Turbo lag. I have been thinking about turbo lag, and response. Its the favorite buzzword.... GT-R guys all want to be like Mines, and have insane "response".

I have been looking at a ton of dyno charts, and I have driven a lot of different configuration GT-R's. I think a lot of what people think is lag, is they feel the big rush/doubling/trippling of power when the turbos really come to life.

I think you are trying to argue what society's perception is of the term "turbo lag" and although you might be right, I'm not sure how you are going to spread the gospel and get it through everybody's head. Society's concept of turbo lag is: the time it takes for the engine to come into powerband (on a turbo car) while disregarding the boost guage and simply going by the butt dyno. It's simple to see why that perception defines the term "turbo lag" - they think before the car starts accelerating hard that the turbo is "lagging". It does take more TIME for the larger turbos to hit XXpsi of boost than the smaller turbos which could also be defined as "turbo lag" (this disregards hp delivery).

Each of these cars may deliver XXXhp at XXXXrpm, but their torque levels/cylinder pressures are vastly different. I totally see what you are trying to say, but I think to validate your idea that you would have to run each of those cars with an accelerometer and measure their acceleration times to prove the point. Otherwise the perception of "turbo lag" as it stands right now is much easier for the average person to "comprehend". Damn that was a lot of quotation marks....

Something to think about. I don't use the term turbo lag, because i firmly believe that 95% of people use the term incorrectly. Same kind of people who want to know how much boost your running, whats safe for them to run. How many bar? How much psi? Without knowing what turbos are on the car. I am not nearly as smart/experienced as most of the posters in this thread... but I like the terms 'transient reponse' and 'boost threshold'. The real subject here is the tradeoff of how high rpm-wise the boost threshold is versus mid-high to high rpm power.

I'm going to throw out some really basic ideas that I'm sure everyone is familiar with, and mix it up with what some of these terms mean to me. Feel free to correct anything thats wrong - I'm no professional engine builder, just a 20 year old car nerd.

Transient response is similar to what you described, but you have to make sure that when you try to quantify it, you need to be talking about being in gear and well above the boost threshold, and its basically doing the same thing as 'measuring' throttle response but by watching the boost datalog instead of just using the butt dyno to feel acceleration- watch the boost gauge or a datalog of boost alongside a g-meter or better yet on a dyno, and see how the car accelerates (or power increases) exponentially more quickly as manifold pressure rises. You can do the same thing for trying to quantify the boost threshold. When you're talking about falling out of boost between shifting or waiting for boost to come back as you roll into the throttle after a shift- this isn't purely transient response. It plays a large part here but so does the powerband of your engine, really, the VE curve of your engine and other factors are at play as well.

Now what you call lag is what we're trying to refer to as the boost threshold. Lag is an incorrect term because the definition of that word involves slowness, delay- a direct relationship with time. Now when you're talking about what you say is "how long it takes for full boost to hit" or "how long it takes the boost to build going from a roll"... Really has nothing to do with time at all.

The amount of boost a given turbo produces (until the point that it produces whatever you consider "enough" that you use a wastegate or VATN to vent excess exhaust manifold pressure, or size the turbine such that it deliberately becomes a restriction to building boost) is directly related to the amount of airflow through the motor. There are plenty of factors here- the motor itself is the biggest one. Dealing with the any single motor in particular, there are of course ways to modify it to improve airflow and thus power, (going back to the old adage "an engine is basically an air pump") and you can target specific rev ranges to improve airflow in low, mid, or high rpm areas via displacement, headwork, camshafts, et cetera.... Sure you can size a turbo to provide a ton of low end power, and it works very well, but it isn't going to deliver the high end power that actually makes the car perform better. Just look at Volkswagen and their trends on turbocharger sizing on the 1.8T and 2.0T. It's very clear that they, like many other OEM's, are more concerned with the false perception of speed and low-end torque than any sort of actual performance- that's just what you get when you size a turbo that way. There are ways around this when dealing with a low displacement motor and turbos, like using a sequential system on the FD3S - direct all the exhaust gas energy into one small turbo when the engine isn't flowing much air, and then after that turbo comes up to 'full boost', direct the extra exhaust into a second turbo, and finally as that turbo spools up use a wastegate to control maximum boost. It well "when it works" - the stock setup nets a 10-8-10psi boost pattern, but it does spool significantly sooner than the exact same setup converted to nonsequential operation. The better solution, in my opinion, is to play to the motor's strengths. I come from a Honda background- there, you're dealing with very small displacement, very well-flowing engines. The correct solution is to forget about any off the shelf turbo kitsm (save for the good stuff from the likes of Full-Race and various copycats like Lovefab and Peakboost,) get a good manifold and size a quick spooling turbo that will make tons of power around 6500-9000rpm. I believe the same thing applies to the RB.

