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Hemi Head?


ESokoloff

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No.

A hemi head is also known as a hemispherical head. They have a half round, or nearly half round combustion chamber.

If you look at the configuration of the valves here, they come down to a near interference position with the piston. The configuration is a conventional shaped combustion chamber.

dc

http://classichemi.com/hemiwhat.shtml

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Semi-Hemi?

The valve stems are not parallel with each other and the chamber is round(ish).

Does a true Hemi have a true/perfectly round(180*) combustion chamber?

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Nice pics. I think the Cam head has an ellipsoidal head chamber, or lenticular as I recall some articles mentioning. The idea being to "focus" the air/fuel mix towards the center of the chamber near the spark plug. The older original Hemis, IIRC, used the tall head to allow for large valves (2), and then use a high dome piston to compensate for the compression ratio loss. This old design also caused problems with the spark plug being "shaded" on one side of the piston (near TDC) so it would take a while for the remaining mix to ignite, requiring dual-plugging to compensate. My old 75 Triumph had a pure Hemi chamber, which looked neat but was prone to detonation and had something like 35 degrees initial advance. I had the pistons polished and smoothed a bit to remove hot spots.

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I'd agree it's not a hemi or a modified hemi like the new Mopar engines or the older Ford Boss hemi engines. (I used to have a 426 Hemi and did a lot of work on it.)

 

With 4 valves it looks a lot like Indy Car engines, or at least what they used to look like some years ago. They were called

"pentroof" combustion chambers.

 

The 2 valves on ea side were pretty much parallel to each other

but were tilted just a little away from the centrally located plug.

 

These appear to be tilted away from each other a little both from side to side and from 1 valve to the other on the same side.

 

It looks like the piston has a quite a bit of quench area considering it's a 4 valve engine, which I would imagine is one reason they can make such a high compression ratio work. With the flat quench areas on top of the piston near the outside edge it would tend to keep the flame in the center of the piston and keep the flame travel short, all which contributes to fighting detonation. That and the centrally located plug and of course the engine management system.

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It would not be a compliment to BMW to apply this 1950s term to any of its engines.

 

We don't call F800s mouse motors and R1200s rat motors, either.

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  • 1 month later...
It would not be a compliment to BMW to apply this 1950s term to any of its engines.

 

We don't call F800s mouse motors and R1200s rat motors, either.

 

Someone's been sippin' the cool-aid...

 

Hemispherical combustion chambers go back a hundred years or more, because they had (have), from some aspects, certain theoretical advantages (no valve shrouding, for one)...though some of their disadvantages have been pointed out here, as well. But what does the chronology have to do with this, anyway? A hemispherical combustion chamber is hemispherical, period. That's why Chrysler called their original HEMI a "HEMI". The new genIII HEMIs are far less hemispherical than their ancestors, but they're still a "cross-flow" design so Chrysler used the designation anyway for the historical tie-in. BMW makes good engines, too...but they're not magical for crying out loud. Gimme a break. JMO...

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Guy, if you need a break, you can take the afternoon off, as far as I'm concerned.

But I do agree with you.

And more, the boxer style as we ride it today dates back to about 1920 or so. Some nice modern features, but a classic design with a long history.

dc

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If my reply came off as a "rant", it wasn't intended to. I was calm the whole time. ;) That "1950s" post just got me thinking about some boxer-related stuff that I've wondered about for a while now.

 

There are a couple of aspects to the current boxer that really are still stuck in the past. One is valves that need periodic adjustment. Where are the hydraulic lash adjusters? They've been around for decades and it's no longer a question of performance or packaging, so why the slavish adherence to antiquated technology? Another anachronism is cable-operated throttle bodies that still require mechanical synchronization. This is the one thing keeping the boxer from consistently being the smoothest twin available, and throttle-by-wire (allowing for computer synchronization of the throttle bodies) has been proven technology for many years so, again, why doesn't BMW utilize the available technology? The current boxers could use some updating of their own...and I don't mean water cooling. Heck, they're already liquid cooled anyway. No, all I meant was, while BMW's engines are very good, they could be great with some meaningful updating...

 

Again, JMO...

 

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So would going back to push rods, even if they are hydraulic, be better than the current design? Not arguing, just asking.

 

As for throttle by wire, I prefer my cables, thank you very much. Too much electro-wizardry is not a good thing to my neanderthal mind.

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There are a couple of aspects to the current boxer that really are still stuck in the past. One is valves that need periodic adjustment. Where are the hydraulic lash adjusters? They've been around for decades and it's no longer a question of performance or packaging, so why the slavish adherence to antiquated technology?

 

Would the use of hydraulic lifters make the engine any wider?

