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Old 06-05-2002, 03:56 PM   #1
Jon Bogert
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Question The definition of turbo lag

Help me settle an arguement about turbo lag. The cars in question are my wife's Audi S4, which makes full boost at 1800 RPM, and the 2.0L Celica rally car, which barely makes boost below 3000 and doesn't get to full boost before 3500.

My wife, used to the S4's tiny turbos, says: "If I step on the gas and don't have power, that's turbo lag. If I open the Celica's throttle at 2000 I get no power for 5 or 6 seconds as it lugs up above 3000."

Now I say that the Celica has almost NO turbo lag. The exhaust is virtually wide open. There is little intake restriction and the total intake volume is tiny--with a straight shot through the A/W intercooler to the manifold. In other words, the instant you open the throttle, the turbo is making as much boost as it can. I say you can't call it "lag" if it only makes, say, 3psi at 2800--that's exactly what the turbo is sized for!

The S4, on the other hand has a ton of lag. Convoluted plumbing, a restrictive exhaust, etc, etc. You just don't notice it because the turbos are so rediculously undersized that they're spinning all the time.

So who's right? Is the term "lag" subjective or objective?
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Old 06-05-2002, 04:21 PM   #2
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Turbo lag (to me) is when it takes a long time from when you press the throttle, to when it makes the boost it's supposed to at any given RPM.

My car makes full boost a little under 2500rpm, but a friend of mine drove the car, and told me that it had a lot of lag when he mashed on the throttle at about 1200 and the car didn't make any boost until about 2 grand (this was in 2nd gear BTW). I told him that it wasn't laggy, the engine just didn't have enough exhaust coming out to spool the turbo. I told him to bring it up to 4k rpm, and mash on the throttle and see what kind of lag there was (obviously there was none, since I'm only running 5pounds for now).

My fiancee has an A4 1.8t, and that thing has a ton of lag. I mash the throttle at about 3500, and have to wait about a sec for the turbo to spool, since it only runs 6pounds (I think?) and has to pressureize a lot more tubing than my car, since the turbo is on the opposite side from the throttle body.

I think that it's a subjective thing...everyone has their own opinion on what it is. But if they don't agree with me, they're wrong
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Old 06-05-2002, 04:27 PM   #3
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IMO, the amount of time from when the turbo starts spinning to when it spins the fastest is called lag.
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Old 06-05-2002, 04:46 PM   #4
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First off, regardless of the truth, your wife is always right.

But I think she has a closer definition of what lag means than you do.

I have an RX with a 1.8 and a little itsy-bitsy IHI, and I can get full boost by putting my foot to the floor at essentially any RPM over 1200. However, you don't feel anything until after the 3k mark.

It is laggy like crazy. (The 7.7:1 compression ratio has a lot to do with that.)

There is more to lag than spool up. The key to it all is TORQUE. An S4 with the 2.7 has boatloads of torque with or without the turbos spinning full on. The 2.0 in the Celica doesn't make you feel it til later.

That's why a turbo RS won't feel laggy, but a WRX will, despite similar hp numbers (its all in the TORQUE)
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Old 06-05-2002, 04:56 PM   #5
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isn't turbo lag the amount of time it would take when you hit the throttle until you start feeling the effects of the turbo--once the engine starts seeing boost?

like if you're cruising and there's no boost, then you step on it and see how long it takes for the turbo to start spooling up?
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Old 06-05-2002, 05:02 PM   #6
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I'll skip the bull*****.

Boost Threshold: The time it takes for the turbo to start producing boost RPM wise.
I.E. my car begins to make boost at 1500 RPMs, that is the Boost Threshold.

Lag: The time it takes for the turbo to respond to throttle inputs, and create boost.
I.E. at 4k, I hit the gas, boost gradually builds after a moment, that's turbo Lag.

Fact: Most modern turbocharged cars do NOT suffer from noticable Turbo Lag.

Myth: Cars that are sluggish low in the RPMs, (I.E. the WRX) suffer from Turbo Lag.

Fact: The car hasn't reached it's boost threshold yet.
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Old 06-05-2002, 05:16 PM   #7
Jon Bogert
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Thanks, Jewbaru, that's exactly what I was after.
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Old 06-05-2002, 06:26 PM   #8
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Jewbaru is definitely correct on that one, I was just about to post that. People always confuse boost threshold and turbo lag. They think that turbo lag and boost threshold are the same thing.

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