Rally Cars tech stuff

Suddenly remembered this post.

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Originally Posted by dimviii

We have to define when we say boost pressure whqat we mean.
Some people mean the overboost pressure,some other the ct boost pressure.
overboost is the turbo pressure we have at midrange rpm when turbo spools,ct boost pressure is when the rpms are at the higher band ie 5000-6000rpm etc. big overboost pressure help to create more torque,while big ct pressure will help to have bigger number at peak horsepower.

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Please explain more. I have always though that when you say overboost you mean the boost above atmospheric pressure or in other words the gauge pressure whatever the revs are. Ct is unknown term for me.

Sometimes people are discussing about same issue using different words, which can cause confusion. When I speak about boost I mean the pressure above atmospheric pressure.

In the book by Graham Bell (Forced induction performance tuning) he defines the pressures as follows.

Gauge pressure: “The pressure you read of a normal pressure gauge. This is the pressure above atmospheric pressure. Usually simply called the boost pressure when referring to the inlet manifold pressure in supercharged engines.”

Absolute pressure: “The pressure above perfect vacuum. Thus, at sea level, normal air pressure is 14,7 psi absolute. Adding supercharged boost of 5 psi would then give a reading of 19,7 psi absolute.”

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Originally Posted by dimviii

example grN evos can create about 2 bar overboost,but cant ''hold'' more than 1,5-1,6 bar at high revs with restrictor.All these are not rule,but depents to external temperatures a lot.

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I recall you once said that a gr N evo has 0,5 – 0,7 bar at 4500 rpm.

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Originally Posted by dimviii

At Finland-Sweden Norway or at mediterannean winter cold rallies you can,but at summer rallies with 30+ temp is difficult.

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I think it’s all about air density that changes with altitude, temperature and to some extent the humidity (100% relative humidity decreases the density by only 0,9% compared to dry air at sea level and 20 ^oC).

As known the wrc cars are said to loose 20-30% of the power in higher altitudes in Mexico. I compared the density at sea level with temperatures + 20 ^oC (1,204 kg/ m³) and - 20 ^oC (1,394 kg/ m³). The density is 15,8% higher with – 20 ^oC. Does this mean that the power also increases by 15,8%? Assuming a car having 330 PS at sea level at + 20 ^oC it would mean 382 PS at - 20 ^oC! Ups!

Air Density Calculations Impact of Altitude Temperature Humidity and Pressure.xlsx

Dimviii, if this is true you should come up north and run your Mitsu in the dyno. You would get good results. ;)

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Originally Posted by dimviii

Wrc cars now can have a 2,5 bar overboost(as the limit) and for sure can hold much more ct pressure from an N4-R4 due to less cc(1600 vs 2000) and different boost pressure hardware system.

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My opinion is still that it’s 2,5 bar absolute (atmospheric pressure + turbo pressure / gauge pressure).

Sorry for late reply.

yes its absolute pressure,thats the way everybody works at engines reffering.
About the density at low temps,yes it helps a lot cause the air density is higher,but no way if 20% more density will have 20% more power.As you drive the car yes you can feel the better response,the more power,but lets say that at a 350 bhp car feels like some 15-20bhp more.I am talking about temp difference from +30 celsious to 5 celsious.
Another point is with less temp you have less intake temps, due to colder air through the intercooler which helps to decrease the hot compressed air from turbocharger.

About the ''overboost'' and ''ct''
lets say that you drive an Mitsubishi evo(or any turbo car with wastegate actuator) you are driving the car with 4th gear and 2000rpms,and you floor the gas pedal.Car will accelerate slowly at first(rpm range out of turbo at 2000rpms) and as the rpms climb to about 2700 turbo starts t build pressure.At about 3500rpms your absolute air pressure gauge reach its maximum pressure which is lets say 1,8 bar.At about 4500-5000rpms if you watch your boost gauge you will see that the boost pressure starts to drop slowly at 1,7-1,6-1,5bar.At 7000rpms your boost gauge will show 1,3 bar.This pressure is the ''ct''
The 1,8bar the maximum pressure your engine reach at 3500rpm is the ''overboost''
Why this happening?
We want more boost pressure at mid range rpms to help the car accelerate faster.As the rpms go up,we cant hold that pressure(1,8bar) for many reasons.Turbo not big enough,at grN cars just 33mm intake at turbo,turbo at continious 1,8bar will heat A LOT the air,and the oem intercooler will not succeed to low the intake temps etc.The reasons vary from car to car,so dont take it as a rule for all cars-engines.
At wrc cars we dont have overboost and ct.We have a steady boost pressure from mid range to high revs.At wrc we have big enough turbos,we have better efficient intercoolers,stronger engines that can cope with 2,5 bar all day(s) long,completely different boost pressure management and hardware.

