(Mods if there is a better category for this post please advise)
Hey guys, I'm working on a complete swap of an L31/Vortec 5.7 efi system into my Nova. The engine is stock with the only mods being long tube headers, and is currently running well with a carburetor and HEI. I am slowly hammering out the details of the replacing the PCM with a Speeduino (simplified Megasquirt knockoff if you are unfamiliar) and intend to put the original L31 intake manifold back on with the MPFI spider.

To start things off I was just going to have the computer take care of spark, just to prove the system works without disabling the car completely/still be able to slap in the HEI back in and run.

From what I can gather a stock L31 table looks like this:


And from a pretty basic calculator I estimate my HEI is running something like this: (I have it hooked to port vacuum so the idle advance is wrong here.)


In an attempt to split the difference I started by looking at a bunch of LT1 tables. I'm aware the head design is similar, but they have the advantage of aluminum heads and reversed cooling over my conventional cast iron Vortecs. Here is a spark table for consideration that I compiled from a couple different LT1 tables and taking the lower values. To my eye it actually seems less aggressive than the Ramjet 350 tables I've seen floating around, but id still knock it down by about 10 or 20%.

The car is not a racecar (if using the awful Vortec intake wasn't a clue,) more of a cruiser looking for reliability/drivability and I am intending to run on 87 octane... so again I think this might still be a bit too aggressive past idle, but i am more looking at the trends here.

I am still a ways away from being ready to fire the car (still having fun in wiring land,) but if anyone has better information than i have gathered/spark maps/other maps they are willing to share that would be a big help.
Sorry for the long/wandering post, but I am curious to hear peoples thoughts.
Thanks.

 

I think I found a more suitable base from someone's iron head LT1 map. Looks like a split between the stock vortec and aluminum LT1.

 

That is a more likely starting point. But the real issue here is what does the engine like!? Vortec heads tend to have pinging issues. So start off with low/stock spark timing, get the fueling in line, then see what the engine likes for SA.

It may sound strange but this isn't a guessing game where we think we know that the engine wants. This is a key to tuning, finding out and using what the engine really likes.

RBob.

 

Thanks! Starting point is all I'm after.

I just want to make sure I'm at least in the ball park so it fires and isn't absurdly dangerous. I'll start with the iron head LT1 table with the high spots knocked down about 10-15% to look about the same as the vortec truck table. I'll also take a closer look and pull some values from the running HEI and work that in.

 

Bob is absolutely right. Get your fueling in line, because you may find yourself pulling spark thinking it is too much, meanwhile it's being pulled because you are lean. Low timing to start, dial in your fuel throughout, then increase timing... and back off when you see timing being pulled in the areas that are showing a little knock. If you start off low and creep upward, you will nip knock in the bud...

- Rob

 

I did not know a Nova 1970 was a thirdgen! I learn something new everyday.

 

This forum has went above and beyond to help people tune and troubleshoot their car and trucks. I routinely send TBI owners here, even if its a 1978 Cadillac a 1985 S10 or a classic car with a L98/TBI/ TPI/ or crossfire.

We do a good job here of teaching and coaching. The forum has had more traffic because of this. I would hate to see this site go like others have gone - 1 post per month or less.

 

Sounds inclusive indeed! You forgot to mention L31 Vortec with Speeduino.

 

As usual RBob is spot on again. I’ve been “struggling” w eliminating a particular area of the SA table free from knock. It happens to take place right where the motor runs during highway cruise. And it seemed that the slightest change to MAP could bring on knock retard. Sometimes minimal, other times 1-2*. Basically, it made me hesitant to even take long trips in the car.
Recently, I have been focusing on AE delivery. Altho I was doing this primarily to eliminate knock, I have found that the process has also produced a real improvement in how the motor responds to accelerator input. Its just that much smoother accelerating and more responsive. I should say that re-configuring and balancing the injectors has resulted in much tighter control of fueling and required
much less “compromising” on the disparity between left and right cylinder banks. And finally, along w a slight reduction in SA in that “cruise” area, I was able to eliminate the KR completely. That was not the case in earlier attempts of reducing SA. It was a matter of getting the AE correct which then allowed me to get the SA right also. Its allowed me to get BLMs throughout VE table to 128 +/- 2. As a result, the car is sooooo much more fun to drive. Motor and exhaust are also noticeably quieter and smoother sounding. Additionally, MPG has improved by 1.5-2mpg. Just as a note, my LT5 has significant mods from the stock configuration, and so the calibration that “works” represents a substantial modification from the stock calibration.

