Is the 40 series torque converter 1:1 gear ratio at 3900 RPM?

[erchuero]

SO i have a go kart with a predator 420, stage 1 with governor still in. It has 19 inch rear tires, 10 tooth sprocket on the torque converter and a 42 tooth sprocket on the axle. I know, here come the comments on too low of a ratio, but it is a split sprocket so i can change any time and i wanted to know top speed. So if the torque converter is truly 1:1 then my final gear ratio should be 4.2:1, right?

I did a top speed run and with a GPS speedo i got 39 mph and the tach (****ty amazon one with the wire around the spark plug wire and set correctly for single cylinder 4 stroke) was reading 3900. Plugging that into the calculator from BMI, i should be hitting almost 52 mph. So if that is truly hitting 3900 rpm then my gear ration is more like 5.6:1.

If the tach is terrible and i am really just hitting 3600 rpm (thats what the governor is supposed to keep it at) then i should still be able to hit 48 mph with a 4.2:1 ratio on 19 in tires.

For anyone saying this is too underpowered for 4.2:1, it shouldn't be a matter of power but RPM, and the tach says i am able to get up to 3900 RPM at WOT. Just wondering what i am missing here.

 

[Kartorbust]

Correct, 40 series tops out at 1:1. If your wanting overdrive, step up to a 780 with a 770 driven and that should give around a 0.68:1 overdrive, this setup is good to 20hp. If you're needing more power, step to the 94c Comet Duster, you'll only get a 0.82:1 overdrive, but it will take to about 40hp.

Its possible that the tachometer isn't accurate. Stock these should be running no faster than say 3800rpm but ideally 3600rpm, since most equipment is ran at that standard.

19" is a bit much for that gearing, you'll want to be closer to about 6.5-7:1 gearing to get the most out of the belt.

 

[panchothedog]

Your language is backwards but the problem is the same. The 42 tooth sprocket
is way too small, giving you a gear ratio that is way too HIGH. So high in fact that it is not letting the engine reach its full rpm, as well as getting the driven clutch to open up all the way. Put a 60 tooth sprocket on it, lowering the gear ratio and you will go faster. The kart will also accelerate much better.

 

[Master Hack]

My guess is the tach is wrong. I believe you have a reading double what it should be.
does your ignition fire on EVERY revolution? You said “ wire around the spark plug wire”

Wasted spark system - Wikipedia

en.wikipedia.org

maybe its actually 1950 rpm? Either that or its set for metric rpm.
and all the above stuff.

 

[panchothedog]

Hey Hack. Kinda highjacking the guys thread here, but it is related to his tach reading. After reading the wasted spark system article you posted. It is clear that all of these small single cylinder engines fire on every revolution. Twice as often as necessary. But then again each spark event would represent one revolution, power stroke events would half of that. To the original O P. Either your tach is wrong, or your driven clutch is not fully opening up, and you are not at a 1 to 1 drive ratio which is how you are calculating your supposed top speed.
Again, I recommend you install a 60 tooth sprocket to get the full potential of both your engine and the clutch.

 

[Master Hack]

Hey Hack. Kinda highjacking the guys thread here,

True. But thats what we do here, right?

if his tach doesnt compensate for the extra spark…
his setting is for single cylinder 4 stroke. Normally one spark every other rev?
just the musings of a lunatic…

but it still could be metric.

 

[Denny]

I agree with the metric rpm theory.