Reed and Piston ports, Apples and Oranges.
The KA runs out of power, by design at a lower RPM compared to older 100cc engines for longevity.
KA will do 17k all weekend.
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What is the bore and stroke of the KA 100?
Has this ever changed to a shorter stroke = more rpm’s?
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Yup late model TaG 100cc’s are 48mm bore. More akin to the 100’s from the late 70’s Early 80’s. Really the only similarity with the reedjet is the appearance.
They also run a larger crankpin from 125cc models vs the smaller ones from the later 100cc days. (I think, but I’m questioning that now after looking at homologation sheets)
I remember the Parilla Swift or the PV 100 pistonport high ref with the starter in the clutch,those engines went easily to 18.500 short stroke.
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Id love to know how the engine was built, that high an RPM is not common here.
Ultra short stroke…
So far in my experience the top RPM I get has been very track-dependent. At a tight, twisty track like Dousman with lots of slow corners we were spinning them up to almost 17k on the straights. Last week at Mooresville which is much more flow-y with more intermediate to high speed corners we were barely touching 15k. I wasn’t at Springfield for Route 66, but the highest RPM I heard they were getting to there was around 17.4k. The engine will do it no problem, it’s all just about what the track calls for and what the stopwatch says.
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I wouldn’t say it’s common either, as Jake mentioned most tracks we run between 15.5k-16.5k probably.
But at SuperNats, basically the entire grid was hitting 17k all week because of the long straight and tight infield.
At some point it becomes useless to spin it more. I can’t imagine anything over 17 is really beneficial on any track.
1000 RPM is about 5 teeth on the axle. I don’t know what gear ratio you have but that’s pretty close. So if you’re turning 16.5 and everybody else is turning 15.5, it would appear you could run 5 more teeth on the axle. That’s a huge advantage.
One thing that I’ve been interested in is average lap RPM. I have a math channel that does the average on throttle RPM for each lap. In theory, the more time you spend towards peak HP/TQ, the more beneficial the gearing is. Too low of gearing you will spend more time below peak HP/TQ. Too high of gearing and you’ll spend more time above. It’s not a perfect math channel because you will never find the perfect formula. I just don’t have a big data pool to test my theory. All I have is my own data of trying different gearing for one specific track. The more tracks and gearing data points the more accuracy to whether my theory is correct or not.
In real life I don’t have much opportunity to mess with gearing. In sim, however, different story.
What I have noticed is running a slightly more tooth gearing over the “correct” gearing lends a very different feel to the kart. Similarly if I drop one tooth below optimal, same but in a more laconic way.
I find it helpful when I am at the edge of my potential laptime on a given track to typically tighten up the kart and go for more accel and low/mid range. I’ll sacrifice some top end for this.(or rather I’ll get to top end sooner).
I dunno why but when you over-gear, at least In sim, the kart gets very tight and unforgiving. You have to get the timing of everything right or it’s a hot mess. But when you do get it right, wow. But then I take that back to the correct gearing.
I think it helps in that as you mess with gearing, the track drives differently. You adapt and learn.
None of this made much sense but what you wrote made me think a bit about what I do and why. Gearing changes how the kart feels and responds. There is no “right” Gearing actually.
I agree and slightly disagree with Dom here. There is no “right” gearing, but there is a Right Range for gearing for a given track. The problem is that gearing, like chassis setup may change with the conditions of the track.
For example, on a green track, you may have to go up in rear teeth to keep the engine from bogging out of the corners. The lack of grip will not let you enter the corner as quickly and your revs drop lower, so you need the engine to pull from a lower RPM. Where as on a rubbered up track, you can carry more rolling speed through the corners and the engine stays up in the range of torque to get a good exit. Taking away teeth can extend your top end further down the track.
Its a balance and only the stopwatch can tell you which is working better.
Oh yeah the “right” gearing is a range I meant to say. Within that range is a choice about wether you want the kart constantly on boil with more limited top end or wether you want a less nervous kart with more zoom.
what you need to consider here is the power curve isn’t a perfect shaped bell curve. KA makes much more power in over-rev than it does below the HP / Torque peaks. Think about how bad the thing bogs at 5k vs how it’s still pulling pretty good at 15k…those marks are respectively ~5k on either side of peak power. That’s why you’ll find alot of people pay close attention to that low RPM mark on each particular track trying to stay in the 7500-8k range for MIN rpm per lap because that seems to maximize power under the curve for the majority of the track. Even then that’s not a perfect model, our home track has one configuration where even with pretty short gearing we get down under 6k for one particular turn. If I were to gear for just that turn I’d be at like 17k peak and way too high in the RPM band to be useful in many sections of the track. So in that particular configuration we just treat that really slow turn as a throw away, try to minimize the bog and make up for the time lost on the remainder of the lap. Still turn around 16k for that config in other portions but haven’t found the need to spin it any higher on that particular track.
I haven’t run MCC with a KA, just once on my old Yamaha but I’m guessing your starting gearing is something in the 11/77 range and you’re probably not turning too much more than 15k there right?
