How to increase cylinder pressure without risking damage

Discussion in 'Performance Mods' started by jaguar, Mar 28, 2014.

  1. jaguar

    jaguar Well-Known Member

    I used to advise people to just go ahead and increase their engine compression for more power and if later on they can afford to get a Jaguar CDI then do so. Not any more. I am too convinced now that if you increase compression you stand a good chance of having catastrophic engine failure like this guy did. The overly advanced standard CDI is only good for non-modified engines with the standard low compression (~90psi).

    With excessively advanced ignition the peak pressure happens too early which is not that horrible on a low compression standard engine but when you shave the head for more compression it takes the whole shebang into the danger zone. That excessive combustion pressure-
    1) wears out conrod and crank bearings too soon
    2) increases engine temperature which can cause detonation and engine failure if there is a lean fuel mixture
    3) causes too much outward ring pressure onto the cylinder which increases possibility of the ring catching on a port and causing unrepairable damage

    The following graph shows peak compression happening 40 degrees after ignition. So for peak pressure to happen between 10 and 15 degrees the ignition has to happen around 22.5 degrees before top dead center (BTDC) which is what happens with the Jaguar CDI.
    Here is my page about increasing cylinder pressure:
    ps- the Jaguar CDI is now called the Performance CDI because of conflict of interest with the Jaguar motorcar company.
    074KU likes this.

  2. Fabian

    Fabian Well-Known Member

    I like the graphs and info.
  3. 074KU

    074KU Member


    It is really good to see information provided in this manner as it allows people like myself (with limited knowledge, a lot of tools and a desire to learn) to make sense of what is being said.

    Ah ha, now the penny drops! (I think)
    Fabian your CDI was killing big end bearings at such a rate because of the combination of load on the engine AND heightened compression with the standard CDI (being to advanced)?

    It is genuinely nice to be able to research this kind of information without inference to the sexual promiscuity of somebody's mother. +1
  4. Fabian

    Fabian Well-Known Member

    From my experience the standard CDI was killing the big end connecting rod bearing and it didn't seem to be related to excessive load on the engine, or excessive compression, because my first 3 engines failed prematurely prior to the installation of a higher compression cylinder head and prior to the trailer being built and towed behind the bike.

    The engines were nursed for their entire (short) life span, with a self imposed 3,500 rpm rev limit and an operation rev limit of 3,000 rpm, but it must be said that those engines used the caged needle roller big end connecting rod design.

    The newer crowded needle roller big end connecting rod design can cope with more load stress due to it's design.
  5. 074KU

    074KU Member

    Okies.. hmm.. :confused::confused: I must just be lucky. I have what you describe as the caged needle rollers, the ones with approx 1 roller gap between each roller and at this point a standard CDI and haven't managed (thank god) to shoot the piston up my pooper yet.

    I am even aware of some models of these engines using brass sleeves instead of roller bearings :confused:
  6. Fabian

    Fabian Well-Known Member

    That design is the caged needle roller version, as shown here (wait for the page to load the photos):

    This is the crowded needle roller version:


    You are correct. The brass sleeve is used on the small end of the connecting rod.

    My experience has shown the small end connecting rod needle roller design to be bullet proof, as i have never had a small end needle roller bearing fail in service.
    Last edited: Apr 2, 2014
  7. Fabian

    Fabian Well-Known Member

    These two photos are the most telling description of detonation problems, where the constant hammering of detonation shatters the needle roller case hardening, which then exposes the softer metal.
    This in turn allows the needle roller to be hammered into a smaller diameter, which allows the big end of the connecting rod to rattle around; increasing the wear on the needle rollers; eventually breaking up into fragments (if you keep riding the bike); flying out of the connecting rod and getting trapped between the top of the piston face and cylinder head; causing pitting on both the piston and cylinder head if you are lucky and complete engine seizure if a needle roller gets trapped between the cylinder bore and the piston.


  8. FurryOnTheInside

    FurryOnTheInside Well-Known Member

    I think you need to check your post Fabian, this picture is named "caged10.jpg" and certainly appears to have roller sized gaps between each roller. Probably obvious to anyone paying attention but just thought I'd mention it in case it causes anyone confusion..
  9. Fabian

    Fabian Well-Known Member

    Revised photos more clearly showing the difference between caged and crowded needle bearing assemblies:


    FurryOnTheInside likes this.
  10. Fabian

    Fabian Well-Known Member

    You can most likely see just how much extra load carrying capacity the crowded needle roller bearing assembly can handle, or conversely, the reduced load per needle roller for a given amount of pressure applied by the connecting rod to the needle rollers.

    The downside of the crowded needle roller bearing setup is reduced max safe rpm, as the rollers are effectively touching each other, thereby creating extra heat by their mechanical interaction.

    I have had only success with the crowded needle roller bearing assembly, with my experience showing that it is completely reliable up to 4,800 rpm as a max safe engine speed and will take 5,200 rpm for brief emergency gear change rpm.

    The engine is much happier when staying around 4,500 rpm as max normal rpm, with regards to the 66cc/80cc engine.

    3,500 - 3,800 rpm is the sweet spot for best usable torque (with a reed valve intake), so i typically operate the engine in that rpm zone with the shift kit.