Thought this topic that I wrote for another forum might interest a few people on here now that people start slowly starting to move towards more serious modifications with their calibra turbos.
Compressor maps look properly scarey the first time you see them, but are actually very simple once you know what you are looking for.
Right, first place to start with for this topic is to post a compressor map i guess!
Here is the one for my turbo:
I will attack this one in chunks
1) How much power can it make
2) What boost can it make
3) What is the optimum BHP to use this turbo to make
4) What conditions will it surge under
5) Will it be laggy
6) How do i know if its the correct turbo for my engine
1) How much power will it make?Well the basic way to work this out is that you can make the fairly safe assumption that for every lbs per min of flow you have, you should be able to extract roughly 10bhp (just over in fact)
So basically whatever the furthest point to the right is on this map, you drop down to the axis, read the value, and multiply it by 10, that will give you a fairly safe figure for what BHP the turbo can do.
So on this map, we can see that the righthand edge of the islands is just past the 60 mark (about 62 i make it) so potentially it can make around 620bhp on the right engine.
2) What boost can it make This is another very staightforward answer as basically its just a case of looking at the extreme of the efficiency islands in the opposite direction, ie at the Y axis.
So in the case of this turbo, its around 3.3 bar at the top of the island.
That is an ABSOLUTE scale though, ie 1 bar on there is essentially equal to ZERO psi of boos.
So this turbo is therefore safe to 2.3 bar of boost in the right conditions.
3) What is the optimum BHP to use this turbo to make Well, its slightly a trick question that one, as there isnt one specific figure.
The key here is to try and keep to the middle effiency island as this is where the turbo generates least heat and hence you can wind more timing on in the engine, which is best for both power and economy.
So basically anywhere witin the 79% island on this map will be cool.
So from that you can see that the middle of that island is above the 40 value on the bottom axis, this represents roughly 400bhp, so thats a good figure to be aiming for your engine to be making in its midrange, so an engine that sees a lot of use around the 400bhp mark would work very efficiently on this turbo.
The centre of the island is at around 2 bar on the absolute pressure scale, so your engine needs to also be making 400bhp in the midrange at around 1 bar of boost.
So this turbo is going to work well for a fairly efficient 16v 2 litre engine, or a more moderately tuned slightly larger one.
So something like the vauxhall LET (calibra turbo) engine will just about qualify with the correct manifold and cams etc, or something like a Nissan RB26 (skyline GTR) engine straight out of the box would also be ideal for it.
4) What conditions will it surge under Well, just in case anyone reading isnt aware what surge is, its basically when the turbo tries to supply more air than the engine is capable of swallowing.
This is observed by a "fluttering" noise coming from the turbo as the air forces its way back through the turbo, this is potentially damaging for the turbo if its a modern ball bearing one as the shaft can bend, and even on older tougher turbos it can present a problem with excess bearing wear or even damaged blades on the fans in the turbo.
As you can see from this picture, the very left hand side of the map (0 bhp effectively) is ALWAYS in the surge region of the graph for this turbo, this means that whenever you back off the throttle, this surge will occur, no matter what the spec of the engine, this is common to all turbos used on cars, and is the reason that manufacturers fit recicrulating dump valves or blow off valves, to allow this pressure to release harmlessly without coming back through the turbo.
So to avoid surge, you need to make sure that at no point in time do you allow the turbo to enter that region, other than as you are closing the throttle where you have dump valve to protect the turbo.
So how do you do that?
Well that is actually quite a complicated question, and its all about the correct spec of engine and mapping of your boost to ensure that you dont allow the turbo to ever create more flow than the engine can swallow at any time.
An example of this would be if you had an engine that was making
[email protected] bar f boost, this turbo just wouldnt be able to supply that without surging as if you look at 10lbs/min of air flow along the X-axis and cross reference it to the 2 bar absolute value on the Y axis you can see that its firmly inside the surge area of the graph.
So effectively, surge will occur if the turbo is put on an engine thats too small in terms of its ability to swallow air, which mainly relates to its size.
You CAN still use this turbo on a very small engine, you just have to ensure that it doesnt make boost too soon in the rev range by mapping your boost controller to stop it trying to.
5) Will it be laggy The simple answer to this is that you cant tell from the compressor map, it just doesnt contain information relating to spooling the turbo it only refers to flow once the turbo is spooled.
Slightly off topic but still relevant:
However while we are on the subject I would to point out that the term "lag" gets misused quite a lot.
The definition of lag that is the "correct" one is that its the time taken for a turbo to spool once the engine is within the boost threshold.
So what is the boost threshold?
Well basically this is the RPM at which the turbo is capable of being spun hard enough by the engine to make positive boost.
So for example, this turbo will require about 3500rpm on a LET engine (i know from experience not from the graph) to get it really spinning enough to see positive boost, now many people would therefore say if they put their foot down at 2500rpm that it was "lagging" for 1000rpm till it started to spool.
This however is a misuse of the term, you are simply outside the boost threshold at this point.
6) How do i know if its the correct turbo for my engine Well if you have read this far, then you are now fairly well equipped to answer this one for yourself hopefully.
The factors mentioned above about peak power, and optimum usage all come into play here, and very importantly so does the surge line.
If you had a 1000cc nova engine, this turbo would be of no use to you, you need an engine that can make at least a couple of hundred BHP by the time its seeing a bar of boost in order to work with this turbo.
Likewise if you had a 8 litre V10 it would be making the 620bhp this turbo can flow before it even came on boost, so the turbo would be absolutely useless.
So you need to make an educated guess (hopefully backed up with data from similar engines) as to what BHP your engine will make at various points in the rev range for different boost values and plot them accordingly on the graph and see where they sit.
Doing this for a high boost 2 litre engine such as the LET will yield a series of guesses on the map that show you if its suitable or not.
Here is Chip's take on roughly where an LET will land you on that map, im using the term "roughly" as many factors influence this, both in the spec of the engine itself and in terms of the exhaust housing used on the turbo and the intercooler used etc and even the outside temperature, not least of course is how the turbo is mapped!
So here are my "educated guesses" at some important points to plot on the map, based on a limiter of 8000rpm and using the engine with a set of cams that are suitable for peak power around 7500rpm
A) 1 bar @ 7000rpm = 400bhp
B) 2 bar @ 7000rpm = 600bhp
C) 1 bar @ 4000rpm = 275bhp
D) 2 bar @ 4000rpm = 400bhp
These figures have been rounded off of course, as we are only looking for estimates here, they dont need to be exactly accurate to get an idea for suitability.
So is the turbo suitable for this use?
Well the answer to that is pretty much a yes, but a tentative one as we are going to need to be careful with how soon we allow the turbo to start reaching larger boost figures, as clearly if our estimate that the engine can make 400bhp by 4000rpm on 2 bar of boost is correct, then it will mean that are risking surge from the engine by trying to make the turbo produce more boost than the engine can consume at that rpm efficently enough to match the flow requirements of the compressor.
The only way to REALLY know if the turbo is correct for this engine, is to build it and see of course!
PS
This is what my turbo that the map refers to looks like by the way:
