VAG 1.8t 20v (why 5v per pot?)

djones · Post 09 Nov 2006, 16:14

Does anybody know why VAG decided to make their 1.8t engine (and a few others) 5 valves per cylinder?

What advatages/disadvantages does this system have?

DaveKillens · Post 09 Nov 2006, 22:08

The logic was .. "more is better...?", and Yamaha first tried it on a 750cc four cylinder sport bike. Less valve mass should allow easier RPM, and maybe better gas flow. They studied to see just how many valves would be practical, and found that more than two exhaust valves led to localized heating problems. And more than three intake valves became ridiculous..... so here we are at five.

zac510 · Post 09 Nov 2006, 23:35

If they can fit more valve into the area/diameter of the bore then theoretically more air can be pulled into the cylinder.
But shrouding (where air flowing past one valve collides with air flowing out of an adjacent one) and mechanical actuation difficulties seem to have caused them to fall out of favour.

djones · Post 10 Nov 2006, 00:16

Ah, so it's just for a bigger intake area.

This will probably sound really silly but why cant they use a horseshoe shaped valve or just a two half circles? Or even 4 quarter circle valves

Tom · Post 10 Nov 2006, 00:22

There are a few variations of combustion chamber, I'm sure someone has made a horseshoe.

I guess the standard type is the easiest to manufacture, the most practical and perhaps provides most air turbulence.

Really the performance advantage from three extra valves can't be measured its so small.

pRo · Post 10 Nov 2006, 07:38

Tom wrote:Really the performance advantage from three extra valves can't be measured its so small.

Did you mean one extra valve?

Anyone remember the Toyota 4A-GE engine, which was quite popular in racing in the 80's? And it's still being used. That engine had 4 valves per cylinder and the latest versions in the early 90's made 130hp. In 1991 Toyota made a new head for that engine, with 5 valves per cylinder. It gave an instant boost of power to 165hp (and more with later versions).

It's not all about the number of valves though, as it also had higher compression (11.0 vs 10.0). But the 5 valve/cylinder head does flow a lot better than the 4 valve/cylinder heads. But you do need high revs to make that difference show. The difference is next to nothing at lower revs, where most grocery getter engines spend most of their time. Just more expensive (and a nice marketing tool, because for many, more is always better, in everything).

luisandregg · Post 10 Nov 2006, 15:55

Before Jean Todt's arrival in Ferrari, they had 5 valves and 12 cylinders engines. Than they passed to 4 valves and 10 cylinders.

The reliability of the engine was increased without the 5th valve.

Maybe it's just a marketing strategy of VAG to sell more engines : everyone want to have a valve more than one's neighboor.

mep · Post 10 Nov 2006, 17:38

You want a ideal shape of the burning camber a kugel whith
the ignition in the centre.
So you have a ideal burning of the gases.
Whith 5 falves you came closer to this solution.

DaveKillens · Post 10 Nov 2006, 18:01

luisandregg wrote:Before Jean Todt's arrival in Ferrari, they had 5 valves and 12 cylinders engines. Than they passed to 4 valves and 10 cylinders.

The reliability of the engine was increased without the 5th valve.

Before Todt arrived in ferrari, there wasn't much reliable with anything in the Ferrari camp, let alone the 5th valve. Not to mention, the FIA changed the regulations, and only 4 are currently allowed.

One advantage of three intake valves is that because of their reduced mass, a more aggressive acceleration of the valves can happen. That means that on the intake stroke, when the intake valves open, they quickly allow a lot of gas/air to enter.

pyry · Post 13 Nov 2006, 17:25

it is stupid to compare an f1 competition engine to a normal engine. the more valves the smaller the mass to control. also five valves in an indirect injection engine allows for better fuel economy due to better turbulences. out of VAG creations i think this is by far more interesting http://www.worldcarfans.com/news.cfm/ne ... rol-engine
would this be allowed in F1?

zac510 · Post 13 Nov 2006, 19:55

pyry wrote:out of VAG creations i think this is by far more interesting

That's nothing special, just VTEC with some extra marketing spin on it.

djones · Post 13 Nov 2006, 20:01

Clever but in typical Audi fassion it's not exactly powerfull. 210 BHP from a 2.8 is quite poor when you compare it to BMW's current 2.5 for example.

DaveKillens · Post 13 Nov 2006, 20:04

FIA Formula One regulations 5.6.3
Variable valve timing and variable lift systems are not permitted.

Not imagine if the Audi variable valve system was incorporated in the 5 valve head, and the engineers went wild by varying each individual valve. For instance, one of the outermost intake valves opened first, before the other two....to promote turbulence. Or one closed later than the rest? Or at low RPM the middle valve didn't open at all?
Hmm..

djones · Post 13 Nov 2006, 20:12

"imagine if the Audi variable valve system was incorporated in the 5 valve head, and the engineers went wild by varying each individual valve. For instance, one of the outermost intake valves opened first, before the other two....to promote turbulence. Or one closed later than the rest? Or at low RPM the middle valve didn't open at all?"

Honda were probably trying that 15 years ago