8-valve vs 16-valve engines

spank_inferno

Hi Guys

I currently drive a 1.4 16 valve skoda fabia & we're looking at upgrading to an octavia. One I've seen is a 1.6 mpi (05, mark 2 model)
This car has an 8-valve engine, but says it delivers over 100bhp.

My question is, in lay-mans terms what is the difference between an 8valve & a 16 valve engine?
Performance? Fuel economy? Reliability?
I assumed 8-valve engines were redundant now?


many thanks for your assistance/advise

peasant

16 valves allow for a quicker gas exchange and slightly higher airflow. More air equals more power (at the same engine size) more power usually means higher fuel consumption also, especially as the advantages only really happen at high revs.

Some of that extra power gets eaten by the higher oscillating masses (more valves need to move).

Double the valves doesn't mean double the airflow and double the power though, as the valves need to be significantly smaller in diameter.

Also 16 V is more expensive and more delicate.

Biro

With 16 valves the engine breathes better at higher revs, so you get a more linear power delivery. Typically 16 valve engines can deliver better mpg, but not in all cases. Performance wise 16 valve engines are a nice bit quicker than 8 valve (assuming same block from same manufacturer), but the funny thing with 8 valve engines is that they tend to be at their best low down the rev range, so to a normal punter driving around town, they could be convinced that an 8v could be noticably quicker to a similar 16v. However once you put the foot down the difference is instantly noticable, over 2,500 revs usually is where the difference becomes marked, and over 3,500 revs there's no contest.
Basically a lot depends on your driving style, but if you don't go far then you will find the 1.6 Octavia will seem a little quicker than the Fabia. When overtaking an artic in 3rd you'll realise the power the car seemed to have disappeared somewhat!

EPM

Have that exact engine in my car. As Biro said, it feels torquey enough low down but there is very little point to revving it above 4000 revs - it doesn't have a lot to give above that.

Its an older engine and gets slated a lot for that reason but its very relable. I get about 34-37 mpg from a tank with the air con on all the time (long spins, very little city driving). The 16 valve probably gives a couple of mpg more to a tank and is def a bit more free revving (driven the 16 valve version in the old model Leon). Still goes well enough for what it is though. I find 5th fine for overtaking (did it about 10 times on the way to Waterford just there), 4th you'll zip by. 3rd you'll never need to do trust me unless a risky passing move

unkel

spank_inferno wrote: »

I assumed 8-valve engines were redundant now?

VW is one of the last major manufacturers to still use 8V small engines. The 1.6 is a solid and reliable engine, but it is very old now. Afaik it dates back to the 80s.

EPM

unkel wrote: »

VW is one of the last major manufacturers to still use 8V small engines. The 1.6 is a solid and reliable engine, but it is very old now. Afaik it dates back to the 80s.

Actually, can anyone clarify this one? I always assumed it was an old one but over on the SEAT forum a salesman said that it was put in because it was a more modern unit than the 16 valve version

It's still in the "new" Mark 6 Golf too so there has to be some reason its still in there

maidhc

EPM wrote: »

It's still in the "new" Mark 6 Golf too so there has to be some reason its still in there

Economics.

EPM

maidhc wrote: »

Economics.

Should've seen that one coming:pac:

Padraig Mor

Heard recently that the 1.8 TDCi engine in the current Mondeo is actually an 8V (as compared to 16V in the 2.0). Is this true?

Dartz

2 Valve engines have better gasflow at low revs and low throttle openings than 4 valve engines. They would have higher torque at lower RPM's

4 valve engines have better high-rev gasflow characteristics, so make more torque at higher revs, henceforth more power, but are a little weaker down low.

5 valve and 3 vavle cylinder heads also exist.

maidhc

Padraig Mor wrote: »

Heard recently that the 1.8 TDCi engine in the current Mondeo is actually an 8V (as compared to 16V in the 2.0). Is this true?

Yes. Same basic engine as was fitted to the Orion and Sierra. The 2.0 is a much more modern PSA HDi, although still 10 years old.

I think most manufacturers get 20-30 years out of each generation of an engine!

ianobrien

Biro wrote: »

With 16 valves the engine breathes better at higher revs, so you get a more linear power delivery. Typically 16 valve engines can deliver better mpg, but not in all cases. Performance wise 16 valve engines are a nice bit quicker than 8 valve (assuming same block from same manufacturer), but the funny thing with 8 valve engines is that they tend to be at their best low down the rev range, so to a normal punter driving around town, they could be convinced that an 8v could be noticably quicker to a similar 16v. However once you put the foot down the difference is instantly noticable, over 2,500 revs usually is where the difference becomes marked, and over 3,500 revs there's no contest.
Basically a lot depends on your driving style, but if you don't go far then you will find the 1.6 Octavia will seem a little quicker than the Fabia. When overtaking an artic in 3rd you'll realise the power the car seemed to have disappeared somewhat!

The reason that 8 valve engines are better (ie more torque) at lower engine reves is air speed. The 8 valve engines generally have smaller port areas than the 16 valve engines per cylinder. The smaller area means a higher air speed and thus more torque.

At high engine speeds, the smaller areas of the 8 valve engines start to restrict air flow (air flow turns turnulent) whereas for the 16 valve engine, this occurs at a higher rpm. This is generally above the rev limiter for a standard engine.

However, this doesn't hold for a performance engine, where cam timing, lift, duration can hold the valve open for longer allowing more air in for both 8 & 16 valve. Then air intake and exhaust manifold come into play for restrictions.....

Biro

ianobrien wrote: »

The reason that 8 valve engines are better (ie more torque) at lower engine reves is air speed. The 8 valve engines generally have smaller port areas than the 16 valve engines per cylinder. The smaller area means a higher air speed and thus more torque.

At high engine speeds, the smaller areas of the 8 valve engines start to restrict air flow (air flow turns turnulent) whereas for the 16 valve engine, this occurs at a higher rpm. This is generally above the rev limiter for a standard engine.

However, this doesn't hold for a performance engine, where cam timing, lift, duration can hold the valve open for longer allowing more air in for both 8 & 16 valve. Then air intake and exhaust manifold come into play for restrictions.....

Well explained, thanks!