Pressure ratio

[Koomorph]

When calculating the Pr for reading compressor maps should you use your peak boost or the boost that your turbo holds?

I.e - I peak at 20psi, but hold 16.....

Thanks
Raf

PS also anyone know the volumetric efficiency of the 1.8T lump?

 

[Songman]

When calculating the Pr for reading compressor maps should you use your peak boost or the boost that your turbo holds?

I.e - I peak at 20psi, but hold 16.....

Thanks
Raf

PS also anyone know the volumetric efficiency of the 1.8T lump?

For the purposes of calculation in the equation you may be using, you could assume 100percent - the 1.8T head flows well

Compressor maps are formulated under strict conditions of temperauture and pressure, so it`s not possible to relate them, with any degree of accuracy, to readings you get on the boost gauge in the car - because, for one thing, the gauge reads only pressure, and doesn`t factor in considerations of the temperature of the inlet charge.

If the compressor is incorrectly sized for the task in hand - if for instance you ask it to operate outside its, often, very narrow area of peak efficiency - It`s very possible to get an indication of increased boost, as a consequence of a large increase in compressor exit temperature ,but a drop in effective power as a consequence of the decrease in the actual density of the inlet charge

According to my calculator, the 1.8T at a PR of 2, 100 pc VE, and a ceiling of 7500rpm generates a flow rate of aprox 470CFM - you then check through the available maps to see which one, most efficiently meets your requirements

That of course is only the tip of the iceberg so to speak! - to make it all work at all well, you then have a lot of thinking to do with regards to flow considerations the hot side of the turbo - turbo development from scratch is only for the brave[wealthy!]

All in all `stuff` which is well outside the available resources of all but the most talented and dedicated amateur - there`s no need anyway because there are a number of well tried confirurations available off the shelf - IHI Garrett,KKK

It`s always worth remembering that what the compressor map says, is not what the engine does on the dyno, which again is not what the car does on the road when driven by Mr `X` as opposed to Mr`Y` - a car`s ability to perform effectively in any given arena, is the sum of many factors only one of which is maximum power output - a car certainly is ,far more than a headline power output figure on a dyno chart - sometimes more is less!!

 

[Koomorph]

Totally understand your points dude - and fully appreciate the numerous variables invovled. Plus that compressor maps are calculated at a static pressure/temp/density etc… and don't take inot account these and other factors. Seconding what you write - power increase isn't just about upping the boost (I for one am a real advocate of head work even on F/I cars - something which a lot of people overlook - you can run same boost, but power levels will go up as charge density and temp will be unaffected). I was just wondering whether I should be using (based on 20 and 16psi) Pr values of 2.4 or 2 when looking at the maps. Is it a case of plotting it on based on Pr at certain rpm? I.e 2.4 x 3250rpm but 2.0 x 6500? Or even less as I'm only running 0.9bar after 6K (not verified by the boost guage, but by CC when they did the mapping).

Raf

 

[Songman]

Totally understand your points dude - and fully appreciate the numerous variables invovled. Plus that compressor maps are calculated at a static pressure/temp/density etc… and don't take inot account these and other factors. Seconding what you write - power increase isn't just about upping the boost (I for one am a real advocate of head work even on F/I cars - something which a lot of people overlook - you can run same boost, but power levels will go up as charge density and temp will be unaffected). I was just wondering whether I should be using (based on 20 and 16psi) Pr values of 2.4 or 2 when looking at the maps. Is it a case of plotting it on based on Pr at certain rpm? I.e 2.4 x 3250rpm but 2.0 x 6500? Or even less as I'm only running 0.9bar after 6K (not verified by the boost guage, but by CC when they did the mapping).

Raf

You should use whatever combination of inputs you require to give the theoretical flow at any particular point - though what practical use this will be to you, i`m not sure - you have a pressure reading but no indication of temperature - that 0.9bar could be at 50+deg C!

When you start a project, you calculate the maximum theoretical air flow potential of your engine - having determined that, you then find a compressor map which most efficiently provides that required airflow, in a manner suitable for your particular use.

How you are best able to maximise the potential of that theoretical flowrate, then determines the practical success or failure of your engine work.

Of course it`s all downhill really - frictional losses in the inlet tract perhaps with poor/restrictive intercooling, bad port design in the cylindehead, inadequate valve area - inadequate combustion due to poor chamber design, failings with mapping and mixture control - restrictions in the exhaust side, poor manifold design - and all the rest of it!!

 

[Koomorph]

Lovely stuff - thanks mate - yor first point is exactly what I want to do .... I love graphs..... ;o) Jsut wnated to see if at 3250rpm I'm getting close to the surge line on a particular map at 2.4 Pr..........

Cheers
Raf

 

[Feel]

I haven't ready all of this thread, but we had a chat about VE etc in here:

http://www.seatcupra.net/forums/showpost.php?p=1181738

About page 8/9 is where is gets good.