Z4 2.5i - not smooth enough?

K

k3ab

Member
I've had my z for nearly a year now. It is a 2003 2.5i with 47k miles on it.

Having previously had diesel cars A4 1.9 TDI and 2.0 TDI I've always found the Z4 to be relatively smooth through the rev range and gear range (once you get to grips with CDV). Just as I'd expect given various references on here to Z4 engines being "silky smooth". :)

I've recently had a loan car from the garage for the day - A3 2.0 Turbo. Seemed a LOT smoother than the z4.

Now I know there is the age difference - my Z is an 03 plate and the A3 was a 59 with 500 miles on the clock. But reading various forums on VANOS seal degradation causing lack of smoothness is getting me really worried.

Now I've never ever driven or even been in another Z4 (let alone a 2.5i which seems a little more rare than the 3.0). So a good experiment and a way to find out of my Z is indeed more jerky through the rev counter is to sit in another Z as a passanger.

The lack of smoothness I refer to is sometimes evident when driving relatively quickly and putting the power down in various gears. It is very difficult to describe it but if you think of power delivery as a nice smooth curve - the A3 2.0T seems to deliver that. Where as the Z seems to start off and have a few minor inconsistencies (dips and re-adjustments). Or maybe I am just not driving it properly?
 
Firstly it's a STRAIGHT SIX, not a V6.

Secondly, a turbo-charged engine can have the power curve smoothed as the ECU can make use of the boost from the turbo to hide/mask any high/low spot. Your Z4 can only do so much with natural aspiration.

Thirdly, when people refer to a 'silky smooth' straight six, they are usually referring to the harmonic balance of the engine, rather than the power delivery. Too smooth a delivery and the car 'feels' slower, too rough and it feels 'lumpy'.

If you try an ///M on light throttle, you will feel power steps at about 8 places through the rev range as the ECU & VANOS try to give you the best at each specific point.
 
Does it have a surge of power above 3k revs?
apparently that's indicative of the VANOS problem, from what I've read.

I worry about this on mine too, as it's a 53 plate and as far as I know hasn't been done, it has the occasional hiccup but I don't see a surge at 3k... might do the VANOS upgrade when I have some time and cash on my hands...
 
Well my 2003 with 45k on it is smooth throughout the rev range. I also drive an X5 with the same engine and that's pretty much the same and that has close to 100k on the clock. Yours is in reality barely run in. Of course vanos could be a problem, but you could equally be hunting a non problem.

I've driven many many cars and the experience from one to the other is so vastly different. Diesels give this amazing endless torque that can be mistaken for smooth and some turbos like the coopes S you just wait for the power then it's there all the way up to the top Rough as hell down low. The zed while the engine is smooth the drive can be anything from as it bounces around on potholes and tramlining, squirming under torque all makes it feel rather rough compared to a 4 door saloon.

I'd certainly echo the sentiment of trying others to see how it compares.
 
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