RE: Mercedes A45 AMG: Review

[kambites]

Dave Hedgehog said:

kambites said:

Any turbocharged setup on a rod car is going to have response issues to some degree or other. The question is how much.

twin scroll turbos have made a huge difference to pick up response in turbo's the A45 produces peak torque from 2250 revs which is not to high

Ah, now I understand what you're talking about. You're talking about boost threshold rather than turbo lag. Completely different thing.

Turbo lag exists everywhere in the rev range and is in fact more pronounced transitioning from closed to open throttle high up the rev range when the turbo is working harder. I'm sure this has a commendably low boost threshold, most modern turbocharged engines do.

[Dave Hedgehog]

kambites said:

I wonder why 50/50? If you're going to go to the effort of powering the rear wheels all the time (with the associated drive-line losses that that gives) I'd have thought that you'd want no more than 50% to the front and a "typical" distribution of something like 30/70 F/R.

Out of interest, why would you consider heavy traffic to be bad for turbo lag? In my experience it makes itself most obvious transitioning onto the throttle coming out of fast corners.

in heavy traffic (commuting) you will not be "on it" you will be going with the flow, that will mean the tallest viable gear and revs near 1k, the worst place to be for a turbo engine with lag. On an old school turbo charged car a 1.0 fiesta would pull away from you in this situation. Mercedes have set the box so that it will drop down enough cogs so that you can build speed quickly if you wish to rather than sitting there waiting for it to come on power.

[kambites]

Yes, I understand now. You're talking about the RPM at which the turbo can start to boost. That's not turbo lag.

[Scuffers]

Dave Hedgehog said:

haldex 4 onwards will split power 50/50 on throttle use (there are dyno traces showing this), its a vastly different animal to the original version and there is always power to both axels, when you have 50/50 power split on the limit of traction steering will apply additional force to the front axel which will cause the car to ultimately understeer. I will say the A45 is vastly better nailed down on the front than any other haldex car i have ever driven. And dont forget haldex is good enough for Bugatti

bugatti does it the right way round, as in permanent rwd with the clutch to the front, much the same as skylines etc.

without this or a real centre diff, it's always going to be fwd biased

[Dave Hedgehog]

kambites said:

Dave Hedgehog said:

kambites said:

Any turbocharged setup on a rod car is going to have response issues to some degree or other. The question is how much.

twin scroll turbos have made a huge difference to pick up response in turbo's the A45 produces peak torque from 2250 revs which is not to high

Ah, now I understand what you're talking about. You're talking about boost threshold rather than turbo lag. Completely different thing.

Turbo lag exists everywhere in the rev range and is in fact more pronounced transitioning from closed to open throttle high up the rev range when the turbo is working harder. I'm sure this has a commendably low boost threshold, most modern turbocharged engines do.

for us none OCD nerds who dont care what the technicalities of it the cars either sitting there waiting for something to happen (in an old school turbo cars that would be when it hit 3-4k rev range) or its starting to accelerate as you would expect, the A45 does have a bit of a flat spot at the base of the rev range but the box swapping down cogs over comes this mostly

to most of us this is lag

[kambites]

Fair enough, doesn't matter what you call it, but that's not what Scuffers and I are talking about. We're talking about the time taken to build boost when you come onto the throttle once you're already well into the turbo's operating rev range.

Manufacturers solved the low-RPM thing years ago. It's pretty much a non-issue these days.

Edited by kambites on Sunday 4th August 17:42

[Dave Hedgehog]

kambites said:

Fair enough, doesn't matter what you call it, but that's not what Scuffers and I are talking about.

probably not, but what counts is how the engine delivers power in the real world, in the A45 you wont be sitting there going "come on, come on, come on ****!!!" like you would in turbo cars of old, but you would in the B8 RS4 until it hit 4k revs.

the 135 does pick up quicker and faster and is a better real world engine (and sounds 100 times better)

the 135 is more for hooning, the A45 is for getting A-B at warp 10, in the dark, when its wet and you tired, with no moments

different car for different people i think

[kambites]

Guess it's jsut personal preference, but I couldn't care less how much power a car generates about <3000rpm because if I want to go fast I wont be at <3000rpm.

What I care about is how quickly the power is available when I'm trying to use the throttle to balance the car mid-corner when I'm pushing on, at which point I'll already be in the peak power band anyway so it's just the time taken for the feedback loop between exhaust and intake pressure to build up. That's what I hate about turbocharged cars, and that's what tends to get worse with higher pressure turbos.

The only way I'm aware of to completely remove this is a rally-style anti-lag system which injects unburnt fuel into the exhaust when off-throttle to keep the turbo spinning, but sadly that's illegal on the road (and probably doesn't do wonders for your fuel economy either ).

[Dave Hedgehog]

kambites said:

Guess it's jsut personal preference, but I couldn't care less how much power a car generates about <3000rpm because if I want to go fast I wont be at <3000rpm.

What I care about is how quickly the power is available when I'm trying to use the throttle to balance the car mid-corner when I'm pushing on, at which point I'll already be in the peak power band anyway so it's just the time taken for the feedback loop between exhaust and intake pressure to build up. That's what I hate about turbocharged cars, and that's what tends to get worse with higher pressure turbos.

which is fine when your on it, but in the real world you wont be driving like that 95% of the time (unless you live in the middle of no where) most people making progress quickly do it in the lower part of the rev band, it makes for effortless quick progress and very easy overtaking

ideally what you want is an engine with lots of low down grunt for normal driving and screams nicely when you are on it, or 2 cars

[kambites]

But the 95% of the time that I don't need it, I also wont care about slow pick-up from low rpm.

Personally, I hate high torque at low revs, although I can understand how people might like it.

Edited by kambites on Sunday 4th August 17:53