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Working on MPG

80' 427

80' 427

1/2 ton status
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High guys, new sight looks good, been trying to keep my head in books so it has been a bit.

So I have been messing with my 84 K10 in an attempt to get the mpg up. Previously it was around 6 and yesterday in a 140 mile trip it was up to 12.34. I have been using an innovative wide band and have a big old vacuum gauge taped to the dash to watch what it happening. I am hoping to get it closer to 14-15 and I have a few more things to tweek. So I guess I was wondering about Aerodynamics. That ugly under bumper air dam, has anyone done a before and after on one? I will include the specs just for fun.

1984, K10 2.5" of lift
Chevrolet 383, 9 to 1 compression, 180 cc Dart Iron Eagles 72 cc chamber, edelbrock eps, 670 holley truck avenger, hei dist 34 deg total in by 3000 rpm, adjustable vac can set at 18 deg.
700r4: 1990, 1550 stall speed
10 bolts:4.10s detroit rear, truetrac front.
33s Uniroyal tiger paw 33x12.5x15

I need to find a vac delay for the tcc to remove the low speed part throttle od w/tcc pinging so I can get the total timing closer to 52-55 deg cruising. Also considering a MSD Streetfire box to help fire the leaner mixture.
 
80' 427 said:
I guess I was wondering about Aerodynamics.
Click to expand...
Oh yeah, these rigs can be aerodynamic just like I can put on a wig and be a pretty lady.

Looks like you're already going in the right direction. What changes doubled your mileage? What is the WBO2 telling you? Have you changed jets at all? I might as well be the first to suggest a Q-Jet. The small primaries are nice for MPG. Yes, EFI could get you further, but you might never recover the cost of the conversion. It's good to mess with timing to see the results, but more advance doesn't always mean more MPG.
 
I'm going to say with your setup, your doing good if you get to about 15MPG.

The truck in my sig got the same economy with a 305/Q-jet/old heads and 350/Q-jet/old heads as it does with the current setup. My '83 K5 with factory setup (305, Q-jet, 700R4, 3.08's, 31" tires) would pull 17MPG freeway at the very best. 50MPH, cool night in the desert, flat ground. I don't speed, as a matter of fact I tend to keep it around 60-65MPH even if the speed limit is 70. It doesn't ride as well at 70.

I get about 25% better fuel economy with my 2015 car (and did with my 2004 as well) than it's rated, so it's not like I can't get MPG out of a vehicle. But the trucks are pretty well limited to around 17 with a gas engine. Eagle Mark said he was able to get 19MPG in a Suburban by running lean at cruise, that's the highest MPG I'm going to believe with a gas engine in these trucks. You have to be careful with lean cruise on a carb though. On the injected vehicles GM set it up to run lean for a specified period/load range, then drop out of lean cruise to ensure chamber/piston temps didn't get too high, then back to lean cruise if conditions were right. Oil is what cools your pistons...I'm not there yet, but eventually I'm going to be able to monitor pan temps to see what running lean does to oil temp. If you run lean long enough, the potential is there to start causing all sorts of issues, and with a carb, getting it to run lean in a very narrow window is unlikely to be possible without a TON of work.

You can try removing the air dam, but it's not likely to matter one bit. One tank won't tell you anything, as you'll be tempted to read into the results because you want to believe it helped. :)
 
The carb comes with 68s up front, 89 rear w/ 2.5 power valve. I had previously dyno tuned it, potentially on crack. It had a 8.5 power valve, 74 front and 64 rears. It was fat nearly everywhere. I have gone all the way down to 62s in the front and now 76s in the rear (soon to be 78s for a slightly lean WOT). At cruising it is currently in the 14.5-15.5 range. As you lean into the throttle the A/F goes to 14s with a 13s right before the power valve. With power valve it is 12.3-12.7 on the primary. With the secondaries it is in the low 13s. I plan on going to 6.5 power valve to delay the enrichment slightly. Right now the idle feed restriction is .310 and it may be slightly rich. To get the idle mixture into the 13.6s (needs to be 14.5s) I am at one turn out. On the transition slot it is in the 12-13s. I had a .005 wire in ifr before but felt is was too lean but may try it again now that I am closer. The timing before was 34 degrees (inertial/mech) coming in about 4000 with a 10 degree vac canister. Now it is all in at 3000 with 36 (initial/mech) and 18 vac, which is about 48 at cruising. The only timing problem is right now is somewhere along the line the vacuum delay on the tcc vac switch was removed. When I am at low speed part throttle in overdrive it will ping. If the tcc would unlock this would prevent this and allow more vac advance. Shooting for 52-55 timing at cruising. I would prefer a Qjet but first I have piles of holley parts, second the eps in a square pattern only and I hate adapters. I have contemplated a tbi but this is an old dirt track engine with enough piston slap to trip the knock sensor all the time. So to recap. At idle it is 13.6-13.8, low speed in town driving 13.7-12.5, cruising 14.5-15.5, on the booster (2500ish up) it is 14.5-13.6, power enrich 12.4-12.7, secondaries WOT 13.0-13.3. Next step is more secondary jet, 6.5 power valve, retry wire in ifr, find a tcc vac delay valve.
 
