redline increase

Discussion in 'Technical' started by MR2T, Feb 4, 2006.

  1. someone told me that new cams could increase your redline. how does that work? anything else that can do that?
     
  2. A redline is a limit where if you go any higher than it, could do damage to your bottom end or valvtrain. All a bigger cam does is allow air to more easily flow into the cylinder. So if you want an engine to go to 8k rpm or higher, it makes it easier for the huge amounts of a/f mixture that is nessesary to get into the cylinder.
     
  3. so how could it be increased?
     
  4. opening the valves, more, less, duration/longer, overlap, etc. yuou can raise your RPMs or lower them, make power higher or lower, curved or flat lines, etc all are factors that a cam can change.
     
  5. Ways to make an engine rev higher:
    Balanced internals (pistons, rods, crankshaft, etc)
    Lighter valvetrain (eg shimless buckets)
    Stronger valvetrain (eg valve springs)
    Dry sump oiling
    Etc...
     
  6. Yes, but you still need larger cams, freer flowing intake and exhaust, etc. to really make the engine spin faster. The lighter components only go so far, you need to reduce pumping losses and friction to really allow the engine spin faster. The lighter components and balancing really just allows the engine to accelerate faster and keeps it from tearing itself apart at higher rpm.
     
  7. Changing to larger cams will not raise your redline. The only way I know to change your redline is to reprogram your ECU.

    Should you change your redline if you upgrade to larger cams? Only if your new cams are meant to breath at a higher RPM level than what your stock redline is. Chances are your Cam provider will tell you whether or not you need to upgrade other parts of your valve train in order to prevent damage to your motor.
     
  8. Yeah, that logic works until you have carburetors, or an ECU without an artificial rev limiter.

    There are a whole host of things that contribute to an engine's redline, you can't just change one thing and expect to increase the redline unless the engine has a rev limiter set artificially low.... there's a reason manufacturers add rev limiters and it usually isn't to protect the valve train.
     
  9. word

    peopl eneed to realize, that so many things need to be considered when building an engine or just increasing performance, I know sometime speople put exhaust and their cars and lose performance, it lost backpressure, so why did it lose power? its because the cams and timing may not be right for that exhasut , if the ex closes at this many degrees after TDC, etc.
     
  10. That's why designing an exhaust for turbo powerplants is so fun. Big + short = success.
     
  11. Its not back pressure that causes the loss of power. It's the loss of exhaust velocity that kills HP in a situation like that.

    A good example is upgrading to larger pipes with ZERO bends, or poorly placed bends. This isnt necessarily true for turboed vehicles though.
     
  12. It's not that the loss of exhaust velocity simply reduces power, it shifts the scavenging effect to higher rpm, which will tend to shift the torque peak to higher rpm, reducing the torque production at lower rpm... which is fine for a racing engine that only sees the upper 1/3 to 1/2 of its rpm band, but for a street engine, I'll take mid-range punch over high-rpm screamers any day.

    With the turbocharged engine, you are trying to maintain as high a temperature entering the turbine as possible, and the longer the exhaust manifold piping, the greater the surface area and the greater the heat loss. Plus, with a turbocharged engine, it's often hard to get much of a scavenging effect because the pressure in the exhaust manifold is usually nearly as high as the cylinder pressure near the end of the exhaust stroke. Also, you want as little restriction after the turbo as possible. The power to drive the turbocharger is the exhaust mass flow rate times the change in enthalpy. Keep the inlet of the turbine as hot as possible and the inlet pressure as high as practical (you don't want it to be much more than the intake manifold pressure), and the outlet temperature and pressure as low as possible to maximize the shaft power that drives the compressor side of the turbo.
     
