A tale of two Hemis: Chrysler's 5.7 and 6.1 V8
In this installment, we'll take a peek inside the Chrysler Hemi siblings - the garden-variety 5.7L, and the rip-roaring 6.1L version found in SRT8 products.
Inevitably, someone will ask, "are these engines really Hemis?" First, we should define the term Hemi - it's short for "hemispherical combustion chamber", and it relates to the distinctive half-spherical combustion chamber shape used in certain Chrysler V8s back in the 50s and 60s. The 426-cubic-inch second-gen version dominated stock-car racing and still has its place in drag racing; it wasn't really a great street engine, but regardless it built name recognition like few other engines.
OK, so are these really Hemis? The modern rendition uses two "squish pads", one each to the front and rear of the chamber; this helps keep the air-fuel charge closer to the center of the chamber and encourage additional turbulence. Ideally, a combustion chamber would resemble a sphere, as this results in the least surface area for a given volume (that means less heat rejection to the coolant) and the smallest burn distance for a spark plug. Domed pistons in a hemi chamber results in a thin skin of charge; imagine a golf ball sitting next to an orange rind and you get the idea. The squish pads improve the situation significantly, with the only drawback being semantics regarding the trademark name. Squabbling about the chamber shape neglects the most important attribute of the Hemi design - the valve placement. We'll get to the importance of that in just a moment, but suffice to say that this new Hemi deserves to carry the name.
There's nothing too special about the engine when viewed from this angle. We can see the size of the valve covers, which are somewhat larger than typical for an OHV engine. We can also see the coil packs mounted on the covers; normally, each coil also sports a plug wire that jumps to a cylinder on the opposite bank. Each coil drives two plugs in series, and each coil fires every revolution of the crank (instead of every two revolutions, as would be typical for a four-stroke engine). This yields a pair of spark plugs for each cylinder that are fired both on the compression and exhaust cycles - likely a necessity to improve the ability to light off the sparsely-distributed mixture in the Hemi combustion chamber.
Note the complex intake manifold geometry. The large plenum and long runners help maximize the torque across the rev range and help the engine to come on strong right off idle without excessively compromising upper-end power.
Ah, yes - the above three shots show the reason that Hemis have built a reputation for horsepower. The valves are arranged perpendicular to the cylinder bank, which puts the intake valve at the inside of the cylinder and the exhaust valve at the outside. This means that air has a nice straight shot from the intake runner into the port and right down into the cylinder, instead of going through a tight radius and across the back of the valve as is the case with a traditional "wedge" chamber.
The angle of the valves allows them to travel towards the center of the cylinder and away from the cylinder walls. This helps prevent the phenomenon of "shrouding", where a portion of the valve becomes ineffective. Due to this, the flow of Hemi heads is quite impressive across the full range of valve lift.
Here's the domed pistons that are required to achieve the 9.6:1 compression ratio with the relatively large combustion chambers. Unfortunately, this peek into the lifter valley doesn't show the hardware responsible for the engine's Multi-Displacement System that deactivates four of the eight cylinders during part-throttle operation. Oil pressure is used to control a lost-motion system in eight of the sixteen lifters; essentially, a pin is pulled that allows the lifters to collapse during cylinder deactivation.
If it has an orange block and black crinkle-finish valve covers, it must be something special...
Here's the high-perf 6.1L version of the Hemi. The first thing to note in the picture above is the sweeping long-tube headers; such geometry was traditionally not found on factory exhaust manifolds, but modern sheetmetal fabrication techniques have achieved wonders. Chrysler claims that they're worth an additional 12 HP over the 5.7's cast manifolds. Not quite so visible is the extra 3.5mm of bore that gives the 6.1L its extra displacement.
In the above two pictures, note the substantially different intake manifold geometry. The runners are shorter, with a longer cross-section; that's a classic recipe for moving the powerband higher in the rev range. Think of this as a modern interpretation of the classic tunnel-ram intake.
The above three pictures show just about everything one needs to know about the Hemi's ability to make power. The airflow paths into and out of the cylinders are smooth and sweeping, with generous cross-sections and no dramatic change in direction. This engine was built to breathe. The 6.1 gets larger intake ports, and the large bore enables the use of larger valves.
