backpressure backpressure

[Mark944turbo]

I remember seeing a formula once that used a bunch of variables to output a maximum backpressure/map ratio, that is, the ratio that if exceeded the motor will start making less torque. At least I think I remember this.

Does anyone have this equation or know if it exists? I ask because on a formum that I belong to we are having an arguement about the limits of our stock turbocharger, and some people are saying that 2x the map is too high, but I am not "feeling" this when I test in my car. It feels faster at 25intake 60psi exhaust than 20 intake 40 exhaust. (not actual numbers, just estimates)

I think the formula that I saw was used more by factory's designing a mass produced engine for regular cars.

 

[Lou Scannon]

If you're making more power and not overstressing the engine than the ratio is not "too high". But there is a lot of top end power to be gained (and reduced temperature stress) by modifying to decrease exhaust pressure. Reducing exhaust system backpressure (e.g. turbine out) is the most obvious, and benefits the entire rpm range, though is tilted toward the top end. Having a turbine wheel trim that is matched to the compressor and the engine's flow range is also very important for minimizing exhaust pressure.
Finally, a larger turbine housing A/R, assuming a wastegated turbo, will reduce backpressure and increase top end power at the expense of reduced spool up performance.
Something to consider when seeking power gains from a given turbo, the process of cranking up the boost, taken too far, will run the compressor into choke, not too mention risking an impeller explosion. In choke, the compressor will not flow any more air, no matter how much power is put into it. Therefore I would expect to see exhaust pressure increase dramatically as you get near this condition, due to the wastegate closing down as boost pressure levels off because of the choked condition.
On my turbocharged vehicle I have observed that my boost and turbine inlet pressures are very similar until near the shift point, where the pressure ratio has increased to ~1.3 (absolute pressures).