BRP Rotax 2cyl 165hp

[John800]

the reliability of these motors in sleds (clean 2 strokes in general)has gotten better in the last few years as brp finally seems to have improved the 800 etec, then they came out with the 850 and they had their share of issues, both burn downs and performance issues. A slight clutch calibration error on the 850s has them getting walked on by sleds with 15-20 hp less because a a few hundred rpm over rev, being that sensitive to a couple hundred rpm its gonna be a dog turd without the cvt. I dont think it would last long at all in a boat either.
I think you would be better off turbocharging the outboard you have, it would give you a much wider powerband

[FMP]

Since we're tossing ideas around, like I said the L5 with port to port pair tuned ex runners and tuner. Would give plenty of low end with typical 6000+ upper and still make 300 I bet if done properly. Before we get into the chamber, what's the exhaust duration on these
?

[John800]

I dont have a numerical answer for you fmp, but they are infinitely variable comtrolled by the ecu. Most are using a temp sensor in the exhaust to monitor temp and vary timing and exhaust valve position to get the pipe hot and in tune quicker. The ecu will pull a few degrees of timing and give it less exhaust port height for a few seconds untill its hot.
To put this i to perspective The difference between running a sled from a standing start with a cold pipe and putting it on a warm up stand and giving it a few wot blasts to get some heat in the pipe to start can be 2-3 sled lengths in 660 on an average stock class sled with no other changes.
Before i go way off track, basically my point is that they are using the variable exhaust in an inbeteen position at mid and sometimes upper rpms solely to get heat in the pipe

[FMP]

Post some pics of one taken apart, if you can find some. I haven't seen one apart and probably most members either. Had a drain guy tell me about them a while back and have been wondering since.

[John800]

i dont have any sleds new enough to have that style exhaust valve brp started that style in 07 ish and polaris in maybee 15. I can post pics of my 12 600 polaris valves that are less high tech than that. I can also measure some port heights on one of my motors if someone can figure duration if i can dig up the rod lenth that should give you a idea for refrence

[Snotrekie]

Typical port duration on 2 stroke snowmobiles are much different than that in 2 stroke outboards. boat won't plane due to lack of torgue off idle. get out your degree wheel you'll see!

[FORBESAUTO]

Curious, what are the normal intake durations and blowdown degrees on these? Suspect biggest difference would be in the blowdown due to the difference in exhaust tuning?

[Snotrekie]

In race version 200 to 208 open to close . 132-134 on transfers and rear transfer is the secret. some go wide some go deep i prefer deep . intake is modified to take bigger cages as close as possible to rotating assembly(shorten intake tract). The head design works with stinger diameter and fuel blend. Typical compression ratio is 16.2 to 16.8 corrected on the average up to 1 liter. most are running programmable total loss ignitions so they can get the leave.

[FORBESAUTO]

So 34-38 degrees blowdown, yes that's definantly more duration than a 2.5. 124 intake is stout and usually set at 27-28 blowdown. What rpm these turn at that port map around 11-12 k?

[FMP]

What's the distance to the far cone in the chamber from the port?

[Snotrekie]

two designs, pancake and hemi. FMP..loaded question. keep in mind squish growth and the ability to remove heat.
Forbesauto, race 9250 to 10k based on head pipe, diffuser and stinger length. generally don't go over that because rods stretch and drive belts become inefficient. Torgue is ET and RPM is MPH and that is the 1st rule I follow when building something. corner to corner 8 to 8400, oval 10200k and drag 94-9600

[FMP]

You probably build your own chambers, pipes. With those high ex ports travel distance must be a way , ball park??? Just going to figure return timing degrees before close and rpm that's all.

[FORBESAUTO]

2.5 with 124 intake and 27.5 blowdown operates in same rpm range (8-10k)with good angles and properly tuned chest with pretty good low end and midrange just for a comparison.

[FMP]

In this instance the originating cyl from where the pulse leaves is also the recipient of the same front as it travels to the bottom of the expansion chamber and rebounds. Gas temp here adjustable with stinger for slight pulse travel speed difference. Hey Snotrekie tells us more.

[perfmarine1]

That big crank dbl roller brg stolen from OMC outboard top crank brg. Also in pipe video I would never use a rubber over, that could be life threating however a grommet I would use over.