V10 RS6 - Excessive turbo lag

[Limited100]

I've got a C6 RS6 V10 and it's really laggy, just wondering what technicians' thoughts are on the possible cause.

Current spec:

Secondary cats removed
BMC filters
Wagner intercooler
Dodgy stage 1 map (non-MRC), seller told me it was stock map

(First run it made 20 PS less and MRC tuning fixed some vac leaks, it pulls stronger at the top end now but won't be driving it much until it's seen to, I'm worried about turbos going and taking the new Wagners with them.)

Should be hitting peak torque at 2.5k not 4k, turbos maxxed out at stage 1 so guessing a boost leak or something else. Because they're maxxed out we didn't try a stage 2 map.



For reference, this is dyno plot of a stage 1 RS6 with stock hardware!



Anyone seen this amount of lag before?

Gutted as I've already put a lot of money into this project and seller lied about servicing, it's needed loads of bits rectifying and despite saying it had spark plugs done a year ago, on inspection they're stamped 08!

Also oil on the spark plugs so some leaking in the cam cover.

Stage 2 will have to wait.

[GreenV8S]

If the engine and turbos are healthy then I'd have thought a boost leak, or leaking wastegate were the most likely suspects. A boost leak would also explain the turbo maxing out. Since you have a non-standard intercooler and the previous owner seems a bit unreliable, it would be worth eyeballing all of that. Perform a smoke test if you can't see anything wrong.

[Limited100]

GreenV8S said:

If the engine and turbos are healthy then I'd have thought a boost leak, or leaking wastegate were the most likely suspects. A boost leak would also explain the turbo maxing out. Since you have a non-standard intercooler and the previous owner seems a bit unreliable, it would be worth eyeballing all of that. Perform a smoke test if you can't see anything wrong.

Thanks, I just hope this inspection can be done without removing the engine, although that's unlikely with these units.

[stevieturbo]

Surely whoever has it on the dyno would have some input ? And how are you determining the turbos are maxed out ?

If it is a dodgy remap, why not have either standard, or a proper map created for the car ?

Is boost control operating as expected ?

And the obvious other things suggested, blockages ( intake or exhaust ), leaks, rag in an intercooler etc etc

[paul_c123]

I'm surprised the charts don't have lambda plotted on them too.

First thing would be to diagnose the issue. you can do a smoke test to look for boost leaks but often you don't need to, as the ECU will log a code and/or show boost pressure on live data. My fear would be if the map is so badly out, the engine is leaning out at WOT and that's an issue for longevity. After all, if the intercooler has been upgraded the mass of air at a particular rpm/throttle/boost level will be greater than the original setup would have been. Sure, there's sensors etc but if the WOT puts the ECU into open loop.....

[TGCOTF-dewey]

Lag or boost threshold.

You seem to be describing threshold rather than lag. Two different issues with different solutions.

[Heaveho]

Has this developed during your ownership, or was it behaving like this from day 1 with you? If you've fitted the intercoolers, the obvious place to start is with the hoses and fittings at the connections there and anything else that was disturbed to fit them.

[Limited100]

Heaveho said:

Has this developed during your ownership, or was it behaving like this from day 1 with you? If you've fitted the intercoolers, the obvious place to start is with the hoses and fittings at the connections there and anything else that was disturbed to fit them.

It's always been a bit laggy, before and after the recent uprated intercoolers. Mechanic didn't see any leaks when fitting them.

[Limited100]

stevieturbo said:

Surely whoever has it on the dyno would have some input ? And how are you determining the turbos are maxed out ?

If it is a dodgy remap, why not have either standard, or a proper map created for the car ?

Is boost control operating as expected ?

And the obvious other things suggested, blockages ( intake or exhaust ), leaks, rag in an intercooler etc etc

He gave loads of input, and managed to fix a vac leak at least. I think they were able to measure turbo 'duty', saying it was '9x%' which is apparently way too high.

Could have a cat blockage although he'd never seen that on these cars.

[paul_c123]

Limited100 said:

He gave loads of input, and managed to fix a vac leak at least. I think they were able to measure turbo 'duty', saying it was '9x%' which is apparently way too high.

Could have a cat blockage although he'd never seen that on these cars.

Its wastegate duty cycle. If its a high value, what it means is the wastegate is open more often than not, resulting in lower boost. However its only a symptom of some other issue, the wastegate is being commanded to open for some reason by the ECU. Live data with some pertinent values would help to progress diagnosis further, but it does sound like the ECU map is wrong and the ECU is falling back to some failsafe mode (not limp-home mode, but a similar idea - and also not illuminating the check engine light). Or it could be a sensor giving erroneous values, or simply not reading due to being unplugged or wiring damaged etc.

Needs a proper diagnostic session rather than guessing here and there.

[stevieturbo]

paul_c123 said:

Its wastegate duty cycle. If its a high value, what it means is the wastegate is open more often than not, resulting in lower boost. However its only a symptom of some other issue, the wastegate is being commanded to open for some reason by the ECU. Live data with some pertinent values would help to progress diagnosis further, but it does sound like the ECU map is wrong and the ECU is falling back to some failsafe mode (not limp-home mode, but a similar idea - and also not illuminating the check engine light). Or it could be a sensor giving erroneous values, or simply not reading due to being unplugged or wiring damaged etc.

Needs a proper diagnostic session rather than guessing here and there.

Pretty much.

Sounds like lots of potential issues, and some proper testing and inspection needs done.

[Limited100]

I've just been through the service history and previous owner's mechanic found, in 2023, that "PCV pipe to turbo nearside blowing both sides needs replacing". I assumed they'd have rectified this but if not, that's the next area to investigate.