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Working on MPG

Nice work with the carb! :bow:

80' 427 said:
The best thing about a carb is that fact that when tuning my 8.1 you cannot go leaner than 14.7 because of emissions (unless I lie to the computer at what lamba voltage is). I have been tuning it too trying to get a little more mpg it but really timing is about all you can play with. With it one of the biggest power gains was simply turning on power enrichment. I started with it coming on at 60% throttle and have backed it off to 75% now.
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This may sound simplistic, but why are you running an 8.1 if you're looking for efficiency? Is it in a dump truck?

You are probably the first person to tell me that carb tuning is easier than EFI tuning. :dunno:
 
Well you need to know I am potentially retarded. I really have no urge to own any half ton any more. Saying that the only choices in a 3/4-1 ton is 6.0, 8.1, and a Duramax. I have driven plenty of 6.0s and the only one to ever impress me was a lq9 in my 06 K1500, and it came with a horrible 4l70 and a weak little girl 9.5 14 bolt. I find the 6.0s in the Hd trucks to be doggy and get horrible mpg. The other choice is a duramax. We can discuss them all day. I think they are great but for the double the cost of my 8.1 I can burn lots of fuel. So that leaves the 8.1, I think it is a great engine, sure the mpg isn't awesome but neither is a 6.0. We have 2 on the farm, my dads 01 reg cab lwb and my 02 ext swb. They pull great and honestly my dad wants to buy and moth ball a crate 8.1 in fear he won't be able to get one if his is ever worn out beyond rebuild. The other fact is I have HP tuners and since we all know just how great this EFI BS is it should allow me to get 25 mpg and make enough power to pull my crawler 80+ with the cruise and ac on still getting 18mpg. Really I was just data logging in hopes that I can eek out a bit more mpg. 1 is 10% of 10 so even minimal gains are huge. The reason I am tuning this K10? I have too much money in it to sell it and I would rather have it rust into the ground farther and keep my "new" 02 in better condition. Plus it needs to earn its justification in me keeping it. I have 3) 73-87 trucks in my yard as we speak. Owning one that is lighter than my 1 tons but get worse mpg seems silly. I do have visions of a AZ body swap and a new vortec 4.3 v6 swap with a turbo but I have many projects before that will ever get done.


So in short, if I could only own big blocks and diesels I would, and I want my cake and be able to eat it too.
 
Well it looks like 12.75 is the best so far followed by a 11.75 from lots of idling and playing farmer. May be short of my 15 mpg goal but not much else to tune. I am sure slowing down to 60-65 would help. Wondering if I shouldn't rebuild my 82 6.2 diesel when I do my 6.5 swap and swap out the 383 in this.
 
80' 427 said:
Well it looks like 12.75 is the best so far followed by a 11.75 from lots of idling and playing farmer. May be short of my 15 mpg goal but not much else to tune. I am sure slowing down to 60-65 would help. Wondering if I shouldn't rebuild my 82 6.2 diesel when I do my 6.5 swap and swap out the 383 in this.
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Should have no problem beating the pants off of those numbers with a 6.2. It's pretty easy to break 20MPG if you are geared well. Higher yet is often seen on stock rigs. Would love to see some specific mileage tweaks applied to a 6.2 rig.
 
I'm kinda surprised no one has mentioned that it's nearly impossible to run over ~43 degrees of timing using a distributor system. The rotor is pointing at another cylinder at that advance. Unless you have some bias built in to back it down from there.
 
How is it impossible? Look up the total advance curve for most any pre emissions v8. Most had 52-55 degrees at crusing. Maybe you are confused by the fact that the dist turns half as fast as the crankshaft so 55 degrees of timing is read at the crank but in reality is only 27.5 degrees inside the distributor. 360/8=45 degrees of separation between posts. Or 90 degrees of crank timing. Believe me I am not treading on new ground here, this commonly known stuff. I guess I may have to reprove what I am doing here since being gone awhile but believe it or not I at least have a clue.
 
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80' 427 said:
How is it impossible? Look up the total advance curve for most any pre emissions v8. Most had 52-55 degrees at crusing.
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Even the CCC carbed setups from the 80's were running 43-45* Can't find any more stock timing maps quickly, but pretty sure until GM started using the fast burn chambers (ala Vortecs), mid 40's was pretty standard under low load conditions.

1983G20VanTimingTable.jpg
 
Strange, I've been reading about timing for weeks now and everything I read says you can't go higher than 41-ish without causing misfires. I guess I was wrong.
 
These are dart iron eagles, more or less junk. When I originally built this engine I was skating in a gray area using turned down 7/16 BBC studs and drilled stock rockers. I didn't think I could get the taller port of a vortec to seal on a stock qjet intake. Should have went vortec heads and had it decked. It would run much better now.
 
bp71k5 said:
Strange, I've been reading about timing for weeks now and everything I read says you can't go higher than 41-ish without causing misfires. I guess I was wrong.
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Now that I'm home and can look at the Corvette timing (L98 Aluminum head motor) the timing map has 47* in it, with a base timing of 6*! Looks like right at what I'd think to be cruise, low load, ~2200RPM.

Obviously AL heads can take more timing, but the point is, theoretically they were run up to at least 53* advanced.
 
80' 427 said:
55 degrees of timing is read at the crank but in reality is only 27.5 degrees inside the distributor. 360/8=45 degrees of separation between posts. Or 90 degrees of crank timing.
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You can over-advance past the terminal in the distributor. I had that problem when I first built my FI setup using an HEI. The real question is the total range of timing you need. You many not have any zones that use less than 15 degrees BTDC, so getting to 55 is a range of 40, or 20 degrees inside the distributor cap. Looking at a standard cap, the gaps between terminals are about 3x as wide as the terminals, which makes the terminals about 12 degrees "wide". Now add in the width of the rotor and that you can spark a little off the edges and that range about doubles. So 55 degrees may be pushing it since you can only choose the overall alignment in 1-tooth increments. That may be the biggest limiting factor - the distributor has 13? teeth, which moves everything 14 degrees (?) crank by moving one tooth.

105347818_new-gm-hei-ignition-replacement-red-distributor-cap-ebay.jpg


I don't know what years/models came with these wide terminals, but that would certainly improve the situation.

hei5a.jpg
 
Here's another question. Have you been verifying the actual timing with a timing light?
 
Yes on the mechanical and I did the first time with the vac. I should check what it is now.
 
The dizzy cap pictured on the bottom is for a 6 cylinder,it only has 6 contacts...I'm not sure they ever had an HEI cap for a V8 with long contacts like that...I think they would be too close,and allow spark to leap to the next contact instead of firing the proper plug..
 
Plus the rotor width is at least a .25 wide if I remember right so it would be part of the the overlap of the 2.
 
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