Note the flat-top pistons. Due to the increase in displacement, it was necessary to eliminate the dome found on the 5.7's pistons in order to keep the compression ratio down to a reasonable 10.3:1.
We can also see the odd lifter angle of the Hemi, as well as the toothed reluctor wheel used by the crankshaft position sensor. What we can't see are the details of the 6.1's camshaft, which utilizes more lift and duration to make the most of the high-flowing intake and heads.
Barely visible here are the oil squirters that keep the pistons cool. This is certainly not a typical feature for a mass-production domestic V8.
This particular cross-section of the head shows the twin spark-plug bores.
Reader Comments (Page 1 of 2)
james 9:49AM (2/20/2006)
excellent write up.
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Michael Karesh 9:58AM (2/20/2006)
Excellent write-up. I especially liked the comparison of the intake and exhaust strategies.
A shame that the 300C and Charger with the 6.1 carry a $2,100 gas guzzler tax. Would it really have been that hard to give the SRT engine MDS? Even if it cut the tax in half, that's $1,000 per unit. Would it have cost more than this to fit MDS?
Of course you can always buy the Magnum SRT8, which as a truck evades the tax.
With the sedans, as much as I like the 6.1's stronger top end the 5.7 is about equally strong up to about 5,000 RPM, and given the tax costs far less. Thus I think a 300C SRT8 with the 5.7 would be a welcome alternative.
To compare the prices of these cars with and without the SRT treatment, go to http://www.truedelta.com . Basically, Chrysler gets an extra $4,000 or so, which seems fair, while the feds get $2,100.
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klaatu 10:16AM (2/20/2006)
"Hemi" V8 engines (overhead valve, 2 valves per cylinder, valves located at angles with intake valves more towards the inner part of the "vee" and exhaust valves more towards the outer, as opposed to most OHV V8's which use valves sitting side-by-side in line with the row of cylinders) are fairly uncommon, but not exclusive to Chrysler, never were.
Studebaker engineers considered this design for their all-new 1951 V8 engineered in house and chose wedge-head instead (a move Chrysler did to save money in 1958, replacing their original "Hemi" V8's with wedge-head engines).
Daimler (the British company) manufactured 155 cu.in. and 273 cu.in. "Hemi" V8's designed by an ex-motorcycle engineer in the late 1950's through the last 1960's, even after being purchased by Jaguar.
The oldest still extant auto manufacturer in the world, Tatra of Czechoslovakia, long have build "Hemi" V8's (air cooled, in the rear of their limousines), the mid-1960's engine being about 155 cu.in.
The Toyota Crown V8 from the mid-1960's (until the "Hemi" V8 was replaced by a Lexus based DOHC V8) started out with about 155 cu.in. and grew over the decades.
Ford had an "overhead camshaft V8" (with hemispherical combustion chambers) in the late 1960's displacing 427 cu.in. (and a very few even saw sales to the general public, in Mustangs).
The 2-valve per cylinder "Hemi" engines (of many types, including inline fours from Toyota for years) have been supplanted by dual-overhead camshaft, 4 valve per cylinder, pentroof combustion chamber designs by virtually everyone. Like almost all Toyota, Honda, Hyundai and Kia engines, for example.
Only "Detroit" manufacturers persist with OHV engines, which can be fine, but aren't exactly "modern". It's like selling "sizzle instead of steak" to brag about your ("Hemi") engine when it is not truly up-to-date.
Example of how far we've come with DOHC 4-valve per cylinder engines; my wife's "humble" Hyundai Sonata V6 is about 168 cubic inches in displacement, the car weighs about 3100 pounds, the advertised "Net" (at wheels) horsepower is 180, zero to 60 in 9 seconds (power steering, power all wheel disc brakes, air conditioning, air bags, etc etc), top speed somewhere north of 130 mph where legal. About 24 mpg.
A 1957 Studebaker Silver Hawk V8 (the most modern looking, low, aerodynamic car of the era) weighed in at about 3100 pounds, the advertised "Gross" (on dyno) horsepower was 225, zero to 60 in about 10 seconds (no power steering, no power brakes, drum brakes all around, no air conditioning, no seat belts). Top speed 115 mph. About 18 mpg.
