Hemi Porsche

[kach22i]

Can anyone add to this story?

http://www.automobilemag.com/news/0407_hemi/
Lee assured me no deception is afoot. The engineers responsible for Chrysler's new engine had better intentions than merely dusting off Hemis in the back of their closet. Early in the research phase, they discovered a combustion chamber that Porsche used for 1965-97 air-cooled 911s offered the ideal starting point for their new design. Porsche's head happened to be a hemi…………………………………… Engineers are not about to leave well enough alone so little from the Porsche design made the long trip to what's now called the 5.7-liter Hemi Magnum V-8. Much of the bowl area of the combustion chamber was filled in to encourage the fuel-air mixture to stir itself prior to ignition. The chamber is shallower than past Hemis so the top of the piston can be nearly flat, thereby saving reciprocating weight. The one vestige from Porsche's classic design is large valves rotated 90 degrees from the most common orientation (in a plane parallel to the crankshaft), and splayed at a 34.5-degree angle in the interests of free breathing.

[jmarenas]

I just forwarded this to my friend whom is the director of Dodge truck marketing, cant wait for his response!

[jluetjen]

It's curious how folks try to make something magical out of something that's common. First off, note that the Hemi dates from 1908 -- not from Porsche. Second, Porsche did not have a single head design from '65 through '97. They made a fairly significant change in the 1970 model year when they changed the shape of the head by reducing the valve angle. 3rd, Having the valves 90 degrees to the direction of the crank was also nothing new -- countless motors did it before Porsche including Jag's, Aston Martins and Ferrari in their 3 liter "Columbo" V12. All of them incidentally also had hemi head designs. Even the 911 head concept is a development of the 356 and 770 F1 heads from the early '60's. So the 911 head was just an evolutionary development of a known technology.

Much as we like to think that everything about our Porsche 911's is radically different, the reality is that it's not the specific features that make a 911 special (they had all been done before), but the way that they were arranged and put together.

The Hemi head did not start or end with Dodge. Only the hype over it did...

PS: It kind of reminds me of the subject of female breasts (which incidently also shares a "hemi shape"). In Europe it's not uncommon to see uncovered breasts on sculptures, on TV, in parks in Munich, etc. Come to the Superbowl and Janet Jackson flashes 1 breast and suddenly it's big news in the US . The Europeans on the other hand are wondering what the whole bruhaha is about. They've been seeing breasts on TV for years!!!

[kmhemi]

I have a 1970 426 Hemi in my 70 Challenger RT, aside from the dome shape the Hemi heads also flow much more fuel and the intake and exhaust path are much shorter and straighter than the common open chamber or wedge heads. The spark plug placement is critical to get the proper fuel burn across the piston. While the new "baby" hemi does have twin plugs it does not have the high compression domed pistons that help make the Hemi magic happen. Not all Hemi engine have domed pistons.
the 426 was advertised at 425 hp for insurance reasons. it was actually right at 500 with proper tuning. that's what hurt them, they are complictaed to work on, solid cams and dual four barrel carbs make it tough to keep in tune.

[kmhemi]

Here is why the Hemi engine is so effecient at making horsepower.

[jluetjen]

BTW, let's be honest here. Even the Hemi is old technology. The Hemi heads are compromised when it comes to combustion quality because they have awful swirl and tumble charactoristics. Having lots of flow isn't everything that it's cracked up to be either, ask anyone who had one of Ford's "Hi Performance" 351 Boss engine's of the late '60's. That head design had huge flow numbers and they ran awfully.

If you want the highest HP head configuration, you need look no further then the work of Keith Duckworth and Mike Costin who refined the 4 valve pente-roof configuration to the point that has yet to be beat. It's got great flow and turbulance. 4-valve heads could also run cleanly from low rev's to peak HP. Finally the 4 -valve configuration also had the benefit of smaller individual valves and associated masses which allowed higher rev's then any 2 valve motor* could achieve. Even the current F1 heads are loose copies of their Cosworth DFV head with smaller valve angles to allow the CR to reach an acceptable level with minimal piston domes (or intruders as they are called) given the tiny strokes of todays V10 F1 engines. While some have tried to improve on the 4 valve pente-roof head with 5 or more valves, no one has yet to develop a better configuration.

So hemi's are nice, but if you really want the goods, get any of the current 4 valve/cylinder engines.