Yep, I’m aware that the graph isn’t a bell curve. I finally got to see someone’s dyno sheet and it looks like it really picks up power at 7500, hits peak around 9800, then falls 2hp between 9800 and 11000, then sort of settles around that power level until 13000 (that’s where the graph is cut off). I’ve messed around with all different ratios at MCC and they all net the same lap time. Literally within a few hundreds for all of them. Everything from a 11/79 to a 10/79. Look at the data and they all fall fairly close in MPH traces. The only thing it does is shifts the top RPM and average RPM forward with the more teeth added and I found that once I got up to the 10/79 it got a lot more difficult to get the lap time. That’s sort of where my frustration and confusion comes in. The engineer side of me needs some sort of mathematical reason behind what works and what doesn’t. My thinking is that the average RPM should mathematically stay rather similar to find the ideal gearing. Whether it’s supernats where people turn 17k down the straight but utilize the gearing in the infield, or New Castle where people lug them down more because it’s a high speed flowy track. The average RPM, theoretically, should stay relatively close because it’s the spot where the engine is in peak power for the longest amount of time throughout the lap. That ideal RPM isn’t going to be at the peak HP on the dyno sheet. It’ll be a bit higher than the peak because, like you said, the curve is healthier on the top end of the RPM range than below peak power. Maybe I’m cookoo. But I don’t buy into the “the gear is right just because it is”.
If anyone wants to try my theory and plug the math channel into their own data, I’ll be more than happy to show them how. I think it could be a useful tool to get rid of some of the guessing.
*Also, not everyone’s ideal average will be the same. It’ll vary from engine to engine.
That is just it. An Average is all of the samples added up and divided by the number of samples. You are likely looking for a Mode. How frequently a sample number repeats. The higher the Mode value, the more true measure you have of how long the engine is operating at a given rpm.
My next question would relate to what else changed when you tried different ratios? Likely you had more over-steer on corner exit with higher ratios than with lower ones from the extra mechanical torque. Did you drive a different line or change at what point your braked and accelerated to compensate? Did you change setup, increasing or decreasing caster or width? There are so many variables that make up a lap time. Maybe the track changed between tests and that affected your numbers.
I bring this up because my last two races at NTK (a relatively short, twisty track) I had to go down a tooth on the rear. Before, the kart was alive out of the corners on a relatively low grip surface. On the last race, the track had been “sealed” and the grip level was way up. Even with the lower ratio, I was carrying more speed through the corners that my lowest RPM didn’t really drop. I ended up picking up about a 1/2 second from my previous race. Problem is, I also made an axle change. So what had the biggest effect? Was it the Axle, the Gearing or the Track. No way to know for sure without back to back tests.
Try not to overthink it. Figure out what you want the most. If the track is Fast and Flowing, maybe you want a good peak RPM. If the track is tight and twisty, maybe you want a lot of punch out of the corner. If the track is a mix, maybe you want to maximize the amount of time you spend at peak Torque or Horsepower. Being an Engineer, I’m sure you can calculate the area above or below a curve (RPM vs Time). That will tell you how much time you spend above or below your Minimum desired RPM value. From there you can make adjustments to suit the situation.
I would guess ideally you want to set your minimum value below peak torque, but at a point in the curve that is not too flat or too vertical. That way you can lay the power down without the rear stepping out every time you apply the throttle or bog the engine from dropping too low. Rate of throttle application can modulate some of the variances on track as well, if you get it close.
I feel like I’m a pretty consistent driver. I hit my marks and let the lap time come to me, rather than force new variables into things. So I don’t think that there’s many outside influences in the data points. I did a back to back to back test between a 10/73, 10/76, 10/79. No other changes to the kart and all within a period of about 45 minutes total. They were all within a tenth. The 10/79 was the worst of all of them. To get the lap time was a lot harder and had no gains out of the corner but lost mph down the straight. 10/73 seemed to be very similar to 10/76. Drove both of them identical and the rpm/mph trace was almost dead on even but the 73 had a hair more top end. Came back the next week with a 11/78 and it bogged out of the slowest speed corner and didn’t net much on top end. Then ran the rest of the race day with 11/79. Came back another day and went back up to the 10/73 just to keep the sprocket off the curbs. Ran that for the morning warm up and was slow (cold and super slick track). I don’t know why but my gut said to go back to the 10/76 (which is what everyone else was on and what my engine builder suggests) and I feel like I made the wrong move even though time wise I was turning almost the same times all alone as they were with the draft. Now I’m going back down in gear to see what happens. I’m just trying to get actual data to WHY something works rather than just say “yep, that’s the gear to run at XYZ track.” So when I go to a new track I have something to base off of instead of doing the blindfolded darts method.
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There is no Magic Bullet when it comes to gearing. All you can hope for is getting it close and driving your way through it. It may have been earlier in this thread that TJ talked about the pro guys having a spectrum of ratios and all of them were within a few hundredths of each other. It all depends on your driving style, your setup and your specific engine. So maybe your 10/79 was out of the park, but sounds like somewhere between 10/73 and 10/76 you were hauling the mail. The pros look for those last hundredths, while most of us deal within tenths. If you can find a few hundredths, then take it and make the most of it. I too have a very analytical mind and try to make sense of how what changes affect what. My brain wants to understand it all. Sometimes there are too many variables to calculate and we just get it as close as we can, then by adjusting our driving it makes it or breaks it.
Just an example at 9000 RPM. 10/79, 11/79, 11/88. Look at the mph! 3 1/2 mph is a bunch!
There’s 8 teeth difference between the first 2 ratios! That’s a bunch, I repeat “a bunch”. (1 driver tooth equals 8 driven)