You "could" buy a 1957 Studebaker Hawk with a 170 cubic inch "flathead" inline six, it had 90 horsepower "Gross" (on the dyno), zero to 60 was about 19 seconds. Top speed about 85 mph. About 22 mpg.
The "flathead" engines were way out-of-date even by the 1950's and by 1965, were completely gone from the new car lists. The design dated from the 1910's.
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JohnV 10:27AM (2/20/2006)
Fantastic morning reading. Thanks for this feature guys....keep em coming.
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Brian Cross 10:31AM (2/20/2006)
I still don't see what's so special. It's same technology Honda has been using for 20 years
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Wally 10:58AM (2/20/2006)
#3 Nobody but Nobody made HEMI engines famous like Chrysler! Ask anybody on the street about a HEMI an they say Chrysler. One of the top names in engines in the world!
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Ted 10:59AM (2/20/2006)
Klaatu, you bring up a good point with the advantages of DOHC 4 valve designs, but you are ignoring the fact that a lot of that power advantage comes also from the computer controlled electronic fuel injection systems as opposed to the carburators used on cars from an earlier era. Also, your understanding of gross and net horsepower ratings is incorrect. Both are ratings at the crank on an engine dyno. The earlier gross ratings were achieved sans accessory drive and emissions controls, the later net ratings included those items on the engines that were tested. That small bit of criticism aside, your statement is true, modern engines are smaller and more efficient than their predecessors.
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Billy C. 11:23AM (2/20/2006)
Interesting article. Interesting engine.
Until now I thought a Hemi was an an engine wanted by either two young idiots with subnormal IQs or by a 30 year old married idiot with a subnormal IQ. Great image building by Chrysler.
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Steve B. 11:25AM (2/20/2006)
"Only "Detroit" manufacturers persist with OHV engines, which can be fine, but aren't exactly "modern". It's like selling "sizzle instead of steak" to brag about your ("Hemi") engine when it is not truly up-to-date."
Not sure, but I think Bentley still uses them in one of their cars.
The "Hemi" design isn't particularly notable, though using two spark-plugs per cylinder eliminated the incomplete combustion problems that plagued classic Hemis. From a competitive standpoint, the design isn't inherently more powerful, with power generally produced by making them freakin' huge... from a marketing standpoint, it is pure brialliance. Just imagine if the V8 in the final Oldsmobile Auroras had been marketed as the new "Rocket" V8.
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John Neff 12:06PM (2/20/2006)
Stay tuned for later in the day for another engine feature on the Ford V8s in the GT500 and GT.
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MikeW 12:51PM (2/20/2006)
The 6.1 V8's camshaft does not allow for MDS, so unless one would willingly give up about 25hp, then MDS could be made available.
The hemi, back in the day-1951 firepower, did not have one innovation that was new. The reverse rocker arm for the intake was done by the germans in the 1938.
The hemis need a dual length intake manifold, which chrysler ruled out because of cost.
Also the 5.7 hemi takes 89 AKI fuel (mid-grade) and the 6.1 take 91 AKI (premium)
and there are two camshafts, one or the LX cars-torque oriented 340hp & 390ft-lbs, another for the pickups 345hp & 375ft-lb.
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Travis Ayres 2:35PM (2/20/2006)
OHC engines were developed before OHV engines, so as far as 'newer' technology goes, we've technically taken a step backwards. Newer is not always better - in fact, date of design has nothing to do with efficiency of design. People take the simple idea that generally things get better over time, and they use it as some kind of sub-conscious mantra that implicitly supports the line of thought, 'Its newer! It must be better!'
This is like those idiots who say horsepower/liter is an effective tool for evaluating a street engine. It is probably very effective in a race class, like WRC, where engines are limited to a certain size. But let me explain why it makes no difference at all whether an engine is OHV or OHC -
Imagine an engine that was 1000 cubic inches, got 40 miles to the gallon, and made 400 horsepower and 400 ft/lbs of torque, and was OHV in design. Lets imagine it was made of a new material, so it also weighs only 200 pounds, with accessories and headers. Now, 1000 cubic inches is roughly 16.67 liters, and at 400 HP, this imaginary engine is going to get a little over 24 hp/liter.
