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Turbo Lage vs. Boost Threshold

B

Barge

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OK, since i'm kind of drunk right now, this is a good time for me to have a little technical discussion on here to try to prevent people from agitating my sense of correctness.

Now I'm not going to expect all of you to understand what a compressor map is for a turbocharger, let alone any type of turbine map you might find (yes, they do exist, although fairly rarely... garret has them for their GT series turbos)... so let's get on to the goodness.

Since i've already stated that most of you probably have no clue how to interpret a compressor maps (you should if you're getting a turbo... it's the sensible thing to do) let's try to avoid using the word "Lag" unless we really mean it.

When most people refer to "lag" they are actually referring to what more correctly would be termed as boost threshold... what's the difference you may ask? Well let's see if we can delve a little deaper into this.

If you were to study a compressor map, or a turbine map if you were lucky enough to find one, you would discover that turbo's don't just magically create "boost". A compressor map can tell you at what flow rates and at what pressure ratio (boost in a sense) the turbo can actually operate. This is what makes the study of a compressor map so critical. If a compressor maps shows that a certain compressor wheel can't flow 300 cfm (yes, technically most compressor maps should be in a more mass measurement but because few people understand the difference and cfm is easier to use and can be converted to at standard temp and blah blah.. we will use cfm) until it's at a certain pressure ratio... or vice versa... then it's just not going to happen... no matter what kind of voodoo magic you want to perform to make it happen.

This, ladies and gentlemen, is boost threshold... if the turbo's not gonna flow enough at at a pressure ratio it ain't gonna happen no matter how hard you try. Although this efficiency can be influenced by the turbine wheel, but then we're getting into trial and error type testing.

So what exactly is lag? You should all be asking yourself this by now...

Lag would more correctly be defined as what happens when you're at a point where the engine can consume enough air that the turbo can supply enough air to create a certain pressure ratio and the "boost" takes x amount of time to actually happen. For most systems this is actually a negligible amount of time. For example.. take a turbo contour and punch the gas a say 1500 rpm... boost pressure will not build until.. ohhhh.. say 2500 rpm (depending on turbo charger).. this is boost threshold... however if I'm over 2500 rpm and punch the gas.... the time for is to develop an amount of boost it would normally be able to create at said rpm is lag.

OK... I think that all makes sense. Time for bed.
 
Maybe he had too much lager on the brain and just forgot an "r".

Thanks for the post though. I enjoy reading the input from you guys in here.
 
nice post, heck i can't type drunk, but back in college i used to think i was the most creative when drinking!:eek:

i do agree though you definately can't beat a 64 Oz. Turbo Lager on a hot day! :laugh:
 
Well let's just hope people read it, the whole thing that is not just the first line. :D
That way they will learn something about the difference between lag and threshold and will stop confusing the to. While you are at it, fix those dumbazzes at Motor Trend so they quit calling cars 'laggy' in their articles that have absolutely no lag! They only know old V8s or something.
 
I like this mention of a turbo lager. Some micro-brewery should pick this up and mass produce it for SZ08.

I spent a lot of time studying compressor maps before helping ShogoFast pick a turbo. I think what you're talking about in terms of lag is the left hand boundary of a compressor map or the surge line. If the exhaust flow is insufficient to generate an ideal pressure ratio for your turbo, you obviously have a mis-sized turbo, probably too large. Or maybe I'm not understanding what you're getting at.

I think what most people think of when the term lag comes into play is the actual engine RPM where max boost is achieved. I don't understand what you mean by your last statement. If you are cruising down the freeway at say 2500rpm and punch the gas with a properly sized turbo the thing should pull like mad. Turbos live at 60mph. I think a common misconception about turbos is they are driven by the raw kinetic forces of the exhaust gases like a windmill, when there is actually a complex thermodynamic interaction between the exhaust gases as they are driven into the relatively small housing and impart energy to the turbine, then expand and cool and continue out the tailpipe.

I have a nifty excel spreadsheet where you can plug in displacement, desired horsepower, and some RPM data and it will greatly help you interpret a compressor map. PM me if anyone is interested.
 
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No, the left hand line of the compressor map, the surge line, is the boost threshold.

Driving at 2500 and punching the gas will do nothing but make more noise if you are on the left side of the surge line still. This is the boost threshold.

For example... on my engine and turbo setup no matter what load I'm at at 2500rpm I will always be on the left side of the compressor surge line, hence none, or very little boost pressure.

Most of that depends on how you size your turbo. Mine is sized to run about 23 psi from 4-7k rpm... hence the surge line is shifted to the right a fair deal compared to a turbo you would use on a contour which might be 5-7psi for the whole rpm range (although most turbo's do not have a broad enough compressor efficiency range to accomodate this... but at this low of a pressure the lower efficiency isn't a huge killer).

I've got an excel sheet I use as well to size turbos... the problem is that is really becomes pretty complex the more accurate you try to make it. I've tried to add a lot of factors to mine so i can play around with it.

And note that the compressor map can be somewhat influenced by the turbine selection. However it will moreso influence what may now be a turbine induced boost threshold and exhaust backpressure which will kill upper rpm power do to lower VE and therefore poorer cylinder filling.
 
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I see your point. I guess at the end of the day you can't have your cake and eat it too. You have to size the turbo how you want to drive your car.
 
Just out of my morbid curiosity what is your displacement and max RPM. I want to run it through my calcs and see what turbo I pick. What turbo are you actually running?
 
It's a 2L engine.. well like 1945cc or somewhere around there.

Max RPM is around 7000rpm.

I'm using a td05H-Evo16G (it's the compressor wheel from an evo3 16G)

My next turbo will most likely be a garrett twin scroll housing but that requires a lot of other custom work and my current engine isn't stout enough to handle 30psi.
 
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