Kick gone at 5.5k revs

[sakimano]

at 5000 rpm the stock S4 starts bleeding off boost. Without it, they make an extra 40-60 hp. You can see the bleed off with the more aggressive fall off in g.

I'd like to get a JHM tune/exhaust RS4 up as well as a TTS supercharged one etc. I started a thread about this on AR. Just fun to look at the stuff...I'm a bit of a numbers geek so I find it interesting.

[adsgreen]

thanks, interesting data
3k to 5k you have ~ 390 Nm
pretty close to the peak of 400 at 5.5-6k, less than 5% more

[/quote]
Thing is with a high revving n/a engine I've found peak torque to be hit lower down as you generally have less of it to begin with.
What really screw up the estimates is the cam profile change.

You can see the last limited line has 380, the next line with the throttle opened wider (but just below the cam change) is actually less torque and then it picks up again after the cam profile has finished changing (and the throttle wide open).

I can't see how having the throttle 50% open can do anything other than slow the car down. Even with a fixed amount of airflow required a more restrictive throttle opening will generate significant pumping losses.

[ArthurPE]

Thing is with a high revving n/a engine I've found peak torque to be hit lower down as you generally have less of it to begin with.
What really screw up the estimates is the cam profile change.

You can see the last limited line has 380, the next line with the throttle opened wider (but just below the cam change) is actually less torque and then it picks up again after the cam profile has finished changing (and the throttle wide open).

I can't see how having the throttle 50% open can do anything other than slow the car down. Even with a fixed amount of airflow required a more restrictive throttle opening will generate significant pumping losses.

a closed throttle (less area) will increase velocity
so for <5000 the air is sped up thru the TB inducing turbulance
per rev 4 cylinders intake air or each cylinder takes in air once per 2 revs (720 deg or 1 cyl/90 deg)
so for 90 deg only 1 cyl of air is passing through the TB, whose area at even 50% is much larger than the intake runner or intake valve(s)
the TB is not restrictive, if it were the engine would not rev to redline so freely under load

imo they limit the TB ang at low Q (~rpm) for controllability/turn down ratio
a TB is nothing more than a control valve, it controls speed and load/torque
too big is bad, poor control/throttling
for example a water system can use 1 large valve for good control, steam/gas, 1 large valve makes the system hunt, too broad a resolution
so they use 2 valves in parallel, one 1/3 size, one 2/3, operated in sequence, once the 1/3 is 100% the 2/3 starts to open
so good control low and high demand
imo the Audi limiting function is the same thing
if driving under low speeds (HP demand) the vlave is a '50%' valve, and in the high demand range (>5000 rpm) it operates 100% range

[ArthurPE]

to add to the debate I think the 317 lb ft is the max torque this engine can produce, I don't think it is limited
using Vagcom maf mass numbers vs theoretical swept volume the vol eff is 1.02 or so, very good for a na engine

T = (Cr x vol eff) x displacement / 4Pi = (12.5 x 14.7 lb/in^2 x 1.02) x 256 in^3 / 4Pi ~ 3818 lb in = 318 lb ft
I do not think the engine is capable of making more than this, and it does make this below 5000 rpm, ie, at 50% throttle, so 100% would not yield more
the only way to get more: higher Cr or improved vol eff (difficult to do on a highly developed engine) or by super/turbo charging

[adsgreen]

That applies in fixed cam setups - I have a race engine that is a dog below 5k rpm and is slower if you mash the throttle low down.
But when you can vary intake and exhaust cam timing this restriction becomes a bit of a sledgehammer. Shouldnt need to do this.

As for tumble... Not so sure. Look how far back the TB is from the cylinders. The air has to go into the manifold where there is a secondary airbox and then individual throttle trumpets for each cylinder. Any turbulence is going to be moot by then.

It's be good to see a before and after dyno plot of just tweaking the throttle position.

The other point.., why does the r8 allow 100% throttle with the identical engine (bar sump)?
(bit a dig, genuine curiosity)

[adsgreen]

Out of curiosity, where did the formula for torque come from?
I'm struggling to see how you can reliably calculate torque based on displacement, compression ratio and volumetric efficiency alone. For example, advancing timing would increase torque but the 3 variables above would remain constant.
Unless you bring in mean effective pressure i can't see how it would work.

