Decoking...

[aidanjaye]

But I reckon all FSi engines (from VAG) might be the same inside. Heard somewhere that Golf GTIs were having similiar probs but VW stated it wasn't hindering performance...but then again they would say that.

[UKS4APR1]

Ever since engine tuning began, the main source of power-gain from normally aspirated engines is in more efficient airflow to the cylinder. This is called gasflowing the head and involves removing metal and reshaping to provide an easier, less restricted flow. How anybody can think that the clogging-up occuring in the FSI engines can not be detrimentally affecting the performance of the engine is beyond me!
Just imagine an athelete suffering from athsma or a heavy smoker exerting himself, clogged-up airways results in lower performance!

[aidanjaye]

Totally agree with you...just wish VAG would.

[pippyrips]

Ever since engine tuning began, the main source of power-gain from normally aspirated engines is in more efficient airflow to the cylinder. This is called gasflowing the head and involves removing metal and reshaping to provide an easier, less restricted flow. How anybody can think that the clogging-up occuring in the FSI engines can not be detrimentally affecting the performance of the engine is beyond me!
Just imagine an athelete suffering from athsma or a heavy smoker exerting himself, clogged-up airways results in lower performance!

Agree 100%

[P_G]

I don't believe there is any doubting the theory, the actual is by how much? This is what no one can quantify because the normal method for comparison, a rolling road, is itself subject to so many variables that any one for those could lay credit or blame to the loss in power originally of the gain seen post de coking.

[SR71]

Not true P_G.

VAGCOM will tell you immediately the difference in airflow due to a clean versus dirty manifold in g/sec.

The difference is ~5-8%.

pippyrips & the MRC crew will be doing us a favour tomorrow and opening up his car again to see what 2000 miles has done to a manifold you could eat your dinner off.

All FSI engines will demonstrate the problem if they use EGR or vent CCV vapours back into the inlet...

Knowing that in some cases the deposition is bad enough to cause a loss of compression, in cars ~3 years old, this is, IMHO, a major issue.

[lengster1]

Dont forget some piccys when shes opened up guys

[P_G]

Not true P_G.

VAGCOM will tell you immediately the difference in airflow due to a clean versus dirty manifold in g/sec.

The difference is ~5-8%.

pippyrips & the MRC crew will be doing us a favour tomorrow and opening up his car again to see what 2000 miles has done to a manifold you could eat your dinner off.

All FSI engines will demonstrate the problem if they use EGR or vent CCV vapours back into the inlet...

Knowing that in some cases the deposition is bad enough to cause a loss of compression, in cars ~3 years old, this is, IMHO, a major issue.

So you say 5-8% difference in airflow but what does that actually mean in terms of power loss? Lets face it the internal combustion enging is not 100% efficient so that amount of air flow loss constitutes what exactly?

[SR71]

P_G,

How much power is sapped as a result of the airflow deficit will depend on ambient conditions etc etc.

The point is, the contamination shouldn't be there in the first place.

What we need in the public domain is the metric by which Audi determine whether such levels of contamination are acceptable...

[P_G]

That unfortunately is the million pound piece of information I imagine we are never likely to get as it will incur all RS and potentially FSI engine owners querying the state of their engines and a looking for the cleaning process against warranty.

[ArthurPE]

imho, the fouling does not impact power much, if at all?
why?
because the engine makes peak power at ~7800, yet revs to 8000+
implying that the throttle is not fully open at 7800, so to get more air, ie, peak power, and overcome the bit of extra pressure drop due to the gunk, all you have to do is crack the throttle (lessen pressure drop) a bit more...
also the cam timing/phasing is variable...but not lift

what are the biggest pressure drops in the engine?
air filter
throttle
inlet flaps
AFM
valves
and frictional losses in the tubing and manifold

side note: the valve is designed to have pressure drop, by virture of its purpose to redirect/swirl the gas stream...
the gunk may help mixing!

