Turbo boost and chamber pressure.

There have been some statements about turbo boost on this forum.
Id like to ask,Does increasing the boost increase the PSI within the chamber? Or does it just make it easier for the air to be forced in? Im pretty sure it would have to increase the chamber PSI, but by how much?

Does anyone have actual figures, comparing non turbo...and the same engine with turbo?
Mainly thinking 240's 940's here. But other car and figures would be interesting.

Then there is the matter of turning up the boost, which must further increase chamber PSI.
By how much? Of course this depends of how much you increase the boost.

If increasing the boost does increase chamber PSI, then would this be comparable to shaving the head to increase PSI in the chamber?

The reason im asking, is that high octane fuel can be gained by water injection and a mix of 50/50 water/alcohol. I know a guy in the USA who compared this to 116 octane. But without changing compression, he gained nearly 20% more power.
Ive been playing with the idea to do this for some time.


  • jamesincJames @jamesinc Oz Volvo Ice Fortress
    In terms of shaving the head, that increases your static compression. Higher compression is generally associated with better efficiency and power, and I think on any given engine, increasing compression should increase torque, but increasing compression changes timing, reduces your margin for error in timing, and forces you to use a higher octane fuel (octane rating being a measure of a fuel's resistance to spontaneous combustion under pressure).

    The way it achieves that is that the higher your compression, the faster your mixture burns, which means you do more burning while the piston is up around the top of its stroke. If the burn is too slow, your piston will already be moving downward, increasing the volume of the combustion chamber and reducing overall pressure, and increasing the surface area that pressure is acting upon, all of which negatively affect engine performance.
  • Vee_QueVee_Que @Vee_Que South Eastern suburbs Melbourne.
    On boost, seriously, there is many websites describing how it works in effectively increasing the engine swept capacity at say one bar(14.4psi) a turbo engine is effectively burning as much air and fuel as an engine double its size.

    Turbo efficiency comes into play. If you have an 740t with a stock turbo and intercooler, you will be exceeding the turbo compressor map ability by 12psi at the turbo and most boost gauges see it at the intake that we see. The job of an intercooler is to get that temperature down to reduce detonation, and reducing heat soak in the intake system from the hot compressed air. So upgrading the intercooler to a thicket one that isn't leaking as all stock ones do with increased boost do, is the first step you take.

    This allows you to run the 12psi at the manifold on a stock turbo on overboost and run a little more ignition timing. From there, fuelling is more important.

    There is a reason not many people use water/methanol injection anymore.

    "The evaporative effects of the water, plus the octane boost of methanol, allows your motor to run more advanced ignition timing and/or boost to create additional horsepower with proper tuning. A general rule of thumb is that an engine will output one percent more horsepower for every 10 degrees intake temperature is decreased. Many enthusiasts have found the benefits of using water/methanol injection equivalent to running race gas, but without the $12/gallon price tag."

    It's called e85, on a turbo motor it has more oxygen and a higher octane level. You need to install bigger injectors and fuel pumps and ideally install an aftermarket ecu. Something that means it's silly to bother with a little injector to put some water /methanol in a little squirter in your intake when you've just gone from 150rwkw to 200rwkws with a change of fuel, fuel pumps and ecu and tune. But you can drive the car hard all day long as long as there is fuel in the car. Unlike with a stock ecu with fixed ignition timing and fuel. Assuming you've already put a larger turbo on with better flow.

    To do it on a non turbo 240 which works at peak ignition of about 16* and will ping on a b230 with 10:1 compression, but on any other motor is around 9.5 c.r, with fixed, non adjustable ignition timing and fixed fuel maps. Well, there isn't anything to cause pinging.

    Not great reading, but this is on a tuned turbo car in the states http://www.superstreetonline.com/how-to/engine/impp-1101-water-methanol-injection/

    Keep in mind, we have much higher quality octane fuel as our top fuel than the Americans do and we run the Ron measurement system.
  • nickmnicko @nickm Sydeneee --- home of the traffic jamb, over priced coffee, and the 2000 Olympics
    edited November 2017
    You are compressing the air. Therefore more O2 is available to create a bigger bag.
    Even though it is "forced induction" that is not so much the function. It is about increasing the amount of O2 and fuel entering the cylinder.
    So if you have 1.5 times the air entering the constraint of getting 1.5 times the power is the engines efficiency.

    A good reason to work in Bar not PSI, .5bar is about 7psi so that is 1/2 atmosphere more air. But you don't see a 50% power increase.
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