Nissan 370Z Tech Forums banner

21 - 40 of 45 Posts

·
Premium Member
Joined
·
35,304 Posts
Is lag really an issue these days? I would think you only see lag in a small 4 banger with an oversized turbo.
:lmfao: Like the EVO in Top Gear?
 

·
Registered
Joined
·
5,719 Posts
So, the Prodrive anti-lag system works by keeping the little propellers spinning and forcing lots of lovely oxygen into the engine even when there aren't enough exhaust fumes to turn them normally. And it does this by dumping petrol into the exhaust, just before the little propeller. As soon as the petrol touches the very hot exhaust pipe it explodes and this explosion keeps the propeller turning
That anti-lag system has been around for years. Practically every turbo race car uses it if the rules don't specifically forbid it. WRC (given their "turbo import" status) are probably the most famous, as well as drag cars.

There are 2 ways of achieving this. 1 is to have a dedicated injector pumping fuel into the exhaust manifold. The other one is to run the engine rich on overrun (the latter is more popular as it has a secondary benefit of cooling the head, and requirng no extra parts).

I'm not sure what it does to your exhaust valves, but you may as well treat your turbocharger as a consumable. It doesn't matter if you've got ceramic, steel or titanium blades - detonating fuel in your exhaust manifold / turbo housing will rip them to shreds in short order. You'll get more life out of a set of Comp-R semi slicks than your turbo if you drive with anti-lag always on.

RaceLogic have been making an antilag / launch control / traction control system that do this for quite some time.


Still, there's nothing like being in a car with an anti-lag launch control. A mate of mine got a ride in a drag-prepped WRX (making around 350hp at all 4 wheels) running it. He pulled up at a set of lights next to a GTO, and the lane had cars parked on the other side of the intersection.

The GTO driver saw the WRX and started revving his motor. My mate's driver hit the launch control button and floored the throttle. The launch control holds the RPM limit at 5000RPM (peak torque on his motor) while working the injectors harder and only using an ignition cutout. End result, all that fuel went into the exhaust manifold and he's running 25psi with the clutch in. All he can hear is this "bang! bang! bang!" from the antilag.

When the guy sidestepped the clutch, my mate swears you could hear the Nitto 555R's trying to rip themselves apart, and the feeling was like being crash tackled by two big blokes.


Apparently the guy doesn't do that too often, and if he's lucky the turbo will last 2 drag meets. :)
 

·
Registered
Joined
·
5,719 Posts
thus we enter the variable geometry as seen on the 997tt.
Variable geometry turbos and other such tricks can only reduce lag, it can't eliminate it. The only way to eliminate it completely is to be able to make boost without load.
 

·
Registered
Joined
·
541 Posts
the 997tt builds boost before 2000rpm. you're idling at around 1000. you barely accelerate into 1st and you're at 2000rpm nearly instantly. for all purposes, there is zero lag on the 997tt. any lag that is technically/mathematically present is irrelevant. wouldn't you say?
 

·
Registered
Joined
·
9 Posts
Lets have a show of hands here. Who has actually driven a turbocharged car before and knows what the **** turbo lag is? I bet you most on this site are scared of the turbo lag boogieman for no good reason. After all, lag is a function of turbo size as compared to engine displacement, for a desired power level. Let's keep in mind Nissan will likely give us a good 3.6 to 3.7 liters to spin our turbos, and the jump to 450 HP from whatever the VQ can turn out in normally aspirated form is really not that much.

Turbo lag is a result of the turbo being passive during cruise/low load situations. The turbo must have hot, voluminous exhaust to drive the turbine. The more hot, expanding exhaust gasses you have, the quicker the turbo will spool. The most common variables to change are turbine (hotside) size and compressor (coldside) size. For every engine, there is an optimum combination of turbine and compressor size for the desired power level (the efficiency range). Depending on the size (pumping power) of the engine, the turbo may have to be oversized and slow-responding to hit the desired power level.

Positive displacement superchargers, on the other hand, are great because they are compressing intake air full time when the engine is running. Crack the throttle and you have instant boost--the supercharger doesn't give a sh!t about your exhaust energy b/c it is driven by the crank. On the other hand, you have to feed the turbo some throttle to get your reward. That's ne nature of the beast, whether variable vane or conventional turbo.

