What does boost numbers tell us?
 

My 1998 A4 1.8T B5 AEB started out with the stock ECU, K03 turbo, 7 psi (?) stock boost and stock 150 HP.
Then came the TAP "12.5 psi +50HP" chip.
The failing K03 was replaced with a an optimized, custom built K04, and a boost gauge added. We logged a max of 17 psi boost (40 degree air temp), but often only got to 15 psi (max at 4000 rpm).
We now added FMIC. With the stock ECU, we still see 7 psi. With TAP chipped ECU, we now see 18.5 psi (in 80 degree air).

a) If the stock has 150 engine HP, how much can be expected from the "TAP +50HP"= chip)? Are there before/after dyno charts out there?
b) Assuming the K03 with the TAP chip had 180 HP at 12.5 psi, to what engine HP could I extrapolate the engine HP now running 18.5 psi?

Is there a rule of thumb (everything else equal) how much HP come with increase in boost for our engine?

since every turbo is different there is no general rule, but a VERY rough estimate would be ~7hp/psi

No matter what the motor ran stock, a chipped stock turbo will put out about 180whp (less for awd) with nothing else and around 200whp with a turboback (again, less for awd).

Gains/psi depends on the size of the turbo. A turbo that can flow 600hp will make more power/psi than a stock turbo rated at 195hp.

to answer your original question. Boost is the pressure of the air being forced into the intake over atmospheric pressure. Naturally aspirated engines can be thought to run negative boost because they create vacuum at atmospheric pressure.

I was looking for some relation between boost pressure (PSI) and engine power (HP) for our (1.8 AEB) engine.

I am sure we have some decent figures/estimates already for the our engine regarding the relationship between airflow (mass, in lb/min or so) and power (HP).

Knowing the "losses" (flow restrictions, compressibility, density, etc.), we could find an estamated relation between intake pressure (PSI) and power (HP).

Or: some empirical data from dynos that show power (HP, Torque) and intake pressure.

Note: the engine does NOT know HOW the pressure (psi) was 'made'.

My issue: its easy to get the pressure, but only a dyno can give power. Having a curve/table that shows torque as a function of psi or data for power at psi would help greatly.

It is really hard to say on a K03/k04 since they dont really hold that peak boost thru out the power band or even to the peak hp rpm.

PSI does not equal air flow(CFM) and it is the amount of air that makes power not how much boost your pushing. So a K03 pushing 18psi is going to flow a very small amount of air compared to a GT28rs pushing 18psi, that is why there is a huge difference in how much power those 2 make at the same boost level.

On a stock MAF setup you can look at the maf G/S readings to figure out your hp and the power curve.

As for your boost going up on your TAP tune when you installed a FMIC, just sounds like your stock SMIC was clogged so bad that it was a huge air flow restriction which would cause the pressure after the SMIC to be much lower then it should have been. Only way to know that was the issue was to measure the pressure at the SMIC inlet vs the outlet.

PSI is in deed directly related to air flow (CFM) and therefore power.

a) If intake pressure is 18 psi at 4000 rpm, the same engine will ALWAYS make the same power, no matter if the pressure was generated by a K04, a GT2871RS or COMPRESSED AIR TANK.

b) Ditto 10 PSI at 6500 rpm. Pressure is pressure. Pressure forces air through restrictions (here: engine), resulted in a flow (which is an almost linear function of torque/power).

c) Certainly, various turbos (large, small) have various BOOST CURVES (and a monster turbo may in deed be capable of generating high boost pressure AND high volume (i.e. high pressure at high RPM), while a tiny K04 may peak earlier and cannot provide very high pressure at higher volume (RPM).

So, aside from the "flow restrictions" that may be slightly different from engine to engine (mine is an AEB, no other modifications on the intake side, exhaust wide open), a dynochart with boost pressure would help me a lot.

A GT2871r can hold full boost to redline, even at 30psi. Now compare that to a GT30r and a GT35r also holding 30psi and I can tell you that your little "boost = air flow" pretty much gets shot to crap. But hey its not like I know anything seeing that I have done everything from K03 to a GT35r making over 800hp.


Even at the same boost level a K04 is going to make more power then a K03 and it isn't like either of them are all that much different in size so the boost curve is pretty much exactly the same.

AEB does not have any restritions, I run a AEB head on my car and am making over 800hp.


If you want dyno charts just do a google search for 1.8t dyno charts, there are tons of them out there.

Here you go, this is a GT30r on a A4.
http://gallery.034motorsport.com/d/2...inal93Tune.jpg

Now please show me a A4 1.8t quattro GT2871r making this power at just 25psi on pump gas.


Same car on 100 octane and a little more boost.
http://gallery.034motorsport.com/d/2...oct_465whp.jpg

The dyno charts I have found so far do not have the corresponding boost curve in it. I would need Torque over RPM AND Boost over RPM in the same chart, or HP over RPM AND Boost over RPM.

Note: Everything has restriction (= friction). Otherwise, an air pump (like a turbo) would not built any pressure but just flow whatever it can move), even a straight pipe.

The shear presence of pressure is proof of restriction. Think of your compressed air tank, open the valve: there is so much restriction that the it takes a long time for the compressed air to flow out of the tank. Naturally, the less restriction, the quicker the air flows out.

Also, the higher the pressure in the tank (=boost pressure) the more air is forced through the restriction (engine). The restriction increases with flow (likely with exponentially with the power of 2?), i.e. you need 4 times the pressure to flow double the air (double the power).

As a rough estimate (purely based on physics, not on combustion engine specifics), an engine making 150 HP at 4000 RPM with 18 psi intake pressure would need 64 psi intake pressure (at the same intake air temperature) to flow twice the amount of air through the engine (and produce 300 HP at 4000 RPM). Certainly, it would be better to talk about air mass instead of air flow (since air mass makes power, not flow per se).