[ArthurPE]

Out of curiosity, where did the formula for torque come from?
I'm struggling to see how you can reliably calculate torque based on displacement, compression ratio and volumetric efficiency alone. For example, advancing timing would increase torque but the 3 variables above would remain constant.
Unless you bring in mean effective pressure i can't see how it would work.

the Bosch handbook of automotive engineering
also here http://search.barnesandnoble.com/bookse ... d=10724546

mep ~ Cr x vol eff x atm P
the formula dictates the max torque an engine can produce assuming it is optimized
it can be moved around the curve, or by not optimizing the system the peak can not be achieved

you can also come up with the same value using the force on the piston mep x area and crank throw (moment arm)

[ArthurPE]

That applies in fixed cam setups - I have a race engine that is a dog below 5k rpm and is slower if you mash the throttle low down.
But when you can vary intake and exhaust cam timing this restriction becomes a bit of a sledgehammer. Shouldnt need to do this.

As for tumble... Not so sure. Look how far back the TB is from the cylinders. The air has to go into the manifold where there is a secondary airbox and then individual throttle trumpets for each cylinder. Any turbulence is going to be moot by then.

It's be good to see a before and after dyno plot of just tweaking the throttle position.

The other point.., why does the r8 allow 100% throttle with the identical engine (bar sump)?
(bit a dig, genuine curiosity)

R8 has 2 TB's each smaller than the RS4
and each will see essentially the same 90 deg flow as the single RS4 one but is smaller so doesn't need restricted

if it is not restricted and has 2 why doesn't it make MORE torque

[sakimano]

to add to the debate I think the 317 lb ft is the max torque this engine can produce, I don't think it is limited
using vagcom maf mass numbers vs theoretical swept volume the vol eff is 1.02 or so, very good for a na engine

T = (Cr x vol eff) x displacement / 4Pi = (12.5 x 14.7 lb/in^2 x 1.02) x 256 in^3 / 4Pi ~ 3818 lb in = 318 lb ft
I do not think the engine is capable of making more than this, and it does make this below 5000 rpm, ie, at 50% throttle, so 100% would not yield more
the only way to get more: higher Cr or improved vol eff (difficult to do on a highly developed engine) or by super/turbo charging

how realistic is 1.02 for a stock engine though? Having spent some time looking at stock exhaust (this car, other cars) I can promise there's some inefficiency there, much of which is to satisfy noise and emissions levels.

[sakimano]

It's be good to see a before and after dyno plot of just tweaking the throttle position.

some of the tunes out there may qualify lol

but seriously I think silverRS4 (no idea what his name is) did something like that. Just eliminated the torque limit with some sort of flash (throttle limit). I may be losing my mind but I think it was a custom tune of some sort? It was a couple of years ago.

Here's something interesting he posted up




This was also interesting...check the 'actual torque' which we can log on vagcom. Difference between 'actual torque' logged on the dyno (GREEN) during a pull vs.actual torque logged on the highway (BLUE) excellent IAT in fourth gear is huge.

[ArthurPE]

how was the vol eff calculated?
can you set up an excel spreadsheet
col 1 rpm
col 2 calculated mass air flow from rpm, displacement and an air density using temp, humid, atm P ~ the same as the following Vagcom run
col 3 actual Vagcom measure mass air flow
col 4 = col 3 / col 4 in %

I have this at home, want to compare

edit: just did it using saki's data from this post, now to figure out how to post it

[klauster]

Needed a new airbox flap solenoid.. all fixed

[P_G]

Good to see it was a simple job. Sometimes is. Saw a black saloon yesterday lunchtime at the Wheel Specialist when picking up the wife's refurbished summer wheels and the owner of the RS who wasn't there at the time had said in conversation with the owners of the franchise that his exhaust valves were broken and there was no kick at 5500rpm. No wonder when the pipe to the right exhaust valve was snapped off (actuator nozzle the hose attaches to still in the hose!) and hissing away like a good 'un.

[klauster]

Good to see it was a simple job. Sometimes is. Saw a black saloon yesterday lunchtime at the Wheel Specialist when picking up the wife's refurbished summer wheels and the owner of the RS who wasn't there at the time had said in conversation with the owners of the franchise that his exhaust valves were broken and there was no kick at 5500rpm. No wonder when the pipe to the right exhaust valve was snapped off (actuator nozzle the hose attaches to still in the hose!) and hissing away like a good 'un.

lol oh dear, owner is a numpty

[P_G]

Just like the EVO editorial, you kinda hope that an RS4 owner knows their car because you do but shouldn't be surprised if they don't!