With your wideband you have enough information to be accurate in what you are doing. Going to injection IMO is not going to do more for you than what you can do with a bunch of tuning with the carb. Since TBI is wet flow in any case, no efficiency to really be gained there due to induction type vs. a carb. Injection only helps try and keep the system at stoich without you having to do a bunch of tuning. That is great for cruise, but if you already know you are in the right AFR range cruising, TBI would do nothing for you.

As far as I'm concerned, anything OTHER than cruise you can just forget for MPG. Take the thing on a purely highway trip with no stops, whatever MPH you normally run, and use that as your baseline. Accelerating hard in a 40MPG (cruising) car, it dips into the single digits, trucks are probably there with even moderate throttle.

Are you measuring MPG by GPS miles traveled, or verified correct speedometer? I assume yes if you are using a WBO2, but never know. :)
 
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I use my phone for mph, then use the tach. I have the corrected miles driven with the indicated mph/actual mph. I am tuning for power and mpg and single digits is good for neither. I have a innovative Wide Band. I used Visard's Holley book (one of my favorite writers) he claims into the 16s is fine for part throttle cruising. As soon as the load increases the emulsion tube starts to richen it, so it isn't that lean at anything but flat land cruising.
 
Sounds like you're doing a good job. Don't try to go much leaner at WOT, that's evil. And definitely get the TCC delay valve if that's all you need to make it work. You'll get more MPG from a locked TCC than you ever will by tweaking timing there.
 
The TCC works it just stays locked all the way down to 2 inhg. The delay slows the vac signal the switch sees so if it drops enough it will unlock and then even though the engine picks up speed and vac it delays the increased vac to the switch so you don't get a yo-yo of lock-drop vac, unlock-pick up vac, repeat. My wide open runs are singular events but anything above 14.7 a/f should be safe. 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.
 
When the cruise gets too lean, you will feel it. Can you run a two stage power valve in that carb.
What I found with holleys, is that if you get the jetting down to where cruise mpg is good, that there will be a flat spot before the power valve opens. It seems like the valves I was using, that the first stage was around 10 and the second 4.
 
80' 427 said:
That ugly under bumper air dam, has anyone done a before and after on one?
Click to expand...

Those only help at all if the vehicle is stock, and not lifted. The undercarriage of our trucks is horribly un-aerodynamic. That thingie is supposed to help push air around.

Once you have a lift, that all pretty much goes out the window.
 
Ya that was my assumption. I kind of assumed that I was doing OK on the engine tuning, just wondered about any changes to pushing a brick though the air. I still wonder about things like tire tread, air cleaner (open vs stock), coolant temp, etc.
 
I can't add much to the mph road because its a road i have never drove down. But I have heard of guys getting the 2wd air dam and using that. I think the 2wd one is like 9 inches tall. Vs the 4wd drive one is 4" I think. It might be worth it as that would be enough to negate the 2.5 lift. In theory.

I think an open air cleaner decreases mpg too. Or so I heard once.
 
I have the small air dam 88 k5 350 tbi 700r4 3.73 285/75/16 15.9- 16.4 mpg is usual for me highway I have a light foot . I think your getting great mpg for the setup you have!
 
