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Tuner Resources

Wideband Tuning for Carbureted Drag Racing

11-sec. Camaro on the Fast Track to a Perfect Tune. (By Stanford Curry)

 
   


Have you ever noticed how the best racers always seem to have it together on race day? Their secret is knowing – instead of guessing – when it comes to their car’s state of tune. Think about it. Do you know what air/fuel ratio your car runs in second gear down the track? Third? How many Gs are you pulling off the line, and how soon after the launch is your foot to the floor?

Eliminating guesswork and variables is the name of the game when it comes to drag racing. Check out these tips to tuning your car with Innovate’s wideband O2 and data acquisition setup. Learn these and you’ll be on your way to more consistent ETs, better trap speeds and fewer broken parts.

 


 
   

This LogWorks chart shows a basic quarter-mile pass. As you read the chart from left-to-right, the black line represents engine RPM, showing four major peaks at 6900 RPM (for gears 1 – 4), followed by a drop into the next gear. The torque you feel in the car is shown in the orange Acceleration (g) line , which is strongest in first gear and trails off in subsequent gears. The Air/Fuel Mixture is the magenta line, and it should ideally hover between 12.5:1 to 13.5:1 in each gear. Also note the blue Vacuum line , showing when the driver’s foot is to the floor (low vacuum) or when the driver lifts, as he did coming off the line in first gear to reduce wheelspin (where the orange Acceleration line dropped and black RPM line spiked). Each line has its scale represented in the left column

1. Recording & Displaying Live Data
2. Downloading & Viewing Recorded Data
3. Idle Tuning
4. Power Valve Tuning
5. Accelerator Pump Cam & Squirter Nozzle Tuning
6. Main Jet Tuning
7. Cruise Tuning

Carburetors are great for wide-open-throttle (WOT) applications like drag racing, thanks to their comparatively low cost and ease-of-setup. Assuming you have the right size of carburetor for your application and that your engine’s in good working order (no fuel leaks, no ignition glitches, no parts rattling around in the oil pan), it’s easy to get up and running and tap the potential of Innovate’s tuning solutions. With an hour or two wrenching and running a few wires, you can capture data that allows you to track your air/fuel ratio, engine RPM, acceleration gs and vacuum (load) as your car travels down the strip. (Installation Tips). The data you capture will help you spot top-end lean-out conditions, highlight traction issues and fine-tune part-throttle / cruise conditions where you can reduce fuel consumption. This overview provides examples of each as we discuss tuning carburetor components including main jets, power valve, accelerator pump and air bleeds (idle and high-speed).

 

 

1. Recording & Displaying Live Data: Now that you’ve installed and configured the LM-1 and LMA-3, it’s time to take your car for a drive, capture some data, interpret the results and make some tweaks where necessary. Like the install guide says, hit the LM-1’s “Record” button once to start recording (“R” will flash on the LM-1’s display) and once again to stop recording – this builds what is referred to as a “Session” in the LM-1. You can record multiple sessions and up to 44 minutes of total driving time before you have to download the data to your laptop and clear the LM-1’s memory. If you still have the laptop attached to the LM-1, you can also maximize the LogWorks Monitor display and see your car’s Air/Fuel, RPM, Vacuum and Acceleration while the car’s running – handy if you’re tuning the idle in the garage or riding along in the passenger seat while someone else drives your car (on the dyno or on the street).

   

2. Downloading & Viewing Recorded Data: Innovate makes it easy to pull recorded data down from the LM-1 to your laptop. With your laptop connected to the LM-1, open up the LogWorks Monitor software and click on the “File” drop-down menu and highlight “Download LM-1 Log” to load your recorded data to your laptop. Once the download completes, you’ll see a new window appear with a graphical representation of your session – similar to the following picture. You can toggle between multiple sessions by clicking on the “Session” drop-down menu.

Example LogWorks Chart

It’s a good habit to immediately save this newly downloaded data onto your laptop before you clear out the LM-1’s memory storage of its sessions. First, click on “File” in your newly created LogWorks graph window and then click on “Save As.” Type a filename that includes something meaningful to you about the session (e.g., “80p88s.log” can denote 80 size primaries, 88 size secondaries). This filename needs to make sense to you when you revisit the information 2 – 3 months later in the racing season. Next, clear out the LM-1’s memory by going back to the LogWorks Monitor and clicking “File” and then “Reset LM-1 Log.”

