Why DOHC and not SOHC ??

[Hydra]

Uhh... I dont thing I saw this anywhere yet but the FE heads are DOHC, just a single belt gear + scissor gear under the cover which allows for the narrow design. In this respect you should be able to get some very nice cam profiles on the narrow head, but you probably wont get much adjustability from an adjustable gear. Not as much as a GE head with twin cam gears.

[3sgte]

One observation is that newer -FE engines with chain-type cam drives no longer have the scissor gear cam drive setup.
Examples would be:
1ZZ-FE http://www.spyderchat.com/1zzfe.pdf
2AZ-FE
2,3,4GR-FE http://www.sae.org/automag/techbrief...1-114-1-17.pdf
1NZ-FE http://www.sae.org/automag/globalview_01-00/03.htm

There are a few 2GR pics at http://www.3sgte.org/photo_gallery2.htm
The site is a rough work in progress, so changes probably will occur.

The PDF in the first link has some beautiful on target information. Especially about the chain/belt and valve angle spacing compromise.

[Hydra]

This is most likely because they can get way more strength and reliability from a chain now, so the use of smaller cogs is possible, thus allowing for a narrow head.

GE heads have been used fairly recently, like in the late model celica. They rev pretty high to make their power though

[Nim]

SR20 valve trains are not very reliable at high RPM.

This is true, but I think that would have more to do with the narrow head angle than the lash adjusters, as a Honda VTEC engines all use lash adjusters, as do Toyota VVTL-i engines, and they rev out quite nicely. Or perhaps it's just Nissan not doing a great job with a cheaply designed engine.

[Norbie]

From what I've heard the lash adjusters in the SR20's are a known point of failure at high rpm.

[Nim]

From what I've heard the lash adjusters in the SR20's are a known point of failure at high rpm.

Perhaps it has more to do with Nissan not making them well then? I don't think it has as much to do with the nature of lash adjusters.

However, having said that, I think I read somewhere that Honda made a motorbike with lash adjusterless VTEC? Said it made it able to rev higher? So there ya go. (assuming I'm remembering correctly).

[Norbie]

lash adjusterless VTEC?

Not possible, VTEC works by switching between different lash adjusters for a single valve. Maybe it had variable valve timing.

[Nim]

Not possible, VTEC works by switching between different lash adjusters for a single valve. Maybe it had variable valve timing.

Okay, found an article. It works like E-VTEC, or kinda like TVIS. It keeps one valve closed at low RPM, and opens it at high rpm.

http://www.motorcyclistonline.com/es.../146_0204_red/

So yeah, not quite the same as standard VTEC. My point, however, is that Honda doesn't like putting lash adjusters in it's motorbikes, for both the size issue and the high revving issues. So my conclusion is that high revs + lash adjusters or rockers = not so great, for the previously mentioned reasons.

Some images:
Hyper VTEC - valve closed (low RPM)
Hyper VTEC - valve open (high RPM)

I wonder if the second valve has a lumpier cam lobe? I wonder if that'd work.

[thechuckster]

motorbike versus automobile engines are two very different beasts - engine that reliablity rev to 12 or 16 thousand rpm are going to have very different wear, reliablity, weight, oiling, etc variables compared to an engine that rarely sees the upperside of 8 thousand.

I'd suggest comparing the tehnologies across similar sized, powered and reving engines.

re: the honda e-vtec - is would be foolish not to have different timing on the 2nd valve - given that it's on/off only (unlike vvti) the last thing you want when travelling past XXXXrpm is to suddenly double the induction characteristics/capacity of your engine. It might not be lumpier tho.

[Nim]

A 4 stroke moter is a 4 stroke motor. Saying 'oh, it revs higher so has no bearing on our discussion' is stupid. We are talking about the revability of an engine. The best way to find out what kind of head revs best is to take it to the extreme!

A SOHC engine is okay for low revving applications, but as demonstrated by the motorbike thing, even lash adjusteres aren't ideal for high revs. It's a compromise based on the fact that you only have to hit around 7500 to 8500 rpm.

[thechuckster]

If you want to know what valve-train revs the highest, get an F1 V8 or V10 engine and post the pics... sur,e it'll only work for 20-30 hours (at best) ut you'll be able to safely rev it to over 16K for at least 2 race durations.

[myne]

What about rotary valve?
Surely that'd be the most likely to handle higher RPM?

Not that I know what rotary valve looks like, but I imagine it to be something like a ball valve that continually spins.

[Nim]

Obviously rotary valves aren't that great, as nobody uses them (or nobody I've heard of). And as for F1 engines, do they run buckets on DOHC? Or are they SOHC with rockers?

[tricky]

Neither. Solenoid actuated = no cam at all.

Obviously rotary valves aren't that great, as nobody uses them (or nobody I've heard of). And as for F1 engines, do they run buckets on DOHC? Or are they SOHC with rockers?

[Nim]

Neither. Solenoid actuated = no cam at all.

THEY ARE? I've been hanging out for that shit for AGES! I didn't think they'd actualy put it in a race or production engine yet! That's freekin' AWESOME!

Okay, end of discussion. Who cares what's better? Solenoid actuators pwnz both of them!