a good indicator on how much influence (pressure drop) the gunk (or any restriction) has is the velocity of the gas flowing through it
Bernoulli states Pd ~ V^2

how much air is moving thru the engine? (I'll use US units )
7800 rev/min x 1 min/60 sec x 1/2 (duty cycle) x 0.147 ft^3 ~ 9.55 ft^3/sec or CFS
or 0.6 CFS/inlet valve
Q or volume = 0.6 CFS = Area x Vel

V = Q/A
the open valve area is a bit difficult to calculate, it's an annulus obscured by the valve stem:
the lift is 11 mm, the diameter ~25 mm
I approximate the area to be ~0.012 ft^2

so V = 0.6 ft^/sec / 0.012 ft^2 ~ 50 ft/sec...that is not really fast...

if the area is gunked up 10%, the same air may flow, but a bit faster 55 ft/sec

what would actually happen is the air Q would drop, and the V would go up, to an equalibrium point so you may flow Q of 0.57 CFS at V of 52 ft/sec, or similar...

now, what is the pressure drop increase?
52^2/50^2 ~ 8%...seems like a lot

but since the valves are only a small % of the overall system drop (maybe 20% at most) the actual overall system change is only 0.08 x 20 or 1.6%...
so in theory (neglecting the over travelof the throtle, etc.) you may lose 1.6% of power, or ~7 HP...maybe

atm conditions (pressure, humidity/density, altitude, etc.), a dirty filter, etc. will have more impact...

[ArthurPE]

I just realized something...the gunk may have NO effect on flow...

area of valve ~0.8 in^2 less the stem diameter, basically the valve seat round opening
this does not get occluded due to the gunk

opening of annulus when valve is actuated... ~1.73 in^2

so the valve seat opening is by far the most restrictive part of the system...it's < 1/2 the area...

so even if the cylindrical opening made by the actuated valve is occluded by 10% due to gunk, it's still ~ twice as large...
it is not the limiting factor, the valve circular valve seat opening is...

the same Q or volume flows thru each opening...but since the area of the valve opening is much smaller than the opening created by the lift the Velocity thru it is twice as fast...

since the Pd is ~ to the square of the velocity, the Pd across the hole is 4 times as much as the annulus created by the lift...

so even if you lose a 1 mm of lift due to gunk build-up, ~10% of the area, it still is far less restrictive than the valve seat area...
like having a 1" orifice in front of a 2' pipe...the pipe is the least of your 'worries' lol

[pippyrips]

But what if the gunk builds up to such an extent that the valves can't close properly, wouldn't performance be affected then?

[ArthurPE]

But what if the gunk builds up to such an extent that the valves can't close properly, wouldn't performance be affected then?

none of the pictures show any gunk on the valve seats
it looks like it puddles on the valve when closed and gets cooked on
when the seat is exposed, it would be swept into the cylinder, most likely cleaning the seats...

and it may not be a big factor anyways, considering the overlap, EGR function and variable timing...

when you look at C & D's test of the RS4 and the e90 M3 it's hard to imaging these cars having different power (they are rated identically at 414)
1/4 ET 13 each
trap speed, the M3 is 1 mph faster, 108 vs 109 iirc

the e90 is 300 lbs lighter...

there is no way a car with 350 crank HP weighing 300 lbs more can be as fast as a car with 414 crank weighing 300 lbs less...not in this inertial reference frame


besides, the arguement made was that air flow is restricted by the gunk, and THAT limited performance...re: VAG afm measurments

[pippyrips]

You are right, none of the pictures shown so far show gunk on the valve seats BUT there are pictures of valves that seem highly likely to be 'leaking' - they are not from my car so it's for me to post them, If the owner is reading this and is willing to put them in the public domain............

Why else would you have two valves side by side, one completely caked in crap (as you would expect) and the one right next to it being almost almost clean in places? I appreciate you need to see the pics to evaluate but it looks highly likely the heat/flame etc is making its way up in to the inlet from the cylinder head, via the leaking valve and cleaning/burning off the buildup in doing so.


To a point, it does look like the buildup can be there without affecting performance to a notable extent. That said, I am certain the buildup doesn't just stop after a set amount - it keeps stacking up on top of itself the more the car is driven - so when does it become a problem? 20k,30k,40k,50k etc