* The only way that today's NASCAR engines can reach 9000 RPM (less then where the DFV began in 1967 ) is through the use of advanced alloys in the valve train and careful computer modelling and design. No small feat, but it really is just trying to make a silk purse out of a sow's ear.

[MikeSid]

Those recent Dodge ads really got under my skin for the very reasons stated above. The one with the father teaching his son to say "Hemi" drives me up the wall. I doubt that most prospective purchasers of that truck even know what a hemispherical head is or what advantage it provides. I think most people just think it's a name like "DuraMax" is for Chevy's new diesel. I guess it's just a pet peeve, but I really think Chrysler is making too much of the word and trying to resuscitate a reputation for the company that was deservedly earned in the '60s by the Plymouth Cuda (it was a cuda right??). As better stated above, Hemi's aren't such a radical design anymore and to make such a big deal of it seems silly.

[Superman]

Yes, I think the domed piston is needed for full power. The domed pistons of the fabled 426 hemi gave the gas pressure more surface area to press against. More power.

No Chrysler did not invent the hemi. But they sure made a lot of them. All through the '50's, for example.

[kach22i]

I learned a lot already, thanks.

Turbulence............I wonder if the 3-valve Ford V-6 going into the 2005 Mustang will have a problem with that.

Odd numbers break up resonance (similar to turbulance/standing waves) right?

I think the stang is one intake valve and two exhaust, allowing the spark plug to be centered.

The four valve stuff works best with two spark plugs per piston, right?

I have a Hemi.
I have a Hemi.
I have a Hemi.
I have a Hemi.
I have a Hemi.
I have a Hemi.
I have a Hemi.
I have a Hemi.

PS: love the feather picture.

[jluetjen]

Turbulence............I wonder if the 3-valve Ford V-6 going into the 2005 Mustang will have a problem with that.

I think the stang is one intake valve and two exhaust, allowing the spark plug to be centered.

Chances are it's the other way around, 2 intakes and one exhaust. In most cases you don't have to try to hard to get exhaust out of the cylinder, the hard part is getting the new air in. Most 3 valve heads that I've seen are simplified 4 valve - pente-roof designs where the 2 exhaust valves were replaced with a single valve. It reduces the cost and complexity of the design for very little detrement in performance at normal (non-F1) engine speeds.

Odd numbers break up resonance (similar to turbulance/standing waves) right?

I haven't seen anything about that in regards to head design. If that was the case, the 5 valve Ferrari F1 engine would still be with us, but it's not. It's not that 3 intake valves isn't better, but it doesn't seem to be a significant enough improvement to merit the extra complexity. Keith Duckworth adopted the philosophy that it can't break if its not on the engine -- so he tried very hard to keep the number of components minimized.

The four valve stuff works best with two spark plugs per piston, right?

Actually, it doesn't seem to need it since the combustion chamber is so compact. Besides, where would you put it since the valves take up almost all of the roof space? Today's F1 engines still have only 1 plug.

[kach22i]

Originally posted by jluetjen
Chances are it's the other way around, 2 intakes and one exhaust.

You are right, I am wrong.

http://www.canadiandriver.com/previews/05mustang.htm
The 2005 Mustang GT's new aluminum MOD (modular) V8 is a single overhead cam design using three valves (2 intake, 1 exhaust) per cylinder.

I must of been thinking about something exotic like a supercharged two stoke diesel or something. I am into wacky stuff at times. Something I read about forced air/fuel intake (at a high rate), and exhaust out via two valves. The V-6 Mustang is not it, sorry.

[konish]

Ummm...I'd like to see more female breasts on TV...

[kach22i]

Originally posted by jluetjen
PS: It kind of reminds me of the subject of female breasts (which incidently also shares a "hemi shape"). In Europe it's not uncommon to see uncovered breasts on sculptures, on TV, in parks in Munich, etc. Come to the Superbowl and Janet Jackson flashes 1 breast and suddenly it's big news in the US . The Europeans on the other hand are wondering what the whole bruhaha is about. They've been seeing breasts on TV for years!!!

I knew this was somehow going to dominate the conversation.

[RMartin]

I have an old Motor Trend with a road test of a 1970 Hemi-Cuda. The performance figures were: 0-60 6.9 secs
1/4 mile 101.2 mph
13.7 secs
WOW! Blazing speed for 425 HP. Any body got stats on the 1970 911S?