So on today's trip to school: I leaned out the .031 (.000755 in^2 area) idle restrictor with a .oo5 wire (.00002 in^2 area)so that is a 2.6% decrease in area and equal to about a .0305 restrictor. I also changed to the 6.5 power valve to delay enrichment a bit and increased the rear jets to 79s (no 78s in my box). So now cruising is mildly leaned so it is more in the 14.7-1.52 region on flat ground. It flickers as low as 18 at times when doing small throttle changes occasionally, but no surge and most of the time 16.5 is as low as it drops. The low speed high vacuum (1000-1500) like when I am driving though town is still very rich 12.7 to 13.6. I am going to try and lean the idle a/f out to the high 14s-low 15s to see if even though the throttle is just open that the needle is still effecting it. The idle now is 13.8-13.9. I am also going to change from the orange acc pump cam in the 1st hole to the green cam in the first hole. looking at the chart it has less pump at 0-15 deg of throttle rotation but is much more aggressive after 20. I think this will help remove or lower the lean flickers I have seen. The WOT with the secondary is still lean (lower 13s) so I am going to jump to 82s to see if that gets us closer to 12.7. This morning was cooler at 29 deg F so we know the air is denser and will affect a/f ratio. I hope to have it so it is near perfect at cooler temps and a bit fat at higher temps.

One of the things I think is coolest doing all this is watching the emulsion system work. At high vacuum, like on flat ground it leans out the mixture, as you increase throttle, decreasing vacuum the mixture enriches. This all has to do with how much air is being pulled though the idle air bleed and high speed air bleed. This makes it so when you are at low load the mixture can be pretty lean and as you increase throttle, like pulling a hill, the vacuum drops and the mixture enriches. So even though this mixture will likely be too lean for pulling a trailer, it will have lower vacuum signal and will enriches the mixture and allow it to pull.

For being just a box with holes drilled in it these things really are a mechanical wonder. I am a nontraditional student in the school of engineering at UNL. The classes help me understand all the things I used to know what they did, now I know more of how. Still they don't teach innovation, they teach theory so it is up to you to figure out how to use the tools.

Marty I hope to go to you-pull-it today to steal one off something there. They can't cost much and maybe i will find a couple different ones so I can experiment. It really needs one, any time the rpm is changed like shifting gears the a/f drops to lean for a second because the signal is changed. It is worst when the OD and TCC come on at the same time. It recovers quickly but sometimes it will ping for just a second.
 
Good job actually tuning a carb. It probably seems like nobody does it because the WBO2 became affordable after everyone looking for MPG was on EFI. I think the hardest parts are the transition areas where you go between circuits and the transients. For the borders of idle/off-idle/primary/secondary, you'll have to settle for something good enough. Same thing with transients - pretty much all you have is the accelerator pump or power valve. In reality even fairly modern EFI systems see the AFRs moving all over the place while driving. As long as there is no smoke, stumbling or surging nobody cares and ignorance is bliss. The brief seconds where it's off are balanced by the minutes and hours where AFR is where you want it.

At light cruise I can only push my 350 to about 15.8:1. After that, the engine just feels a little weaker and I subconsciously start driving in other load ranges, so it's not just a game of going as lean as possible without surging.

Boy, this tuning business sure is easier by just typing in numbers.
 
Under power I rarely see leaner than 15.5. I may stay there and just work on getting the whole curve in that range. Honestly it runs a lot smoother down the highway.
 
How do you like that innovative wb? any good/bad with it?

As far as aero, the more air you can block from going under it the better you'll be.... Except that doesn't help w/ clearance 4x'ing... New cars have them and they make a measurable difference. One other thing is on a lot of cars when the air leaves the back top edge, they've been finding that it ends up more efficient to let the air leave off as straight as possible rather than trying to move it back to where it came from. The turbulence involved in trying to put it back ends up being worse off. I seem to remember reading that from some MB testing.
 
Mine is a lm2. It is the basic kit with just a o2. It can do far more than I use it for. Data log, dual sensors, collect ecm data. I just wanted a simple meter I could use to tune carbed engines like this truck, hopefully lean out my camaro 1050 intermediate circuit and get it more street able as a whole (swap to a 750 now). Maybe even a international letter tractor or 2. I would bet it is going turbulent underneath despite the fact this truck is no were near .7 mach. I have 4 trucks and this is now just transportation and hunting rig.
 
K5.jpg
 
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