Over the weekend I went out to Oregon for a wedding. While I was there, I visited Brent and his little home electronics lab to see about modifying my MicroSquirt to be able to run a stepper motor for idle air control. Previously I had determined exactly what modifications were needed. Since Brent is more skilled with all things electronic, I asked him to do the modifications.
As Brent was working on connecting wires to pins 9, 10, and 12 on the MicroSquirt CPU (a MCS12C64 Freescale chip), disaster struck.
We ended up with a broken pin on the CPU and a lifted trace on pins 9 and 10. The lifted traces aren’t such a big deal but the broken pin on the CPU is too small to fix with the tools we have available to us. I contacted Matt at DIY Auto Tune for his thoughts and to see if we could possibly buy a new chip from him to put onto the board. Alas, we would need a BDM cable to program the chip once it is replaced. Those cables run a couple hundred bucks. At that point, I might as well buy the updated MicroSquirt V3 to replace my V2.
I have an email in with Peter at PF Tuning, on the recommendation of Matt at DIY Auto Tune. Perhaps he will be able to fix the MicroSquirt and add on the circuitry for stepper control for less money than it would cost for me to buy a MicroSquirt V3, the spark controller that DIY Auto Tune makes (the V3 can’t support direct drive of spark generators), and some solution other than stepper control for idle air control. We shall see soon.
After going down several paths (Bosch 3 wire PWM IACV, decode PWM signal with an Arduino and then run a stepper from the Arduino, etc.) to look at how to do idle air control (IAC) for my Honda Pacific Coast PC800 fuel injection project, I’m coming back around to the idea of modifying my MicroSquirt V2 to include a stepper motor control. Stock the MicroSquirt ECU doesn’t come with stepper control because there wasn’t enough space on the printed circuit board for the half dozen components needed and there wasn’t enough room in the ECU wiring harness.
I’ve talked previously with Matt at DIY Auto Tune about adding stepper control to a MicroSquirt. It is possible but it’s also hard because of one specific solder connection that needs to be made to the MC9S12C32 IC that is the brains of the unit. The DIYPNP Upgrade Kit for stepper motor control from DIY Auto Tune has the parts necessary. There is a schematic showing how to assemble the parts on a DIYPNP ECU board. From the schematic of the MicroSquirt V2 board, two of the three necessary pins are available from the IC. The third pin, IOC6/PT6, is soldered to the board but doesn’t have a trace coming away from it that I could tap into somewhere else.
Based upon the Motorola datasheet for MC9S12C32, it looks like the pin pitch on the 48 pin IC package that appears to be on the MicroSquirt is only 0.5mm. That’s TINY. I will have a very hard time finding a fine enough solder tip and solder to be able to work with a very thin wire and solder it to that pin. The pin on the 48 pin IC package is #9 I believe.
Luckily I have a couple motorcycle-specific IAC valves that are stepper motor-driven in my vast inventory of spare parts. One is off of a CBR600RR throttle body and I believe can be modified to work with my specific setup.
Another problem will be implementing vacuum bungs below the butterfly valves in the throttle bodies. I’m already using the two vacuum ports for my MAP sensor. If I were to double-duty those vacuum ports to also serve the IACV, I believe that I would probably get erroneous MAP readings. Based upon the pressure difference here in metro area Denver with sea level and with the top of mountain passes in Colorado, I need rock-solid IACV performance to make the bike rideable across that wide range of altitudes and atmospheric pressures. There is very little room to install another set of vacuum bungs. I am going to have to find some good stainless steel bungs that are bent at appropriate angles that I can press fit into place or perhaps JB Weld into place. It will be tricky. But then again… what part of this project hasn’t been tricky?
I’m going ahead and ordering the kit to have stepper motor control. I also have to order a new MAP sensor for the broken pressure sensor that I am using for barometric correction.
10am: After fighting with a surging idle yesterday, I re-evaluated where my MAP sensor was getting its vacuum data. Yesterday I had changed the placement of the MAP sensor’s vacuum pickup from inside the front of the airbox near the IAT sensor to the combination of the front and rear cylinder vacuum pickups where the venturis are located below the butterfly valve plates on the throttle body. This was giving me an incredibly fluxuating MAP reading by up to 20KPa. Now I have moved the MAP vacuum pickup back inside the airbox and close to both intake runner trumpets. Hopefully this will help matters and give me a monitorable vacuum source that will both change when it should based upon engine load and also not have such wild fluctuations.
