Audi v8 down on power??

[Donick1980]

Hi there, my name is nick, I’ve owned my s5 for about 9-10 months now and I’ve genuinely had trouble free time with the car so no need to pic anyones brains lol

so to my problem... I’ve just had car dyno’d basically to see where car was at After a carbon clean (proper one).
Car seems to be about 50 bhp down from the last time it was dynod 5 years ago.
I have a graph from MRC from 2016 when car made 372ps and 480 nm of torque, I think this was achieved after a carbon clean and map tweak.

so tonight it has made 318 bhp and 440 nm @ power station in Tewkesbury.

I know there different dynos but 50 ish bhp difference seems excessive and to be honest the car has never really felt (to me) like it had 370 bhp...

First thing I’m gonna do is get it back to MRC for health check where I believe it will be dynod again.
i’m just wondering if anyone had any ideas what could possibly be the cause for loss of power?

only probs I’ve had with the car is one time last august car completely lost power while driving on motorway, pull on to hard shoulder then tried to restart.... would just turn but not fire. Until 10 mins later sprung back into life then drove perfect again for 5 or so miles then did it again. Did this another 3 times before I got off at services and brimmed it with fuel (already had half a tank). Had diagnostics done when I got home and not a single fault??? Until today it hasn’t done it again, but when operator of dyno pulled it on to the rollers it cut out ...EXACTLY the same as before, would not fire after numerous attempts ?

so guy plugged diagnostics tool in while it was sat there, surprise surprise no faults...
So we left car for 5 or so mins then tried again and fired up no probs.

only Other thing I’ve noticed with car is it seems to take quite a few cranks to start and this is the same whether hot or cold.

which is leading me to think fuelling prob somewhere? Car seems to run perfect though with no misfires and no codes ever popping up??

any input will be greatly appreciated 👍

nick

[Leon R]

Carbon build up on the valves would be a good place to start again since you have no faults.

[catman]

Leon R said:

Carbon build up on the valves would be a good place to start again since you have no faults.

He said that he's just had that done, prior to the dyno test.

[Leon R]

He did indeed. I miss read that it was before he purchased at the MRC dyno.

[samoht]

Crank position sensor could be on its way out?

[Donick1980]

Hi guys to confirm it has just been carbon cleaned, it was previously carbon cleaned at MRC and that was when it dyno’d approx 370...
I have a feeling it has been down on power since I’ve had it, the carbon clean made it a little livelier but it’s never felt quick... had a remapped 335i previously and that would’ve destroyed the s5 in a straight line.
Strange thing is the s5 feels quicker than my old s4 v8 ??
I could do all this digging around, looking at/for possible fuelling issues that may not be there and it could just be I had some st fuel in it🤯

I’ve got my mechanic to check fuel pressures this tues and also change fuel filter (which is in tank so may well have been neglected by not changing it) and give it general service.... prob get him the also check coil packs while he’s there...

I’ve also got it booked in at MRC on the 29th for a health check on their dyno, the one where it originally was mapped.
Should know what’s going on a bit more then?

Some pics of carbon clean before and after:

[Donick1980]

samoht said:

Crank position sensor could be on its way out?

You’re not first person to suggest this.... eerrrmmm??

[Donick1980]

[AJB88]

Literally could be Dyno lottery or inflated figures by the tuning company in the first place (I'm aware that MRC are well respected though).

Back in 2009 I had a Leon FR 2.0TDI PD170, had JBS do a Stage 2 DPF delete on it, made 238bhp on their rolling road, it then made 204-205-206 on 3 separate independent rolling roads.

[Donick1980]

Yeah know these dyno numbers shouldn’t really be compared, it just 50 bhp seems a great deal to be out? There’s a small part of me hoping it is 50 horses down coz if I can find them, then the car will be all I’ll ever need ...

I remember when I just added a straight through from turbo 2.75” exhaust to my fiat coupe 16v turbo back in the day and it added 50 bhp and the difference I felt was phenomenal. 😍

[Kawasicki]

How quick is it in gear compared to the magazine tests? I usually dyno my cars by checking whether they will hit the expected top speed... but you need a lot of space for that.

[Donick1980]

Yeah was gonna try this, so measuring say 60-100 ? Torque stats quotes stock car at 6.5 sec 60-100... I actually think mine is quicker than that.... Dry toMoz aswell

[Kawasicki]

Donick1980 said:

Yeah was gonna try this, so measuring say 60-100 ? Torque stats quotes stock car at 6.5 sec 60-100... I actually think mine is quicker than that.... Dry toMoz aswell

Yep. I’m sure there’s an autocar report with in-gear times available. I’ll have a look at the German mags in the morning, if I remember!

[Donick1980]

Let me know what you find .... if you remember lol..

[thebraketester]

I would start by getting someone to do some data logging on it.

[ghost83]

Any chance the mrc dyno was bhp and this one is wheel horse power?

Transmission losses would see around 50bhp

[njw1]

Donick1980 said:

samoht said:

Crank position sensor could be on its way out?

You’re not first person to suggest this.... eerrrmmm??

Not on an Audi but I've had cam sensors be that far out of tolerance that the engine would bog and stall when pulling out of junctions but not actually log a fault code. Crank sensors I've found either work or they don't...

[stevesingo]

You are doing the right thing by going back to MRC, comparing results from different dynos, on different days, with different operators is pointless.

Be aware that a dyno operator could, by changing the test procedure, how the vehicle is tied down or any of the variables, could easily get two results from the same car on the same day which are15% different.

[Donick1980]

ghost83 said:

Any chance the mrc dyno was bhp and this one is wheel horse power?

Transmission losses would see around 50bhp

318 was deffo power station’s quoted flywheel power, there dyno (Maha) gives massively low wheel hp reading apparently due to there being 2 contact points for the each tyre .... it’s twin roller front and back.

Wheel power was actually 218.5, this is due to claimed drag loss of 105bhp using there coast down method?🤯

[Arnie Cunningham]

Absolute numbers from dynos can be quite variable depending on the operator and the piece of kit it is. I’d really only pay attention to comparative numbers on the same dyno.

You’re talking to MRC. They have an excellent reputation on these motors, I would just work through it with them. On my RS6 there’s basically only 2 or 3 places worth going to and MRC are one of them.