somebody give me some insight; I'm dizzy from continually looking at that strobe light.

And another thing - I don't have a working tach. The cable must be broken or something. And even if it did work, the area where the car seems to run best, the cable won't fit on the distributor, because a hunk of manifold gets in the way. Is this common?

And advice, would be greatly appreciated.

 

The timing is based on the engine, not the distributor, so you should still time the engine the same. However, different distributors have somewhat different timing curves. As such, you should really be timing your engine for the total mechanical advance, not simply the advance at idle. The "A" range you describe is advanced, the "R" is retarded. Advanced = BTDC.

Most people recommend about 36 degrees total mechanical advance all in between 2500-3000 RPM. The initial can be as far advanced as about 16 or so degrees (with vacuum advance disconnected).

I have heard a lot of complaints around here about the Pertronix ignition systems. You may be better off with a stock HEI style if you don't want points. Otherwise several other companies make good products (MSD, Accel, etc).

I can't answer your question about the Tach hookup as I have an HEI without the mechanical hookup myself. I also don't have a working tach, but that is because my tach is the mechanical style and the HEI I have isn't

Lars has a good writeup over at http://www.corvettefaq.com/lars.asp about setting the timing curve for optimum performance. Definitely worth a read.

-dath

 

Thanks for the answers dath. I think it might actually be timed correctly, if "A" = BTDC. I have it at 4 now, and it seems ok.

To do it right though, you're right, I do need to look at the whole. I just need to get my tach working again. I read something somewhere about an "elbow" that you can link up, to make that angle less harsh; I'll go back and look through the after market catalogs again.

Thanks again.

 

No problem. I'm pretty sure you're right, I think I heard about the same elbow at some point along the way. Never was of too much interest to me as I really just want to get an electronic tach and stick with the HEI I have. . .

 

Get a new dampner?

 

You don't need an elbow, you just rotate the dist so that the cable goes straight in... Move the plug wires if you have to...

A means Advanced which means before...

So yes you want 36 degrees Before top dead center which is the same as 36 degrees advance

 

A stands for advanced

R stands for retarded

the real question is each mark one degree or 2 degrees. on most SBC each mark is 2 degrees. the vette is the only BB i own so i got nothing for comparison.

ive got my 427 390 horse at 4 marks "A" right now. either stock or twice stock depending on what the marks mean. much more and the car pings on the highway when its hot. and any more and it vapor locks when i try to restart hot. but if i run more timing it runs better cold. dont let good cold running fool you. what matters is when the engine is at very much operating temp.

also at the 4th tick mark the car makes more power up top than with more timing. and by more power i mean it doesnt ping. i wont claim my car is in good shape. its possible im leaning out or any number of other problems. im also experiancing some sort of intermittant problem yet to be sorted out. (not pinging, probalby spark cut out of some sort) but more timing is definately not a good thing right now on my engine.

does anyone know what the total advance is on a stock 390 horse dizzy is just for funnies?

i dont buy into this 36 degree stuff one bit. its physically impossible for every engine on this board to run best at 36 degrees total timing. more compression means less timing. higher flowing heads mean less timing. good squish/fast burn means less timing. hotness means less timing. and other crap like "im a hick with race gas in a 9:1 motor means way more timing. being overly rich means more timing. running a 160 thermostat (which is bad for everything IMO) means more timing.

therefore it is only coincidence if our 427s run best at the same total timing as a POS late 70s small block or a built high output motor.

you want your timing to have only as much advance as your worse case scenario can handle. pretty much really really hot outside with alot of WOT and the worse gas you will ever run. (after some trial and error i have found stock to be my current answer) this is probably because in 1969 higher octane gas was available.

so besides all my ranting what is the stock total advance on a 427 390 horse?

 

if you do need the 90 degree adapter .corvette america has one pn 4415 $ 49.00

 

Pull the distributor out and rotate gear one tooth in direction you'd like to turn dist ... that should get you some more clearance while permitting you to set the timing where you want it.

 

You don't need to pull the distributor. The GM Service Bulletin that addressed this exact issue in 1969 describes the process just as I demonstrate it on my Tuning Tours: Simply rotate the distributor 45 degrees clockwise (each plug tower is 45 degrees). Then, swap all the plug wires in the cap 1 position counter-clockwise. This will straighten out the tach cable and give plenty of timing adjustment.

Don't set the timing to 4 degrees BTDC. You need to set it to 36 total, which will generally result in an intiial timing in the 12-18 degree range. Go to Sears and spend the $69.95 for an adjustable timing light so you can set your timing correctly and get some performance out of the car. 4 degree timing is going to make your Vette run like a dawg... and we can't have that..!

 

As Lars has noted, consider this solution for the tach problem.

 

Moving the wires is probably an easier solution ... rather than turning the gear as I'd suggested earlier.

 

You guys are awesome. Thanks for all the replies.

As for the last few suggestions, I'm terrified of removing any wires or the distributor in general since the car is running OK now. It's not incredibly fast, but considering it has the original carb on it, I'm not too concerned.

What happend is, as I go to spin the distributor, where the tach cable screws in, the screw fitting on the cable hits against a chunk of the manifold that sticks up. With the cable unplugged, it spins all the way. I was seriously considering grinding it down, but thought that might be a horrible idea. Since the tach doesn't work anyway, I'm dismissing it for now.

Eventually, I'll probably try one (or both) of those suggestions, and hopefully the car will still start (I have a tendency of making things worse). But as for the "moving the distributor a tooth" idea, I thought on Chevys the distributor could only go in 1 of 2 ways, because of the groove on the bottom of the distributor (which I believe goes into the oil pump). Is this true? Is this only on small blocks?

 

an adjustable timing light is one of the best (and cheapest) investments you can make. eliminates the hassle of reading faded or non-existent timing marks. a real time saver.

 

Yes & No. With the distributor out, you can reach down in there with a long screwdriver and key into the oil pump intermediate shaft ... and turn it to any position you choose. There are thirteen teeth on the dist gear ... you COULD drop the dist into 13 different positions so long as you first turn the intermediate shaft 1/13th of turn each time. It is commonly required to do this in order to get the dist aligned where you want it. It is unlikely that you need to grind anything ... it is not needed if you have OE intake & dist. Same for sbc or BBC.

 

Dunney, whatever you do, follow Lars's advice here, he is correct, that is the simplest solution to your problem. If you are really concerned about it and don't want to mess with it much, just get a timing light and check your timing now (with the vacuum advance disconnected) and then set it back to where it was once you turn the distributor and move the wires. It really is no big deal. It shouldn't take more than an hour, maybe an hour and a half tops.

Messing with the timing is fun!