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No. There would be no packaging penalties, and push rods are not a requirement for their use. All the current automotive engines that utilize more cams and valves than you could ever wish for already have hydraulic lash adjusters. They could easily be employed at the pivot points of the followers in the new cam heads. I've seen them as small as pencil erasers. They would eliminate all the reciprocating mass of the shims and any shim retaining components, so there's one performance improvement right off the bat, even if having perfect lash all the time doesn't float your boat.

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...As for throttle by wire, I prefer my cables, thank you very much. Too much electro-wizardry is not a good thing to my neanderthal mind...

 

Normally I couldn't agree more. The adoption of CAN-Bus has made life easier/cheaper for BMW, but how has it improved our lives? Scanning these forums, I'd say that it has caused far more grief for the end user than it has alleviated. However, switching the boxer from cable-operated throttle bodies to throttle-by-wire (TBW) is one instance where "electro-wizardry" could actually improve our lives. How would you like to have your throttle bodies in perfect synchronization...all the time. And, BMW should just love the complete elimination of all the expense and mechanical hardware now required for the implementation of cruise control. That's right, TBW makes the addition of cruise control a mere question of software, with no additional hardware whatsoever. Without any hardware to screw up, cruise control reliability couldn't be any worse than it is now. Of course, I'm sure that they'd still continue to charge us just as much for the option, even though it would cost them nothing to "install" it. The same would be true for an improved/more-accurate version of Traction Control.

 

Maybe it's just me, but I would be willing to accept this proposed round of "electro-wizardry" if it meant that we, the end users, would finally, actually benefit from real-world improvements in performance and reliability...

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Would the use of hydraulic lifters make the engine any wider?

 

Morning Eric

 

With proper engineering & a very complex system it wouldn't have to be any wider.

 

Lots of motorcycles have in the past or have now used hydraulic valve lash control systems.

 

On overhead cam setups like the angled cam lobe, splayed valve arrangement on the current camhead BMW Boxer it would be very expensive & quite complex.

 

To keep the reciprocating weight down & the packaging small it would almost have to have a floating rocker with the hydraulics on the rocker fulcrum (like some Hondas use). That would mean a total new head design in the valve train area, a significant change in the oiling to that area, & a lot of more expensive parts.

 

Due to the high RPM's the boxer can operate at & the distance from the oiling source the oiling to the hydraulics would have to be much improved with some sort of small hole flow restrictors close to the hydraulics. That would probably then require some type of inline oil screens & even anti foaming chambers to keep high RPM oil aeration out of the hydraulics.

 

Maybe now that BMW has gone to a lighter engine oil & is going to a new design engine block with water cooling you might see some sort of hydraulics on the valve train but I seriously doubt it.

 

As long as the engine design is like the current design & periodic valve adjustments are super quick & easy I just can't see BMW putting the engineering & complexity into putting hydraulic valve systems on the BMW boxer.

It would cost them a lot up front, bring on a whole new set of warranty concerns & for the most part not gain the customer anything useful.

 

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With proper engineering & a very complex system it wouldn't have to be any wider.

 

Lots of motorcycles have in the past or have now used hydraulic valve lash control systems.

 

On overhead cam setups like the angled cam lobe, splayed valve arrangement on the current camhead BMW Boxer it would be very expensive & quite complex.

 

To keep the reciprocating weight down & the packaging small it would almost have to have a floating rocker with the hydraulics on the rocker fulcrum (like some Hondas use). That would mean a total new head design in the valve train area, a significant change in the oiling to that area, & a lot of more expensive parts.

 

Due to the high RPM's the boxer can operate at & the distance from the oiling source the oiling to the hydraulics would have to be much improved with some sort of small hole flow restrictors close to the hydraulics. That would probably then require some type of inline oil screens & even anti foaming chambers to keep high RPM oil aeration out of the hydraulics.

 

Maybe now that BMW has gone to a lighter engine oil & is going to a new design engine block with water cooling you might see some sort of hydraulics on the valve train but I seriously doubt it.

 

As long as the engine design is like the current design & periodic valve adjustments are super quick & easy I just can't see BMW putting the engineering & complexity into putting hydraulic valve systems on the BMW boxer.

It would cost them a lot up front, bring on a whole new set of warranty concerns & for the most part not gain the customer anything useful.

 

I couldn't disagree more with most of this, though yes, it would be expensive for BMW to change the current cam head design to the one that I proposed...major engineering changes are always expensive. The change from the hex-head design to the cam-head design was, itself, very expensive...and far more so considering how little they actually gained from it and how short the amortization will be before possibly doing it all over again when the water-cooled version comes out.

