Quote:
Originally Posted by stevesrt8
I also would like you engineering types to express your views on the various forms of supercharging, turbo, mechanical, and nitrous/chemical. Please advise us how we factor these into said corrections.
Nitrous is especially interesting with its unique combination of cooling and pressurization of the manifold with no parasitic losses. I'd like to see that quantified!? The main interest I have in this thread is comparing the D/A adjustments between N/A cars and boosted cars. I maintain that boost or nitrous "create their own" air and thus are not entitled to the same correction factor that an N/A engine is. What do you think?
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Steve, this turned out to be a lot simpler that I had imagined. It was more of a concept solution than any nuts and bolts, so to speak.
Atmospheric corrections for chemical and mechanical forced induction...
Turbochargers and Supercharger:
Let’s agree that superchargers and turbochargers mechanically maintain an artificially elevated level of air pressure in the intake manifold. This elevated intake manifold pressure is maintained regardless of the ambient air pressure. It follows that a 6 psi supercharger at sea level is going to provide 6 psi at 5,000 feet. It also follows that there would be no correction for altitude for a supercharged or turbocharged engine as the available air for the engines combustion cycle would be the same regardless of the altitude.
This is not to say that there is no correction necessary for varying weather conditions. Humidity affects the percentage of oxygen in air and temperature affects the mass of the air as always. It would then follow that to normalize separate mechanical forced induction engine tests one would still use the SAE formula but run both calculations with an elevation of 0.
Nitrous Oxide:
I think we can agree that the injection of N20 in liquid form at approximately 1000 psi into or immediately before the manifold does several things. It drops to manifold pressure of approximate 15 psi and evaporates causing the surrounding air to cool significantly. It also increases the oxygen concentration of the intake manifold because it is of a higher oxygen concentration than the air. As a result your intake manifold charge has a higher percentage of oxygen content because of the nitrous oxide and an increased mass because of the cooler temperature. This benefit is regardless of altitude, temperature or humidity. If it cools 70 degree air 30 degrees then it will cool 40 degree air 30 degrees as well.
Now here is where it becomes interesting. A N20 injection system commonly doses out its charge in units of horsepower and to achieve that 75 shot or 100 shot it injects a given amount of nitrous per unit of time. Therefore a 100 shot will add 100 horsepower at sea level or 5,000 feet over and above whatever horsepower the engine is making without it at the time. In other words, the amount of available oxygen in the intake manifold will be whatever the ambient amount is plus the additional effects of the N20 injection.
It now follows that to apply atmospheric correction to a nitrous injected engine one must figure the correction factor the same as a naturally aspirated engine but subtract the added horsepower from the nitrous before applying the correction factor to the horsepower.
In summary, it is my opinion following the above thought process that to apply atmospheric corrections to supercharged and turbocharged engines one must use the SAE formula with 0 elevation. To apply atmospheric corrections to nitrous engines one must use the naturally aspirated horsepower of the engine.