The example below shows a basic chassis dyno pull from 3000 RPM to 7000 RPM (black line) with the pink Air/Fuel Ratio line indicating a slight lean-out condition after 5500 RPM. Spikes like these may point to a fuel delivery problem, so check your fuel bowl levels. Otherwise, the lean-out can be addressed by changing to a size or two larger on the main jets for both the primaries and secondaries. The blue line is Vacuum , showing that the driver’s foot is to the floor all the way through the dyno pull. You can also add a note to the session by clicking on the little yellow note icon, just below the drop-down “Channels” menu and dragging it down to the Timeline area where you wish to append the information. The note information you add to that box will be displayed when you “mouse over” it with your cursor later.

Screen Capture of Carb Dyno Pull with Lean Air/Fuel Ratio

As you can see in the following diagram, richening the jets two full numbers in the primary and secondary sides richened the mixture and reduced the high RPM lean-out as noted by the flatter pink Air/Fuel Ratio line through the pull.

Screen Capture of Carb Dyno Pull with Richer Air/Fuel Ratio

We tuned for a mid-13 Air/Fuel Ratio here as a baseline. Even with the dyno pulls showing a flat Air/Fuel Ratio in the 13:1 range, a run down the track is the next logical step to test your baseline tuning. The conditions are usually different between dyno and track, providing the opportunity to further refine the jetting of the primary & secondary main circuits. Dyno vs. track conditions can vary in elevation, temperature, headwind, etc. You may also find the track passes sometimes highlight a lean-out condition caused by fuel bowl starvation – too small of a fuel line, too low of a fuel bowl level, not enough fuel pump volume, etc. As the log from a track run illustrates, the jetting was initially too lean in third and fourth gear, but richened with RPMs which at least rules out fuel delivery issues. Therefore, the next objective is to jet the carb in wide-open-throttle conditions for 13 – 13.5:1 Air/Fuel in third and fourth gear.

Carb Drag Test

The preceding graphic shows that the car is running too lean in third and fourth gear ( Air/Fuel in the 14:1 range) with 80 size jets in the primaries (plus a power valve) and 88s in the secondaries. Standard practice recommends changing up or down 2 jet sizes front & rear at a time, which would have put us just about perfect if we had a set of 82s and 90s with us. We only had 83s and 92s at the time of this test which richened the Air/Fuel Ratio just a little too far – low 13s and high 12s as shown in the following diagram. What’s important to note here is the sensitivity of Innovate’s O2 readings. You can easily see that the Air/Fuel Ratio dropped from dangerously lean 14.5:1 to safely rich 12.5 – 13:1 through richening the main jets by a factor of 3.5 jet sizes (up 3 in the primaries, up 4 in the secondaries).

Carb 83-92 Jets Rejet 4-Gear

Innovate’s point and click interface also makes it easy to stretch the timeline (click the magnifying glass and then click on the timeline across the bottom of the chart). You can also add the rectangular data boxes you see in the picture by clicking on the gauge icon and then clicking anywhere on the chart.

Innovate also provides you a facility to look at your session data as an aggregate of all the sessions instead of looking at run-by-run views of each line graph. You can also easily combine all of your sessions (dyno pulls, drag strip runs or street driving) and look at them in a single table. As shown in the graphic below, simply click on the LogWorks “View” drop-down menu, and then click “View Chart” which will bring up a table.

When the Table view comes up, click on the “Table” drop-down menu and select “Table Setup” to choose which variables you want to place on the vertical and horizontal axes of the table. A commonly used approach is to select RPM as the Horizontal X axis and Vacuum (or MAP) as the Vertical Y axis, and select Air/Fuel Rati o to fill in the Chart Content. The table’s tabs allow you to look at the data points’ Average, Standard Deviation, Number of Data Points, Maximum and Minimum values.

