Results 1 to 11 of 11

Thread: Why does the RS125 GP bike have a rear-exit exhaust?

  1. #1
    apriliaforum Junkie killerofangels's Avatar
    Join Date
    Feb 2005
    Location
    Virginia Beach, USA
    Posts
    328

    Why does the RS125 GP bike have a rear-exit exhaust?

    I was browsing some pics of the RS125 (RSW125?) GP bike previously used in professional racing [RIP]. I can't help but wonder why the exhaust exits the rear of the cylinder. This may be the same on other GP bikes as well. I always thought the exhaust usually exits the front because that's the area with the best cooling potential. I know this isn't really an RS50 issue but it's still a single cylinder bike. Any of you tech experts have an answer to this?
    Name:  RSR125LHnaked1.jpg
Views: 10729
Size:  56.4 KB
    Ringa-ding-ding-ding! 2-strokes kick ass!!!

  2. #2
    apriliaforum expert nathan k's Avatar
    Join Date
    May 2012
    Location
    England, New forest
    Posts
    683
    I don't know but when you think about it a straight exhaust will create less resistance than one that bends under the engine. That could mean more flow? definitely less weight.

  3. #3
    apriliaforum expert apriliamandan's Avatar
    Join Date
    Jan 2012
    Location
    England (Cambridgeshire)
    Posts
    3,128
    Quote Originally Posted by nathan k View Post
    I don't know but when you think about it a straight exhaust will create less resistance than one that bends under the engine. That could mean more flow? definitely less weight.
    Good thoughts Nathan, but its a 2 stroke, resistance is needed for back pressure, no?

  4. #4
    apriliaforum expert RS_Roadrage's Avatar
    Join Date
    Aug 2012
    Location
    United Kingdom
    Posts
    827
    Its because the engine gets the air forced into the cyclinder instead of it having to suck it up/using energy and gets blown straight outbthe exhaust port, the chamber is where most of the back pressure is built up, this is my current knolage although i hope to work on more engines like this in the future
    RS 50 2004
    new stainless steel brake lines
    Giannelli race exhaust
    0.30 reeds
    stock air box
    polished Cylinder
    (BR9EIX)
    (82 main jet)

    tinted screen

  5. #5
    apriliaforum Junkie killerofangels's Avatar
    Join Date
    Feb 2005
    Location
    Virginia Beach, USA
    Posts
    328
    Quote Originally Posted by apriliamandan View Post
    resistance is needed for back pressure, no?
    From what I understand about 2-stroke exhausts, the back pressure is created by the reduction in cone size towards the end of the exhaust. It creates a pressure wave that travels back up the exhaust and packs a little of the charge back into the cylinder right before the exhaust port closes. It's kind of a messy science because a little of the fresh charge is always lost out of the exhaust port then -maybe- at the right RPMs, it's all pushed back in by the return wave. That's why 2-strokes are being phased out by emissions regulations and the whole push towards direct injection (which only injects fuel after the exhaust port is closed).

    Quote Originally Posted by RS_Roadrage
    Its because the engine gets the air forced into the cyclinder instead of it having to suck it up/using energy
    Not sure exactly what you mean. Are you referring to the rotary valve or maybe the ram-air system? All 2-strokes have air forced into the cylinder by the pressurized crankcase. There is no intake cycle as with 4-stroke engines.

    My main concern about the exhaust at the back is for cooling. The exhaust side is typically the hot side and creates some kind of distortion at the piston or cylinder when compared to the relatively cool intake port side. I know most of you guys aren't into engine design but I think the smaller capacity engines are a great learning/test platform for this kind of thing as the cost involved are pretty low. I hope to someday create my own cylinder variant much like Athena did with their injection system or exhaust valve system. I think they're really headed in the right direction and I'm excited to see how successful their cylinders are. Have any of you guys tried one on your AM6?
    Ringa-ding-ding-ding! 2-strokes kick ass!!!

  6. #6
    apriliaforum expert ryanperkins's Avatar
    Join Date
    Dec 2011
    Location
    kettering
    Posts
    717
    sorry but this just looks the nuts!!
    what do you think off mine? comments please!!

  7. #7
    apriliaforum expert
    Join Date
    May 2010
    Location
    Plymouth
    Posts
    769
    Could also be to do with keeping heat in the exhaust to improve scavenging. aswell as keeping the bike more streamline and using more space thats availible inside the frame.