Some fool is going to say, THATS A TERRIBLY INEFFICIENT ENGINE! Well, you're right, when it comes to hp/liter, aren't you? But if it gets better gas mileage than your engine, puts out more power than your engine, is lighter than your engine, and it isn't in some limited racing class, then apparently its MORE EFFICIENT in every way that counts!
Basically, OHV or OHC is totally insignificant, its just the status of the relevant designs and materials, which is always changing so fast it is STUPID to argue over. Those who say OHC are for revving higher, need to think that if they design a material to make pushrods and cams super light weight, you've negated that benefit of OHC (the Corvette Z06, with the OHV LS7 engine that has pushrods and revs to 7K is a perfect example). So you're arguing for a design based on its aptitude, which in application has so much to do with design specifics and materials that your original argument is null.
So you're arguing the current status of designs that have nothing to do with what you can get your hands on (unless you have infinite funds, of course) for the bragging rights for your street cars that are already outdated (essentially) once they reach the market. BRAVO. Way to think!
Lets all get together, and rate engines based on the relevant efficiencies, shall we? PLEASE? MPG, HP/TQ, engine size, engine mass, distribution of engine mass (as it is pertinent to the vehicle's center of gravity), and power at given RPM (a real dyno chart would be best, something that shows the power under the curve, so to speak).
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Bob F 3:15PM (2/20/2006)
Quote: "With the sedans, as much as I like the 6.1's stronger top end the 5.7 is about equally strong up to about 5,000 RPM,"
Just a reply to this - as a past owner of a 5.7 and a present owner of a 6.1, I can definitively tell you that the 5.7 is no contest for the 6.1 at any RPM level. They are 2 different beasts and they act as such.
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mike 5:31PM (2/20/2006)
HEMI is what you get when you own one of these. Gas mileage is terrible compared to Ford and GM. If you get one save some extra money for your preperation H.
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Art 7:31PM (2/20/2006)
GREAT, GREAT post. The exact terms and design principles allude my feeble 16 year old mind. Still amazing, and fairly layman.
Now, in regards to a hig revving motor: at the sacrifice of torque, won't a large piston connecting rod/stroke ratio cause a given combustion cycle to yield more rotation of the crank than a small rod/stroke ratio?
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NATHAN 10:33PM (2/20/2006)
REP. MIKE DON'T THINK 14 CITY 22 HWY IS BAD FOR A 1500 QUAD CAB 4X4. NONE OF MY FRIENDS WITH FORDS OR GM'S ARE GETTING THAT.AND ALL OF THIS ON A 04 NON. MDS TRUCK IN THE ALTITUDE OF COLORADO!!!!!!
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Jim 3:37AM (2/21/2006)
I just love the reference to " the garden-variety 5.7L" HEMI. Just think, during the dark days of my high school years (late 70's, early 80's), a Corvette had to make due with less than 200 HP. Now my 120 cubic inch Honda engine makes that, and the 5.7L HEMI with 345 HP is the "garden-variety", and comes is a station wagon. I love it!!
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Gary 8:19AM (2/21/2006)
The 426 wasn't a "true" hemi, either. They had to tilt the heads some 15-20 degrees to shoehorn it into the intermediate bodies.
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Michael Karesh 10:11AM (2/21/2006)
Bob F (#16):
I posted the above comment while away from my office. Upon reviewing the power curves in the 2006 Magnum brochure, it appears that the 5.7 and 6.1 are only very, very close up to 4,000 RPM, not 5,000. At 5,000 the most powerful 5.7 produces 350 horsepower, the 6.1 about 390.
The two might still feel much different at lower RPM, but unless the power curve Chrysler provides is inaccurate this is due to different final drives, throttle calibrations, exhaust notes, etc. These could also be applied to the 5.7.
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CrunchyCookie 2:27PM (2/21/2006)
Travis, HP-per-liter mostly covers your ground because size varies almost directly with gas mileage anyway.
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