monitoring intake temps

[twospoons]

is anybody doing this? what temps are you seeing at the plenum?

obviously cooler intake charges = more power... we all know that...
and air temp increases as pressure increases... so
more boost = higher intake temps...
but, more boost = higher compression ratios for the cylinders...

at what point are we loosing power because we are trying to run too much hot air boost instead of just the right amount of cooler air?

is there any kind of a formula (even very generalized) that we could use to help tune with? such as boost at temp = compression ratio... for instance 12psi (in the plenum) @ 150* = x.0:1 CR and maybe 15psi @ 250* = x.0:1 - x.5:1 CR... not that these numbers are in any way, shape or form accurate... just an illustration...

[Bob The Great]

that's the whole point of using different turbos. 9b's can produce 20psi, but they are way out of their efficiency island, and are producing 20psi of super-hot air.

To find the effective compression ratio, you can use this formula. It is dependant on boost pressure and static compression ratio.

Cf=(B/14.7+1)Cr

where Cf is the final ratio, Cr is the static ratio, and B is the boost pressure in psig.

[twospoons]

right... but exactly how much of a factor does heat play into that? just calculating the compression ratio base on boost pressure measured at the plenum isn't entirely accurate... just with that formula and not considering thermal expansion of air at different temperatures you should be able to produce more power at 18.8:1 (20psi) than you do at 16.0:1 (14.7psi), right... but if the colder air at ~15psi is able to expand more than the 20psi of hot air... then wouldn't you actually be running higher compression ratio than 16.0:1 at 14.7psi and maybe not quite the full 18.8:1 at 20psi? and yes i know that it would depend on what turbos are pushing that pressure... but i believe that the intake temps would be somewhat indicative of that (higher temps = less efficient/smaller turbos at higher pressures)... maybe 20psi does create more power than 14.7psi... (not that 9b's would ever stand a chance at holding that)... but maybe 15psi creates more power than 17psi???

i would just like to know how to factor air temp into that equation.

[Bob The Great]

then, maybe I don't understand your question. Compression ratio is completely independant of air temperature, type of turbo, hell even type of engine.

It almost sounds like you're asking more how to calculate power. Compression ratio is a factor in calculating brake specific fuel consumption, which is used along with injector size and duty cycle to calculate maximum possible power.

You are correct that hotter air is less dense, and so an engine ingesting air at 200d. will be less powerful than the same engine ingesting 150d. air, simply because it is taking in less mass of air, even if both engines are operating at the same final compression ratio. The mass of air entering the engine is what limits the injector pulse width, thus limiting power. But, it has no bearing on compression ratio at all.

Is this what you were talking about?

[twospoons]

Originally Posted by Bob The Great

You are correct that hotter air is less dense, and so an engine ingesting air at 200d. will be less powerful than the same engine ingesting 150d. air, simply because it is taking in less mass of air, even if both engines are operating at the same final compression ratio. The mass of air entering the engine is what limits the injector pulse width, thus limiting power. But, it has no bearing on compression ratio at all.

ah... density... yes maybe that's what i'm thinking... anyway my question...

is there any kind of a formula (even very generalized) that we could use to help tune with?

what i meant was would it be beneficial to monitor intake temps at the plenum and use that (along with boost pressure) to tune with? i wasn't specifically asking about compression ratios, although i'm still having trouble understanding how density and/or temperature (at whatever boost level we are measuring) doesn't affect the compression ratio... i'm no physics expert by any stretch of the imagination... i was just curious how intake temps exactly factor into this...
thanks

[Bob The Great]

well, I suppose you could, but it's a lot easier to measure the result of the combustion (O2's and EGT) and then adjust then to try to precisely measure the air going in and then match it with just the right amount of fuel. That's the ECU's job.

[twospoons]

yeah i figured i'd get an answer like that...

[Multiades]

I know I've seen devices like this before, but where can I purchase an inexpensive fast-reacting air temperature gage, or possibly a differential temperature gage (obviously with two inputs).

It would be very useful to be able to quote peak air temp in the plenum.

-Chris

[95red3000gt]

I know on a DSM, you can use DSM Link to monitor intake temp. Not sure were the sensor is though, but maybe you could just use that sensor and modify it to read to an LCD or something on our car...just a thought

[twospoons]

Originally Posted by 95red3000gt

I know on a DSM, you can use DSM Link to monitor intake temp. Not sure were the sensor is though, but maybe you could just use that sensor and modify it to read to an LCD or something on our car...just a thought

well that's not exactly what i'm talking about... the MAF sensor does have a way to measure air temp... however the maft alters that signal to the ecu so it reads a constant 80* and the ecu uses that to pick a certain timing map... i have a datalogger and this has been confirmed...

Originally Posted by Bob The Great

well, I suppose you could, but it's a lot easier to measure the result of the combustion (O2's and EGT) and then adjust then to try to precisely measure the air going in and then match it with just the right amount of fuel. That's the ECU's job.

why is it easier? just because that's what the majority does?

i'm going to do some more research on my own... i just thought maybe someone else had already considered this possibility and could save me the trouble of looking it all up for myself.