Typically, that is how I see ours. If I am going to do a gear change it needs to be 3 to 4 teeth. The motors actually have such a wide powerband that running them to 14k or 16k doesn’t do much to affect lap times. I’m typically gearing it to the low RPMs that my daughter hits.
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Speaking of powerbands, is anyone willing to share a dyno sheet with me? If you want to block off any power numbers that’s fine with me. I’d just like to see the shape of the curve and RPM.
If you take a tooth off the axle and pick up about 200 RPM, (in that RPM range) you’re going about the same top speed, approximately. If your times don’t change, it’s either because of your driving or the gear ratio is just not right for the track. There’s no way ever to get away from this.
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I can confirm as well. For X30 and I assume other single gear karts, a single tooth doesnt lead to a significant change. It is noticeable, but probably too subtle to be worth the effort.
I can’t speak brothers, but in my experience, at my level of competitiveness, I could definitely see and feel the difference one tooth would make.
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replace brothers with 4 others
Michael, that is the situation we are trying to figure out (gear for max RPM or keeping RPM in the power band). We have a fairly tight hairpin at the end of the longest straight. I have found that if my rpms fall much below 6200 I am slow / boggy coming out of the hairpin, so I am gearing more for that. I have seen mid to high 15K at the end of the straight which is doing nothing but making noise (or at least that is my understanding).
I too would like to see a dyno report. Where does this engine make peak power?
@alvinnunley I guess what I meant is if you are going to the trouble of changing a sprocket, unless you are fine tuning, 1 tooth is pretty subtle.
I once had a dyno, with which I tested a fair amount of engines, and in my testing, I found that most 2 cycles, with a pipe, reach peak torque in the neighborhood of 10,000 RPM and peak horsepower in the neighborhood of 11,500 RPM. Some higher, some lower, but not by a great deal. In the case of the KA100, I would guess that they follow the average.
How much of a difference in port timing and/or port configuration did those engine have from each other?
You’re on the right track in your approach. That’s basically the approach you should take in most cases. Stopwatch has final say (look at time for multiple laps vs just a single fastest lap) of course.
But generally you’re gearing for acceleration out of the turns and let it rev out on the longest straights. It’ll always be a compromise.
Are you familiar with the term “area time”? Meaning, in essence, measure the area of the port and plot it against the time the port is open. I wouldn’t be surprised if there are rules governing it. It’s really noticeable in the early Pirelli and Komet engines. The long stroke Pirelli has long narrow ports while the short stroke Komet has short wide ports. Just guessing, I’ve never really plotted it, but they appear to have similar area/Time. I wouldn’t be surprised, if you put a degree wheel on either one, they would have very similar port openings and blowdown.
Its not a true Dyno Chart, but you might be able to interpolate a similar curve based on the X30 by shifting the graph down to match peaks.
Using:
&
X30 has thicker lines.
I would bet IAME’s engineers designed the engine with similar specs on porting only scaled down for the 100cc class. If so, that would mean the KA starts making Power around the 8,000 mark and peaks out on Torque at 9,750 and Hp at 11,250. Secondly, you could aim your gearing to not drop below 8,750 coming off the slowest corner to maximize exit power and acceleration.
Now this is just theory and based on some assumptions (we all know about those 
). Maybe @tjkoyen could weigh in more with all of the experience he has with the KA.
I understand what your saying. It’s been a long time since I thumbed through Jenning’s Tuner’s handbook, but I recall the term “area time”.
I still have my copy, right here in my desk. Pristine condition. Gordon Jennings wasn’t always right, but he was always interesting.
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Pipe design theory was interesting…
He’s book along with with Obert’s Internal Combustion Engines was part of our curriculum.
I don’t have a dyno sheet for our motors, but since the clutch stall test threshold is 6500 RPM, I always try to keep her geared where she’ll be above that. Typically, I am aiming to have her lows on average be around 7500. Some laps are lower, some are higher, but I don’t think we have ever turned more than 15.5k rpm with her hitting those numbers, and that was with an extremely long straight.
I don’t know if it was in this thread or another, but I believe TJ spoke of some drivers being up to 6 teeth different at the same event, turning the same lap times.
A professor Blair, Belfast University, did some interesting stuff that was published in SAE. My first Mayko pipes, in 1973, used his design theories. Hard to believe that was almost 50 years ago. How time flies when you’re having fun.
Is that for Jr or Sr? If Jr, would Sr be slightly higher due to added weight?
Lots to reply to here…
@mtbikerbob at 6200 minimum RPM, you’re just not carrying enough speed I would imagine. I never see anything below 7.5k, on any track. So that’s not a gearing issue, more a driving/chassis issue.
Top RPMs probably will never be below 15k. I think 15.2k or so is generally the lowest we go before the negatives start to outweigh the positives. Don’t be afraid to rev the thing either. I’ve mentioned before, but we ran up to almost 17k at SuperNats with no problem. Making lots of noise on the top end, but the tight infield made it necessary.
Regarding gearing, I don’t think I said “6 teeth” but definitely you see a range of about 3 teeth among the top guys. I definitely see differences in 1-2 teeth changes, and I don’t feel I need to change 3-4 to see a difference. 4 teeth is a big jump.
In summary, run the max RPM between 15-16k and you’ll be in the ballpark.