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Discussion Starter · #1 ·
Hoping that someone who is more well-versed in engines can explain this to me.

What makes the Hemi design different and/or better from the Chevy LS/LT engines and the Ford 5.0 Coyote?

How is it that it makes more power per cubic inch compared to the Chevy LS?

How is it that it makes MUCH more bottom-end torque, and a little more power, than the Coyote even though the Coyote's heads have 4 valves per cylinder and (I think) flow more air than the Hemi?

What makes the Hemi more or less reliable/durable than its counterparts?

What makes the Hemi even more torquey than the LS at low RPM with a crisper tip-end throttle response?


Conversely...

Why is it harder to squeeze more power out of the Hemi with bolt ons and a tune than it is with the LS and Coyote? Why are Hemis harder to tune than the LS?

Why can the 6.4 Hemi in particular not take very much boost? I know about the piston ring landings but why are they where they are?
 

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Remember, it takes a 6.4 motor to "sometimes" beat a 5.0 Mustang. That Coyote motor is pretty darned good. It also gets a better MPG rating with NO fuel management system. 420 torque is also good for a 5.0, when the hemi 5.7 only has 410 and 85 less hp.
 

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Discussion Starter · #3 ·
Remember, it takes a 6.4 motor to "sometimes" beat a 5.0 Mustang. That Coyote motor is pretty darned good. It also gets a better MPG rating with NO fuel management system. 420 torque is also good for a 5.0, when the hemi 5.7 only has 410.
That's true, but I think that's more reflective of our car's weight and aerodynamic profile compared to the Mustang, and the under-tiring from the factory, than the engine itself. I think its MPG is also due to the weight and aerodynamic difference as well as simply having a smaller displacement.

Yeah, that's true, but the 5.7 is not tuned as aggressively and doesn't have as high of a compression ratio, or heads that flow as much air. Its cam is also fairly mild.

With higher compression pistons, a more aggressive 93 octane tune, a more aggressive cam, ported heads, ported 6.4 intake, larger throttle body, and short or long tube headers, you could probably come damn close to 500 crank horsepower with that thing. In its current factory form, it's just a bit lazy. Doesn't rev high either, I think its limit is 5700 RPM.
 

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All HEMI heads flow better than any stock LS head
 
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Here's some information about the Hemi engine:

The Hemi engine is named for its hemispherical cylinder head. It provides an efficient combustion chamber with an excellent surface-to-volume ratio, with minimal heat loss to the head, and allows for two large valves. However, a hemi-head allows no more than two valves per cylinder, and these large valves are necessarily heavier than in a multi-valve engine. The intake and exhaust valves lie on opposite sides of the chamber and necessitate a "cross-flow" head design. Since the combustion chamber is a partial hemisphere, a flat-topped piston would yield too low a compression ratio unless a very long stroke is used, so to attain desired compression ratio the piston crown is domed to protrude into the head at top dead center, resulting in a combustion chamber in the shape of the thick peel of half an orange.

The hemi-head design places the spark plug at or near the center of the chamber to promote a strong flame front. However, if the hemi-head hemisphere is of equal diameter to the piston, there is minimal “squish” for proper turbulence to mix fuel and air thoroughly. Thus, hemi-heads, because of their lack of squish, are more sensitive to fuel octane rating- a given compression ratio will require a higher octane rating to avoid pre-detonation in a hemi engine than in some conventional engine designs such as a wedge.

The hemi head always has intake and exhaust valve stems that point in different directions, requiring a large, wide cylinder head and complex rocker arm geometry in both cam-in-block and single overhead cam engines (dual overhead cam engines may not have rocker arms). This adds to the overall width of the engine, limiting the vehicles in which it can be installed.
 

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the major item - cross flow heads on Hemis - this allows larger valves as they're opposed to each other vs. inline like most 2 valve layouts. [2.14" intake / 1.65" exhaust]

Shorter, and more straight valve runners for both intake & exhaust - allows higher flow rates that most of the competitions' heads.

The drawback for the HEMI combustion chamber is the quench area lead to higher emissions that have to be dealt with by way of tuning.

One of the steps was moving the rings further up the piston. Compared to the 5.7 - you have more displacement and all passenger car engines (regardless of displacement) are limited to the same grams per mile emissions limts.

So its more a challenge that a small(er) engine can run 'dirtier' relative to its displacement while the larger engine requires more measures to stay within the limits.

