Hi all. I didn't want to hijack the header thread, though
this topic relates. I'd like to discuss the meaning of lag,
the causes of lag, and the measurement of lag.

What piqued me to start the thread was some posts that
claimed a certain mod would "decrease lag by 500 RPM".
Lag cannot be measured in RPM. Let's imagine two motors,
both turbocharged, both able to make the same maximum
power at the same RPM. Also, let's say the rotational mass
is the same etc. The only difference is the turbo setup, such
that if both are running at 2000 RPM and the gas is floored
till they reach 6000 RPM, one of them takes 4 seconds to
get there and the other takes 3 seconds. Then the slower-
to-rev motor is suffering more lag. Lag needs to be measured
as the rate of change of RPM, eg: RPM/sec. This would need
to be measured at a given starting RPM, and be measured by
setting the motor at starting throttle (just enough to keep
the motor at the starting RPM) and flooring it. This is because
lag will be different at different starting RPM, and will only
be significant in the transition from off-throttle to full throttle.
Ie: if you start at 4000 RPM, the lag (time to get) from 4000
to 5000 RPM will be greater than the time it takes from
4000 to 5000 if you start the test at 3000 RPM. So lag would
be expressed as:

RPM/sec @ XXXX RPM

This is a little distant from the proximate cause and effect of
turbo lag. The real thing we're concerned with is the rate of
change in boost, or fiinally, the rate of change in the RPM of
the turbo impeller from as slow a speed as it needs to keep
the motor at the starting speed until the turbo impeller reaches
the RPM to produce maximum boost.

Because some turbo impellers spin to 120,000 RPM, a
typical lag measurement might be:

(120,000 rpm - 2,000 rpm)/ 1 second (one second lag)
= 100,000 rpm/sec.

100,000 rpm/sec might sound fast initially, but it sounds like
really bad lag to me. I guess the most intuitive practical
measurement should be time-to-boost @RPM, eg:

.75 seconds to max boost at 3000 RPM.

Note that this is independent of what the max boost is. That
is a good thing to isolate the lag as a measurement. Note
though that if you have two different turbos, and one has a
lag of .75 STMB and another that has a worse lag of 1.0 STMB,
but it produces more maximum boost, it might be producing the
same actual boost (and therefore power) at .75 seconds as the
less laggy turbo, so it would be preferable for any car that spent
any more than .75 seconds accelerating... That would be if max
power were the goal. If the goal was instead more instant
throttle response, which you would feel much sooner than .75
seconds, you might prefer the quicker turbo.

My two cents,
Joe Weinstein

 

Hi. thanks.

The torque curve doesn't help because it's plotted at full
boost. There is a difference in the programming, I believe for
first gear as opposed to 2-6, but lag is time. As to 'significant
boost' you can get maximum boost at any RPM (maximum
allowed by programming at that RPM etc). The question is only
how long it takes to get to that maximum power at that RPM.
There need not even be any change in RPM to measure lag!
A dyno could be programmed to modulate the load to keep
the RPM constant. You would start with minimum throttle, and
then crack it open, and see how long it takes to get maximum
power at that RPM. That's the lag.
Joe

 

Joe,

I almost didn't respond because this subject is complicated and can be rather long.

Turbo size selection starts with choosing the compressor housing. Cars operating with high octane fuel usually base this on how much hp is required to be competitive in their venue. Street cars operating on pump gas are boost limited, so their primary selection criteria is dictated on how much turbo the engine combination can accept at a specified boost level.

Whether you're seeking to reach a desired power level; or specific boost level , first determine how much air flow is required to reach your goal at a given engine displacement and rpm.

Next, you need to add boost into the equation. Turbo designers use pressure ratio instead of an outright expression of boost pressure.

In the turbo world, engine airflow is measured in pounds per minute. Generally, on a hp EFI engine every pound per minute is worth about 10 hp.

Every compressor has a definite combination of airflow and boost pressure at which it is most efficient. When choosing a compressor, you want to position the point of maximium efficiency in the most useful part of the engines operating range. , As efficiency drops off, heat transfered to the air induction side of the turbo goes up. This is bad for power and durability.

Turbo manufacturers publish compressor maps that establish the peak efficiencies of every turbo unit and it's variations. These maps are important part of the compressor selection because popular turbo series like the TO4 and it's custom aftermarket derivatives have many different available wheel trims - a classification system that defines the relationship between the compressor's inducer (inlet orifice) and the compressor wheel overall diameter and tip shape.

