Andy.F

People say things such as "I find the vfxx turbo is quite laggy, it doesn't produce 1 bar until 3900rpm"

Now, that to me is meaningless. The above statement is dependant on many variables such as, which gear it was in, what the car weighs, what the diff/gear ratios are, uphill/downhill, weight of flywheel, number of passengers, boost mapping, what the ignition timing is, AFR, what type of exhaust is fitted, induction system, air temperature, EGT .....and probably much more.

The example above is actually a combined measure of turbo spool up time (lag) and the 1 bar boost rpm threshhold of that particular installation.

To compare my short geared lightweight RA to a laden long geared UK scoob in this way, is a pointless and misleading exercise.

To find the 'boost threshhold' then the car needs to be held back on the brakes, in top gear, with the foot to the floor. Allow the speed to increase slowly until you register a boost pressure of, in this example, 1 bar. That is the minimum rpm that your turbo produces 1 bar.

Turbo spool up time "lag" is a different thing, lag is due to the rotational inertia of the turbo. Bigger turbo's have larger diameter wheels which require more energy to increase in speed.
As the turbo may need to spin up from 2000 to 80,000 rpm to produce boost there is a time 'lag' associated.
You can measure turbo 'lag' easily on a bench dyno or during gearshifts with a device such as an AP22 accelerometer. I have on file a few interesting comparisons between VF and TD series turbo's

On the road, one way to identify 'lag' is to come off the throttle at a rpm point above maximum boost threshold, for a few seconds, then hit full throttle again and monitor the delay until full boost is achieved.

Lag is also an issue during gearshifts, if you can shift quick enough, the lag can be minimised as the turbo is unable to slow down so far, before power is applied again.

The boost threshhold is largely dictated by the exhaust turbine wheel and casing design.
It is possible for example, by CAREFUL selection of parts, to fit a bigger compressor wheel and actually reduce the boost threshold !!
This is where I hope to go with the 20G hybrid. The 20G compressor wheel is a more efficient design than the original big 16G. As a result, the exhaust turbine will need to spin slower and transmit less torque to produce the same airflow from the compressor. Now assuming a similar exhaust gas velocity, the force on the slower rotating turbine will be increased, offsetting turbo spool up time.

I will be most surprised if my maximum boost threshhold of 1.6 bar(never mind 1 bar) is any higher than 3500 rpm. My calculations show it to be closer to 3200 The limiting factor being the surge line on the compressor.

As for true turbo 'lag' then a bigger compressor may not be such an issue during gearshifts after all, as the inertia of the bigger wheel will help retain the rotor rpm better during the gearshift

Andy

Pavlo

Agreed.

Lag between gears or during throttle control, is rather different to whatgear you have to be in on the motorway to accelerate from 70mph or whatever.

I will have some alternative results of my own to add to the mix in a while.

We will see.

Paul

ozzy

To me lag is below 3,000 revs and as you say trying to get it spooled upquickly enough. It's only noticeable if I'm plodding along in traffic and then decide I want a quick buzz.

John Banks, AlanG, The_Gza and myself all drove our cars along the back roads near Johns and although we all mentioned 'turbo lag' in certain conversations Alan quickly pointed out that none of us had ever been below 3,000 revs on the runs

These were all different cars from std Turbos, Hybrid Turbos and a VF (IIRC) so guess lag is only a real complaint when you want to show off by going from 10mph to 60mph without dropping gears

Stefan

[Edited by ozzy - 1/1/2003 8:28:32 PM]

john banks

Agree Andy. I hate lag far more than I hate a high boost threshold. I dislike roller bearing turbos which are supposed to be less laggy than sleeve bearing turbos but probably aren't, are not rebuildable and don't easily hold top end boost without an actuator as tight as an ant's chuff whilst making similar noises

And yes, I think we all slip from time to time and say lag when we mean boost threshold. You can't deny there is usually a correlation between the two though.

[Edited by john banks - 1/1/2003 8:37:09 PM]

Andy.F

Yes, the key word being efficiency. I don't think the VF's have a very efficient exhaust turbine, so any hybrid'ing, although improving on standard, is on a slippery slope IMHO of course

Bob Rawle

I use a relatively simple test to verify spool (I call it this as lag implies "not spooling" whereas its rate of spool we are interested in, least I am).

Start with the car in 4th gear at 1800 rpm, hold the throttle wide open and monitor revs and boost, make a note of what revs the car achieves 1 bar boost and then max boost, repeat in third, at the same time you can check for boost stability, overshoot and ability of turbo to hold boost at high revs.

Car should be on level ground but if done on a slope then run both ways and average the results.

This is a consistent car to car way of comparing differing setups and settings and allows the generation of a database of info relating to differing turbo's and mod states which then can provide meaningful perception about the potential effect of future changes in practical terms.

In effect this is simulating a rolling road power run "on the road" ... if done in conjunction with the use of an AP22, for example, meaningful comparisons can be made of power and torque.

The surge line has to be watched for of course but rarely becomes and issue at wot ... now part throttle boost performance is another thing and its relatively easy to step over the mark in that area. I have more problems at the other end of the map these days, turbo choke rather than stall.

Trout...

Andy,

don't disagree with your thoughts - although I see that your hypothesis has been moving forward - you recently said a bigger turbo would reduce your 1/4 times

The other post from whence this came - clearly was talking about boost threshold - which in common idiom people, especially people who drive Scoobs, call lag. Techically it is not - so thank for the update

On your car you have worked wonders with the boost threshold - getting results that are not often seen elsewhere - 3,200 for 1.6bar would be very impressive indeed - look forward to seeing on your car sometime.

Trout

Andy.F

The reservation I have with the 'in gear' boost points is that you can't really compare car to car. Same car you can compare different turbo's ok but take the example I mentioned earlier, my RA 5th gear is exactly the same overall ratio as a UK 4th, the RA is some 100kg lighter.
If they both done a 4th gear test, the RA would pick up rpm much quicker than the uk therefore spool time would feature more in the results than boost threshhold.
I can watch the boost needle in J bank's car climb and pick the rpm from the dial at the same time, in my RA it all happens much quicker and is difficult to record.

Trout...

Andy,

but the reality is - the only real test of appropriate threshold is what works for the car that the turbo is going in - if you see what I mean - and I understand what you are saying

Trout

PS What makes you think if I do build an engine I will keep the boost turned down

Andy.F

Yes, I'd agree with that. Ironic then that I have an ultra close ratio box which would work well with a big laggy turbo........and I have a small responsive one !!

I was not suggesting (on other thread) that YOU would turn the boost down, merely that my drivetrain would not take much more than current, so not really any point in me going 2.3,4 or5 ....Yet

Trout...

Quote 'In effect this is simulating a rolling road power run "on the road"'

Isn't it the other way around - don't rolling roads try and simulate what is happening on the road - and not necessarily successfully? It all depends on how the RR operator chooses to apply load. IMHO PE for example choose to a lighter load - and so the boost arrives quite late compared to a run on a level surface in fourth. Cerainly on my runs there the boost arrives 3-500rpm later than on the real road.

PS and Well Lane also have different approaches to applying load and so the peak torque point moves around quite considerably.

Trout