The other part is the 392 / 6.4 is 10.9:1 CR - trying to add boost that high static CR with pump gas - normally boosted engines run 9 - 9.5:1 CR to be able to run under varying fuel quality [premium grades range from 90 - 93 octane across the 'states. CA and high altitude areas have lower max octane due to the blends required for those regions)

The ring gaps need to be larger to allow for thermal expansion - under boost the combustion gases run hotter, all the parts (pistons, rings, exhaust valves, heads) are subject to higher heat levels.

On a 392 / 6.4, have those ring gaps close up and butt up and the ring lands will break off. If there's lean detonation conditions, its really touch and go.

The 5.7s run 10.3:1 CR and the ring lands are little farther down, so there's a bit more leeway - but you're still running low amounts of boost vs. doing a proper build for forced induction.

Now the Mustang comparison - you're dealing with a lighter vehicle. So it can run faster with less power.

Drop a Challenger down to Mustang weight and it will be faster yet. Simple physics.

Back to the Coyote - 4 valve engines breath well for high rpm operation. They'll make high hp numbers, but torque will be lower than similar power ratings with a two-valve engine.

I've owned two vehicles over the years with OHC engines - one was 4 valve V6 (Chevy) and the other is 3 valve (Benz). They're both high revving engines and are responsive. Below is the comparison of similar power, but the lighter car is faster.

The Chevy was about 200# heavier than the Benz with power ratings similar.
The Chevy was 9.25:1 with 3.4L and the Benz is 10:1 with 3.2L but the Benz had a bit more torque and 5hp higher.
(210hp / 215tq vs. 215hp / 234tq)

0 - 60 mph Chevy = 7.1 / Benz = 6.9

1/4mi Chevy = 15.4 / Benz = 14.9
 

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Discussion Starter · #8 ·
the major item - cross flow heads on Hemis - this allows larger valves as they're opposed to each other vs. inline like most 2 valve layouts. [2.14" intake / 1.65" exhaust]

Shorter, and more straight valve runners for both intake & exhaust - allows higher flow rates that most of the competitions' heads.

The drawback for the HEMI combustion chamber is the quench area lead to higher emissions that have to be dealt with by way of tuning.

One of the steps was moving the rings further up the piston. Compared to the 5.7 - you have more displacement and all passenger car engines (regardless of displacement) are limited to the same grams per mile emissions limts.

So its more a challenge that a small(er) engine can run 'dirtier' relative to its displacement while the larger engine requires more measures to stay within the limits.

The other part is the 392 / 6.4 is 10.9:1 CR - trying to add boost that high static CR with pump gas - normally boosted engines run 9 - 9.5:1 CR to be able to run under varying fuel quality [premium grades range from 90 - 93 octane across the 'states. CA and high altitude areas have lower max octane due to the blends required for those regions)

The ring gaps need to be larger to allow for thermal expansion - under boost the combustion gases run hotter, all the parts (pistons, rings, exhaust valves, heads) are subject to higher heat levels.

On a 392 / 6.4, have those ring gaps close up and butt up and the ring lands will break off. If there's lean detonation conditions, its really touch and go.

The 5.7s run 10.3:1 CR and the ring lands are little farther down, so there's a bit more leeway - but you're still running low amounts of boost vs. doing a proper build for forced induction.

Now the Mustang comparison - you're dealing with a lighter vehicle. So it can run faster with less power.

Drop a Challenger down to Mustang weight and it will be faster yet. Simple physics.

Back to the Coyote - 4 valve engines breath well for high rpm operation. They'll make high hp numbers, but torque will be lower than similar power ratings with a two-valve engine.

I've owned two vehicles over the years with OHC engines - one was 4 valve V6 (Chevy) and the other is 3 valve (Benz). They're both high revving engines and are responsive. Below is the comparison of similar power, but the lighter car is faster.

The Chevy was about 200# heavier than the Benz with power ratings similar.
The Chevy was 9.25:1 with 3.4L and the Benz is 10:1 with 3.2L but the Benz had a bit more torque and 5hp higher.
(210hp / 215tq vs. 215hp / 234tq)

0 - 60 mph Chevy = 7.1 / Benz = 6.9

1/4mi Chevy = 15.4 / Benz = 14.9
Thats the kind of detail I'm looking for!

