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Discussion Starter #721
The theory behind the airflow under the car, providing downforce is false. The air going under provides front lift and this is providing rear downforce or more precisely, it pivots the car towards the rear and provides more pressure on the rear tires as the front is lifting, but that is not OK. It is a bad solution.

I have rerouted the air in the front now, but let's wait with these mods a bit as I have given a lot of insights for today. ?
 

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Our new GMS 3" exhaust is performing above expectations. The backpressure is immensely reduced, improving response, quicker turbo spool, better flow and far less heat. ............. Next step is increasing the air flow into the IC and we have a nice solution on the way.
Great news! Would you be willing to share some numbers, especially pressure, or at least a % decrease?
 

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Will post dyno graphs. I'll just let you wait for a bit more... :)
Honestly, I'm just as curious what turbine inlet and outlet pressure is (vs. RPM or through a full throttle pull to redline). This will give a good indication if system is running comfortably, for example if pressure increases a lot after ECU work, it tells us that something is saturating. And if I remember correctly, pressure can change when you move on to replacing IC, and these pressure data, obviously together with pressure before and after IC, provide valuable data. Remember, if you change the IC, it has the potential to affect stability - everything affects everything. Here from a exceptionally knowledgeable "Kiwi" testing a new IC(Note; Not a 4C!):
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And one more thing i really miss is transient response figures, everyone say "response increase", but no one actually bothers to measure it.

Below shows what I mean, but maybe at other RPMs (than a 2.2 diesel engine).
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Here's another similar graph, showing torque, something that might be easier to relate to:
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Throttle response/turbo lag/transient response is for me one of the biggest shortcomings of a turbo car - yes, also the 4C. And worse on petrol engine than diesel, since diesel is pumping displacement all the time, giving some speed to wheel. Petrol engine pulls vacuum at closed throttle(= no exhaust gases), so you have close to zero initial turbine speed when you open throttle, and you therefore need to wait because of inertia of wheel.

I think your less restrictive exhaust will improve transient response, but it would be nice to show it, rather than just give a subjective opinion. It also proves that proper engineering is done, not something many aftermarket suppliers can say in my opinion. Most is 70% engineered, then the rest is dependent on luck/trial and error, if you understand what I mean.

Porsche by the way has a system that opens throttle when you completely go off throttle, exactly to get some initial wheel speed.
 

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It's my understanding that Alfa use the same technique in the 4c, but also increase the valve overlap to increase the effect.
 

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It's my understanding that Alfa use the same technique in the 4c, but also increase the valve overlap to increase the effect.
Never heard anything about Alfa keeping the throttle plate open, do you have some more info/references?

I have too little knowledge about effects of valve overlap, but if throttle plate is closed, it does not help with overlap, as the air is stopped regardless of what's happening downstream. On partial throttle, its a different story, but our engine however has plenty delay, its very far from instant. Have not compared it with other gasoline 1.75l though. Compared to the 2.0l BMW diesel, response is a lot slower, but with bigger displacement and smaller turbo, that's not a fair comparison.
 

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@RodE Many thanks, have not looked in the cloud before, excellent!

I cannot see anything regarding throttle plate, but improving response compared to the previous 1.75l engine i presume. But I have a hard time taking it seriously when I see writing like this:
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"Approaching", OK. But "closely" is at best a huge exaggeration. I have a NA car as well, and its not comparable - by far. Must be a salesman written this.....
 

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Well, it is from an Alfa publication so there's bound to be an element of self promotion..!
But, I don't find turbo lag to be a problem at all.....maybe I just drive around it.
To do a fair comparison with a n/a engine I would say that you need to compare like with like i.e. same capacity and power. So, a 1750 n/a producing 240hp will be pretty responsive but if you tune it to produce, say, 300hp you will have very 'cammy' characteristics while the turbo unit will have barely changed with regard to response. Plus, the torque difference will be enormous.
Don't get me wrong, I would always prefer n/a over turbo. My reflections above are based more on competition engines.
 

