How efficient are hemi-heads and rollercams vs 4 valves per cylinder?

[Frank]

Since there seems to be several that work with classic american cars in real life I thought I might get some more knowledge on this subject I've though about for years since I read it.

In some carmagazine, not sure which one or when, then it was written that classic american V8s weren't so bad since they used roller-camshafts which to a fair extent was as good for efficient combustion as 4 valves per cylinder. I think it said that the rollercams could give a faster time between opening and closing the valves and could thus enable more air/fuel in the chamber than with normal camshafts.

A different time I read that you can't have hemispherical combustion chambers with 4 valves per cylinder. However I also read, from a comment on the Porsche 968 Turbo, that with forced induction you don't need 4 valves per cylinder to ensure good filling of the combustion chambers.

Frank

[rommaster2]

First off you can have 4 valves per cylinder with a hemispherical head, in fact quite a few japanese DOHC motors use hemispherical head designs. Now you cannot fit four valves per head in a regular motor because the plug in the side gets in the way.

Second it depends a little more then just the number of valves, displacement is important too as well as port design. Just look at the canted valve cleveland heads from ford, the 4V iron heads flow ridiculously well once ported, as good as most aftermarket heads for fords.

Finally the roller cam did not come into play until the eighties and even then it pales in comparison to the speed of an overhead cam. This is because an overhead cam has much less valvetrain weight and so it can move faster naturally. That said modern camshaft profiles allow for alot of power to be made with OHV or DOHC, and OHV is a bit better for torque and low end power.

Also just as a sidenote at one point in the eighties ford produced a very small amount of experimental heads for the 302 motor that had 4 vavles per cylinder and special valvetrain. Obviously to run one of these heads you would need all the parts which is why nobody runs them as people only have hodgepodges of parts from them plus there were only a few made.

Basically if you account for smog regulations, 4 valves per cylinder is much much more efficient as you can have hemi heads and fast cam profiles along with good flow characteristics of 4 valve, just look at the honda s2000, it produces about 120-140hp per litre, name an american v8 that does that easily and daily driver while passing smog.

[victoria]

Basically if you account for smog regulations, 4 valves per cylinder is much much more efficient as you can have hemi heads and fast cam profiles along with good flow characteristics of 4 valve, just look at the honda s2000, it produces about 120-140hp per litre, name an american v8 that does that easily and daily driver while passing smog.


I can easily Since I own 3 of them. Mustang Cobra's have a 4 valve 4 cam 4.6 with hemi chambers... the non supercharged versons made in the range of 350-400 HP stock. the supercharged ones well over 400 even though ford only claimed 390. I have one with some mods thats turning in the 680 HP range at the rear tires on a dynojet. and passes MA's smog check every year. Still completely OBD2 ready. and yes I drive a non supercharged but cammed 2001 cobra everyday including in the winter (with snows) and its in the 400 HP range, its alot of fun :)

4 valves = more airflow
overhead cams = less moving parts
hemi chambers = more even burn

forced induction = more of everything and just plain ole fun honestly 4 valve cars LOVE BOOST of any kind because they can take advantage of it more.

pushrod v8's work well. look at any of the new corvettes.
just multi cammed multi valved motors do more with less basicly.

Just a little link to see a ford 4V DOHC 4.6 crate motor

http://www.buyfordracing.com/shop/product_info.php?products_id=653cPath=11_27

[rommaster2]

I'm well aware of the cobras, they are a somewhat flawed example as the ones that make the 100-130HP per litre are boosted and are also 4 valve\hemi headed, so kind of the best of everything :P impossible to make much of a comparison. Great cars though a bit overpriced on resale *shrug*.

Also your figures on the naturally aspirated cobras stock are a bit inflated but still it is an impressive motor. GM makes great ones too but they are starting to tap out on what they can do and if you notice the only one that is again as efficient from the factory is boosted.

[camarokid]

i got a 91 lumina z34 with the 3.4 dohc/ it has the hemispherical heads. great design in my eyes

[Stang Seller]

Having an engine dyno and flow bench at the shop I work at, I've done quite a bit of testing on both the Hemi 426 crate motor and LS1 motors in the last few weeks. Out of the box, the crate Hemi made 450HP and roughly the same torque. We did some testing with the stock heads and valves, and I must say it's a very inefficient combustion chamber. It makes that power because of the valves and ports, really. You need a pretty big dome piston to up the compression, and that leaves hot spots and causes some unburnt fuel to go out the exhaust.
The Ford Cleveland uses a canted valve design, which in my opinion, is superior overall in overhead valve engines. The 4V head (not 4 valves, just denotes Carb openings) has immense flow potential but lacks good velocity. It is a great head to use if you have LOTS of cubic inches or if you want to make power up to 8000 RPM.
The LS motors are a great achievement. With a near stock motor, with some minor NHRA Stock Eliminator modifications which include a camshaft (stock lift) and valvetrain components through open headers we've achieved over 480HP at the crank!!! That is just nuts for a stock motor with a cam!
Soon, I plan to get a DOHC Ford on the dyno and flowbench to see where I can improve it to make a streetable N/A high HP car within Stock Eliminator guidelines. I'm just curious but it'll take a while to get this info. I still have to convert and dyno test my 351W.
All in all, I think the OHV Hemi chamber is weak. In 4 valve OHC form, it works much better. Also, no factory HEMI OHV engines came with roller camshafts.

[Frank]

Thank you very much for the replies (sorry for the long time to reply but I had some issues in my family). I see I actually slightly misunderstood the concept of roller-cams, but that's been corrected now and I got even more information than I hoped on efficient engine design :)

Nate, you mentioned canted valves in a Ford V-8, would you happen to know what a "shrouded" valve is? The european designed Ford Zetec had this in the the 1.6 and 1.8L form while the 2.0 variant had normal valves according to data I've read but I've never seen any diagram or picture showing the difference. Btw, all three engine-sizes had the same stroke, 88.0mm so the 1.6 was really a long-stroke engine.
Interesting note btw, the first-gen Zetec engine had nice progressively torque-curves without any drops at mid-rev unlike many other early 4-valve engines, but the updated Zetec-E engines however did.

Speaking of cylinderheads, has anyone heard of the Coates (not sure if the spelling is right) horizontal rotary valve cylinderheads which don't even have camshafts? I think they had both a V-twin motorcycle engine and a car-engine (looked like an 80s MB E-class if I remember correctly) running. Such a concept could give a much more compact cylinderhead (at least compared to OHC heads) and maybe lower inertia and fewer moving parts, they also claimed better combustion efficiency.

Frank