  8. #8
    apriliaforum prov-nov
    Join Date
    Dec 2012
    Location
    Swansea, Wales
    Posts
    34
    My thoughts on this are that the exhaust gases have a direct path to the chamber, the chamber being where the pressure is built, with a direct path more gas will travel through the chamber causing more pressure, which then gets to the rear can, causing more back pressure than an exhaust at the front of the cylinder. As for cooling im not a 100% as i can see you're point of not having as much cool air as if it was situated at the front of the cylinder.

  9. #9
    apriliaforum expert mattology's Avatar
    Join Date
    Jun 2009
    Location
    Saint Petersburg, Florida
    Posts
    2,916
    The cylinder is reversed

  10. #10
    Racing SERVICE Competition Steve / AF1 Racing's Avatar
    Join Date
    Sep 2003
    Location
    Austin, Texas USA
    Posts
    3,341
    I've owned the bike below since late 2008, while ownership does not qualify as expertise, I'd be most inclined to say that the reason for the rear exiting exhaust was mostly down to packaging. Rotary valve engines have to have roughly 1/4th of cylinder available for the placement of the rotary valve assembly. The ideal place to mount a rotary valve would be the front of the engine, but there is no room for an airbox, and carburetor, and radiator, so Rotax's designers did the next best thing and moved the rotary valve assembly to the rider's RH side of the engine. This allowed Rotax/Aprilia to mount a large airbox, with useful ram air effect on the RH side of the bike and still have a pretty good pathway for intake charge to enter the cylinder, but this also has some drawbacks, because you then end up having to mount the clutch basket AND stator assembly on the same side (the rider's LH side) of the engine.

    By the time that Aprilia was purchased by the Piaggio group in 2005, the RSW engine design (rotary valve on RH side) was already considered obsolete in favor of the Piaggio group's own 125 GP racer, which was called the RSA. The RSA mounts the rotary valve and carburetor on the rear of the cylinder and the exhaust is once again exiting out of the front of the cylinder, in the conventional way.

    Complete engine diagrams for the RSA can be found on the Aprilia Corse website at the link below:
    http://www.serviceaprilia.com/public...010_110926.zip

    Click image for larger version. 

Name:	RSW125-2011May.jpg 
Views:	2039 
Size:	425.6 KB 
ID:	213867
    AF1 Racing, inc / ApriliaForum.com
    9900 North Interstate 35
    Austin, Texas 78753
    direct:512-459-3966 fax:512-482-8319

    Currently:
    '97 RS125 Two Stroke

    Previously:
    '00.5 Mille & '97 RS250.

  11. #11
    apriliaforum Junkie freedomgli's Avatar
    Join Date
    Sep 2009
    Posts
    479
    Quote Originally Posted by Steve / AF1 Racing View Post
    I've owned the bike below since late 2008
    Awesome bike Steve! Thanks for taking the time to explain some of the engine differences between RSW125 and RSA125. I've seen rotary valve inlets on the back of the engine before driven by a crankshaft powered toothed belt. I've also seen the rotary valve inlets on the side of the engine directly feeding the crankcase. I'm sure designers have many pros and cons when deciding which path to choose. But can anyone give me the highlights why one would mount the rotary inlet valve on the rear vs. the side?

Similar Threads

  1. Why does the 2013 ZX-6 have a 190/55 rear tire
    By The Alien in forum General Motorcycle Talk
    Replies: 46
    Last Post: 10-04-2012, 07:45 AM
  2. Why does the RSV4 have a ABS light... with no ABS?
    By dastrix in forum 2010-2019 RSV4 - All Variants
    Replies: 7
    Last Post: 02-20-2012, 02:43 PM
  3. Rake/trail on the 250 gp bike?
    By Olof Eriksson in forum RS250
    Replies: 7
    Last Post: 10-04-2008, 12:43 PM
  4. Why does the oil look blue
    By Gtrain in forum SL1000 Falco Forum (all years)
    Replies: 7
    Last Post: 04-18-2006, 02:51 PM
  5. rs125 gp bike on ebay
    By daz19 in forum RS250
    Replies: 0
    Last Post: 11-01-2003, 09:44 AM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •