Unofficialbmw.com

[keiko]

Hi Everyone,

Just been pondering about these two questions:

1. Supposedly, hemi engines are more efficient and powerful.
Is there a good reason why most car manufacturers have stayed
with the flat piston design? I cannot see a good reason at this
day and age with super accurate CNC lathes doing all the
manufacturing work. Of course, it may just be a conspiracy....
Is it because A. car mfg'ers are lazy B. there is a good reason?


2. My friend and I were arguing about the following:
He says that turbocharged or supercharged engines
are inefficient, period.

I say that, provided that the engine maximum HP/torque
are the same (i.e. very similiar dyno graphs), _strictly_ naturally
aspirated will have less fuel efficiency than turbo/supercharged
engines. The reasoning would be that the gas burns more efficiently.
What do you gals/guys think?


Keiko

[donaldan]

Are you confusing the shape of the piston with the shape of the combustion chamber?

Hemi engine has flat piston.

Hemi refers to the hemispheric shape of the combustion chamber. Your assumption that it is "more efficient" may be oversimplification. There are in fact many more efficient designs around, for example, the very popular four valve overhead cam engines found in many of today's cars.

The hemi has to do more with design philosophy than "Laziness" or "Not Laziness." It uses the simpler pushrod design and is in fact easier to manufacture and is very reliable, but may be surpassed by the higher speed and power potential of the overhead cam design. The overhead cam design is more complicated to manufacture. Therefore the reverse may be true. To manufacture hemi is in fact "lazier."
_________________
Don

[panzerkeil302]

I think I remember watching the show "trucks" a while back and they said the hemi was more of a marketing campaign than anything else...and a good one at that.

[peerless]

Hmmmm. Let me throw my $.02 cent's out here. First your poll makes no sense concerning the topic at hand. First lets get the different designs classified and squared away.

1) Closed or 'Wedge' chamber head design. This is generally a 2 valve design. Good for high compression and performance in 2 valve application.
Good mechanical resistance to detonation. Great for N/A applications, (all throttle)

2) Open or 'Semi-Hemi' chamber head design. This is usually a 2 valve configuration. Lower compression design. Lower manufacting and design cost. Not something you would find in a 'Performance' application. You can add forced induction to boost power becuase compression is usually lackluster.

3) True Hemi chamber design.
True hemi designs will have the valves outboard and spark plug in the middle. Mostly a 4 valve configuration. Supposed to provide the best 'swirl' properties enhancing fuel efficiency. Compression is usually decided in piston design which is usually dictated by end goal of the vehicle it will be placed in. IE: Performance car or Commuter car. Both N/A and boosted performance can have significant gains with this design.

All head designs may use any variation of piston design. Flat-Tops, Dished, Domed depending on performance level or economy desired.

You can make any of these designs good or bad, again depending on intended application. You and your freind can analyze all you want. Efficiency can be built in to amost all head designs. In the end it comes down to intended application. Is a turbo engine more efficient then a N/A application? Well it depends doesnt it. For the most part Forced Induction is less efficient. More air requires more fuel which messes with emmissions which means its detuned from the factory. Low compression so you dont destroy the engine under boost when a lazy consumer doesnt change the fuel filter in 100,000 miles. Not what I would call efficient. You have to think about emmission standards and fuel milage too. It has absolutely nothing to do with manufactures being 'lazy' or one design being more efficient then the other.

How about the little 4 cyl. hemi N/A with decent compression and good power to wieght ratio. They get killer gas milage. Its a hemi. Look at Honda engines, they dont make a whole s pot of power but they get good gas milage and have almost been hemi design for a couple decades now. Toyotas, Nissians, most of the asian makes are hemis. A lot of european cars are hemi's. I would say for effeciency hemi would be the way to go.
4 valve hemi's really respond well to forced induction.

For N/A performance its definatly a closed chambered wedge head with 'canted' valves. You can really crank the mechanical compression up there and still be safe on standard pump gas. What this setup gives you is really crisp and immediate throttle response. By far my personal favorite of the designs.

I have driven a few turbo cars and ya they can be fast and powerfull, their initial throttle reponse sucks and are generally doggy and inefficient off boost.

