Race Engine Forum UK

Hello Guy !

I've discover recently that my car is now very sensitive to air temperature .
Driving at day when it's warm outside (approx. 30-32c) my engine don't want to pull so hard as when I'm driving at night when the air is cooler and denser (approx. 24-25c at night). At night it feels much more faster and accelerate strong like that the engine has 5-10bhp more when driving at night when it's cold outside .
However i know that hot air reduces power but i didn't fell this change in air temperature so strong and sensitive with my previous non modified engine , also we have 4 cars in family and driving all this other cars i can't fell this power output differences regarding air temperature .. well maybe a little bit but not so drastic .

Is this because of the increasd compression ratio ? or because of the modified head for some reasons ?!
The air filter element is standard with standard air box (I've try to fit K&N induction kit on my engine but i didn't like the results ¢‚¬Å“ air/fuel mixture was all over the place ,exhaust was surging and popping on over-run fuel mixture was lean , also the car was misfiring and was a bit sluggish .. so i remove that induction kit and fit OE filter and air box back ..)

If you have some spare time can you please explain me how this hot-cold air reflects on engine power output ? I've heard something that for every 7c increase in air temperature the engine loose 1% or 1bhp , is this correct ?

Thanks !
Damir

Damir, hi

no reason why your engine should be more sensitive to air temperature than any other other, except to say that a unit more highly tune than standard has superior throttle response and so you are more likely to sense the change.

In view of the behaviour you have described I would be checking for detonation at the higer intake temperatures. I do not know what the K&N induction kit looks like but it may be that it flows more air than standard and makes the engine run lean and detonate?

GC

Hi Guy !

Yes true ! The engine was detonate now on this hot summer air (was fine at night when the air was colder) so i back off the ignition timing a few degree to compensate this driving on different air temperatures at day and at night .
What about the air/fuel mixture , should i increase or decrease mixture when driving on hot air ? and the opposite does engine running on cold air need more fuel ?

Well this K&N induction kit makes the inlet tract very short compared to the standard OE inlet pipework , it ends around 20cm from the throttle body (butterfly) so when fitting this kit i must remove all the inlet pipes and connect just one 5-6cm adapter pipe in between the throttle body and the air flow meter , cotton filter fits directly on the end of AFM !
I will make some pics later so that you ca see this as standard inlet pipework and with K&N kit .

However the engine was running lean for sure with that K&N kit (I've check this with an lambda sensor) but i didn't wish to bother with the mixture adjusting because the filter was sucking hot air together with under bonnet temp. as being behind the driving light and it's simply impossible to fit him somewhere else , theres not enough place to reroute the inlet pipes ..

Thanks !
Damir

Damir,

If you've found an airfilter that allows so much more air in that it alters the characteristics of the engine then I would be thinking about bolting it on and adapting the fuelling to suit as (if this is the case) you are going to see a substantial gain in power.
This is suggesting that:

1. Your original set up is restrictive.

Or:

2. Your original airfilter is blocked with dirt or water.

Don't fall into the common trap of thinking that more noise = more power, quite often different air filter assemblies will sound a lot different and you can actually now here something that was already going on, but was silenced by the OE set up (they are purposely designed this way).

You can always run a cold air feed to a different type of filter.

Aftermarket filters whether they be cotton-gauze or foam generally flow more air than paper ones, but do not filter the fine particles out as well.

Martin.

Hi Damir

there is a clear relation between the engine power and the air temperature.

Your engine is actually breathing a volume per cylinder with every second piston stroke, so there is a volume flow of air into your engine.
The fuel (where the power comes out) is injected proportional to the air mass flow.
The air mass flow is equal the air volume flow times the air density.
The air density is nealy invers proportional to the air temperature, thus:
Higher air temperature -->lower density --> lower mass flow --> lower fuel flow -->lower power.

On a graph it looks like that:
20‚°C is the reference temperature, so for example 30‚°C temperature results in 3.3% less air mass flow.

So despite the risk for detonation there are good reasons to let the engine breath cold air....

Thomas

Hi Martin !

Well me thinks that the OE inlet setup on Peugeot 205 GTi is very good ! but don't get me wrong i do not think that the K&N filter is bad or useless i know that these are the best aftermarket filters which you can buy today !

Unfortunately this warm under bonnet air is the main problem here when we speak about this induction kit or any other aftermarket mushroom filter which fits outside of the air box , me thinks that this system can be improvement only when a good shield protects it from the engine bay heat, and it seals airtight as the bonnet shuts.
Panel filter element which fits inside the OE air box might be much better choice than this induction kit at least for this car/engine..

Some pics of my engine , inlet pipework and K&N kit :

Thanks !
Damir

Hi Thomas !

Thanks for reply and that graph , i agree with you !
Anyway it's logical hot air - low density , cold air - high density .

Regards !
Damir

Hi Damir,

I don't see any problems with this K&N set up (apart from the possible filtration problems noted earlier)

It takes cold air from the front of the car (true, a shield would help, but only when stationary) and the exhaust is a long way from it. You would have thought that disposing of what looks like around 1.5 metres of tubing and bends would see a measurable power increase.

To sum up:

K&N 'cone' advantages:
More power, flows air even when dirty, very long service intervals.
Disadvantages: Doesn't filter the air as well as paper, noisy (induction roar)

Paper element disadvantages:
Doesn't flow as much air, especially when damp or dirty (even slightly)
Long intake tubing lowers power output, needs replacing regularly.

Advantages:
Filters air better, cheap to replace, easy to source locally.

Those are the facts as I see them, but it is your choice. Nothing is ever easy eh?

Martin.

Damir, hi

Immutable Rule:

When you have a brilliant (fully sealed, cold-air ducted) OE-designed intake system like the one on your car never, ever strip it out and fix any filter straight to the throttle body.

If you want to run a K&N - get one that fits the OE filter housing.

GC

Hello Martin !

Yes but one more thing , when fitting this induction kit the inlet pipework become very short (around 20cm) compared to OE setup and me thinks that this upsets air flow measurement , so this can be one more reason for lean mixture with this kit fitted .
What i mean is maybe it's possible that flap inside the AFM vibrate/bounce at certain engine speeds because of the short inlet throat , more air flow entering at greeter speed through the AFM then before , also maybe some turbulence occur inside around the AFM flap ... and because of this AFM gives false readings/signal to ECU ?! It may be needed to adjust-tight the spring tension inside the AFM because of this , and tightening the spring tension means less fuel so it's not good again ..
Does this make any sense ?!

Regards !
Damir

Hi Guy !

Mine point exactly ! panel filter would be much better choice for this car/engine .

Thanks !
Damir

Hi Damir,

Thanks for taking the time to explain the theory behind your reasons, yes it makes sense to me. My background is more from carbs (years ago) and more recently turbo cars which are using map sensors, so I never took into consideration this problem with the AFM.

Martin.