You can also enable color-coding by clicking on “Colors” and selecting the color scheme of your preference. Cells in the table are then color coded based on their numerical value. The color scheme in the following diagram is the “Wobniar” (“Rainbow” spelled backwards) which intuitively uses blue for the rich / cool side of the Air/Fuel Ratio spectrum and red as the lean / hot side.

Color-coded AFR Chart Example

You can also view the table’s data as a three-dimensional graph by clicking on the table’s drop-down menu choice of “View” and then click “View Table as a 3D Graph.” The following diagram shows what the 3D graph looks like:

3D Chart Example- Carb Cruise

 

 
   

3. Idle Tuning: Get the car started and fully warmed up to operating temperature for at least 10 minutes – take a trip to the local store and back (this avoids tuning the car “cold” and subsequently running too rich when it is warmed up on cruise night). With your car in neutral or park, the laptop hooked into the LM-1 box and running the LogWorks Monitor software, you should see your Air/Fuel , RPM idle speed and Vacuum (MAP as PSI) on the laptop display, similar to what’s shown in the following diagram.

LogWorks Dials (Virtual Gauges) at Idle

Adjust your idle speed screw(s) for primaries (& secondaries if so equipped) to achieve your desired idle speed. Then adjust the idle mixture screws. Some carburetors only have two idle mixture screws for the primaries, while others offer a “4 corner idle system, using the primaries and secondaries. The goal is to achieve an Air/Fuel Ratio somewhere between 13:1 and 14.7:1 while maximizing your Vacuum reading. Looking at the Air/Fuel , RPM and Vacuum , you will find that barely moving your carb’s idle mixture screws – maybe 1/16 th of a turn – can change the Air/Fuel Ratio over half of a point without any change in your perceived idle speed. This is just one area where Innovate’s wideband really shines – sensitivity.

If the idle mixture screws don’t have enough impact on the Air/Fuel Ratio or Vacuum , you can try incrementally changing the carb’s idle feed restrictors and idle air bleeds (if the carb is so equipped). If the car idles with the idle mixture screws closed or nearly closed (less than a half turn from full clockwise / bottomed out), you can lean out the idle by substituting a smaller idle feed restrictor for the one currently in it, or you can try using a larger idle air bleed. If the car requires turning the mixture screws out from fully closed / bottomed, an abnormally large number of turns (4+ turns out from fully closed), you can try richening the idle circuit by installing either a larger diameter idle feed restrictor or a smaller idle air bleed.

For cars with automatic transmissions and no “idle-compensation” solenoid triggered by the trans or A/C, you may face a tradeoff of idle quality when the transmission is in Park compared to when it’s in Drive. Once you set the idle for Drive (optionally with the air conditioner running and lights on as your “highest load” worst case), you may wind up with a little lower Air/Fuel Ratio in Drive or higher idle speed in Park than you might otherwise prefer. Chock the drive wheels, set your emergency brake AND have a friend hold their foot on the brakes while keeping the engine running and transmission in Drive for this exercise. Try to achieve an acceptable idle speed while maximizing Vacuum and getting Air/Fuel Ratio between 12.5:1 and 14:1.

 
   

4. Power Valve Tuning: Once you have the idle speed and mixture set, take note of the Vacuum reading using your LM-1 and LogWorks software. If you have a carb that uses a “power valve” to enrich the mixture under load conditions for the primaries or secondaries, your should try using power valve with an opening level 2 – 3 inches below the idle vacuum (in Drive for automatics or Neutral for manuals). This gap prevents dumping too much fuel into the engine during either idle or cruise. You can see the power valve add fuel on the LogWorks chart by looking at the Vacuum reading (blue line) and the pink Air/Fuel Ratio reading. When the blue Vacuum line dips below the setting of the power valve (5.5 inches of vacuum, for example), the pink Air/Fuel Ratio will drop a point or two (from 14:1 down to 12:1) because the engine needs more fuel under that heavier load. In the same situation under load, if you were to lift back off of the throttle, this would raise the vacuum above 5.5 inches – and the Air/Fuel Ratio would rise back up from 12:1 to 14:1. At light loads above its stated rating (most are available from 2.5 inches up to 8.5) – the power valve is no longer active since the cruise and/or mains are providing enough fuel.