  13. The reason for trying to make the exhaust manifold of a turbocharged engine as small as possible, except for reducing the surface area (heat loss) is because that reduces the volume inside the manifold. When the volume between the engine and turbocharging decreases the pressure in the manifold will see a larger change during an engine cycle. With zero volume the exhaust manifold pressure will be equal to the pressure in the cylinder which at the time the exhaust valve is opening may be say 10 bar but only around atmospheric late in the exhaust phase. Say that you have an intake manifold pressure of 2 bar absolute, that together with the exhaust pressure of 1 bar absolute when the exhaust valve is closing will give you a good cylinder filling anyway, without using tuned exhaust pipes. The low exhaust pressure will also result in decreased pumping losses.

    The energy availible in expanding that 10 bar pulse of exhaust gas is also much greater than using a constant pressure of say 2 bar. The downside is the reduced turbine efficiency of a pulse system.

    Using exhaust scavenging is however still good, but is hard to combine with a small exhaust manifold.
     
  14. "The reason for trying to make the exhaust manifold of a turbocharged engine as small as possible, except for reducing the surface area (heat loss) is because that reduces the volume inside the manifold. When the volume between the engine and turbocharging decreases the pressure in the manifold will see a larger change during an engine cycle."

    Good point. But given that we have pulsed flow going on, you end up with some mass of burned gas in the exhaust manifold runners when the exhaust valve shuts until you start scavenging some of that with another cylinder's exhaust flow, so for most of the rpm band, you aren't necessarily "refilling" the entire volume of the exhaust manifold with each exhaust puff. So, wouldn't you think most of the pressure loss through the manifold is a result of pipe friction and not a result of filling?

    "The energy availible in expanding that 10 bar pulse of exhaust gas is also much greater than using a constant pressure of say 2 bar. The downside is the reduced turbine efficiency of a pulse system."

    Yeah, unfortunately piston engines have pulsed flow, so when talking about mass flow rate through an engine, we pretty much have to use the "quasi-steady-state" mass flow rate in order to make any calculations bearable.

    "Using exhaust scavenging is however still good, but is hard to combine with a small exhaust manifold."

    I think that's what I was trying to get at. <A BORDER="0" HREF="http://www.supercars.net/PitLane?displayFAQ=y"><IMG BORDER="0" SRC="pitlane/emoticons/smile.gif"></A>
     
  15. There is always a loss of pressure due to wall friction. The thing with a large volume manifold is that it is allowing the exhaust gas to expand before reaching the turbine.

    It's also desirable to separate the pipes in such a way that there is no overlapping of exhaust pulses.

    The following is a text on the subject taken from MHI:

    "For the conventional constant pressure system, the exhaust manifold pressure is stable at a relatively low value. There are major flow losses due to the big difference in pressure between the cylinder and the exhaust manifold. These losses are admissible because of the simple construction and the fact that all the cylinders are connected to the same manifold.

    When using a MHI Twin Scroll Turbocharger System, the exhaust manifold pressure is at a low value for most of the time, which is very advantageous for the scavenging process. Because the exhaust manifold has a very small volume, the pressure in the exhaust manifold will rise quickly as the exhaust valve opens.

    The pressure in the cylinder and the manifold then both drop during the blow down, while their difference remains small. The blow down losses are far less with the Twin Scroll Pulse System and the cylinder pressure is utilised as much as possible to drive the turbine.

    Where it concerns using the pulse energy, it is not possible to connect all of the cylinders to the same exhaust duct. During the scavenge period immediately following the blow down, no pulse of another cylinder can be allowed. This would cause a back flow of exhaust gases into the cylinder and even into the inlet manifold where the pressure is lower than the exhaust pulse peak pressure.

    As it is only possible to connect cylinders that are more than 720/3=240 degrees apart to one exhaust duct, three cylinders at one duct is the maximum. Concerning the automotive industry, a MHI Twin Scroll Turbocharger can significantly improve the performance of 4 and 6 cylinder engines."
     
  16. to raise your redline you just raise your redline, to not destroy your engine while doing so is a whole other story.
     
  17. Increasing your redline isn't good for the long haul.
     

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