I also verified that Injector 1 wiring goes to the front cylinder and Injector 2 wiring goes to the rear cylinder. While I had the airbox off, I also confirmed that I have a good airtight seal in the throttle bodies. Oh, and I also rewired the TPS so that it doesn’t read upside down anymore. I had miswired the high and low sides before although that didn’t seem to make a difference to the MicroSquirt. Now I’m waiting for the RTV goop to dry in the airbox before I put it all back together and try firing up the bike again.
While I’m waiting, I’ll ruminate a bit on future spark control for the bike. Based on what I’ve deduced from the diagrams of the PC800 pulse generator wheel and the MicroSquirt V2 information (I have a MicroSquirt from circa May 2011 which means it is a V2), I believe that I can 1) get an adequate signal from the stock hall sensors and pulse generator wheel and 2) directly drive the spark generators on the bike. One problem I might encounter is that each coil drives two spark plugs on the PC800. The information on my MicroSquirt says that I should only be driving one coil and one spark plug per wire on the bike. Maybe that’s a problem… maybe not. I do know though that the spark generators on the PC800 only have two wires going to them. One wire is +12V and the other goes to the stock ignition control module where it is grounded to charge the coil and then I believe is let float to discharge the coil. The ignition and fuel only have a 10 amp fuse on the stock PC800 between them so I believe that the two spark generators are well below 7.5 amps each. Once I have a stable fuel map and whatnot, I think I am going to embark on controlling spark with the MicroSquirt as well.
Noon: After much futzing around, I ended up using the Coors 16 ounce screw top beer can that I had originally planned for an oil catch from the crankcase breather hose (don’t need that anymore since I’ve gone back to the stock air box) to make a vacuum surge tank. I tried taking MAP vacuum from the air box again but it just wasn’t working. Now I have MAP running from the two vacuum ports on the throttle body via the beer can surge tank. I’m getting nice, stable, and reasonable MAP readings now.
Coors 16 ounce screw top can to the rescue as a vacuum chamber to buffer vacuum from the two vacuum ports on the PC800 carbs. Note that the can crushes in quite a bit when vacuum is pulled. This is only a temporary solution. In the future if this path of investigation pans out, I will experiment with reducing the volume available in the vacuum chamber until I find the lowest volume I can safely run to buffer the fluctuations from the vacuum sources. Then I will make that chamber out of PVC pipe to resist against crushing forces of the vacuum.
My next problem is saturation of the IAT sensor. I placed it on the bottom of the air box right over the engine. As anyone with a PC800 can tell you, the space around the air box really roasts regardless of if the cover is on or not. I am moving the IAT to be in the ambient air stream now and take data from somewhere more in line with the actual temperature of the air the engine is receiving.
Plugged up hole where the IAT sensor used to be located in the air box.
1pm: I moved the sensor to be in free air and now I am getting realistic readings from that. However, that didn’t fix my idle problem. I’m still getting either very high idle (~3500RPM) or very lean AFR (~16-17). I then decided to remove the air box to see what happens. Lo and behold, the problem is exactly the same. Next, I tried closing up the air intake stream using some plastic lids. The more I closed the stream, the more the idle came down and the more the AFR started correcting itself until I got to the point where I had 1100 RPM and 13.5 AFR which is pretty good considering the VE table I’m using is absolute garbage and a complete guess on my part.
A blurry picture of the IAT sensor hanging off the clutch lever in the free air.
