appletree wrote:You can not see the surge on the digital pressuer reader as you said but i rigged up a spare turbo gauge and you could see it on that, pulsing when i held part throttle with the dump valve shut to make surge (you can hear it to)
but not a hint when i used full throttle the take off for this experiment was the one on the charger outlet
Mine and e30_turbos intercoolers are the same, wonder if hes try'd anything like this on his to see what the drop is
Excellent work Matt! Brilliant stuff. Get ready for a long read as this is going to be a long answer. Please take the time to read this carefully.
The simple fact is, the only way a smallish engine can make a whole lot of horsepower is by flowing a whole lot of air. Sadly, the pressure drop across your intercooler is an exponential function of the amount of air you pump through it. I must however say that I am quite surprised at the magnitude of the pressure drop across your intercooler - a whole 3.7 psi at 5500RPM! That's just over a 1/4 bar of boost that you are losing. Or looking at it another way, your compressor is having to work 1/4 bar harder to give you the same hp without the pressure drop. I expected to see closer to a 1psi drop at 5000RPM.
However, I don't believe that this pressure drop is due to any restriction imposed upon your flow by the intercooler.
Now for the science bit:
When you compress a gas in a closed volume it gets hot. We call this
adiabatic heating (assuming no other heat transfer in or out of the volume of gas). Put simply, the air coming out of a turbocharger or supercharger compressor is
hot because the compressor has
squashed it.
The converse is true if you heat a volume of gas in a closed volume - the pressure rises. And likewise, and this is the point,
if you cool a volume of gas you will lower its pressure. Matt, do you see where I am going with this? The air coming out of your intercooler is at a
lower pressure because it is at a
lower temperature.
So, what does this all mean? It means that, if you want to lower the
pressure drop across your intercooler you need to lower the
temperature of the air coming out of the compressor. Since you can do nothing about adiabatic heating, your only option is to do something about the efficiency your compressor. This is why, in every compressor installation I have been involved in, and in every text book I have read, the objective has always been to run the loadline up through the zone of maximum efficiency on the compressor map. This reduces the temperature of the air coming out of the compressor for two reasons:
- 1) The compressor puts less heat into the air as a result of improved pumping efficiency, and
2) because of the reduced pressure drop across the intercooler the compressor doesn't have to work as hard in the first place to achieve the demanded manifold pressure.
Since you already have a nice hole at your compressor outlet, I would suggest that your next task is to insert a low heat capacity thermocouple and measure the temperature of the air coming out of the compressor. I say low heat capacity because you don't want to have to wait all day for the thermocouple to warm up while you drive around at 5000RPM
appletree wrote:What dose this mean in real terms in relation to surge? I'm guessing it woun't be helping any as its more of a restriction.
The only way I can really comment on what this will do to your problem with surge is to re-plot your loadline using the new data you have presented (possibly you don't realise just how valuable this data is, so good on you for making the effort).
However, at this point I will say the following. Closing the throttle is effectively reducing the air flow through the system. Doing this while making boost is a bad idea since it pushes your loadline right over to the far left of the compressor flow map (deep into surge). The situation is exacerbated by the fact that you are already operating your compressor just to the left of the surge line. Ideally, by operating from the middle of the compressor map you have a reasonable margin for error to the
left of your loadline. This means that you can make some boost while you take your foot off the throttle, meaning that the blow-off valve doesn't have to activate instantly. In your case, however, you have no choice. Your blow-off valve
must engage the instant you back off the throttle to prevent your loadline from moving to the left.
And by the way. The sort off surge that you can actually
hear over the engine noise and
see on a boost gauge is the sort where the air flow around the compressor wheel has actually begun to stall. This is
chronic surge. Don't do this, its deadly. The sort of surge I have been describing to you largely goes unnoticed and occurs physically as a small amount of turbulent air flow along the aerodynamic surface of the compressor blades. The subtle vibrations that this turbulent airflow causes in the blades will eventually cause them to fatigue, resulting in catastrophic failure. It might not sound like much, but believe me, when those blades are turning at 100kRPM, that small amount of turbulence is like an elephant massaging your head with its bum. You wouldn't attack your wheel rims with a jackhammer while they are spinning, so why do it to your compressor wheel?
Food for thought. Thus endeth todays sermon.