 

Lol, hear me out:
This forum seems to be the the authority on modified engines running Vortec heads, pretty much all the information I find for them routes me back to here. Most of the truck forums where the L31 if from are pretty quiet/not super helpful for fringe cases such as this. You guys also have new enough cars that folks here will be messing with ECUs that aren't the 0411/other stock GM PCMs, so I figured of all the forums I know this one had the closest combination of things going on that someone here would be knowledgeable. I appreciate all the help even if I don't have a third gen... Always thought they were rad though.
I figured I'd catch some flak eventually... Surprised it took this long.

My intention was to change as few variables at a time and fade in computer control one system at a time. SO I was originally intending on starting with spark control and letting the carb continue to handle the fueling. The idea that I should at least establish a working/reliable timing map similar to the HEI, then take on fueling. Seems like this is the opposite of the way that most folks go though... Probably for reasons I have yet to discover.

 

What works in trucks is a bit off in a car.

 

Why I was looking at the LT1 stuff over the stock L31. I figure my 3400ish lb would be more like the f/b bodies.

Fast, I've seen your spark table all over the place, but have no idea how to use/translate those EQ tables you provide with it. Any insight?

 

Stoichiometric ratio value divided by the EQ ratio = commanded air/fuel ratio. 14.7/1.15= 12.78:1

 

Thanks!

 

Start by finding the bin file of a vortec truck. Download it and review it. Assuming your engine is stock as you said. The factory tune files would be the best starting point.

 

Actually it is pretty weak for emissions. The table I have floating around that was referenced above worked even in a heavy Express van. I would start there rather than the factory tune that does not even give 24° of timing at WOT.

 

The other upside to Fast's table is it is already in typical MAP values in KPA. Now that I (at least semi) understand what the extra spark adder table/PE table do I can start there and probably work it up for my much lighter car. The MS/Speeduino seem to handle power enrichment/acceleration enrichment differently, and I haven't found a similar spark table, but I'd rather err on the side of retarded than mess with too much knock off the bat.

Edit: I added Fast's table to the previous post where I mention it.

 

Yeah I know, back from the dead. Just posting this for people searching for help. Fast's spark table posted above works good in my 5000# K10, HT383 with automatic, 4.11 gears with 33 inch tires. Works better than the b-body LT1 stock one and WAY better than the L31 stock table.

I used his spark tables 14 years ago when doing an L31 so I knew just who to look for when I needed a table for this truck. Thanks for helping people out.

My simplified spark table for MSD DIS attached.

 

Good deal! I had a lot of time into refining that. It is pretty much my go to if I ever tune a Black Box which are few and far between now. You might be able to sneak a little more into it, especially above 4,800 rpm but literally will not gain much. IIRC GM calls for 32* @ 4,000 rpm using their 22* HEI unit and a initial timing setting of 10* BTDC.

I should have also stated before the EQ ratio stuff is equivalent to PE. Take the 1.15-1.25 row that is all the same value and put it into the PE table. Some like to eliminate the PE table by zero'ing it out, then wonder why they get knock retard under load outside of PE and back off the higher map spark advance because they are getting knock retard when the chambers are burning the hotest at stoich. The added fuel in PE cools the burn and allows a bit more timing. Typically at a Equivalence of 1.15, most engines like 2-4* more timing than they can run at the same load operating at stoich. Funny but true the 11:1 aluminum head 383 currently in my Express van runs a similar timing map. It has a touch more timing up top, saw 32.5* this evening at 5,500 right before it shifted 2-3 @ 90 mph in 100F ambient temps detonation free on 91 octane that has been in the tank for 5 months. Drove it to burn some old gas out of it and get everything hot and moving.