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Well, it is from an Alfa publication so there's bound to be an element of self promotion..!
But, I don't find turbo lag to be a problem at all.....maybe I just drive around it.
To do a fair comparison with a n/a engine I would say that you need to compare like with like i.e. same capacity and power. So, a 1750 n/a producing 240hp will be pretty responsive but if you tune it to produce, say, 300hp you will have very 'cammy' characteristics while the turbo unit will have barely changed with regard to response. Plus, the torque difference will be enormous.
Don't get me wrong, I would always prefer n/a over turbo. My reflections above are based more on competition engines.
Me too. Keep it to over 2 grand or so and response is fine. Maybe it’s that I like to squeeze the throttle on rather than suddenly mash it to the floor that I don’t detect lag. Whatever it is I’m satisfied with the throttle response of the car. Being in the right gear helps too.
 

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"Being in the right gear helps too."

My biggest bugbear. I make sure I'm in the right gear entering a corner then finding I've inadvertently nudged a paddle, up or down it's just as bad.
 

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Being in the right gear helps too.
Definitively, thats why i attached this graphs earlier, found it informative, even if its a diesel engine. I redlined the turbo response below, to get to approx 80% of max, it need 2.5 sec at 1500, while at 2000 time's down to 1.25sec, then 1 sec at 2500RPM. And at 1500, its building the last 20% really slow, I assume this is better with the adjustable AR cams and overlap mentioned. Dark blue line is supercharger, so should be close to NA.
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Note: target(green) is changing slightly, and I did this by eye, so numbers not that exact.
Note2: THIS is turbo lag, often turbo lag is said when the person really means turbo threshold. For anyone who's not following me:

Driving techniques reduces the impact(I agree with your squeezing out of a corner is best), but response is nevertheless poor compared to NA, technique/training is just a way of getting around the lag problem.
 

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FCA talked about eBooster technologies a few years ago, coming from the same turbo manufacturer. Afaik the principle is you take away the lag by spinning the turbo with an electric motor. A bit of a hybrid* between turbo and a compressor. Why doesn’t anybody try that?
(* funny, as I thought a ‘hybrid turbo’ was this in the first place, and not just a reassembled one from different parts).
 

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To do a fair comparison with a n/a engine I would say that you need to compare like with like i.e. same capacity and power. So, a 1750 n/a producing 240hp will be pretty responsive but if you tune it to produce, say, 300hp you will have very 'cammy' characteristics while the turbo unit will have barely changed with regard to response. Plus, the torque difference will be enormous.
Don't get me wrong, I would always prefer n/a over turbo. My reflections above are based more on competition engines.
In my opinion that is not a fair comparison to take power into consideration. Remember, NA only have atmospheric pressure(1Bar absolute) available, while we are ramming air down its throat. With say 1Bar(2Bar absolute), very ideally/simplified that means 200% Air/fuel = 200% torque = 200%hp at that RPM. So a engine producing 100hp NA, will produce 200hp at 1bar overpressure. Remember, extremely simplified....

Highly tuned production NA cars are around 110hp/l. They dont have very much over 100% filling(= torque) it cannot compete with the 200% torque of FI, so mainly they get the extra power by revving higher and shift torque peak upwards in the rev range. Racing engines take RPM even further and volumetric efficiency(by intake, cams and more I assume) to the extreme, but as you say; with sacrifices.

So I think a fair comparison would be another 1.75l engine with 170 - 180hp revving to 7000RPM, and guesstimated peak torque of 200-250Nm. Remember, it will not be peak torque at redline. If you have [email protected], you roughly have a 250hp engine. Edit: Here is new MX5 2litre 184hp torque(vs old 160hp). Peak torque of 205Nm, but at 7000 we have - surprise - 184Nm:
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Edit: Upper Volumetric efficiency NA limit is said to be around 130%, very impressive, but that would be a racing engine. And still far from 1.5Bar over pressure.
 
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