4-valve v's 2-valve heads

[perthdrivers.txt]

Sorry Bill - it occured to me after I left work that you're not stupid enough to think I wouldn't notice it was a 7L donk.

Back on topic - I was using a 2.1" intake valve.

Bills results kinda prove something else - with more rapid lift rate the only real benefit was at higher rpm.
Which basically says that up to 6500rpm both 2V & 4V engines are injesting the max air they can anyway - so helping breathing is not making any more power coz the damn things just won't swallow more air anyway.

Which also proves that a 2V engine can have enough flow that there is no advantage to 4V untill revs get quite high - ie. outside the usual operating range of a large bore engine.

So it's basically irrelevant that the 4V flows more - no benefit can be gotten from that extra flow.

The only power increases achievable in the given rpm range (up to 6500rpm) would come from better combustion efficiency - something the 2V is definately equal to the 4V in, in some cases superior.

I've learned a bit.

Basically, if you're running a bore size up around 4" you'll see no benefit of a 4V over a 2V untill it's revving past 6500rpm.

Forced induction would be a different story - the ability to shove more air in than the engine can aspirate on it's own would see a benefit from the increased flow of the 4V.

There must be more to it than that - why would 4V engines be so dominant in the market if the only advantage was in high rpms when 90% of passenger car engines would be lucky to see those kind of revs in their lifetime?

[perthdrivers.txt]

double post 4 $

[TK]

It's probably a simplicity/durability thing, no side loading on valves/guides so they last longer.
Also a bunch easier to design a 4V head that'll flow 'enough' air for a stocker - no fancy casting/machining required to get them up to scratch.

Probably many other advantages from a manufacturing &/or reliability view - new dohc 16v donks don't wear out like the old stuff - they only die due to poor servicing/overheating from split hoses etc, the engines themselves if everything else is maintained will usually do at least double the milage of a pushrod engine before needing work.

[hamish]

Talking about 2 valve motors (blatant thread hi-jack) - TK, do you have any experience with the old Alfa 2litre twin cam heads?

Depending on how my budget is going, I may need one re-co'd in the next few months, and I'd like to be able to send it to someone who can do a good job on getting the seats right, & maybe get a pocket port done.

[TK]

Probably as much experience with them as most - ie. SFA.
But hey, it's got valves, I can port it.

[Cameron_Datto]

Found this Forum about 6 months back , There is some good reading . Pics and Graphs . http://www.guy-croft.com/viewforum.php?f=2

[Tripper]

Probably as much experience with them as most - ie. SFA.
But hey, it's got valves, I can port it.

What did you know about the 2jz head before porting it ?

the results speak for themselves with that head

[hamish]

The only real quirks I can think of with the Alfa head are the 30degree valve seats they have from the factory, and the inlet:exhaust valve ratio is too big on the exhaust. Apart from that they a conventional old school 2 valve 'hemi' style head. I'm not looking for mega power increases, but it would be nice to wake it up a bit over std.

Anyway, I'll get in contact when the time comes.

[GTSBoy]

The "How to Power Tune ALFA Twin Cam" book has a good discussion on what is needed and the sort of (fairly radical) cams that can wake them up.

[hamish]

Gts, got that book right next to me

Unfortunately, the big valve conversion, full porting & big 12mm lift C&B cams I'd like are probably quite a bit over my budget

Also, if I was really chasing power, I'd do what the book suggests & get a Twinspark to drop into it, again more money I don't really have to spend on it - the 2 litre I've got lined up for it is a freebie from a mate. I just want to do the basics properly.

[TK]

What did you know about the 2jz head before porting it ?

Nothing, I'd never even seen one....

The only real quirks I can think of with the Alfa head are the 30degree valve seats they have from the factory, and the inlet:exhaust valve ratio is too big on the exhaust. Apart from that they a conventional old school 2 valve 'hemi' style head. I'm not looking for mega power increases, but it would be nice to wake it up a bit over std.

Ok, fairly easy, we'll find some 45 deg seat valves that are a bit bigger for the inlets & re-cut the seats to suit, do some work on the intakes & tidy up the exhausts. Should be fairly straight forward.

[GTSBoy]

Gts, got that book right next to me

Unfortunately, the big valve conversion, full porting & big 12mm lift C&B cams I'd like are probably quite a bit over my budget

Also, if I was really chasing power, I'd do what the book suggests & get a Twinspark to drop into it, again more money I don't really have to spend on it - the 2 litre I've got lined up for it is a freebie from a mate. I just want to do the basics properly.

Yuh, fair enough. I did the same thing when I built mine. Got the ports sorted as best as possible without major changes to the seats, big pistons, pissed off the Dell'Ortos etc etc. Never even got around to buying the big Autodelta profiled cams that it needed to make it work properly (was running it on 105-48s) before I sold it, but that was a nice enough setup anyway.

[hamish]

TK, sounds good

Alfaholics in the UK have 1mm OS valves, but at ~$55 each I think I'd want to try & find a cheaper alternative. Pair of billet performance cams are around $600 plus shipping, pretty spendy for the performance gain I think they'd give me, especially if you did the upgrade to bigger carbs or EFI to make full use of them, and even then it'd struggle to make 200hp. Reminds me what a bargain parts for a SB chev are.

[260BC]

A discussion on BMEP and a comparison between F1 and Nextel Cup engine outputs.
http://www.epi-eng.com/piston_engine..._yardstick.htm

[Ze]

It's probably a simplicity/durability thing, no side loading on valves/guides so they last longer.
Also a bunch easier to design a 4V head that'll flow 'enough' air for a stocker - no fancy casting/machining required to get them up to scratch.

I always thought it was emissions related as well as being driveability related.

Sure if you want to go to hero efforts you can get a 2-valve head to flow pretty well and burn pretty well but if you go to the same efforts on a 4-valve head , you'll get it flowing better or have a wider rev range.

If you want to open the valves further in a 4-valve head you can , you just don't need to as aggressive profile.

Your not really making too much sense , you want to limit revs for both + lift only for the 4-valve head yet let the 2-valve head be a free for all apart from revs.

Then we've got the variable valve mojo which is much easier on an OHC head (especially a DOHC head) compared to an OHV head.

Sure you can get pretty good performance out of a 2-valve head if you want nobody is going to argue with that, but a 4-valve head can be designed better.

It's an art form optimising within the variety of constraints whatever they are but it doesn't make the constrained version better than one with less constraints it's just another challenge.

Probably many other advantages from a manufacturing &/or reliability view - new dohc 16v donks don't wear out like the old stuff - they only die due to poor servicing/overheating from split hoses etc, the engines themselves if everything else is maintained will usually do at least double the milage of a pushrod engine before needing work.

I think that's more materials and machining rather than the number of valves though TK.

A pushrod engine is cheaper and going to be slightly easier to package so if the manufacturers could get away with it they would I reckon.

When it comes to the LS series engine as to why GM went with a OHV design , I think it's a case of them catering to their market perceptions rather than any technical merit.