As the RPMs rise the amount of power the engine is producing "on it's own" is increasing because the VE of the motor is going up, the motor is able to intake an increasing amount air per revolution and then you add a correspondingly larger amount of fuel in a revolution, so you make more power and the revs keep climbing.

Another big factor here is the turbo itself, as the turbo starts to spool and compress the air, obviously more air comes out the exhaust ports and then directly into the turbine providing MORE force to power the turbine blades and drive the compressor... you get the point. Eventually you get past the "threshold" in the rev range where there is enough airflow through the motor to provide enough exhaust gas energy to spin the exhaust wheel with enough force that the compressor wheel can start compressing air, and everything compounds from there.

6 months or maybe 8 months ago I would have been pretty uneducated on the subject of sizing turbos for RB's, and today I still have 1/100th of the knowledge as some of you. But in those 6-8 months then I've done a lot of reading, a few days of wrenching, a couple hours of dyno tuning, and a far too short stint behind the wheel of a 2530 powered R32.. Now, I don't understand why 99% of people would go to the trouble to do a turbo upgrade and go with anything less than the HKS 2530s or the Garrett equivalents (GT2860's) or even be concerned with the response from them vs stock- if thats something that's ever even come up.

this next tidbit has probably been said by dozens of people hundreds of times, but I think maybe sometimes people look at the fact that since the turbos are noticeably bigger with noticeably more power potential there will be less transient response and a higher boost threshold, when really the transient response is far and above stock due to the vast developments in turbo design in the last 15+yrs and the boost threshold is about the same or better, depending on the car (as evidenced by Sean's dyno charts) to stock. I'd be curious to know just how much more improvement there is with these 2 important (and finally defined) points on the smaller turbos like the GT2860-7 N1 equivalents and what purpose you would need to build such a quick responding car for?

another nugget of info regarding the boost threshold. If you ever get the chance to build a factory NA car and build it into a forced induction car, put the car onto a dyno in the NA state. If you use the exact same motor and just throw a turbo on it, its very interesting to look at the NA dyno graph's torque curve (which is really also a representation of Volumetric Efficiency on a NA engine) and correspond that to a dyno graph of the now-turbocharged engine that has HP and TQ- and manifold pressure/boost.

So to summarize the boost threshold - it's not a time related issue, but an airflow related issue, and since airflow is related to engine speed, you can connect the dots here and put the car in the correct gear & therefore be at an RPM above the boost threshold. Also, like Sean said - if you can't catch a lower gear since you're sitting there at the Christmas tree at a dead stop, launch the car at a higher engine speed. you'll start out above the boost threshold at 0mph and your ET's will thank you. Voila - no more imaginary "turbo lag" problem!

Fantastic stuff Sam. I only wish you dropped that post in the old GT-K thread (waste of time debacle that it ultimately was) when Mr. Knowitall, previously civictuner, insisted that turbos "don't care what rpm your engine is at."

I want to know why 400whp at 6krpm is more/less air than 400whp at 10krpm. If my assumption is incorrect then please explain what is unique about revving the motor to 10k.

I see it like this..

We know the exhaust housing can be a limiting factor in power. This I am sure we can ALL agree on.

W/that said...an engine that rev's into the upper rpm's needs something that won't be a restriction. If your motor moves X amount of air @ 5k rpms, but moves XY cfm's @ 10k you need a turbo that will be able to flow XY cfm's through the exhaust housing. If not the turbo will bottleneck and power will fall off..

Thats why a .48 a/r dies up top on a b series motor. The motor outflows the turbo.

On a BB unit, you can "cheat" and usually go w/a .82 a/r because they have very similar spool characteristics to a.63 journal bearing unit.

The point I'm trying (and failing) to make, Adam, is that if your motor flow X at 5k and xy at 10k, none of it matters at all once to turn on the b00st. (turn on the bass!)

The limitation is based on how much air your compressor is sending in. Turbo's don't die because motors rev high, they don't die because motors make too much power, they die because they can't flow the air that comes along with both of those things.

The motor doesn't outflow anything. The compressor outflows the turbine or vice/versa. In closing, there is no difference at all between 400whp at 5k vs 10k. The same turbo will work very well. Turbos are very nice and friendly, they don't care what your engine looks like or what you rev to.

I bet a t3.48 stg1 wheel will flow enough air to make the stock 140whp that a b series makes no problem. No choke, almost all natural VE dropoff. Just plot the flow on the turbine, I should check right now.

EDIT-the last paragraph is implies the horrible choke and die effect common to .48 t3 housings is not because the RPMs.

The biggest problem w/the GT-K post was misunderstanding. Alof of you guys (myself included) were talking about the same thing but using different terms to explain ideas...