[jluetjen]

R&T tested the 2.2S in the March '69 issue with a 0-60 time of 7.3 seconds with only 200 SAE gross HP and a quarter mile in 14.9 at 88.5 mph.

Lesssee -- more then 2x the HP and only a second difference. If you were to consider the braking time from 100-0 I suspect that the numbers would be even closer. If you were to look at the lap time at LRP I suspect that the numbers would easily be favoring the 911.

The 2 liter 911's actually raced with Detroit's ponycars in the early Trans-Am. I'll have to check some of the results to see how many of V8's they normally beat at the end of the race.

[kmhemi]

a 70 Hemi challenger or cuda will dip into the low 11's with ease. I will post some stats in the morning. in 1970 running 13's with a Hemi. that means boiling the tires to about 90 mph. ..I love my 911 guys don't get me wrong.. But in 1970 there were no good tires...I could go on for hours.

[island911]

Originally posted by Superman
Yes, I think the domed piston is needed for full power. The domed pistons of the fabled 426 hemi gave the gas pressure more surface area to press against. More power.

Like so many things, maybe on the surface that idea may sound right. . .but it's not.

"More surface area to press against" does NOT provide more power. It doesn't even push harder on the piston, at any time, due to more piston surface area. The bore defines the force available from cylinder pressure.

[Quicksilver]

Island911 beat me to it.
To take it one step further...

The valve angle, the room required for the valve train, and the angle of the port runners are what defined how much dome there was to the shape of the Hemi's combustion chamber.
The dome of the piston was actually dictated by the desired compression ratio.

A high domed piston is actually inefficient. The most efficient piston shape is flat. That way you don't impede the flame travel and you have the smallest surface area to lose the combustion heat to.
The reason that your V8s have domes is that it would be insanely expensive to move the valve train and intake/exhaust ports around to get the most efficient combustion chamber design. The power improvement of a higher compression ratio is so large that it makes it override the consideration of a fractional loss of efficiency from inefficient chamber design.

Still if you look at the base design of all the AmericanV8 engines you will find that the basic engine design will have been for an engine with flat-topped pistons and a large production volume. Later in the engine families life you will find limited production motors where the basic design is tweaked to give a result outside of the original design.

When building high compression V8s, one of the problems that you have to deal with is flame travel and detonation problems that are caused by domed pistons. That is why if everything else is equal we will select heads with smaller chambers and sometimes we will weld up the chamber to reduce the volume. (and get a good quench area for more turbulence!) If we can keep from poking the piston dome up into the chamber we get better flame travel and nothing to impede swirl flow.

This stuff gets pretty complicated but remember: An engine doesn’t know what brand it is and it doesn’t care!!!


Wayne

[kach22i]

What about "domed" or convex pistons?

Are there concave/saucer pistons, and why?

[jluetjen]

concave/saucer pistons, and why?

Yes. Those engines have what is called a Haron design. The original Porsche 924 engine as well as the common Ford Kent crossflow engine both had this design. In an engine of this design the head is completely flat and the combustion chamber is in the piston. Some benefits/issues with the design are...

Benefits
* You can change the CR just by changing the pistons.
* The combustion chamber is compact with good quench charactoristics.

Issues
* The pistons don't do a good job conveying the heat out to the block.
* The valve sizes are limited by diameter of the cylinder far more drastically then in other designs.
* The heads don't flow that well.

For a street engine the issues are not major, but for developing big HP they are almost insurmountable. The best non-turbo'd 924 2.0 that I've heard about put out about 195 HP with MFI, big CR's, etc. Pretty weak actually if you consider how easy it is to get 195HP from a 2.0 911 motor.

People have been able to get surprising HP from Kent motors just by virtue of so many people working on it. (Sounds like a Chevy V8 again!). But once again, being proud of getting 100 HP/liter from a Kent pales when you see how easy it is get that kind of HP from other designs.

But it took my old favorites Duckworth and Costin to come up with a 4 valve head for the kent motor which allowed it 210+ HP from 1600 cc's in the fairly well known BDA of the mid-60's. (For comparison by those in a V8 state of mind, this state of tune is comparable to a 788 HP 6 liter engine). Note that in the BDA the dished pistons of the original Kent engine were replaced by flat-topped pistons.

From there they took that basic design (bore, head design, etc) and made it into a V8 with a 180 degree crank (basically 2 4cylinder BDA's sharing the same crank and spread 90 degrees apart). This was the concept of the DFV.