Help Me With Some Turbo Related Maths

[B-Man]

It means that the pistons are sucking in air faster than turborcharger can blow in air (yet).

[R33LIN]

sorry to hijack this thread, but on an aftermarket gauge on idle for rb25 how much vacuum should you see?

and is there anyway you can tell that you have a vacuum leak or a leak of some sort!

hopefully someone can help!

cheers in advance!

[Richieb]

Here's my answer.

From what I can remember, F1 cars have a choice of the turbo or not.

They have a calculation for it, but am not entirely sure what it is.

[discopotato03]

Wasn't going to get involved in this one but here goes .

For a start please delete all notions of "negative pressure" and pistons or engines "sucking" . Firstly negative pressure doesn't exist - its as simple as that . You have a total vacuum or zero and everything above that is positive pressure . We have been brought up with this brain dead notion that atmospheric pressure at sea level is zero and anything less - is negative ?

The technical terms are PSI"A" meaning referenced from absolute zero and PSI'G" meaning PSI Gauge pressure referenced from sea level atmospheric ie ~ 14.7 PSI/1000 millibars .

Your Skylines manifold gauge is whats called a compound guage because it measures either side of 1 BAR or 1 barometric which is 1 atmosphere at sea level .

If you look at turbocharger compressor maps you'll generally see a reference on the left hand side of the map and the units used are pressure ratio (PR) . This is because 1 PR = 1 atmosphere = 1 BAR = 14.7 lbs/square inch = 1000 milibars .

The map really only tells us anything useful once the turbo can generate pressure greater than atmospheric and thats why the scale starts at 1 PR .

Start two . Engines can't "suck" . What they can do is generate an area of lower pressure than atmospheric so the atmospheric pressure being higher forces its way into the cylinders - minus any restrictions that hinder its progress . This is the reason why its so important to have low restriction filters and everything else between the atmosphere and the cylinders themselves .

NA competition engines can have high volumetric efficiency built into them but it doesn't fit in with the noise/emissions/power characteristics in road car engines . Supercharged engines (forced induction) can easily get good cylinder filling because they have greater than atmospheric pressure at times in their inlet manifolds so the restrictions down stream of the compressor are less of a problem innitially anyway .

The whole reason to supercharge a piston engine is to get higher torque that the same sized NA engine can develop . These days engines are far more complex than they used to be so turbocharging is not as simple as it once was .

In my mind I think a better measure of forced induced vs NA performance is torque and all engines are different on that score . For example a 4.5L 4 cam Jap V8 may make better overall power than a Holden 5L iron nob but both will be way ahead of your RB25T at lowish revs .

Turbo or mechanical supercharging is not a free ride and they create as many problems as they solve in high performance tune . One of the biggies is that once you get more air and fuel in to make higher cylinder pressures you get a higher volume of exhaust gas to have to deal with - bigger engine exhause flow . In some ways this is the most important aspect to deal with

because even though the engine only operates in a supercharged state some of the time its exhaust tract has to cope with high mass flow all of the time . Its a big ask to drive a healthy sized turbine that won't create unacceptable restrictions and have the engine responsive as well .