Because of the turbochargers modular nature, in many instances it is possible to mix and match different turbo housings (the exhaust side of the turbo) with given compressor housing. This permits tailoring the turbo specifically to the engine's operating characteristics and the car's intended use.

The turbine must make the compressor spin fast enough to produce the required airflow at the specified boost level. A small turbine spins faster than a larger turbine (which reduces lag), but develops more backpressure (which restricts the exhaust flow). The goal is a turbine that spins fast enough to generate the necessary response and air flow while minimizing backpressure in the exhaust.

The turbine wheel's overall diameter and the housing exducer bore (the turbine outlet's id) basically determine the turbine's ability to generate the shaft power needed to drive the compressor at the flow rate required to create a given boostor power level. Simply put...... larger turbines make more power than smaller turbines.

However, brute force and size is not all that matters. The turbine's A/R (area/radius) ratio basically determines where the turbo starts to accelerate. A turbine housing looks kinda like a big snail shell. Unwrap the shell and it resembles a cone. Cutting off the tip of the cone leaves a hole - the cross sectional area of this hole is the A in the A/R. The hole size is important since it determines the velocity at which the exhaust gases exit the turbine scroll and enter the turbine blades. For a given flow rate the smaller the hole, the higher the velocity - but the greater restriction to the exhaust gas flow.

The R in the A/R is the distance from the center of the cones cross section to the center of the turbo shaft. A smaller R imparts a higher rotating spead to the turbine; a larger R gives the turbine shaft greater torque to drive the compressor wheel (because the lever arm R is longer).

Why is A/R ratio important? Consider two extremes. Land S p e e d Racing (LSR) versus quarter mile drag racing. In the LSR application, the turbo's rate of acceleration is not critical, the set up can be lazy off the line, but the overall acceleration rate, once it begins, should be smooth and linear - this application generally calls for a high A/R ratio. At the drags (and on a street car) , you need more aggressive, instant response, which tends to lean to a lower A/R ratio.

Unfortunately, there is no easy scientific method for selecting the proper A/R ratio. Seat of the pants feel is important. If boost rise is sluggish, the A/R ratio is too high. In extreme cases the ratio gets so big the turbo can't turn fast enough to produce the required boost. But if the ratio is too small, the turbo gets into boost so quickly that the vehicle becomes almost undrivable - and on top it will feel like a choked-up normally aspirated engine that's under carburated. Also, what equates to a low or high A/R ratio varies by turbine series and engine displacement.

Given an equilivent turbine trim and A/R ratio, as engine displacement increases, the operating rpm range charasteristics of the turbine decrease. Then there is also the heat the unit will see from the engine and exhaust gases, which change the units efficiency curve.

*Marlan Davis, Part 1, Science and Selection

 

Chad,

Keep typing buddy, VERY informative.................

TIA

Lou

 

Thanks to both Joe and CJV!!! Very interesting stuff. Keep it coming.

 

Damn Chad ... thanks!

Joe, I'm thinking about what you are saying and I agree, you need the changes (or derivatives) as you are describing.

 

1) The formula to determine required air flow is:

Cfm = CID/1728 x rpm/2 x VE

*VE is at least 100% for a turbocharged engine so use 1.0.
* CID is cubic inch displacement.
* Cfm is cubic feet per minute.

2) Next you need to add boost into the equation:

Pressure Ratio = (14.7/boost pressure) / 14.7

3) Therefore the cfm requirement under boost is:

Cfm boosted=Cfm unboosted x Pressure Ratio

4) Convert airflow to pounds per minute:

Lb/min = cfm boosted x 0.07

* A good rule of thumb use 80 degrees at sea level is to multiply cfm by 0.07.

5) Generally on a high powered EFI engine, every 1 pound of air flow is worth about 10 hp, so to find the required lb/min for a race only application, start with horsepower requirement, then divide by ten:

Lb/min = hp/10

 

Thanks Chad. I wasn't going into all the details of which
turbo to choose or how they work, because I was talking
only about lag, but with your valuable expansion of the
thread, this will become a much referenced one.
Tim, thanks for getting the exact point and saying so!
Joe

 

This is one thread for Kevin