Another big function of turbo lag is the volume of your intercooler and associated piping. Not only do you have to develop your p.s.i. in the intake manifold, you have to develop the p.s.i. first in the intercooler and piping before the engine. This can take a few milliseconds, but is certainly perceptible part of the lag equation.

Variable vane turbos have been around for years. They were incredibly popular for a time in the 1990s, but they had a high failure rate and were expensive and complicated to repair. There is nothing magic about variable vane turbos. They still have lag because, after all they are still driven by exhaust gasses, but the ability to hit full boost is shifted further down the powerband due to the added efficiencies allowed by the variable vanes.
 

·
Registered
Joined
·
541 Posts
Lets have a show of hands here. Who has actually driven a turbocharged car before and knows what the **** turbo lag is? I think I'm reading a bunch of misinformation from a few on this site.

Turbo lag is a result of the turbo being passive during cruise/low load situations. The turbo must have hot, voluminous exhaust to drive the turbine. The more you have, the quicker the turbo will spool. The most common variables to change are turbine (hotside) size and compressor (coldside) size. For every engine, there is an optimum combination for the desired power level, at full boost. Depending on the size (pumping power) of the engine, the turbo may have to be oversized and slow-responding to hit the desired power level.

Positive displacement superchargers, on the other hand, are great because they are compressing intake air full time when the engine is running. Crack the throttle and you have instant boost--the supercharger doesn't give a sh!t about your exhaust energy. On the other hand, you have to feed the turbo some throttle to get your reward.

Another big function of turbo lag is the volume of your intercooler and associated piping. Not only do you have to develop your p.s.i. in the intake manifold, you have to develop the p.s.i. first in the intercooler and piping before the engine. This can take a few milliseconds, but is certainly perceptible part of the lag equation.

Variable vane turbos have been around for years. They were incredibly popular for a time in the 1990s, but they had a high failure rate and were expensive and complicated to repair. There is nothing magic about variable vane turbos. They still have lag because, after all they are still driven by exhaust gasses, but the ability to hit full boost is shifted further down the powerband due to the added efficiencies allowed by the variable vanes.
there is no misinformation here. i have and drive a GVR-4 with a big 16g (i also have an R32 GTR). as well, my friends drive Evos, and i have driven theirs. i know what turbo lag is. my car has it, as do the Evos. i can dial down lag to a very small amount and it always seems like too much. and the newest version of the 911 turbo has zero perceptible lag. any "actual" lag that can be detected on a dyno, for example, is between [email protected] and sub 2000rpm --a negligible area of the power band. the variable turbos of the Porsche begin boosting at sub 2k rpm. this is so soon that the boost is on before any "wait" is considered relevant. i urge that you look into it yourself. read up on your Porsche stuff, as that is a roadcar often forcing other makes to bend to it's will. the GTR must heed this to remain a relevant force.

it has been stated in this thread, and in another entirely devoted to variable turbos, that such technology is not new. but Porsche has created an extremely refined roadgoing version of this technology. and i maintain that if the GTR is turbo'd, which it probably will be, it is going to have to match this responsiveness in the stock consumer model, else it will be an embarrassment to Nissan and the GTR heritage that is a symbol of national pride, power, and prestige for the Japanese.
 

·
Registered
Joined
·
5,719 Posts
the 997tt builds boost before 2000rpm. you're idling at around 1000. you barely accelerate into 1st and you're at 2000rpm nearly instantly. for all purposes, there is zero lag on the 997tt. any lag that is technically/mathematically present is irrelevant. wouldn't you say?
No, I wouldn't say.

Good turbo sizing choice, and this variable vane / variable scroll goodness, and a well matched blow off valve can make lag when going between gears in a drag race (i.e. at WOT) negligible.

But boost isn't built as a function of RPM. Its built as a function of thermal load. I can make a turbo car hit redline while having it never hit full boost, with a light enough application of the throttle. It might only just get into positive pressure at redline.


