Turbo "Lag"

[Ako]

Ok, this is a wee argument I'm half having with someone, and much as it amuses me there's some damn intelligent people on here who can help me out here.

Now, ok. Say you have Car X, which is stock in every form. It comes on boost at 3000rpm.

Now, adding a big huge "replace my bumper" intercooler to car X will increase lag, right? By which I mean it will take an instant longer to come on boost, and won't be as responsive when you hit the throttle. The driver might think its coming on boost later since in 1st 2nd its taking a bit more to ramp up...

However, in a 5th gear pull - it should still come on boost at 3000rpm just the same right? Its hard to quantify this, but in a nutshell I'm trying to explain the difference between boost threshold and boost lag...

Thoughts please

[rage]

the affect that the intercooler has on response is because it increases the amount of volume that the turbo has to pressurize before being able to supply boost to the engine. thus this results in the boost hitting slightly later. think of that in terms of 1st gear vs 5th gear. the affect would be the same, i would think there would be load differences though, 5th gear pulls are usually very high load and would build boost faster, and make the changes to response of the intercooler less noticible.

those are my thoughts. feel free to challenge if you feel otherwise.

[Dell'Orto]

I thought the obvious explanation was lag is between gears, threshhold is how long it takes to come on boost?

[Ako]

Yeah thats what I'd say as well - I'm just trying to get it worked out in my head if, and how, going to bigger pipework etc will change that threshold.

[Malcolm]

here's the way I see it.

Let's assume there's so little volume of air between the turbo and the engine that it can be ignored for starters.

In this situation, boost will be generated once the amount of air being produced by the turbo is more than the amount of air being consumed by the engine. This should be pretty obvious when you think about it. So, once we reach the point where the turbo is producing more air than the engine is consuming, we will begin to see positive intake manifold pressures. To me, this would be the boost threshold, and would be dependant on a number of variables, basically I would expect it to be at a fixed engine CFM, (does CFM vary with revs and load, or just revs, anyone?)


Now because we do infact have a volume of air between the turbo and the engine, the instant we reach the engine's boost threshold, we wont instantaneously see positive manifold pressures as there will be a delay due to the time taken for pressure wave to travel from the turbo to the intake manifold. This, IMO is the "lag"

I'm not an expert on such things, but I'd expect the pressure wave to travel faster through smaller diameter (and hence cross sectional area) piping, so the time it would take would depend on the diameter of piping, and the length that needs to be travelled. So small, sort piping (ie small volume) would allow a small amount of "lag", and large, long piping (ie large volume) would create a larger amount of "lag".

So, how does that relate to your question? Well I'll answer that when I get back from uni, now I must leave

[Santa'sBoostinSleigh]

as above seems correct in my thinking methods -

big intercooler = more 'space/air' to pressurise up to 'boost' levels (ie +0BarG)
same deal with a larger volume inside piping - big lengths and/or big ID (internal diameter) piping = more 'space/air' to pressurise up to 'boost' levels (ie +0BarG)

(FYI for those not in the know - BarG = pressure in Bar - gauge pressure)

[fivebob]

there will be a delay due to the time taken for pressure wave to travel from the turbo to the intake manifold.

I'm not an expert on such things, but I'd expect the pressure wave to travel faster through smaller diameter (and hence cross sectional area) piping, so the time it would take would depend on the diameter of piping, and the length that needs to be travelled.

Careful there, pressure waves move at the local acoustic velocity, so they aren't affected by volume, but simply by distance. The amplitude and strength of these waves are affected by the number of bends which cause the wave to be reflected, and changes in diameter which can increase or decrease wave strength and amplitude, and also cause reflections.

Having said that I'm not even sure where the presssure wave are originating, at the turbo or at the point of restriction, or if lag is a wave phenomenon or is flow related. In theory if the pressure wave originates at the turbo then in a very short time that pressure wave would arrive at the manifold, and in theory no lag would be felt provided the turbo had enough flow to maintain that pressure.

[Ako]

Thats not quite my question.

I mean... You can have a large turbo, with a high boost threshold - say 4500rpm before full boost is seen - but it can still be responsive - punching the pedal at 6 grand will spool it instantly on a good setup.

By the same token you can have a setup which will come on boost 3000rpm, but it can be laggy as all hell if you have big long pipework and a massive intercooler. By which I mean you punch the throttle at 5 grand, and still have a half second delay while it comes on boost, and thats the characteristics everyone hates, and where turbos get their bad name from.

What I was wondering - since I've never owned a car which can demonstrate this well enough, the VR4's came on boost if you looked at the accelerator - is will making a "laggy" intake setup - long, big diameter pipework, big cooler, blah blah - cause that boost threshold to move up in the revs? Not "it will seem to since it takes longer to come on boost" - will it actually change the RPM at which things are starting to get interesting?

My way of thinking says it shouldn't change a thing - but the common thought with a lot of people is that it will. Whos right here?

[strx7]

boost Threshhold is the engine RPM require to spool the turbo.

