vRS acting strangely in summer?

[sam_b]

I recently saw a post on the octy forum from a guy in India talking about low power on his RS and the realtion is any to the ambient conditions.
Initially i dismissed the relation, but the past couple of days my car doesn't seem to have the same crackling performance and I'm missing the punch. As I bought my car in Sep last yr, this is the first summer i've had the car.
The turbo spools right on cue, everything looks fine. So it lead me to the temp. It's nearing 44 deg Celcius. Now obviously that means hot air and low density of air...which leads to less air being charged into the engine. On the face of it it explains lesser power, but here's my prob with this theory
An Otto engine is basically a thermodynamic system which is operating between two pressure limits......the intake pressure and the exhaust pressure (and adiabtaic compression and expansion thrown in the middle), with the area of the graph giving us the power of the engine. Now arguably the intlet pressure will go down in summer heat as the air is hot and density low. but also the back pressure has also lowered as the atmospehere pressure will also be lowered again due to low density air. So the two pressure limits are lowered and hence the area of the graph should remain unchanged and no redcution in power should be observed.
The only flaw in my theory here could be that the back pressure is not lowered to the ambient pressure due to the muffle in the middle...and then this would explain observed/imagined power loss.
Could someone else put in their two cents worth and ease my confusion....?
BTW I've been wrestling with this question since my themro classes in college.
And the same my explain power changes, if any, at high altitudes.

[Goochie]

Sounds like the "pikey mod" season is upon us.

Would an intercooler water spray kit be worth fitting if the temperature is 44c ? that'd solve your problem.

[sam_b]

Originally Posted by Goochie

Sounds like the "pikey mod" season is upon us.

Would an intercooler water spray kit be worth fitting if the temperature is 44c ? that'd solve your problem.

Goochier: somehowI thought you'd be amongst the first few to respond
So you think it's the temp then?
But doesn't fit well with wat I've read about engines.

[Goochie]

This web page may be of interest:

http://auto.howstuffworks.com/turbo7.htm

Some people here (particularly Fabia drivers) find that even with the relatively moderate summer temperatures we get in the UK that power is reduced. Given the temperature difference between here and India I'd put money on that being the cause.

[Nigel26]

from my limited knowledge (not in the slightest scientific).

cool air = high density = good combustion = good power

hot air = low density = poor combustion = low power.

also take into account the level of heat soak to the inlet manifold and air intake, these also contribute to poor performance.

ultimately in hot conditions you need to be able to reduce the heat going into the engine as much as possible. CAI and induction kits with sealed enclosures, FMIC or water sprays and venting to the IC can help this.

one thing i have fitted to my vRS is a "power gasket" from Awesome, some may think this is mearly a gimmick but after running the car for a few minutes the other day after an hours drive home in warmer than usual conditions for this time of the year i could actually touch and hold the inlet manifold. i couldnt do that before. this suggests that the heat soak from the head to the manifold is greatly reduced with this gasket thus lowering the inlet temps dramtically.


hth

[Goochie]

On the plus side, a hot exhaust will flow better and help your power figures.

[SkodiRS]

Going on the theory that exhaust backfire is also lowered...does this not have an effect on how the exhaust gases are "sucked out"?

A certain amount of exhaust back pressure is required...I think.

I'm guessing that even though there's less exhaust pressure and requires less power to push the gases through.....the high ambient temps will still give less dense air and hence less power.

*Note: all my usual disclaimers apply.

[Neo_VR]

Yep...

as said, the by-product of a turbocharger is heat, and lots of it!!!
this is the reason why turbo cars have intercoolers, to cool the compressed gas. but if theres a high ambient temp, it cant sufficently cool the compressed air as it passes thru the intercooler, so the temperature rises, air density drops and you dont get as big a bang... both petrol and diesel engines will start to reduce boost pressure to drop this temperature, hence the drop in performance..

a remapped fabia RS can hit 90c intake temp in about 15 seconds in warm weather.

[sam_b]

Right...here's wat my net search got me. This explains it totally. SkodiRS and NeoVr...you guys were on the right track. Even though back pressure is reduced, it is not sufficiently lowered to compensate for the loss of air density which leads to less molecules of air in the engine. The thing i missed was that it's not the air pressure but the air mass at inlet..(no. of molecules of air). this page explains all but a warning to the mathematically challenged.....it has a lot of equations. But it will appeal to all the mechanical/auto engineers here
http://wahiduddin.net/calc/density_altitude.htm

[Goochie]

Originally Posted by SkodiRS

Going on the theory that exhaust backfire is also lowered...does this not have an effect on how the exhaust gases are "sucked out"?

A certain amount of exhaust back pressure is required...I think.

I'm guessing that even though there's less exhaust pressure and requires less power to push the gases through.....the high ambient temps will still give less dense air and hence less power.

*Note: all my usual disclaimers apply.

Obsoloutly. The minimal gains from a hot exhaust are far outweighed by the high ambient temperature flowing through the IC.

[sam_b]

As an aside let me tell you how hot it is here....
My a/c requires me to pour water over it's compressor every hour or so, else it overheats and loses cooling. I think my a/c will benefit from the intercooler and water spray kit.
And I lay my suffering entirely on the door of the British. Couldn't your ancestors choose a better place to build their capital in India.
There are cool places in India too....

[sam_b]

oops posted twice...sorry

[andy ellis]

Must admit mine never seems to be greatly affected by temperature even when I ve done a lot of manouvouring on these hot days. Many moons ago I use some expensive cool blanket to totally cover all my air pipes in the engine bay. This ensures that the air getting to the intercooler is not far off ambinent, as opposed to prewarmed by the engine. Therefore the intercooler stands a chance of doing its job. Pipes also covered post intercooler. In addition have put a vent in the wheelarch liner to get a good flow of air through the intercooler. (Fabia)

[andy2407]

Originally Posted by sam_b

Right...here's wat my net search got me. This explains it totally. SkodiRS and NeoVr...you guys were on the right track. Even though back pressure is reduced, it is not sufficiently lowered to compensate for the loss of air density which leads to less molecules of air in the engine. The thing i missed was that it's not the air pressure but the air mass at inlet..(no. of molecules of air). this page explains all but a warning to the mathematically challenged.....it has a lot of equations. But it will appeal to all the mechanical/auto engineers here
http://wahiduddin.net/calc/density_altitude.htm

That's a litttle heavy going for a Friday afternoon.
Anyway, my (perhaps slightly less technical) opinion is that the loss of power is probably not noticed so much here in the UK, as we are all slowing down to look at the ladies who decide to wear less clothes once the sun comes out - further explanation here