How much BHP lost through heat.

[KTR]

Ok, looking for a bit of info as to how much bhp can be lost through raised inlet temps, specifically relating to cars on a rolling road. I am sure I read somewhere that an increase in inlet temp of 2 degrees would lose 1bhp. Does this sound right?

[kambites]

I can't imagine that a given temperature rise would lose a set number of BHP, but rather that it would lose a proportion of the BHP?

Since power is basically proportional to how much air you can shovel into the engine, what I think you need is the formula for how the density of air varies with temperature, and power will vary proportionally to that?

Edited by kambites on Tuesday 17th August 19:59

[kambites]

According to wikipedia, at 20 degrees, air has a density of 1.2041kgmmm; at 25 degrees is has a density of 1.1839. So that five degree shift will multiply power by 1.1939/1.2041 which is 0.9832.

So you'll have 1.7% less power at 25 degrees than at 20 degrees.


Unless, of course, I'm completely wrong. Which this evening, I think is quite likely.

[anonymous-user]

That would account for the power lost via reduced intake charge mass, but spark efficiency in a gasoline engine can have an even bigger effect. Especially on say a heavily boosted engine. Something like a scooby turbo will have BLD ignition slopes heading towards 5 to 8 bhp per degree BTDC at peak torque rpm. So, if the hot air charge leads to the onset of detonation, the knock control will retard the ignition, and then that forces the fuel controller to richen up the AFR to prevent exhaust EGT's exceeding the limits. Thats a tripple whammy, reduced massflow, retared igniton AND enriched AFR's (assuming it was rich of LBT in the first place, which in 99.9% of cars it's going to be)

Rolling road experience would suggest these factors could add up to ~5bhp per degC on a typical 2.0l turbo car (of around 260bhp) and possibly higher for bigger capacities/powers.

[KelWedge]

All of it if the fuel lines get hot and petrol turns to vapour !!

[Garett]

I had my Saab on a rolling road in late May, it was quite a warm day but I got on early at around 9:30am.

Here is a pic of my graph, it shows the bhp and the ambient temp, not the inlet temp.

HTH

Edited by Garett on Tuesday 17th August 20:56