I think this means that the PC800 carbs when turned into throttle bodies let too much air through. I’ve gone back through and checked for leaks everywhere but there are none. Everything is sealed as it should be. The cross sectional area of the PC800 carbs where air can go through (ignoring the butterfly valves) is about 2.66 square inches across both cylinders. I calculated that number as follows:
pi * (diameter of one cylinder intake / 2)^2 = area of one cylinder intake
area of one cylinder intake * 2 = area of all intakes
sqrt(area of all intakes ÷ pi) * 2 = diameter of all intakes
And for the PC800, this is:
pi * (1.3 inches / 2)^2 = 1.327 square inches
1.327 square inches * 2 = 2.66 square inches
sqrt(2.66 square inches ÷ pi) * 2 = 1.84 inches for the diameter of all intakes
For comparison, on the VT1300CXA, the area of the throttle body is only 1.65 square inches (the diameter is 1.45 inches for those who want to do the math). Looking at the butterfly valves on the PC800 carbs versus the VT1300CXA throttle body, it appears the VT1300CXA seals a little more than the PC800 does as well. Is that enough to really make a difference? I don’t know but I’m guessing it might be…
At this point, I’m trying to decide what I could do to rectify this problem, if it really is a problem. I could try to hack together a way for the VT1300CXA throttle body (not the intake runners and injectors) to mate with the PC800 throttle body and remove the butterfly valves from the PC800. I could try to put in some sort of restriction on the air intake for the PC800 although that would probably starve the engine for air at high loads and high RPMs.
2pm: During lunch I thought about my problem more deeply. In looking through the excellent Performance Fuel Injection Systems book written by the guys at DIY Auto Tune, I found some information in the fuel delivery section of the book (page 21 in the first edition) on how to calculate a proper throttle body size. The formula for the cubic feet per minute is:
(maximum RPM * VE * engine displacement in cubic inches) ÷ 3456 = cubic feet per minute
I don’t know what volumetric efficiency will be on the PC800 but I’m going to guess it’s about 85%. The engine displacement for the PC800 is 800cc or 48.8 cubic inches.
On the PC800, the formula looks like:
(7500 * .85 * 48.8) ÷ 3456 = 90 CFM
Next, the Performance Fuel Injection Systems book tells me to convert the cubic feet per minute calculation I just did into a maximum throttle body area with the following formula:
(CFM * 2.4) ÷ 300 feet per second = maximum area of throttle body (square inches)
For the PC800, that formula is:
(90 * 2.4) ÷ 300 = 0.72 square inches
Converting this into a diameter using this formula:
2 * sqrt(Area ÷ pi) = diameter
Using that formula, we get a total effective diameter of:
2 * (sqrt(0.72 ÷ pi) = 0.957 inches
That is much different than the 1.84 inch effective diameter of all intakes for the PC800 throttle body that I produced from the PC800 carbs. I double-checked all of this with the simple calculations that can be done on this page and came back with 1.2 inch effective diameter. Close enough, especially since these formulas are most likely all empirically derived rather than based on any sort of fundamental theory.
I suspect that the slides in the PC800 carbs block a large amount of airflow at low RPM and open up more at high RPM based upon the vacuum bellows that they attach to. In effect, the slides are almost like a RPM-actuated damper valve found on some of the higher end V-twin muscle bikes put out by Honda and Yamaha. Interesting!
For the PC800 carbs to work as a throttle body, I would need to reduce the overall intake diameter to just under 1 inch. I believe that my options are either to block part of the flow at the top of the PC800 carbs-turned-into-throttle-bodies or block the flow at the air box intake. The air box intake will be a bit tricky as it is a triangular shape rather than a circle or square. However, if I can reduce the area out at the end of the air box, then I could pipe in the IACV into the air box. Otherwise I might have to drill new holes in the PC800 carbs below the butterfly valves in order to pipe in IACV air.
2:30pm: I did a test where I took off the top-most piece of the air box that is removable (rectangular-looking thing) and blocked air flow with my hand. By blocking air flow there, I was able to change the speed of the engine and the AFR as well, even with my bad VE table. I’m pretty sure this means that I need to constrict the amount of airflow that the throttle bodies receive. One potential problem I observed with restricting airflow where I did is that the air box actually started to collapse in on itself a little bit due to the vacuum that started building. That isn’t particularly desirable!
The rectangular opening where you can see the K&N air filter through is what I was using to adjust how much air was coming into the air box.
What I might try doing is getting some relatively thick tin or sheet metal and cutting it into circles that could slip in above the PC800 carbs/throttle bodies and below the air box connector. I would drill holes in the sheet metal of the correct diameter to allow the proper amount of air through. That just might work.
Now it’s time to go down to the river for some afternoon summer tubing. I’ll come back to this project hopefully later tonight. I’m pretty sure I have some sheet metal sitting around my garage somewhere…
