Mortorq

Greetings all,

A friend of mine h1200 miles away down in scenic Austin has the factory one year only(near as I can tell)computer controlled Quadra jet L81 350 in his 1981 Corvette.



Seems to be the 350 version of our ubiquitous LG4 305.



He is only putting in mid length headers, and a true dual exhaust.



Is there any way to mildly adjust the advance curve in the crude 1981 ECM to bring it in slightly earlier, like the analog version of putting in lighter advance springs in a standard HEI?

Thank you!

sofakingdom

iTrader: (1)

Rotate the dist body counter-clockwise. Done.

The CPU chip is one of this family; 6809 if memory serves, though it may very well not. https://en.wikipedia.org/wiki/Motorola_6800 Not sure if its ROM is copy-protected like some later ones (Microchip for example https://www.microchip.com), if not, wouldn't be all that hard to download it and disassemble it since it's only 1k bytes or something, and re-write the tables. Problem then would be re-flashing it.

Mortorq

Understand Sofa thank you. I wasn’t specific enough again. Wanted to bring the advance in earlier without altering the total amount of advance. If that makes sense.

sofakingdom

iTrader: (1)

Yes it does, of course.

See Motorola 6800 series. Learn all you can. Pretty sure you're going to come to the same conclusion everybody else has for the last 30-some years.

Rotate the dist body counter-clockwise. Pretty sure that's as close as you're gonna get in the near term. Not sure there's much else in ANY term.

I might offer though, if this is a LG4 car, get the "chip" out of a L69 car. Probably doesn't have to be a F-body. A MCSS would probably work exactly the same. After all, the "chip" doesn't particularly care about the sheet metal wrinkles.

Mortorq

File this under, “Now why didn’t I think of using an L69 chip?”
Perfect idea.
Thanks again Sofa, I owe you one.

BadSS

The 1981 Computer Controlled Carbed Corvette is a one year only 8.2:1, 350cid. I'm not familiar with this "one of" CCC system - I did a quick search for the knock sensor part number and couldn't find one. Reason I was looking, and you need to continue to look, is that if it does have a knock sensor, and I would think that it would, more than likely it will be different from the one used in the L69 305s. That could be problematic if swapping "chips", assuming that the "chips" are physically the same. If you find the chips are swappable, pretty sure it will need to be one for an ECM (1227169), or earlier than 1987 "8079" ECM.

Now, if you can find a pinout of the ECMs of the 81 corvette and the L69, it's possible you might be able to swap out (plug and play) to a later 1987 and up L69 "8079" ECM. If so, you could get a 1987 305 knock sensor and wrap it rather generously in Teflon tape and tighten it no more than it takes to prevent the knock sensor from leaking. That's what I've done to get the L69 ECM/chip to work OK with larger engine swaps.

Saying all that (and you're going to need to research and make sure I'm not leading you astray), the ignition curve on the L69 is fairly conservative. So,,, I'm not so sure it would be any more aggressive, being a 9.5:1, 305, than the one they would have used for the 8.2:1, 350.

If it were me, no more mods than just exhaust, I'd disconnect the 4pin connector at the distributor, bump the base timing to 10-degrees and set the rich stop on the carb up 1/8 turn. Probably needs at least a hanger change if not one bump up on the rear metering rods. I'm going to say the minor carb tweaks to get it right for the freer flowing exhaust will help more than swapping to some differences unknown ignition curve.

Edit: Not that you asked, but the heads and cam for the L81 are pretty anemic - a decent set of 64cc heads and even a small performance cam would really wake up that thing.

Vader

It seems like I've been through this deja vu once before, but maybe it's just a dream...

Nope, That's another '81 Y-body from a couple decades ago. Is this also a white '81 with a saddle interior?




I still have all I need to read and write/burn old EE and UV PROMs, but never disassembled the '81 code due to its lack of market.

Just as BadSS suggested, the middle and top parts could stand some upgrading, and even stock parts could be a great improvement. This one got later (reworked) '416 heads to bump compression, and a new stick.



And the recommendation from RB about twisting the spark spinner is about as good as it gets with the stock ECM. This later got an older big cap unit with its very own springs and weights to wake it up even more.

Eventually it got a separate TCI control to deal with the spark and run the 6L60E that replaced the old TH350 with the also one-off lockup scheme. Ultimately, it ended up with a Dart block, and was dubbed the "E85-385" since that's what it burned and how much it displaced.

Mortorq

Vader & BadSS,
Thanks so much for the assist, this site has to be the biggest treasure trove of knowledgable guys outside of NASA lol.
This friend of the family, a non-mechanical 58 y/o anesthesiologist just wants to reliably cruise in his bucket list Shark looking out between those overarching front fenders haha.
That orphan '81 CCC 350 did have me scratching my head.
The ceramic coated mid length headers, retro dual exhaust kit, general tune up, & a twist of the distributor should be adequate enough improvement is my impression of this fella's desires. His "guy" down there in TX will be removing the smog pump & other clutter, whatever help that's worth.
Thanks so much gentleman, you NEVER disappoint.