 

If, however, the Hydraulic Lash Adjuster (HLA) had been employed in the initial design of the new cam head, it would have saved both money and design/manufacturing complexity. Several of the precision features in the cam head could have been eliminated from the head to more than offset the "expense" of HLAs. There would no longer be any need for precision shims or their precision retainers, reducing the reciprocating valve mass. There would no longer be a need for a precision bore in the follower pivot for the follower pivot pin, since the follower would, as you have suggested, now float with the HLA as it's pivot point, making the follower lighter (further reducing reciprocating mass), simpler, and cheaper. As a result, the precision tool-steel pivot pin will also disappear, along with it's retaining clips...and the two required precision pivot pin bores in the cylinder head. The HLA can now be located where all the eliminated follower pivot hardware used to be, with plenty of room to spare, and it requires but one precision bore: the actual nest for the HLA itself. It is a "blind" bore; a mere pocket, actually. That's right, the HLA just sits in its nest and never moves. In fact, it is the one active element in the entire valve train that does not move, ever, no matter what the rpms are. We're not talking about a pushrod engine with reciprocating hydraulic lifters here. The HLA is fed with oil at whatever pressure the engine happens to be supplying at the time, and after its initial pump up to a zero-lash condition, all it requires for continued operation is continued exposure to pressurized oil. Yes, it will shed some miniscule amount of oil, but far less than one of the plain bearings on the nearby cam, and that oil will follow the same (slightly enlarged, if necessary) route back to the sump as does the rest of the used oil from the head. In the cam head, the pressurized oil supply for the HLA is already available (cam feed), requiring only a feed to be drilled from the cam feed gallery to the HLA nest/bore. Since all that is required is pressure without a lot of flow, the size of the feed hole isn't even all that critical. But oil foaming? How? If it doesn't move, and it flows virtually no oil, how the hell does the HLA cause the oil to foam? And filter screens? What for? Is something going to break? And even if it does, the cam head has an oil filter that filters far finer particles that any screen ever could, right? Warranty concerns? How will eliminating several parts (machining operations?) from each head result in an increase in warranty concerns? Remember: this is decades-old, proven technology we're adding here. It's only "new" to BMW...

 

I just don't see how completely eliminating those "super quick & easy...periodic valve adjustments" could "not gain the customer anything useful". Think about it: you would never have to adjust your valves! If this would cause some kind of a trade-off in performance then I could easily understand any objections, but it won't. The resulting constant valve lash would also make it even easier for the also-proposed TBW to keep the throttle bodies in perfect synchronization. The only "drawback" I can think of is that the dealers would no longer be able to sell you valve shims or valve adjustments, no matter how "periodic" they might be.

 

I do agree with you that we probably won't see these improvements any time soon. Looking at the design of the cam head, you can see that BMW's engineers are really good at thinking "outside the box"...they just suck at thinking outside BMW's box. The "not-invented-here" syndrome may well be a factor in this...

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Bob

 

With the angled cam lobes & splayed valves it would still need to have the sperical shim (just not in thickness variations) so those would have to stay or the rocker arm wouldn't be able to follow the valve travel through articulation.

 

It seems strange that Honda uses anti foam chambers & flow restrictors & BMW wouldn't need them. Lots of oil foaming with all those chains in the lower crankcase. I doubt they would work correctly with full high RPM cold oil pressure but still remain properly inflated at hot curb idle in 115°f ambient unless metered.

 

I guess I also can't see how the rocker arm will be lighter but that would be a design/engineering problem to work out.

 

Seems like a valid reason or reasons that modern sport bikes still use mechanical cam hardware. If it was cheaper & more advantageous to the MoCo or better performance then why aren't they on all performance bikes now?

 

That new BMW 6 cylinder would be a candidate for some form of hydraulic valve lash control. That thing is a bugger to get to the valve shims.

Even my KLR should have them first as the cams have to be lifted to access the bucket shims.

 

But on the simple old BMW boxer with the ability to check the valve lash in under 10 minutes per side I guess I can't see adding the complexity of hydraulic valve lash control.

 

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I guess I have trouble understanding how reducing the parts count and machining requirements in each of the cylinder heads constitutes "adding complexity". As for the new BMW 6 cylinder having shim-type valve adjustment, for the reasons stated previously I suspect that it may be because it is a BMW...

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Morning Bob

 

I guess I am having a difficult time understanding that (if) hydraulic valve control is what you say it is--

 

Cheaper-

Less parts-

Lower weight-

Lower machining requirements-

Better for the MoCo-

Better for the customer-

 

Then why didn't BMW use that type of valve control on the re-designed camhead boxer, or on the new high tec 6 cylinder.

 

Or why doesn't Honda, or Kawasaki, or Suzuki use it on their performance bikes?

 

They can't all have stupid design, engineering, & marketing departments.

 

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