Difference between 8v and 16v (For a report)

[Laine_16v]

Ive sorta got a project to detail into the main differences between the two, why one is more powerful e.t.c

If anyone could give any main points i could write about i would be very appreciative but i cant really think of much to write about lol.

I posted this on WC and Stan gave me a great paragraphs worth of information for me to quote in the report which is brilliant but i would prefer a range of sources to beef out the report.

If someone like Chris H (or anyone else who knows what they are talking about) could give me a summary paragrapg of the main differences that'd be awsome!

Cheers!!!

[Chris H]

gas speed

a thing called curtain area (the area the gasses pass through with an open valve)

combustion chamber shape (thus efficiency)

These are tbh the main differences, no doubt stan mentioned them but its not our place to do your work, but if you pay me I will

If you want mopre specific answers then ask more specific Q's

[stan]

Posted: Sun Feb 04, 2007 10:32 am Post subject:

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why 4v head produces more power (in brief..im sure you can elaborate!):

more valve area for piston area,

increased peak flow, whilst retaining good velocities,

better in cylinder movement as air enters chamber,

more even cylinder filling, so burn can propogate faster/better,

reduced time for flame front to reach cylinder walls (due to positon of spark plug in relation to inlet valves), so end-gas has less exposure to peak pressures and temps...so more ign advance can be run with reduced risk of detonation,

they will rev higher also.

Americans will tell you the inherent hemi-head design is bad for combustion..in terms of reduced squish, poor piston crown design due to larger valve cut -outs etc...they may have a point on certain aspects, but as always its a comprimise.


from williamsclio.

come on lad...your not gonna learn if we give u the answeres

[Laine_16v]

No, im not copying it, im simply quoting what the key points and then from that i will research each element you spoke about in greater depth.

The more attributes i know about the more i can research. I am supposed to not know anything about it either (sort of research from scratch).

[stan]

well it depends how much you are required to know , and how indepth you are required to go...

Friction losses due to extra valve movement...increased cam loading due to more pressure per length etc...all add to overall mechanical losses on 4v set-up..as parascopic as they maybe....but not major contributors.

If I were you I would elaborate/research the points above esp re effect on volumetric efficiency and effect on det limitation....

[Chris H]

4V chambers are technically not hemi heads though, they are pent roofs with shallow angle, hemi is a close relative but the chamber of a pent roof is superior to a hemi.

Losses via froction on a 4V head are no higher than an equal performance 8V head, but no doubt as stans trying to say its comparing standard to standard so not an issue.

Generally cam lobe loadings, friction etc are lower on multi valve engines as the smaller valcves have less mass, less powerful springs are then needed to control the smaller valves.

However in a like for like situation where a 2 litre 200bhp 16V is compared to a 150bhp 8V loadings will be similar.

However a 2 litre 200bhp 8V vs a 200bhp 16V the 8v will have loading maybe 5 times as high on valve train, simply due to the mass of the parts, cam lobe required and the spring rates to keep the valve under control.

The rev higher reason has been touche don by Craig, the reason is as he pointe dout airflow (if can't increase revs if it can get the fuel in to make the power to do it), the valve train mass and control is better on 16V setups.

Valvetrain is another area, 90% of 16V setups use twin cams and direct to follower activation, theres no rockers, theres normally a tappet bucket on the valves end which means its direct. Thus superior control.

On 8 valves you can have direct to valve head activation (example RSi engines), but generaly there are rockers involved which add considerably to mass lessening control over the valves as the revs rise, When you factor in pushrods etc the mass, flex and heat build up tells a sad tale.

[Laine_16v]

This is all great stuff cheers. Ill sort all that info out and ask more specific questions.

But the pressure is off because the report isnt due for a while yet.

[stan]

yeah my bad,...i meant pent not hemi.