Boring Technical Stuff

4.0 V8 40v biturbo TFSI - 591 bhp
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welwynnick
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Boring Technical Stuff

Post by welwynnick » Sun Dec 08, 2019 3:06 pm

I never paid much attention to the C8 RS6 when it was launched, assuming it was just a re-skin of the C7 with some more technology. There's a bit more to it than that though.

The Audi website had little to say about the engine apart from the addition of mild hybrid capability. However the pictures were a bit more interesting. The C7 (first picture) had a hot inside-V turbo layout, and a right dog's dinner it was. Audi abandoned side mount intercoolers and went for a single water cooled charge cooler in the front of the V. That pushed the turbos to the back of the V. Everything was compromised - the intakes were restrictive, the charge cooler was small and a long way from the inlet manifolds, and the exhaust and catalysts were tied up in a reef knot.

I assumed the C8 (second picture) was the same, but it isn't, and I haven't read a single word about it anywhere. Audi retained the hot-inside-V turbos, but went back to large 22-row side mount intercoolers, which sit close the inlet manifolds of course. It frees up space inside the V, the turbos move forwards to the centre (like everybody else's V8's), leaving room for the catalysts at the back of the V. The exhaust downpipes now resemble uncooked spaghetti, rather than cooked, and the air filter appears to sit on top of the radiator, with short, straight inlets to the turbos. It looks very much better than before.

The peak power is no higher than before, but torque is higher, and that gives more area under the power curve, which helps acceleration. I think transmissions are part of the answer. When the C6 RS6 and Bentley Continental were launched, they were limited by the torque capacity of the 6-speed 6HP26 gearbox. I believe the C7 used the 8HP55 which allowed Audi to use more boost. Since then, large capacity super-and turbo charges engines have become the norm, and ZF now make the 8HP90 with even more capacity. That allowed Audi to dial the torque up to 800Nm, and acceleration benefits (or it would if the car wasn't heavier). Anyway, it all looks more tuneable than before.

Nick
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MikeFish
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Re: Boring Technical Stuff

Post by MikeFish » Sun Dec 08, 2019 6:11 pm

Very interesting, thanks for that.

welwynnick
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Re: Boring Technical Stuff

Post by welwynnick » Wed Jan 01, 2020 1:22 pm

One other thing I just noticed, which has been staring me in the face for years.

The C7 4.0T engine has the throttles immediately after the turbos and before the charge cooler, rather like the BMW 760il and the original Jaguar XJR.

The new C8 4.0T engine (like the V10 5.0T) has the throttles on the inlet manifolds, which is the preferred position for a couple of reasons.

Firstly, this minimises the inlet volume between the throttle and the cylinders, so the intercooler and the inlet pipe don't have to be pressurised when the throttle is opened, which helps to minimise lag.

Secondly, the intercooler is always running at the turbo outlet pressure. When the throttle is part open, there's a pressure loss and a temperature loss due to the expansion of the air downstream. If the charge air is cooler, the temperature difference will be lower, so the heat loss will be lower. In other words the charge cooler is much more efficient at part throttle if the throttle is downstream of the cooler.

This won't make much difference on the dynamometer, but it will help on-road driveability and response.

Nick

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Re: Boring Technical Stuff

Post by 2 5HAN » Wed Jan 01, 2020 3:49 pm

Very interesting Nick and thanks for the info.
Clearly you are technically minded and understand the engine design etc
Have you compared it to the Porsche Panamera and the drive train that it runs? I was told that the 2020 RS6 Engine was going to be a detuned version of the engine found in the Lamborghini Urus and would be using the same drivetrain but with the hybrid inserted as it is in the Porsche. The Urus has a blanking plate over the Hybrid input section whereas the Porsche and Audi are using the Hybrid Motor
If this is the case then the engine should be good for well over 750bhp with ease

welwynnick
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Re: Boring Technical Stuff

Post by welwynnick » Wed Jan 01, 2020 7:05 pm

VAG engines have typically used a couple of cylinder bores (82.5 & 84.5mm) and a couple of strokes (89.0mm & 92.8mm) to cover a very wide range of engine capacities. They are all under-square. The C6 & C7 RS6 actually share 84.5 x 89mm dimensions.

The new engine seems to have nothing in common. The bore and stroke are 86 x 86mm, and these are the exact same dimensions as the new Panamera. My understanding is that all new V8 engines used by Audi, Bentley, Lamborghini & Porsche are essentially the same.

Bentley Continental GT and Porsche Panamera use the 8-speed PDK gearbox, while just about everybody else uses a variant of the ZF 8-speed torque convertor box, and that includes the Bentayga and the Cayenne.

I don't know anything about the hybrid stuff.

Nick

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Re: Boring Technical Stuff

Post by welwynnick » Thu Jan 06, 2022 10:10 am

I realise this is the least interesting thread in the forum, but it's useful to post reference information online because I often loose it at home. So here goes. VAG produce some interesting training documents called Self Study Programmes. They're not manuals, but just high level introductory descriptions of any significant new product. There are lots of them and they pack in a lot of information.

I recently found SSP 920493 for the NEW Porsche / Bentley / Audi 4.0 V8 engine, and it was a good read. I noticed that it makes a break from the previous generation of 90 degree v engines, and doesn't use a block girdle any more. The block is now a deep, single-piece casting, and uses individual iron bearing caps with six bolts apiece. The crankshaft is also fully counter-weighted, which is a step forwards. Tuners have been quick to extract 1khp from it.

It's available from the usual places:

https://www.vdveer-engineering.nl/en/in ... e_vignette
https://procarmanuals.com/vag-ssp-92024 ... 8-engines/

Nick

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