crcain

So if the max airflow of the turbo doesn't change, why does a stock Evo with a boost controller bleed exactly the same way as a 400 hp Evo with every bolt on imaginable plus cams?

If what you were saying was true, the 400+ hp Evo would bleed boost WORSE than the stock Evo because for the same boost pressure, the 400+ Evo has a lot more airflow.

l2r99gst

iTrader: (2)

Again, you aren't giving me specific numbers so I can only guess.

But, also, there are other factors with boost taper. A crappy manual boost controller can be one of them. Programmed boost taper can be another. Weak wastegate pre-load can another if back pressure is sufficient (I wasn't disagreeing with you on this).

But the ultimate/max airflow that the stock turbo can and will produce is limited by the compressor. It doesn't matter how many mods the car has.

Have you ever plotted points on the compressor map to see where you are landing? Even a mildly modded Evo (close to stock) can taper boost because of the riding the compressor choke line. Mods will matter on the exact psi number that you will taper to on a maxed out turbo, but it will be relatively close (23ish +/- 2 psi).

Let me give you a link of a trace I just did for my car. I am maxing out the stock turbo on E85. This will give you a good picture of what I am talking about.

Edit: It's actually from this thread, page 1:


In a situation where taper wasn't occuring, that line would be horizontal across the map. It's not, because I am maxing out the compressor. My 30+ psi tapers to 22ish because of this.

A mildly modded or stockish car would do the same thing if trying to run high boost.

Appauldd

iTrader: (22)

All turbos will bleed down as they approach the surge line if boost is set as high as the turbo can accept. Look at any efficiency map. All turbos will become less efficient as compressor speed increases beyond its maximum efficiency limit.

Even though you are on the verge of the surge line, wouldn't it be possible to lower the boost to make heat less of a factor? Then you could turn up the timing and, in theory, achieve the same power level as before?

I know there are 2 trains of thought here.
1) High boost and less timing
2) Low boost and high timing

Not only should you be able to run more timing, but your car should run cooler as well. The turbo will be more efficient and create less heat to make the boost. Then because the turbo will take less time to get to the desired boost you should see improved mid range power.

Just a thought.

crcain

To l2r99gst:

I get what you guys are saying, but if the choke limit is airflow related, I think I make a valid point, which is, as we add cams, turbo elbows, etc... all things that increase VE, the factory turbo would begin to bleed more and more, because for the same boost, you are running a lot more airflow. But I've not seen this... in my experience a stock evo with boost controller bleeds the same as a fully modded car with the same OEM turbo.

And yes I have plotted some points on compressor maps. I used a compressor map for my 35R from Garrett and plotted some points and was way out of the efficiency islands.. just like your stock turbo compressor map. It makes me think compressor maps aren't that helpful. By the way, how are you calculating your airflow for that compressor map you posted?

To Appauld:

You have to remember, increasing boost provides an increase of _average_ cylinder pressure. Increasing ignition angle mostly increases _peak_ cylinder pressure. The net effects are that duh... boost is better than timing. And of course, at some point that doesn't hold true and each car is different.

Appauldd

iTrader: (22)

Eric made a really cool Excel Efficiency Calculator. Shoot me a PM with your email....I will send it to you. Makes it really easy to see where you should be in the realms of boost for your specific turbo.

JordanS4

iTrader: (11)

Maybe you have some different experience as me, but modded cars certainly hold more boost to redline than OEM Evos.

crcain

Does that compressor map you posted have a choke line on it? Or a surge line?

Appauldd

iTrader: (22)

The left side of the table is the Surge limit. The right side is the Choke limit

l2r99gst

iTrader: (2)

You are correct, but I think you are getting hung up on 'a lot more airflow'. The VE of our engines, even in stock form, are about 100% in the mid-range, falling to roughly 85ish percent up top. Certain mods simply shift that VE curve or add to it at certain RPM ranges. Adding 5-10% VE will allow you to have more volumetric airflow at the same boost level, but not as much as I think you are thinking.

This is why different cars using the stock turbo have different max boost levels though. That's why I gave a range of 23 +/- 2 psi or so. It depends on several factors...VE (for volumetric airlfow) and temp (for mass airflow) are the two big ones.

(All this is assuming coming from a higher boost level...I'm not sure if this is what you are talking about though, since you never gave any specific numbers of what boost taper you are referring to. Also, I do agree that there are other reason for tapering boost as mentioned previously. This is just the main reason for tapering boost for high boost applications. For example, why the stock turbo can't hold 30psi to redline.)

I didn't make it...I forgot who did. It is a cool spreadsheet though and it saves time from doing manual calculations. I have simply added some maps to it.

Typically a compressor map will have the surge line as the left most curve and the choke line as the right most curve.

Appauldd

iTrader: (22)

In the realms of VE....I think it proves for itself that porting helps in all areas of engine performance. More air in and out greatly increase VE (well, not greatly, but quite a bit).

Now that we have beaten compressor maps to death, what about turbines? After all, it is the turbine that spins the compressor.

l2r99gst

iTrader: (2)

Turbine maps are just as, if not more, important. The proper turbine and housing is what controls spool and back pressure.

But, I think that's for another thread. There is plenty of info out there to research. I'm done typing for a while.

crcain

How are you guys getting your airflow numbers to plot points on a compressor map?

Also, aren't our cars running well over 100% VE because they are turbo charged... I thought a perfect NA car might get close to 100% and FI can go well over 100%.

Appauldd

iTrader: (22)

we use a cool excel spread sheet.

Danieln

iTrader: (1)

Evo 9 turbo 01580 series can go up to 419 potential crank horse power with 65% efficiency potential.
Since the exducer bore of the turbo is between 61mm and 70mm.... 400HP can be achieve also at 22psi top end.
22psi boost can be achieve at 3000rpm with a good fuel and tuning.

On another hand if we will try to calculate the actual airflow for a given power (400HP) than seems that we need 44lb/min of air.This converted to cfm will be 627cfm.

Seems out of the range of the map after my calculation.

I use the formula Wa=HP*A/F*BSFC/60 where Wa is actual airflow in lb/min

BSFC (brake specific fuel consumption) must be 0.50 to 0.60 for evo`s so race fuels and aggressive tuning are required to reach low end BSFC.

IF we will try to calculate required manifold pressure required to meet the horsepower, or flow target means that we need at least 25psi of boost to get there.

If you want 400 hp with an A/F of 12 and BSFC of 0,5 the airflow would be 400*12*0,5/60 = 40 lb/min of air.

I`ve made some calculation between a Evo9 with 32 mm restrictor for a GruppeN car with 290 hp 28psi peak boost. Power with a 33 mm restrictor would be more or less 308 hp on the same boost levels.