Hope to helped with my s$ite English. :dozey:
if you didn t understant something dont hesitate to ask again.

By WRC cars do You mean 1.6T cars with lower boost compared to 2.0T? Because I have a turbo pressure curve from an early Fabia WRC Evo I engine and it looks like the one of Gr.N cars just has much higher values but the decrease is same (I think it must be similar with the restrictor).

It starts around 2700 rpm with something like 2 bars at 3000 rpm. It reaches maximum cca 3,3 bar at 3500 rpm but than it starts to decrease to 2,9 bar at 4000 rpm, 2,3 bar at 5000, 1,9 bar at 6000 rpm and finally some 1,4 bar at rpm limiter at 7500 rpm. Numbers are approximate, the grid is quite large.

The engine has max. torque at 3700 rpm and maximum power constant from 4000 to 5000 rpm.

Mirek the dynoplot i ve seen from escort wrc, was almost steady pressure to 6000 rpms and after that started to drop.From 3000 rpms had already 3 bars.Unfortunately haven t seen from later wrc cars,or the new 1,6 ones.
Of course restrictors doesn t help to keep high pressures at high rpms,but depents to turbo design,engine efficient,for how many rpms will manage to keep the boost.

ps the escort wrc i ve seen is with 38mm restrictor

How's the ALS affect all this, Mitsubishi's ones sound lot agressive to me, unlike Subaru's and WRC/RRC?

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Originally Posted by stefanvv

How's the ALS affect all this, Mitsubishi's ones sound lot agressive to me, unlike Subaru's and WRC/RRC?

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every als can adjust to aggresive or less agresive.Driver will choose how aggresive to his likes at every surface.

I saw a dyno from Subaru that had 2,9 from very low (think it was 2000 rpm) to almost 7000. I also ask PS if that was right, and he said yes. Think it was the 12B.

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Originally Posted by Coach 2

I saw a dyno from Subaru that had 2,9 from very low (think it was 2000 rpm) to almost 7000. I also ask PS if that was right, and he said yes. Think it was the 12B.

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yes its possible from so low revs with agressive antilag to build plenty of boost pressure.

Thanks, now I know the difference between overboost and ct but what does ct stands for?

The explanation you gave I assume is about an evo without a restrictor because otherwise I can’t understand 1,3 bar @ 7000 rpm.

Below is a dyno chart for a evo9 I got from a Finnish company that tunes cars with turbo (Google Translate.)

[img]MitsubishiEvo9Stage1_21_zpsc5d4ccdf.jpg%20photo%20 by%20OkdF%20|%20Photobucket[/img]

The lowest is a standard car. The next one is as they call it stage 1 with a power of 326 PS @ 1,5 bar. The most upper one have 355 PS @ 1,6 bar (Stage 2).

On an evo with restrictor I assume the pressure will decrease rapidly after torque peak because of the restrictor. On the chart below the torque decreases rapidly after peak torque -> pressure drop in the inlet manifold. As I said I recall you said about 0,5 – 0,7 bar at 4500 rpm.

[img]Mitsu_Evo9_grpN_Dyno-1_zps3780d166.jpg%20photo%20by%20OkdF%20|%20Photob ucket[/img]

Max rengas teho = max power from the wheels
Max moottori teho = max power from the engine
Max momentti = max torque
Max nopeus = max speed
Ilmapaine = atmospheric pressure
Lämpötila = temperature

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Originally Posted by stefanvv

How's the ALS affect all this, Mitsubishi's ones sound lot agressive to me, unlike Subaru's and WRC/RRC?

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I have always wondered how other Mitsubishi did their anti-lag as compared to the "rocket" system that Prodrive used. For that matter how do the modern systems from vw Citroen and ford work?

(really loving the technical discussion, is there any way to start another thread about it so we don't anger brother John and lose all this great info?)