Dohc Hemi
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Dohc Hemi
I’m sure this has been brought up some time in the past but here goes.
Given the combustion chamber design of the LT5 is it considered a hemi?
In other words does my Vette have a DOHC HEMI?
Given the combustion chamber design of the LT5 is it considered a hemi?
In other words does my Vette have a DOHC HEMI?
Instructor
Joined: May 2006
Posts: 107
Likes: 1
From: Tennessee
A hemispherical head design is one where the combustion chamber is more round, the 4 valve heads on the ZR-1 and just about every motorcycle I've ever worked on or owned are the dual overhead cam 4-valve type (except the air cooled, pushrod 2v HD and my 5 valve per cylinder Yamahas 
) which I don't believe have a special name. Suzuki used to call theirs "TSCC" for "Twin Swirl Combustion Chamber".
) which I don't believe have a special name. Suzuki used to call theirs "TSCC" for "Twin Swirl Combustion Chamber".
Le Mans Master
Joined: Jul 2005
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Originally Posted by We Gone
I’m sure this has been brought up some time in the past but here goes.
Given the combustion chamber design of the LT5 is it considered a hemi?
In other words does my Vette have a DOHC HEMI?
Given the combustion chamber design of the LT5 is it considered a hemi?
In other words does my Vette have a DOHC HEMI?
It is considered to be a pentroof combustion chamber like other 4-valve heads. In the thread Neat started, somebody said the LT5 has a hemishperical combustion chamber, but I was so sick of that thread, I didn't bother to post a correction as there were many other errors in that particular post.
A lot of people don't know the "new" Chrysler "Hemi" really isn't a hemi. It has the intake and exhaust valve across from each other like the old Hemi, but the combustion chamber is not a true hemi since it has a squish/quench area (a hemi has zero squish/quench).
A true hemi looks like half of a sphere...thus hemispherical combustion chamber. While this allows for much bigger valves for a given bore size (with the advantage of more flow of course), from a thermodynamic standpoint, they are the most inefficient ohv design. They have the highest combustion chamber surface area to bore ratio meaning a lot of heat is transfered to the cylinder head jacket water. The piston has to have a crown/pop-up to get compression which again gives more surface area to transfer heat versus a flat top piston. And since heat is horsepower, the more heat rejected to the coolant means less to drive the wheels affecting economy, emissions, and power. That's why Chrysler had to add a squish/quench area to the new hemi...to get the fuel economy numbers up and the emissions numbers down.
The pentroof design is widely recognized as the best. It has the lowest combustion chamber surface area to bore ratio, the highest valve "window" area (since it has 4 valves), lightest valvetrain mass, central spark plug location allowing the least amount of ignition advance (causes less "negative" work as the piston continues to TDC) etc, etc
Sorry this got long, hope it helps.
Instructor
Joined: Oct 2003
Posts: 142
Likes: 0
From: Algonquin IL
I knew this had to written by an engineer. No one else would go into that much detail.
Originally Posted by glass slipper
In a word....NO.
It is considered to be a pentroof combustion chamber like other 4-valve heads. In the thread Neat started, somebody said the LT5 has a hemishperical combustion chamber, but I was so sick of that thread, I didn't bother to post a correction as there were many other errors in that particular post.
A lot of people don't know the "new" Chrysler "Hemi" really isn't a hemi. It has the intake and exhaust valve across from each other like the old Hemi, but the combustion chamber is not a true hemi since it has a squish/quench area (a hemi has zero squish/quench).
A true hemi looks like half of a sphere...thus hemispherical combustion chamber. While this allows for much bigger valves for a given bore size (with the advantage of more flow of course), from a thermodynamic standpoint, they are the most inefficient ohv design. They have the highest combustion chamber surface area to bore ratio meaning a lot of heat is transfered to the cylinder head jacket water. The piston has to have a crown/pop-up to get compression which again gives more surface area to transfer heat versus a flat top piston. And since heat is horsepower, the more heat rejected to the coolant means less to drive the wheels affecting economy, emissions, and power. That's why Chrysler had to add a squish/quench area to the new hemi...to get the fuel economy numbers up and the emissions numbers down.
The pentroof design is widely recognized as the best. It has the lowest combustion chamber surface area to bore ratio, the highest valve "window" area (since it has 4 valves), lightest valvetrain mass, central spark plug location allowing the least amount of ignition advance (causes less "negative" work as the piston continues to TDC) etc, etc
Sorry this got long, hope it helps.
It is considered to be a pentroof combustion chamber like other 4-valve heads. In the thread Neat started, somebody said the LT5 has a hemishperical combustion chamber, but I was so sick of that thread, I didn't bother to post a correction as there were many other errors in that particular post.
A lot of people don't know the "new" Chrysler "Hemi" really isn't a hemi. It has the intake and exhaust valve across from each other like the old Hemi, but the combustion chamber is not a true hemi since it has a squish/quench area (a hemi has zero squish/quench).
A true hemi looks like half of a sphere...thus hemispherical combustion chamber. While this allows for much bigger valves for a given bore size (with the advantage of more flow of course), from a thermodynamic standpoint, they are the most inefficient ohv design. They have the highest combustion chamber surface area to bore ratio meaning a lot of heat is transfered to the cylinder head jacket water. The piston has to have a crown/pop-up to get compression which again gives more surface area to transfer heat versus a flat top piston. And since heat is horsepower, the more heat rejected to the coolant means less to drive the wheels affecting economy, emissions, and power. That's why Chrysler had to add a squish/quench area to the new hemi...to get the fuel economy numbers up and the emissions numbers down.
The pentroof design is widely recognized as the best. It has the lowest combustion chamber surface area to bore ratio, the highest valve "window" area (since it has 4 valves), lightest valvetrain mass, central spark plug location allowing the least amount of ignition advance (causes less "negative" work as the piston continues to TDC) etc, etc
Sorry this got long, hope it helps.