Just my experience.
_________________
Robert

E30 Repair and Maintenance Services
714-398-8405

[donaldan]

Maybe, just maybe, the original posting refers specifically to the Chrysler/Dodge/Mopar design of "Hemi." In fact, Chrysler uses the name "Hemi" as a registered trademark for marketing purpose but DOES NOT own the design. It is a pushrod engine with hemispheric combustion chamber, 2 valve per cylinder with a center located sparkplug. It is not particularly sphisticated compared with today's multi overhead cam 4 valve per cylinder penthead combustion chamber engine. Many variations of the latter are sold by the different manufacturers. Some have 3 valves per cylinder. Others have 5 valves. Some have double camshaft and others have single camshaft etc. etc.

Because of the simplicity of the Chrysler's "Hemi" it can be easily built, easily maintained and made to be quite reliable. But overall, it is not particularly more efficient than the overhead cam designs, nor can be made more powerful per cu. in.
_________________
Don

[peerless]

[donaldan]

Simple mathmetics:

Your supercharged hemi produces 1.0hp per cu. in. - 426hp divided by 426 cu. in.

BMW M3 with 3.2 liter engine (=193.3 cu. in.) produces 333 hp, or 1.72 hp per cu. in. - 333hp divided by 193.3 cu. in.

Note that the BMW engine is naturally aspirated without supercharger. Even without a supercharger the BMW's overhead cam engine produces 72% more power per cu. in. than the 426 hemi engine.

Another example, the E39 BMW M5 with a 4.9 liter (=296 cu. in.) produces 400 hp, or 1.35hp per cu. in. That is 35% more than your 426 hemi. Again, it is done without supercharging. With supercharging, the 4.9 liter BMW engine is known to go over 500 hp. And that is done with an engine 30% smaller than your hemi.
_________________
Don

[donaldan]

Simple mathmetics:

Your supercharged hemi produces 1.0hp per cu. in. - 426hp divided by 426 cu. in.

BMW M3 with 3.2 liter engine (=193.3 cu. in.) produces 333 hp, or 1.72 hp per cu. in. - 333hp divided by 193.3 cu. in.

Note that the BMW engine is naturally aspirated without supercharger. Even without a supercharger the BMW's overhead cam engine produces 72% more power per cu. in. than the 426 hemi engine.

Another example, the E39 BMW M5 with a 4.9 liter (=296 cu. in.) produces 400 hp, or 1.35hp per cu. in. That is 35% more than your 426 hemi. Again, it is done without supercharging. With supercharging, the 4.9 liter BMW engine is known to go over 500 hp. And that is done with an engine 30% smaller than your Chrysler hemi.

Furthermore, the BMW engines will use significantly less fuel to produce equivalent horsepower than the Chrysler hemi.
_________________
Don

[peerless]

Ok Dan I think I see your point. I wasnt really defending the 426 hemi, but lets look at some actual numbers.

1965 426 Hemi stock trim, Corrected HP@6,000 rpms = 463HP & 472ftlbs TQ@4,000rpm.
http://www.allpar.com/mopar/hemi/dyno-test.html

Simple Mathmatics:

463Hp divided by 426cu inches = 1.08HP per CI

That nice yellow one I posted, Lets try along the lines of 890HP

890HP divided by 426 CI = 2.8HP per CI

----------------------------------------------------------------------------------

Regardless of all this junk, thats not really what the topic was about. It was relating to cylinder head design and the end result on effective combustion and volumetric efficiency.

Stay on target
_________________
Robert

E30 Repair and Maintenance Services
714-398-8405

[keiko]

Knowing that the BMW engines have lots of torque, I still want to say that HP per cubic in. seems like an inaccurate yet useful measurement.

[scrat]

ok i got to add my 2 cents. to. Anyone thinking that a bmw m3 engine is better than a hemi engine from the 60's is nutts. Truth to the matter. The Hemi engine was the elite engine. With no other engine that can compare to it. Thats what got it banned from most race tracks. As no other engine manufacture could produce a more powerfull engine. This is true to torque and hp. You just cant compare them. I love bmws. I really do. But i would not be dumb enough to try and race a hemi on the track. Maybe an m5 can take on a car from that error. but not an m3. a superbee, daytona, roadrunner. would kick the s out of an m3.

Case closed.
_________________

[panzerkeil302]

Case closed?

Okay, for it's time, the hemi was great...the same way Elizabeth Taylor was great (in her 20's)...




But scrat, that was 40 YEARS AGO!,,,would you want to nail Elizabeth Taylor today?

[donaldan]

Aman. Penzer, you couldn't have said it better! But then, there are those who are nostalgic and want to return to the old days - big hulky engines slurping in gallons of fuel, belching thick smoke and resonating with pounding exhaust. Blessed are those who can live the youth that has past many of us, me particularly.
_________________
Don

[peerless]

[scrat]