So, is this emissions thing the reason why Dodge can't release something like a modern 426 Hemi? Does the Hemi design just make it too hard to meet emissions requirements to the point that the extra displacement isn't useful?
 

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Thats the kind of detail I'm looking for!

So, is this emissions thing the reason why Dodge can't release something like a modern 426 Hemi? Does the Hemi design just make it too hard to meet emissions requirements to the point that the extra displacement isn't useful?
its probably the trade off in efficiency and emissions - the lower the MPG, the higher the Gas Guzzler Tax and that is borne by the buyer.

The fact that the 392 + A8 can get by without Gas Guzzler Tax is a big step
 
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The biggest difference besides being hemispherical head between the ford and dodge s the cam layout. Ford runs an overhead cam set up, 4 cams and no pushrods. Both dodge and gm run pushrod engines. The overhead cams allow the ford to rev higher. Valve train is a more stable at high rpm's. The gm runs a pedestal style rocker arm that has a roller fulcrum. Dodge runs the shafts and no roller rockers. The gm creates less friction for the rockers then the dodge. Push rod engines have been around a LONG time and the overhead cam motors are more current design. Ford changes engine styles frequently to try and gain the edge, but this also has consequences. Reliability for the long run comes into play, look at the 3 valve setups on the 5.4 with phasers. Problems. Now with the gm and dodge the same basic concept is carried over every year and the latest ideas are incorporated like the phaser. Also means the aftermarket can design improvements for them a little easier and will be cheaper. The gm guys are lucky because they have the same basic ecu that isnt hard to play with. Dodge keeps changing it making it harder for the aftermarket to keep up. Look at the newer dodges requiring a communication module now. Dodge might build high hp, but they always try hard to keep us out of the ecu to build more.
 

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Discussion Starter · #13 ·
The biggest difference besides being hemispherical head between the ford and dodge s the cam layout. Ford runs an overhead cam set up, 4 cams and no pushrods. Both dodge and gm run pushrod engines. The overhead cams allow the ford to rev higher. Valve train is a more stable at high rpm's. The gm runs a pedestal style rocker arm that has a roller fulcrum. Dodge runs the shafts and no roller rockers. The gm creates less friction for the rockers then the dodge. Push rod engines have been around a LONG time and the overhead cam motors are more current design. Ford changes engine styles frequently to try and gain the edge, but this also has consequences. Reliability for the long run comes into play, look at the 3 valve setups on the 5.4 with phasers. Problems. Now with the gm and dodge the same basic concept is carried over every year and the latest ideas are incorporated like the phaser. Also means the aftermarket can design improvements for them a little easier and will be cheaper. The gm guys are lucky because they have the same basic ecu that isnt hard to play with. Dodge keeps changing it making it harder for the aftermarket to keep up. Look at the newer dodges requiring a communication module now. Dodge might build high hp, but they always try hard to keep us out of the ecu to build more.
See, this is an issue I have with Dodge (as a company, not the cars they make). They make it a lot harder and more expensive to mod these cars, which doesn't jive with their image of being the "brotherhood of muscle." This ECU unlocking didn't used to be such a pain, it was only post-2015 that it became a pain. Which is, ironically, when Dodge started really going ham with their current marketing strategy.

So the GM engines naturally spin higher due to less friction? Why hasn't Dodge incorporated that kind of design? Is it just not compatible with a Hemi combustion chamber?
 

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So the GM engines naturally spin higher due to less friction? Why hasn't Dodge incorporated that kind of design? Is it just not compatible with a Hemi combustion chamber?
The gm will spin a little higher than the dodge because of the valve train. This is actually the weak link on the dodge. The ford will spin even higher than both dodge and gm. Ya the hemi head design is why the spacing on the rockers. The shaftmount allows for the weird rocker arm assembly. You can get roller tip rocker arms and do upgrades to the valve train and it will spin higher. Also the higher rpm will let the newer head designs flow a little better.
 

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Here is a real Hemi. If you go thorough the pictures you can see how the halve train is set up, our Gen III is very similar.
Notice that think makes 700 HP without a blower!!!! That engine will eat Coyotes and SH** Bow Ties!
 

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Discussion Starter · #17 ·
Here is a real Hemi. If you go thorough the pictures you can see how the halve train is set up, our Gen III is very similar.
Notice that think makes 700 HP without a blower!!!! That engine will eat Coyotes and SH** Bow Ties!
No picture is showing up for me
 
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