RS4 with milltek, without remap feels slow?

[Dom81]

Agree with Dave that it's a good idea - I'm just waiting for the right time (which possibly involves a sticking airbox flap) but I'd do that irrespective of the exhaust

Admittedly mine was fitted at the same times as a service and new plugs so it's impossible to benchmark, but I was running 0.5 secs faster over 3-8k. I'll probably settle for the fact that it might dyno slightly less do to the change in back pressure, but "driveability" offests this - the gain itself would then be the oil & plugs...

[neckarsulm]

what do you do with the vacuum pipes that go to the original exhaust valves when you fit a milltek?
Can some people leave them unblocked and lose power that way?

[Dom81]

what do you do with the vacuum pipes that go to the original exhaust valves when you fit a milltek?
Can some people leave them unblocked and lose power that way?

You'd need a pretty clueless fitter, but I guess every town has one! I'd already blocked mine and clipped them away when doing the exhaust mod, so never got to see MRC's approach.

Obviously if you have a valved Milltek then it's just a straight swap.

[ArthurPE]

what do you do with the vacuum pipes that go to the original exhaust valves when you fit a milltek?
Can some people leave them unblocked and lose power that way?

bingo

make sure they are plugged up tight

[adsgreen]

I thought a remap was only needed if you changed the cats?

Any fitment of a freer flowing exhaust will tend to weaken the mixture (due to the greater purge of spent exhaust gasses), so remapping after fitting a Milltek or any other freer flowing exhaust is "a good idea".

Posts here previously have suggested that fitting a Milltek can cut the power by 5-10bhp, which a remap more than restores.

Milltek don't say a remap is essential because the aim of their exhaust is not to increase power.

I'd be surprised if the ECU couldn't adjust the fueling to compensate as the difference a slightly free flowing exhaust should be well within the tolerances of the ECU's capabilities. Maybe its when back to back tests are done without having the ecu time to adjust the differences show up.

[adsgreen]

what do you do with the vacuum pipes that go to the original exhaust valves when you fit a milltek?
Can some people leave them unblocked and lose power that way?

bingo

make sure they are plugged up tight

YEah, thats why I hate vacuum systems... a problem anywhere in the system can cause havok everywhere. It's like old style christmas lights

[P_G]

I thought a remap was only needed if you changed the cats?

Any fitment of a freer flowing exhaust will tend to weaken the mixture (due to the greater purge of spent exhaust gasses), so remapping after fitting a Milltek or any other freer flowing exhaust is "a good idea".

Posts here previously have suggested that fitting a Milltek can cut the power by 5-10bhp, which a remap more than restores.

Milltek don't say a remap is essential because the aim of their exhaust is not to increase power.

I've never quite worked out why that is on an RS4. On the B6 V8 S4's a cat back would give marginal gains but never robbed power. Maybe it is the Milltek design for the RS4? In reality the largest restriction to airflow, the cats are still there.

I'm going to try and take mine to a r/r in the next couple of weeks to see what effects my non res cat back has; but the 3-8k tests are pretty much the same / improved.

[ArthurPE]

especailly on this car, it generates no/little engine vacuum due to internal EGR
it has an electric pump and a venturi across the TB

[sonny]

Maybe we should have another rolling road day just for old times sake lol

[Dom81]

Yes - about time too. Perhaps at MRC this time?

Interestingly, the view from Dave at MRC was that the B7 likes back pressure - hence him recommending 200 cell over 100 cell cats. I'll probably end up with neither but that's another story...

[ArthurPE]

mine is running like a scalded ape or the holy ghost in spiked reeboks
25F
low humidity
high atm pressure

I bet I have >30 more HP than a 80F/70%/low atm P day

I was shifting down from 6-5-4-3 and back just to hear it and feel the surge
hitting about 5k in 3rd, then up to redline
much to my wifes consternation

[mark758]

Generally it's a trade off; a free-er flowing exhaust fitted to a standard car will normally improve top-end power but also gas laws normally rob the car of some low end torque.
This last bit is the symptom that normally makes people describe the mod as making the car feel slower. It may well feel slower but in a time trial it most likely isn't.
Remapping n/a cars is usually addressing the fact that one of the input factors to the ECU is now always going to be different (ie rate of exhaust gas flow) so it starts off with the right base data (instead of trying to correct itself from what it expected), bingo better drivability.
Milltek always aim to increase power with their systems but I believe the power improvement claim got entangled in the RS4 vs RR bhp controvesy and then ran away

[adsgreen]

I don't follow why a 4 stroke engine would want any back pressure. Surely the ideal is for all exhaust gases to be expelled by tdc. Any restriction here is going to cause pressure in the cylinder making this process harder and less efficient. Secondly I'd have thought you'd want the exhaust gases moving as fast as possible so when the intake valve is opened early the exhaust gasses help draw in fresh intake gases. Again any restriction will slow this down making it less efficient.
There's no reason to have any exhaust (apart from noise, emissions) - could follow aircraft and drag racers and vent straight to the atmosphere - this surely can't be less efficient than having an exhaust? (unless other cylinder exhaust gases are creating some negative pressure effect... Not sure

[ArthurPE]

the back pressure should restrict exh flow during the overlap (intake begining to open and exhaust almost closed) in order to prevent any of the intake charge from making it to the exhaust, ie, short circuiting...
the cylinder vacuum (relative, actually pressure should be a bit less than the exh back pressure, so the intake will tend to go there instead of out the exhaust, thereby filling the cylinder and not losing air/fuel charge...

just like flow thru a control valve (the TB, intake/exh valves are just that) pressure drop is the primary variable and is controlled by the valve position...without a bit of diff pressure drop thru the system you would have no ability to control/regulate the system...

even an F1 exhaust, although open, is longer than it needs to be to promote flow and introduce a bit of press drop

[adsgreen]

I think f1 is a bit if curveball as there's lots of factors at play.
Heat is a real big problem (hence why all the problems with blown diffusers) and secondly the aero can help with exhaust flow as you say above.

Taking another extreme example, drag and tractor pulling engines vent straight through ridiculously short exhausts and the only objective is ultimate objective.

Be interesting to find out the amount of valve overlap on the rs4- I'm guessing it's an interference engine so makes scope for overlap small.