Running Rotax Evo Sr and getting to 13200 rpm max on the straight right before the first corner. My gearing is good for the size track. Should I be hitting 14000 or does the performance drop over 13000?
I think for any engine, performance drops are going to happen at some point in the rpm band. The Real Question is how is it out of the Corners and does it still Pull all the way down the straight. If going up a Driven Tooth causes the rear to step out on corner exit, then you are close. If the Pull down the Straight is lacking, then maybe go up a tooth or two on the Driven sprocket. Its always a balance and can change with track layout, grip levels and driving style. You can always adjust your throttle application to compensate for rear grip (or lack of) on corner exit, but you cannot change your gear on track for straight line pull. You can change your line to roll off the corners for higher exit speed and allow you to run a lower driven gear and hit peak rpm. Just have to play with it and see what it does. Some of the fastest guys will be banging off the rev limiter at the end of the straight, while other fastest guys never touch it. See what makes you Fast and work from there.
Itās still pulling at the end of the straight and feels good all around so Iāll try a tooth up.
most probably gearing. In Australia we run a restrictor plate and I seldom get over 12000. Also donāt just mash the pedal on the final corner, feed it in slowly.
Iāll try a tooth up considering the straight is uphill. Is your restrictor in the exhaust pipe? I thought the evo allows you to mash the throttle unlike the pre-evo?
yeah in the exhaust. Nope you still have to feed it in gently.
Do consider track conditions have an impact on high RPM also. I normally hit 15900 at my local track, last week after some sandstorms the track condition was less than great, I barely reached 15k, just because I couldnāt carry so much speed through the last turn. A few days later after they ran a 60 minute rental endurance Iām hitting 15900 again.
Thatās not a Rotax Max though I assume?
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Lol no, same would apply though just different numbers
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Iāll have to get use to being light on the pedal. I came from 4 cycle where you can hit it hard. You scared me for a second when you said 15,900 but I get what you mean, thank you
I canāt remember from whom I got this tip but it works for me.
Imagine your go pedal is a wet sponge, press on it as though youāre squeezing the water from it.
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As you can see HP drops off at 11500
Interestingā¦so I shouldnāt be too concerned about not getting to 14,000 then. Thanks for sharing
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I think what you should be trying to find out is if others on the same gearing are getting more revs than you.
The short answer is yes, the engine should easily be able to go beyond 14000 rpm if geared and jetted properly. If you have uphills etc then all this throws in different combinations.
On my pre-evo rotax I can easily hit the limiter at 14200 so I see no reason why the evo canāt do the same at 14800.
Hey Andy
Well in fact you should be āworriedā. Most tracks are set-up as such that there is an optimal gear ratio, especially for the rotax max which have a relatively narrow power band.
You want the gear ratio set up in such a way that you get just above 14000rpm at the end of the main straight, but stay above 6500rpm in the slowest corner.
Below 6500rp, the rotax max (senior) has no punch so you want to stay above that.
What you want to do is to use a gear with more teeth (to get quicker accel, but lower top end) until you reach 14.000rpm at the end of the main. Then, as you improve your times, lower the nr of teeth to the rear sprocket while still trying to hit the 14000rpm each run. When you get to 14500 rpm or so consistently, drop another tooth etcā¦
I have been working on the GPS data (Unipro) of my 2 sons who run with a rotax max senior, and I can tell you, you can win valuable 10ths of seconds with the correct gearing. Iāve been cooperating with the guys at GKS Genk track here in Belgium, and have applied the learned methods to my boysā data. Will post some charts and plots later on this to demonstrateā¦
Rotaxās in general like tall gearing in general but as always it depends on the track, most of my experience has been on the larger mid-range sprint tracks (3/4 to 1 mile). Of interest, is on race day, the difference in gearing was 4 or 5 teeth between competitors but the fast guys were all running less teeth. If the kart is handling well, the driving is excellent and the carb is tuned right then a tall gear will work. There will be slightly less RPM but net result is higher top speed and better mid-range power. It is track dependent but I always tried to take teeth off and get the carb and driving right. Sometimes the times would drop 1/4 sec taking one tooth off.
Here some examples of the RPM data of 1 of my boys at the Genk GKS Track (Belgium).
Longest ratio (12/72T) leads to a lot of zoneās with too low RPM for a Rotax (<6500RPM, low torque). By increasing the teeth on the rear sprocket (72>74>75T) leads to less of those zones, so stays more in the torque zone (6500-12500rpm).
You also see that with the 12/75T, he has only 1 āredā zone (<6500rpm) but he also reaches >14000RPM at the end of the main straight (dark purple area). At this moment and with the current tire (Apex blue), this is his best setup. The times with 12/75T are >1sec faster than the 12/72T timesā¦ The 12/72 was way off. He only got to 13250 RPM that wayā¦
When switching to Mojo D5 (softer than the APEX blue), we can try to drop again to 74T as he then can carry much more apex speed through the last corner before the main straight, allowing him to still reach >14000RPM at the end of the main straightā¦
In general, and after talking to some experienced mechanics / team bosses, this is what you want. To stay above 6500rpm in the slow corners and to reach >14000rpm on the main straightā¦
So then weight also plays a role: the data above is from my oldest son who weights 7kg above the min weight limit for the series weāre racing in (170kg total weight, kart+gear+pilot).
My youngest son is lighter and can race at the min weight (163kg), so he is geared at 12/74T, i.e. 1 tooth less than the oldest one. The youngest is lighter, so can do with a bit longer gearing as his acceleration is bit better, so he still reaches 14000RPM with the lower gearing.
Weight , tires, etc all play a role in determining gears for a trackā¦
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