a few quick questions on turbochargers and how to fix lag

[MrSmall]

okay, i've been looking at these turbochargers, i'm not sure, but about how much performance increse do they give? somewhere around 30-50% right? does that mean i'll be getting 413 horsepower on a 310 horspower car? (40% of 310 is 124)

also, in order to elimate the "lag" you get when you first start out? i know there are ways to lower it with getting a small turbocharger and a larger one, and a few other ways too... but do any of these ideas actually eliminate it? or just reduce it

[454Casull]

DP

[454Casull]

okay, i've been looking at these turbochargers, i'm not sure, but about how much performance increse do they give? somewhere around 30-50% right? does that mean i'll be getting 413 horsepower on a 310 horspower car? (40% of 310 is 124)

It's not that simple... It depends all on boost pressure, and if your fuel system/engine internals can handle it. If your boost is low, then you won't get much of an increase in power. OTH, if you have too much boost and not enough fuel, you'll run lean, with lower power. And god help you if your cast block windows on you.

And as for turbo lag, you'll never be rid of it completely unless you have an electrically-assisted turbo (and I don't think that it spins up to full boost without help of exhaust), or you have an anti-lag system (AKA bangbang system) installed.

Features such as air/ball bearings and variable geometry will indeed decrease lag, but as with the question whose answer is "You can't get there from here.", these only ease the lag. (BTW I'm referring to the question "If every step halves the distance between here and there, how many steps will it take to get there?".

[YellowMaranello]

With an electrically-assisted turbo, does it just keep spinning and keep putting air through the bov until you open the throttle (that would sound really wierd)? Or does it just get it up to speed alot faster then the exhaust would otherwise?

[YogsVR4]

I think a supercharger is the only way to eliminate any lag on forced air systems.

As stated above, you have more concerns with your setup to actually get power from the turbos. Also remember, unless the car was designed for a turbo (or supercharger) you will need to limit the amount of boost you put into the car or you'll be replacing your engine sooner then you'd like.













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[ivymike1031]

my understanding of the electric-assist turbos is that the electric motor just helps to reduce lag by accelerating the turbine faster than the exhaust alone would.

[Deakins]

Or, you can do it like this

[454Casull]

Superchargers will still have lag. The engine must bring another component up to speed.

Accessory/flywheel-less NA engines have the least lag.

[ivymike1031]

I think you guys had better be more clear about what you mean by "lag" before the discussion proceeds any farther.

454, your version of "lag" would seem to be something along the lines of "the time it takes for the engine to reach max rpm from idle." (engine angular acceleration is the key parameter)

I had thought that lag was more along the lines of "the time it takes to reach full intake manifold boost after the throttle is opened." (manifold pressure is the key parameter)

Yogs, your version seems to be about the same as mine.


454, if I'm right about what your version of "lag" is, then I've got something in mind that probably has less lag than "Accessory/flywheel-less NA engines": a low-inertia engine with an integrated electric motor assist (like the honda insight), if the controller is properly configured.

[BeEfCaKe]

My understanding of "Turbo lag" is the time it takes before the turbo really starts to spin up to speed(require higher RPM) and cause any major power gains?
Superchargers don't really have this "lag" because it spins up at low rpms.
Yes? No?

[SaabJohan]

"Lag" is defined as the time needed to reach full boost from the time the throttle was opened.
There is also a point called "boost threshold" which i the lowest rpm the turbocharger can produce above atmospheric pressure.

You can't eliminate lag, but it can be reduced. Under the turbo era in racing something called Anti Lag System was introduced, this was in the mid eighties. Basicly ALS let air/fuel mixture go into the manifold when the driver reduce throttle, and when the throttle is reapplied the air/fuel goes with a bang and spin up the turbo almost immediately. Today this system are built in to some aftermarket ECUs, and it's also used in WRC rally. The downside is that this wear out the turbo in a short time.
In WRC rally they also don't use a blow off (under short time gearshifting this can actually reduce spool up, and specially if it's used with ALS) and this causes the compressor to reach the surge limit when the driver lifts of the gaspedal. This can be recognized with a chirping sound.

VG or VNT can't be used with gasoline engines since the exhaust temperature is too high.

How much the turbocharger increase power depends. An easy way to get a approx. hp is by multiply the NA engine power with the pressure ratio over the compressor. The pressure ratio, PR is around the same value as the absolute pressure in bar. This means that to reach 400 hp with a 310 hp engine you will need around 0,4 bar (around 5.8 psi) of boost pressure.
But you can of course get more power if you want. In the mid eigties the 1.5 litre engines used in F1 run really high boost pressures, up to 5.5 bar (around 80 psi) was used in race contition and the strongest of them all, the 4 cylinder BMW engine could deliver up to around 1500 hp (remember that it's only 1500cc).

[454Casull]

Got any more info on that BMW?

[SaabJohan]

Originally posted by 454Casull
Got any more info on that BMW?

BMW M12/13 F4t
The engines were based on used stock blocks in cast iron, but were modified for racing. M12/13 F4t was first used in F1 under 1982 and the engine then developed around 600 hp. After a few years the power had reached around 1200 hp in race trim, and even higher under shorter periods. 1500 hp is based on an acceleration measurement since the BMW dyno only could handle around 1300 hp.

The engine was used by Brabham, Benetton, Arrows and maybe a few more which I can't remember.

The engine was developed by BMW motorsport (Paul Rosche and his people).
One single KKK turbocharger was used, but later they used a single Garrett instead.
Bore and stroke are 89,2 x 60 mm.

Earlier versions of this engine have been used in IMSA and F2.

A few of theese engines are today for sale, but they are not cheap.

http://www.race-cars.com/carsold/oth...6/benx86pe.htm