Rotax Pre-Evo Rev Limit


(Christopher Ramnauth) #1

Rotax gurus,

I need your help, can anyone confirm from personal experience using race studio analysis what their maximum rpms were on a pre-evo senior rotax using the green plugged ignition coil?

I have a number I am seeing from my data but experts across the pond in the UK say it ought to rev higher before hitting the hard limiter.

Can anyone else shed some light?


(Tony Zambos) #2

12.7K was our usual max during a road race. Heard that they would rev to 14K on a sprint track, but can’t prove that.


(Christopher Ramnauth) #3

I’m currently getting 14150 but being told they should go on to 14600 before hitting the limiter on the green coil, the black coil is said to have no hard limiter yet I’m not seeing this either…my black coil seems to hit a hard limit at 14050


(Marin Vujcich) #4

14150 is spot on. Technically it is advertised at 14200. 50 rpm tolerance is normal


(Christopher Ramnauth) #5

Marin,

What’s happening is when i hit the 14150 on the green coil and 14050 on the black coil the rpm seems to oscillate up and down about a mean point which ends up being around 13750. From this I’ve deduced that it’s better to not hit the limit at all ie get as close to it as possible but not hit it.

The guys from the uk however disagree with me and insist they should rev more


(Alan Dove) #6

This was from the Prevo engine manual

“At engine speed above 13800 r.p.m. the ignition timing will be controlled such that the engine performance will drop significantly (see diagram 1)”

So it never had a strict rev limit afaik (people had reported of getting 14.9). 14.2 was usual


(James McMahon) #7

Important to note that fuels are different in the UK as well. Lastly, the peak RPM isn’t a lot of use without knowing the gearing too… which of course depends on the track.

Outside of indicated RPM, what performance area do you seem to be lacking compared to the other races?


(Marin Vujcich) #8

You have to have the jetting and needle spot on to reach 14200. We would do at least 3 jet changes a day with a prevo. Slightly too rich and 13800 is it. You need to speed up flame front by running at lean limit to offest timing retard.
There is a rev limiter at 14200 with green coil. Whether it is hard or soft is debatable


(Christopher Ramnauth) #9

Alan, it is true none of the official documentation mentions a rev limit of any sort but I’ve done the on track testing using two coils and my findings suggest that there is indeed a limiter on both coils (green and black). The data i am looking at is not off the mychron device itself, rather the traces when viewed on race studio analysis.

James I don’t see fuel type being the cause of this. I’m geared incredibly short at 11*81 and should be easily smashing past 14150 rpm but the engine clearly hits some sort of limit and oscillates up/down while the gps speed plateaus. The reason I’m after this is so i can gear shorter to benefit off the incredibly tight corners on our little track while not sacrificing top speed.

Marin jetting is spot on i am hitting 14150 all day long lap after lap with the green coil. On the black its 100 rpm less but same deal rpm just zig zags and the speed plateaus.


(Alan Dove) #10

I think we both agree the engine has a limit, it’s the type of limiter - hard vs soft. My understanding was the Rotax products used a soft limiter system as opposed to the hard limiters you have in something like OK/KF when the engines just stop at the set rpm.

btw I hate limiters of any sort. I’m a 20,000rpm kinda guy :slight_smile:


(Marin Vujcich) #11

Just remember if it is a soft limiter it will randomly kill the spark count by a percentage that will incresase the more you go past the set limit
This will show a swing in rpm, as some of the spark is not there too count. Look at speed trace and peak rpm. If speed is constant at rpm fluctuations, its the count as descibed above