I hate Rotax so I never check these topics, but I think you (@chris1388) chris need to open up your entry into that hairpin a little bit. It looks like you’re low on entry and turning in slightly early. Fixing that will help you carry more speed through there. Also, a general Rotax tip is they really hate being off-throttle and they really hate aggressive throttle input. So if you’re struggling with bogging off the corner, try to roll into the throttle a little sooner and more progressively, and try to always keep a little throttle on, just enough to stay under the clutch engagement.
I don’t really have any specific gearing tips for Rotax, it’s been about 4 years since I raced it, but @Marin_Vujcich’s gearing and RPM targets sound about right.
Side note, that parking lot is so bumpy in turn 1 I almost got motion sickness from watching the video…
From my observation the casting is different, but more from a manufacturing perspective rather than strength, plus the ignition pickup mounting has been sorted for the evo. Lots of people run both and mix and match on fast engines. Its barrel and reed block set up that is the most important.
Hi guys, thanks for all the tips and feedback. I just managed to get some data from one of the faster guys but his data is running that track in the opposite direction, I don’t have any data that way yet but I was able to do some analysis. He was on 12*86 and averaging 13200 max revs, I am really starting to think my engine is letting me down. I used the engine rpm analysis feature on race studio and it’s showing from around 10000-13000 rpm he has 3 sometimes more hp over me. Is this a feature that can be used to compare two engines? See attached photos
Is gearing the same? Wouldn’t that make a big difference? I recall being baffled at how my kart was topping out on a straight to discover that the gearing was still set for a different track.
No gearing isn’t the same but it ought not affect the calculated hp and tq since it looks at rpm and speed. Basically this guy is geared way longer than I am, he’s not even hitting the second power bump after 13500rpm while I am geared way shorter and revving to 14100rpm…I should be quicker off the slow corners as well as benefitting from the second torque/power bump at 13500+ yet I’m still slower. I know my chassis has part to play but based on the above and the engine rpm power/torque analysis on race studio, I’m leaning to believing my engine might be letting me down as well
Hi, gearing does play a part as 2 strokes power in affected by the rotational acceleration of the engine. As the engine can sometimes not sit at desired RPM long for the pipe(heat) and timing to sync properly. So without identical gearing and weight it may be a bit misleading.
What would be far more beneficial to us is if you overlay speed traces of the two karts (once you are going the same direction)
Marin, I’ll soon get data in the same direction so we can compare then but I also have some data of my kart with 3 teeth longer gearing and the engine RPM analysis is more or less the same…my engine makes around 23hp peak. The other guy is making 25hp consistently.
In the situation I mentioned before, you could see that the gearing was the culprit as the data showed a flat line at towards the end bit of the straight. (Throttle down but revs topped out)
RPM’s are used to calculate HP. IMO, if you are not using the same gearing as the engine you are comparing your engine to, the data is going to be skewed. Other elements involved here are aero and mechanical drag. To obtain better information, you have to use a dyno or test both engines on the same chassis with the same driver.
I understand, but I looked at data of my kart with two different gearing setups and the analysis wasn’t very different, near identical in fact. The other engine does show a significant hump around 11000rpm while my engine sort of plateaus
With all 2 strokes the weather plays a big part. Was the data on the same day? And was the jetting the same? Jetting is super critical
Also was his motor consistently showing the hump?
Marin, no it’s not on the same day but the weather was comparable. I agree jetting can affect it but like I said, I’ve looked at multiple sessions of my own data and what I’m seeing is pretty consistent for my engine at least. I have three different races on the same day for the fast guy and yes the hump is evident consistently in his data. He is using a 155 jet with 36:1 oil fuel ratio, I am on a 165 using 32:1 oil fuel ratio.
Chris, that’s a massive difference in jetting.
You can’t have 2 engines that far apart and get the same performance. A 155 would suit a 30C day at around sea level. 165 at that temp would really hurt upper power range.
Marin, I know it’s a huge difference, but my engine is popping even on a 160 hence why I went up with the jet. I can’t run anywhere as lean as he was, the engine just does not like it.
So some more info, remember I told you they’re all geared longer? They’re all on 12*86 and from the faster guys data in the opposite direction (yes it’s not same direction, but min and Max speeds/revs should be similar) he’s revving to 13200 max revs on average and is hitting more or less the same top speed that I am in the opposite direction. Could the reason he and the rest of them are jetted so lean be because they’re not revving very high?
This would point to longer gearing paired with lean jetting being faster than me who’s trying to gear very short. I know my jetting is not overly rich because with my short gearing I’m still able to pull to 14200 revs cleanly …if it were too rich it wouldn’t rev to 14000+.
I’m baffled as to how the pairing they’re using is actually quicker.
If the others can run that lean, you need too. Otherwise you are leaving large power gains on the table. This is critical for upper RPM power. One jet size can be the difference of passing or being passed. You might be pleasantly surprised.
Yip, some do some don’t. Carbs/engines are all a bit different. But what I am saying is if you don’t run it just before it pops then you are losing power. If it pops at 160, run a 162. Also he may have the needle set at max height (max rich) and/or his floats could be at a different height. Also a little trick that helps top end is run the plug gap down to min. I used to run 0.45 on pre-evo not the standard 0.7.
This encourages a slightly earlier spark/flame kernel which give you a slight timing advance. Makes no difference down low, but at 14000rpm it does. Of course spark plug fowling is the downside if you idle around to much.
So in a nut shell, try all these things before you through money at a new barrel, then at least you will have a good base.
Also as TJ said above, Roll a bit harder into the turn with a later apex and earlier on the gas. Your engine matches his down low so you should be able to make gains off the turn.
Marin, he is running the needle on the richest setting and his float height is set 3.5mm leaner than mine…this is however confusing me. If his float height is set so much leaner, then won’t that mean he needs to run bigger main jets? I am on p2 needle position and 165 main, the leaner I go with my needle the better performance I feel when going on to the gas.
So what do you suggest I do then? If I leave my gearing short to reach 14200 rpm and jet leaner than 162 it’s gonna pop and I have tried this…they fly by me when I’m popping down the straight. The only other option is to gear it longer (go from 1183 to what they’re using 1286) and then try jetting leaner.
The reason I’ve never tried the above is because I am barely matching them down low off the hairpins as is with my short gearing (where my thinking is I should be significantly quicker) my whole point was to kill them off the hairpins and then give up some top end on our relatively short straight…this way what I gained in slow bits would be more than I lost on the straight but this is not happening. I’m afraid going so much longer would see me slower by even more.
