Page 1 of 2 1 2 LastLast
Results 1 to 10 of 11
  1. #1
    Mr Bill's Avatar
    Join Date
    Aug 2005
    Location
    Macon Georgia
    Posts
    451

    Question RPMs vs Detonation Question

    I've read over and over that if your rpms are not over 7000 (give or take a bit) that you are subject to detonation. So why is it the guys running tripples fear detonation if their rpms are not much higher, like 7500 minimum? Why the difference? I am trying to understand why detonation happens at different rpms.

    Thanks!


  2. #2

    Join Date
    Jan 2006
    Location
    Australia
    Posts
    62
    +1
    1
    Bill,

    I'm no guru in 2 stroke detonation but can offer a small insight ...well my take on it anyway ...

    In this case the guys are virtually using the tacho as a "load meter". Craft with propellers/impellers are always under load, you can use the max rpm/WOT as indicator of load so to speak.

    Too much load equals too much cylinder heat ..too much cylinder heat for a given air/fuel mixture (octane as well) can cause detonation.

    To over-illustrate it ..imagine putting an impeller on a GP that was so steep it only rev'd out to 6000 at WOT and you kept it there for a few long topend runs.

    OUCH !!!

    So I imagine the guys are recommending a rpm to not go BELOW with a certain pitch prop (or state of tune) to keep maximum protection from high speed detonation (which is hard to hear/feel til too late).

    So its not so much the rpms but the WOT load at those rpms ... and if it (max rpm) comes down too much ... more power, more fuel, less pitch I hear the engine cry ....

    The never ending chase for speed ... addictive huh !!!!! (Smile)

    Regards
    Anthony

  3. #3
    XL1200Ltd-Keith's Avatar
    Join Date
    Aug 2005
    Location
    Arizona
    Posts
    506

    detonation

    Read "About Exhaust Valves" .

    http://www.groupk.com/y1200gpr-xl.htm

    Last edited by XL1200Ltd-Keith; 09-20-2006 at 07:14 PM.

  4. #4

    Join Date
    Apr 2006
    Location
    South MS
    Posts
    1,095
    You asking why the difference on tripples? I beleive most run a totally different timing curve for tripples and also tripples move the power band up higher than a single pipe runs at.

  5. #5
    ABBOTT's Avatar
    Join Date
    Jul 2005
    Location
    Birmingham, AL / Lay Lake
    Posts
    3,736
    +1
    96
    Tripple Pipes are so much more in "tune" with each cylinder --whereas single pipes are not. The charge is so much stronger and produces massive HP and therefore gives you a different "power band" with different timing too as Ron said.

    Tripples tend to run lean at certain rpms too which can be cured with Jetting and even some race gas -------(just learned this).

  6. #6
    YudLuz2's Avatar
    Join Date
    Jun 2005
    Location
    Flowood MS
    Posts
    1,648
    +1
    112
    Quote Originally Posted by Mr Bill
    I've read over and over that if your rpms are not over 7000 (give or take a bit) that you are subject to detonation. So why is it the guys running tripples fear detonation if their rpms are not much higher, like 7500 minimum? Why the difference? I am trying to understand why detonation happens at different rpms.

    Thanks!
    Deto occurs at different rpms because of a number of things. Carb signal, lean conditions, squish clearence, load on motor, porting....... I think one reason you see more problems on a loaded motor below 7000, is the timing advance curve on the ski. Take for instant the stinger curve in the R&D ignition, it maintains a 26 degree advance off idle until 4800. From 4800 rpms it slowly drops down to 16 degrees timing at 6900 rpms. Then from 6900 rpms to 9000 rpms it maintains the 16 degree total timing on all three cylinders. Now 16 degree on each cylinder is a lot for a pump gas motor, now less say you over proped and you could only turn 6700 rpms. Your motor now would be running around 19 to 18 degrees of total timing. You could easily now put your motor over the edge running this rpm, get it over 6900 and the timing pulls back down to 16 degrees. Now using a Advent you are a little less aggressive and they pull addition timing out of the 3rd cylinder, so most likely your motor might live, when another ski
    would blow quicker. On a triple most of your timing on the over the counter boxes pull most of the timing back out of the ski around 7500 rpms, above that most curves only give 8 to 10 degrees of final timing. So keep it above that rpm range and you will be safer too. For best performance on each setup, one should write your own custom curve. For my triple pipe ski it was a must, its probably the
    most important thing on a triple for high end performance numbers.

  7. #7
    Mr Bill's Avatar
    Join Date
    Aug 2005
    Location
    Macon Georgia
    Posts
    451
    Thanks! That certainly helps.

  8. #8
    if you look at the design of the gpr engine you will see a n engine thats designed to rev and not to lug
    when you lug the engine the build up of heat is alot more and therefore leads to detonation

    you are better off letting it rev

  9. #9

    Join Date
    Nov 2005
    Location
    All over
    Posts
    716
    Let's clarify on thing. You don't need to run at WOT all the time in fear of damaging your engine. We are mearly talking about WOT events and overlaoding your engine. You can run your boat around the lake/river at 40mph (as long as your mid range is tuned correctly,) without an isssue!

  10. #10
    yes you are right
    i am talking aabout wide open throttle loadings and not loading the engine
    if you are at half throttle then the ski is not making the heat energy it is at wide open throttle and deto is not so much of an issue

Page 1 of 2 1 2 LastLast

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. 15f rpms vs 100 degree air temp question
    By bandm hx in forum Kawasaki PWC Performance (4-stroke)
    Replies: 2
    Last Post: 07-17-2011, 08:35 PM
  2. Replies: 0
    Last Post: 03-02-2009, 01:29 PM
  3. RPM vs. Speed question
    By big83bronco in forum Yamaha PWC Performance (2-stroke)
    Replies: 25
    Last Post: 08-04-2006, 08:06 AM
  4. rpm an speed question ???
    By Ty in forum Sea Doo Open Discussion
    Replies: 2
    Last Post: 07-19-2006, 04:15 PM
  5. RXT handling stock vs. mods question
    By Mark S in forum 4-Tec Performance
    Replies: 2
    Last Post: 03-22-2006, 08:04 PM

Posting Permissions

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