 
   

5. Accelerator Pump Cam & Squirter Nozzle Tuning: The accelerator pump system handles most of the “increase-in-load” fueling needs and can be seen on the LogWorks chart as a drop in Air/Fuel Ratio coinciding with a drop in Vacuum . If your mixture is too rich (e.g., 10:1) right after you floor it, try using a smaller-lift accelerator pump cam or retarding the pump cam by moving it to a different position on the throttle shaft. If you find that you have a lean spot a half second to a second after flooring the throttle, your accelerator pump nozzle may be too big. Try reducing the accelerator pump squirter nozzle size, which extends the duration of the accelerator pump’s discharge – you should then see a smoothing or flattening a given Air/Fuel Ratio on charts after flooring the throttle. This effect is easily seen on charts for manual-transmission cars accelerating through the gears.

 
   

6. Main Jet Tuning: The main jets are the focus of most tuners working with a carb, and Innovate’s wideband system makes this the simplest of all the tasks. Drive the car to a dyno shop, record a session (from 2500 or 3000 RPM through redline in third or fourth gear) and open LogWorks to view the RPM and Air/Fuel Ratio lines. The Air/Fuel Ratio may show a drop at the same time you floor it (the blue Vacuum line confirms this with a drop), but for now focus on the Air/Fuel Ratio and the shape of the line roughly 1.5 seconds after you floor it until redline. If the line is relatively flat and horizontal, hovering around 12.5:1 – 13.5:1, you’re set. If it’s above that range (too lean for wide open throttle), try going up two jet sizes (on a Holley or Demon carburetor – Edelbrocks, Carters and Quadrajets may be similar). If the Air/Fuel Ratio is below that range, it’s richer than optimal unless you’re either trying to combat detonation or are running a turbo/supercharger – so try leaning it out with a size or two smaller jet size. Once you’re consistently pulling between 12.5:1 to 13.5:1 you should be ready for some tuning at the drag strip.

You should test your car both on the dyno and at the track, since the dyno doesn’t offer the same variations in load, traction, wind resistance and temperature as the track. While at the track, pay attention to the Air/Fuel Ratio in each gear – the Acceleration / g reading (orange line) will come into play here as well. If your Air/Fuel Ratio is between 13.0:1 and 13.5:1 in all gears, you’re in great shape. If it’s too lean in first gear off the line but fine in the others, check the g reading – you may be pulling so hard off the line that you’re uncovering jets. Possible cures include checking the float levels or getting main jet extensions for the secondaries. If your Air/Fuel Ratio is fine in first and second but starts to get too lean in third or fourth, you may be draining the fuel bowls faster than your fuel pump & lines can fill them. The diagram below shows a good bit of wheel spin modulation in first gear (RPM spike + drop in orange g line) and flat Air/Fuel Ratio lines in gears 2-4 which will require slight richening from 14.4:1 down to 13:1 via the main jets. Another track tip includes logging the RPM drops between gears. Previous logging showed that we could stay on the fat of the torque curve after each gear change by narrowing the gear splits, so we changed from a Tremec wide-ratio 3.27:1 first gear trans to a close-ratio with a 2.95:1 first gear.

4-Gear Pass

Instead of a run-by-run view of each line graph, you can very easily combine all of your passes (sessions) and look at them in a table or as a chart. Create a table in LogWorks’s graph by clicking “View,” then “View Chart” and configuring your table axes (Horizontal = RPM, Vertical = Vacuum or MAP, Chart Content = LM1_02). To collapse all the sessions, click on Sessions. The table below is based on the same data as the dragstrip run, and it highlights the lean Air/Fuel Ratio condition during wide-open-throttle (low Vacuum / under 1.58 PSI) over 5000 RPM as orange or yellow color-coded cells.

4-Gear Table

 
   

7. Cruise Tuning: Many tuners think that main jets control most of the fuel flow during cruise RPM – 2100 to 3400, depending on gearing. The reality is that the idle circuit on most carburetors can control most of the Air/Fuel Ratio during steady-state cruise, yielding better fuel economy. Using the Innovate LogWorks software, you can find the transition period between the idle circuits and the mains fairly easily, and then optimize the idle circuit as an “idle/cruise” circuit and let the mains & power valve specialize on high load, wide-open-throttle situations. A dyno is the safest and most repeatable place to do this type of tuning, but you could drive your car on a long, flat stretch of road to capture the data as well.

Assuming you’ve properly set up your idle circuit in the 13:1 to 14.5:1 Air/Fuel Ratio range for zero load (parked) idle and partial load (under 1900 RPM), try temporarily jetting the mains over-rich. Go up 4 - 8 jet sizes just for this exercise, giving you between a 10:1 and 11:1 Air/Fuel Ratio at 4000 RPM 3 rd or 4 th gear steady state cruise. With the car on the dyno or on a flat road, run it up to 4000 RPM in 3 rd or 4 th gear, then start recording the session on your Innovate equipment. Begin dropping RPM in 250 RPM increments from 4000 RPM and then holding that RPM for 10 seconds each time: 4000, 3750, 3500, 3250, 3000, 2750, 2500, 2250, 2000, 1750. Download this data to your laptop and look at the Air/Fuel Ratio for each 10 second RPM sample. When the Air/Fuel Ratio begins to rise on that chart (from the 11:1 range to the 13:1 range), the mains are transitioning back down to the idle circuits. On many cars this occurs between 2000 and 2500 RPM.

An additional way of looking at this data is through the LogWorks Table and Chart facilities described earlier. The chart below shows a good cruise Air/Fuel Ratio (in the 14:1 range) attained at varying load ( Vacuum ) conditions between 2100 RPM and 2400. Note that lower vacuum (higher load) conditions brought the Air/Fuel Ratio down just below 14:1 while lighter loads had it near 15:1.Not bad for a 560 HP pump-gas Camaro that runs 11s in the quarter and gets 21 MPG.

Good cruise Air/Fuel Ratio at varying load conditions

Depending on what RPM and Air/Fuel Ratio your car cruises at on the highway and if you have adjustable air bleeds for the main jet circuits, you can begin incrementally trying larger air bleeds to delay the RPM onset of the mains. This may slightly lean out the mains’ Air/Fuel Ratio as well, so be careful during this phase if you choose to work with the high speed (main jet) air bleed circuits.

Use the LogWorks View / View Chart facility to build a table for each set of high speed air bleeds you data log. The table facility in LogWorks allows you to highlight and copy these tables into tabs on a spreadsheet, so you can calculate the difference of your setups on a third tab to see the effect of your changes. In the example below, we changed the high speed air bleeds on a Demon carburetor to delay the onset of the main jets during cruise. This tweak improved Air/Fuel Ratio in the car’s cruise range (13 PSI through 16 PSI, from 2000 RPM through 2500) as shown in the green highlight. (This table is the difference in AFR between the two runs).

Carb Table Comparison Showing Delta between "Before" & "After" AFRs

Once you have the mains coming on closer to the RPM level you want, you then need to double check Air/Fuel Ratio through the full range that the mains are operating to ensure you’re not getting too lean. If you are too lean, increase the size of the main jets to achieve a decent Air/Fuel Ratio . Be careful and incremental here, listen for any signs of detonation and remember to not delay the onset of the mains too far (resulting in flat spots). 2800 RPM is the upper limit for delaying the onset of the main jet system, especially if your engine combo is set up for a low torque peak with high compression & a smallish cam.

Remember to make one change at a time, measure the results, and repeat. Soon you will be "finding" horsepower and efficiency in all sorts of unexpected places!

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Standford Curry owns and tunes a '69 Chevy Camaro, an '02 Honda S2000, and a '97 BMW M3. He has won numerous SCCA and Club events, as well as placing first in the 1997 North California Solo II complete series. If you found this article helpful, check out Stanford's article from Chevy High Performance Magazine "Old School Meets New School :How to Get EFI-Quality Mileage From Your Carburetor."

The Innovate Forum is a MASSIVE source of useful tuning information. Visit and search for threads on your particular tuning challenges (or add your tips to the community).

 
     
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