Lag is the time taken between mashing the pedal and seeing +ve manifold pressure when you ABOVE the threshhold

[Ako]

Read my question more carefully. I know the difference, I'm just trying to work out how changes to the inlet side of things can affect that threshold, if at all.

[fivebob]

It all depends on whether lag is a wave or flow phenomenon, or even a combination of both.

If it's pressure wave related, then the difference in lag would be undetectable, unless there were lots of bends and changes in diameter of the piping or the distance between turbo and intake is very long. If it's flow related then it would depend on the compressor map as to how long it would take to fill the volume of piping & I/C. If it's a combination, which is the most likely explanation, then it would depend on all these factors, though what weighting each has would be impossible to say without doing some experiments on that particular turbo and engine combination.

[SUBARUCONVERT]

On my legacy i went from a VF10 and top mount to a TD05 and front mount, the lag has increased dramaticly, the VF10 was touch the throttle once in the power band and u would get instant boost and power. the thing i have noticed is that with the current set up if i nail it at 5k (or from when ever i get full boost) is that the boost gauge will sail out to 17psi but then there is a .5 sec delay untill i get power. so i think its a flow related thing

[ENT1CE]

fitting an electronic boost controler will help your lag problem
by increasing the gain setting it keeps your waste gate shut longer bringing boost on sooner.

in some cases as much as 700-1000 rpm earlier

[Lith]

Can I throw a spanner in the works and say its conceiveable if you are going from a crappy small and restrictive intercooler to a higher flowing intercooler that you could conceiveably reduce boost threshold?

I've heard (I haven't seen it myself, so I can't guarantee this) that people have done absurd things on Skylines like putting a boost gauge just after the outlet of the turbo compressor, as well as have the "normal" one - and compare the boost levels between the two while running at full boost. Apparently there often is a drop of pressure of a few psi between the two with the factory intercooler setup.

Now, doing the same thing - but with a nice front mount kit instead of the horrible oil-cooler like thing on Skylines... *APPARENTLY* there is a smaller difference between the two gauges. Again, this is something I've read on the net (and everything you read on the net is true...) - but if its true, would it not mean that the turbo itself needs to reach a lower pressure at its outlet, meaning the distance to boost threshold is slightly lower??

Just musing in the irony that it could be possible to increase lag and reduce boost threshold with one cunning stunt

[RS13]

Yeah, the factory RB20DET intercooler is awful, with a 1.9psi pressure drop. I'm making 12psi at the manifold with a boost controller, which theoretically means that the turbo is producing 14psi, which is at the limit of its' efficiency. This is why guys running 1 bar constantly shit turbos, they may see 14.5psi on the gauge, but the turbo is making almost 17psi..

On that note, if I was running factory 10psi, and installed an aftermarket intercooler with a 0.2psi pressure drop, I would see 12psi at the gauge, although this would affect throttle response (which shouldn't be confused with throttle lag!) but with a cooler charge, would add reliability, and look cool!

does CFM vary with revs and load, or just revs, anyone?

The way I understand it, is both. Revs, and load, hence why when you drive up a steep hill, or are full throttle in high gears, the extra load created adds to the engine load, which in turn works the turbo harder, and creates more boost.

I thought the obvious explanation was lag is between gears, threshhold is how long it takes to come on boost

Yeah, thats what I thought, except I've heard it referred to as throttle lag, and throttle response.

Moreso, if you had larger intercooler piping, and a large intercooler, the speed of the air is going to be lower, with higher volume (ie, less throttle response, more lag), whereas with smaller diameter piping and a smaller intercooler, it would respond faster, but would lack the volume, so less top end power. Does that make sense? Thats' the way I see it anyway.

[SUBARUCONVERT]

fitting an electronic boost controler will help your lag problem
by increasing the gain setting it keeps your waste gate shut longer bringing boost on sooner.

in some cases as much as 700-1000 rpm earlier

Its not really a problem, i get full boost in first at about 3300, but can get it about 2900 in 5th, i only used 5k as an example as its well in the thick of the power band. And a boost EBC wouldnt do much for the lag i get as it makes "boost" instantly, but takes time to get the air flowing

[Si]

Yeah, the factory RB20DET intercooler is awful, with a 1.9psi pressure drop

Not as bad as a MA70 cooler... 10psi in... 6psi out......

rock on!

[Lith]

So that suggests my theory of boost threshold dropping with a better intercooler??

[Steve123]

Intercooler and piping etc may affect lag a little, but ive aways believed that it is minimal. I believe different turbos can cause more lag than pipes etc can.

I have upgraded the turbo on my car to something with approx twice the flow capacity. I have not changed anything elce. Threshhold has gone from 2.5k to about 4-4.5K. I can deal with that ok, but the lag is terrible. It seems to take about a second to come on boost.

I have another car. It has the stock turbo but a large FMIC and 2.5" piping. Threshold is about 2.5k but it has pretty much no lag compared to my other car.

Gearing makes a large difference too.

[Santa'sBoostinSleigh]

big turbo takes longer to spool, = longer till on full boost