Am I that bad?
Sam, on your next motor I want to supply the oil pan only! Leave the controversial stuff to Mike?
Nobody makes modified torque converters for the Hondamatic cb750A. The torque converter that Honda used came from the early Civics parts bin. I contacted over 20 car transmission companies trying to find someone to modify the Honda converter. Yes, that included most of the "name" performance companies who advertise in National Dragster. (NHRA) Only one midwestern company was mildly interested in trying. They did modify my cb750 converter. The stall speed improved from the stock 2,800 (achieved) to 4,200 rpm (achieved.) An exact copy of what they did is in Sam's motor. The internal fins of the torque converter are cast metal. Most car torque converters use sheet metal fins.(aka bendable) Changing the angle of certain rows of the fins will increase the slippage without hurting the power transfer at the higher rpms. You can't do that with the factory torque converter! The fins break, not bend. You can make the converter slip more, but it won't still effectively transfer the power. Kinda like a CVT setup - The motor revs high, but it takes a while for the wheels to turn fast.
Ok, what about a different torque converter? The shaft splining will have to be adapted, either by using a different shaft spline to mate with the car's converter splines or machine a new hub for the converter that will fit the stock Honda spline. If the thought of that expense doesn't discourage you, I'll add this one. What oil goes through most torque converters? (You know, red, thin stuff) The car transmission converter shops have
no clue how a converter will work with 20W-50 mineral oils! So we want to buy a high stall converter, modify it, and run a heavy oil through it and achieve a higher stall speed? Converter, $6-800, machining to fit ?$$$, and then try it to see if the stall speed is anywhere near where you want. If it isn't, rework the converter stall speed $$$ and try again. Sounds like a $1,000+ lottery.
I did have a converter reworked by milling out the stock cast fins and hand welding in 16 fins on one side of my converter. I was out over $600 for this modification. The tranny shop wouldn't guarantee
any stall improvement for what I spent. Fortunately, I gained stall speed up to an observed 6,000 rpm. So, yes, a converter can be reworked at least that far. That's what I'm running on my 10 second drag Hondamatic.
Now, if I were to give this converter to Sam to run, what would happen? Virtually
no change in slippage rpm on that bike. Just because the converter can be made to slip to a higher rpm, doesn't mean that it will. The converter eats up the torque,(converting it to oil heat) limiting how high it will slip in rpms. Without the torque necessary, the slippage rpm doesn't rise. Make more torque in Sam's bike at around 2,500 rpm and it will slip to a higher rpm on the starting line. The lower the motor's power, the less that the torque converter will slip. We don't need a "looser" converter; we need more torque around the rpms that we want to launch at.
How do we make more torque in the 2,500 to 3,500 rpm range? More compression, more stroke, and/or a change in cams. A slotted cam gear may shift the torque curve enough to help with the launch rpm. It may not be enough of a change at the low rpms in question. It is the most cost effective method to try if the upcoming changes on the dyno don't help enough (including the oxygenated gas). In my book, the second most cost effective change is a change in cams (vs new pistons, or another crankshaft). The other alternative is to live with what we've got. My worry with that is the repeatability of the bike. If it varies like it has at Rockingham, you might as well park it. Winning rounds will be due to good guesses, not data of many passes. This can be an ATM machine. It won't be until the 60' times get consistent.
I'm not trying to have anyone's head. Everybody has tried their best to make this bike work well. All suggestions have been positive. Anybody with car torque converter knowledge will verify that the amount of slippage of a converter changes with the power of the motor that it is attached to. It doesn't matter if it goes 120 mph in the quarter mile. It doesn't matter if it goes in the 11's once. It matters if it will repeat on the track. Some Hondamatics do repeat. That's why at least one is called an ATM. I want Sam to have an ATM.
JW