And as psicho, what happens in the off-throttle / on-throttle transition? If you're trying to balance a car on the throttle around a bend, you're constantly feathering the throttle. And this is where you'll still get lag. As soon as you back off, the BOV vents the pressure back into the intake, which depressurises the air between your turbo and your throttle body. If you get back on it, even if your turbo is at its maximum speed and making maximum boost the moment you touch the throttle (which it won't be since there's no exhaust load to drive the turbines prior) there's still a delay when you open the throttle closing the BOV, and when the intake pipe / intercooler gets repressurised.

And so you get lag.

Of course, if you've got no BOV to vent it then the air in the intake stays pressurised for a while...but then backpressure and cavitation slow your turbo down (especially since you've got no exhaust load to keep the turbines spinning) and you get lag anyway.


I don't doubt the lag will be minor in the 997 Turbo, and you can just adjust your driving style to suit, but there's no such thing as a "zero-lag" turbo without using an antilag kit, which isn't even remotely street legal even if you could afford to constantly replace your turbos and exhaust manifolds. And a keen driven who is used to a responsive NA engine will still notice it even if they don't find it a problem.
 

·
Registered
Joined
·
541 Posts
scathing states:

"I don't doubt the lag will be minor in the 997 Turbo, and you can just adjust your driving style to suit, but there's no such thing as a "zero-lag" turbo without using an antilag kit, which isn't even remotely street legal even if you could afford to constantly replace your turbos and exhaust manifolds. And a keen driven who is used to a responsive NA engine will still notice it even if they don't find it a problem."
----------------------
i respond:

although your knowledge of the ins and outs of boost are impressive, it appears, at least to me, you speak specifically of the 997Turbo without knowledge. or at least, in due respect, you have not fully heard my point: the lag of the 997tt is extremely minor and so small as to be nearly irrelevant. the boost power is nearly, almost exactly, instantaneous upon acceleration.

insofar as rpm, you seem to miss my point. by 2000rpm, the boost is full-on in the 997tt, with an amount of it available prior to this. such an rpm is barely above idle speed. and to the driver, this is "felt," the physics of thermal dynamics notwithstanding. never did i ever postulate boost to be a function of RPM. boost in the turbine is created by the buildup of exhaust pressure. you can go higher in rpms and not necessarily build enough of this pressure to be in boost.

but regardless of how any of it works, if you are a spirited driver of the 997tt, and you have nearly full-on power when you've barely begun to accelerate, this creates virtually negligible perceptible lag, as i have stated previously, regardless of actual lag. and the lag present is very, very --very-- small.

therefore i maintain that the lag is zero, regardless if you dynoed the car and actual lag was found. and that what you did discover, or maybe even feel yourself provided you were keen to do so, would be so minute as to be virtually none. the sensation from idle speed to acceleration under boost will be perceptibly instant. and that is extremely innovative for a mass-production road car. the GTR will need to answer this with an equivalent setup or it is no dice.
 

·
Premium Member
Joined
·
6,474 Posts
Lets have a show of hands here. Who has actually driven a turbocharged car before and knows what the **** turbo lag is?

:wavey: There are 3 turbocharged cars sitting in my garage right now. I sold another one last month.

I have driven quite a few setups in my life. Both on and off the track. Tarmac and dirt.
 

·
Registered
Joined
·
5,719 Posts
but regardless of how any of it works, if you are a spirited driver of the 997tt, and you have nearly full-on power when you've barely begun to accelerate, this creates virtually negligible perceptible lag, as i have stated previously, regardless of actual lag. and the lag present is very, very --very-- small.
Yeah, I'd agree with that. I just won't call it "zero-lag" because it will be there, but certainly it'll redefine what people think is possible with a turbocharged car.

Mind you, the auto 997 Turbo is the quicker of the two making it the driver's choice. At that point you can just left foot brake while throttling to keep the engine load up, so in the hands of a good driver it can be completely lag free (given that it makes boost so easily) by my absolute definition earlier, as long as you drive to suit. It'll just chew through pads faster.

I just can't wait for Porsche to sort out their DSG and slot that baby into the Turbo.
 
21 - 40 of 45 Posts
Top