| From | Message |
TonyC
Dodge Dakota
JOIN HERE
11/19/2002
22:14:43
|
 Subject: Hemi Engine Question
IP: Logged
Message:
What's the difference of a hemispherical combustion chamber verses a regular combustion chamber. I know it is much more efficient, but physically what are the mechanical differences? Is it more in the shape of the heads or are the cylinders different? I was trying to find a web site that would answer my question, but there were about 19,000 hits and it would have taken too long to go through all of them, so I figured I'd ask here first. Thanks
TonyC
|
50
Dodge Dakota
JOIN HERE
11/19/2002
23:23:14
|  RE: Hemi Engine Question
IP: Logged
Message:
Tony, just go to the "Dakota V6" forum. Look for the title "hemi" started by xxx. It is about #5 from the top as I write this. You will find info from several people that should answer your questions.
|
hh
Dodge Dakota
JOIN HERE
11/20/2002
17:24:17
|  RE: Hemi Engine Question
IP: Logged
Message:
Hot Rod magazine has a nice tech article this month on the new 5.7. Basically, a hemi-head has a dome shape above the piston inside the cylinder head. More explosive force on the slug.
|
JasonT
Dodge Dakota
JOIN HERE
11/20/2002
19:45:35
| RE: Hemi Engine Question
IP: Logged
Message:
Hemi engines are different because of the hemispherical combustion chamber. Where as a 3.9,5.2,5.9,@4.7 have a wedge style combustion chamber with both the intake and exhuast valves side by side in the chamber and on the same angle. On Hemis, the valves oppose each other and are on different angles with the spark plug located in the center of the chamber for efficenticy. the valves opposing each other it promotes great flow from the ports which makes more power than a wedge where the incoming/outgoing air has to make more turns in its path through the head which makes it less efficient.
|
Lab Rat
GenIII
11/20/2002
20:09:34
| RE: Hemi Engine Question
IP: Logged
Message:
Adding to that a sphere has more surface area than a flat combustion chamber which allows larger diameter valves to be used.
Gibson Dual Sport, AirForce One air intake, Jet II, Jet TBS, 180 stat, IAT, TPS, no viscous fan, Lakewood traction bars, "Wilson" antenna ball...when are we gonna see some real parts??!!
|
TonyC
Dodge Dakota
JOIN HERE
11/20/2002
22:22:49
| RE: Hemi Engine Question
IP: Logged
Message:
Thanks guys! Excellent explaination in terms I can understand. I appreciate the info.
TonyC
http://www.geocities.com/tony_cricco/techie.html
|
Hersbird
Dodge Dakota
JOIN HERE
11/20/2002
22:49:38
| RE: Hemi Engine Question
IP: Logged
Message:
really the best benifit is as Jason said, the opposed valves. The heads just will flow much better with intake coming directly from the manifold on the top and the exhaust going directly into the header on the bottom. This is why the valve covers are so wide, the valves form a 'V'. A 4 valve per cylinder design will also accomplish the same thing including the hemi cumbustion chambers but that is much more complicated, less reliable, and much more expensive to produce. Remember the new Hemi costs less to build then the 4.7. Really the old DOHC neon motor was a hemi, with hemi cumbustion chamber, opposed valves (although 4 of them not 2) and a center mounted spark plug with the wires going through the valve cover. That motor did good also making 150 hp from only 2.0 naturally aspirated liters.
|
brianman
Dodge Dakota
JOIN HERE
11/21/2002
01:17:36
| RE: Hemi Engine Question
IP: Logged
Message:
the 4.7 litre is almost a hemi, it has opposed valves and a centre mounted spark plug, but has a flat side in the chamber making it not a real hemi.
hemis have beter positioned plugs, straighter ports for better flow. wedge chambers are to much comprimise between cost and performance, the ports have to find their way around head bolts and pushrods. hemis dont.
|
Texas Todd
Dodge Dakota
JOIN HERE
11/21/2002
12:36:09
| RE: Hemi Engine Question
IP: Logged
Message:
Jason T said:
,@4.7 have a wedge style combustion chamber with both the intake and exhuast valves side by side in the chamber and on the same angle.
brainman said:
the 4.7 litre is almost a hemi, it has opposed valves and a centre mounted spark plug, but has a flat side in the chamber making it not a real hemi.
I tend to agree with brian, sorry Jason.
I seem to remember Opposing valves on my 4.7, when changing my cams.
Jason, I'm not ragging, I could be wrong here. Do you have a 4.7, VERY different motor. Are you sure they're not opposing valves as Brian, and I think? It is OHC.
|
brianman
Dodge Dakota
JOIN HERE
11/21/2002
14:34:21
| RE: Hemi Engine Question
IP: Logged
Message:
i cant remember what site it was but it had pictures of the 4.7 litre chamber, its not wedge or hemi but referred to as having polyspherical chambers. hemi or pentiroof chambers are the easiest to make into overhead cam configuration.
|
Hersbird
Dodge Dakota
JOIN HERE
11/21/2002
21:11:38
| RE: Hemi Engine Question
IP: Logged
Message:
The 4.7 does have opposed valves, and it also has very close to a hemi shaped cumbustion chamber. The spark plugs go right down the middle. They should have just made this motor capable of greater displacement in the first place. Then they wouldn't have had to make the completely new Hemi, although personally I like the reduced cost and simplictiy of a single cam with pushrods over the overhead cam setup.
|
50
Dodge Dakota
JOIN HERE
11/21/2002
21:53:37
| RE: Hemi Engine Question
IP: Logged
Message:
I'm with you, Hersbird. 2 valves/cylinder and pushrods may be old tech but the LS1 and HEMI are proving that it is still a viable design. Overhead cams and multiple valves causes most engines (4.7 excluded) to be low on torque. 2 large valves per cylinder allows us to have hp and torque.
|
JasonT
Dodge Dakota
JOIN HERE
11/21/2002
22:45:15
| RE: Hemi Engine Question
IP: Logged
Message:
Sorry to be misleading guys, But the 4.7 has a non-wedge,but non-Hemi combustion chamber. sorry for the confusion.
|
brianman
Dodge Dakota
JOIN HERE
11/21/2002
23:25:59
| RE: Hemi Engine Question
IP: Logged
Message:
valve actuation has no effect on torque, not even number of valves per cylinder. first look at the ford two valve mod motors, they make more low end torque than a vortec 350. engines are like air pumps, the more air in and the more air out give you more power. two valves do good because they keep air velocity up which help fill the cylinders quickly. put some big port/valve heads on a street motor and watch the low end go right out the window, because big ports/valves kill velocity. number of valves and cam location dont necisarily.
|
50
Dodge Dakota
JOIN HERE
11/22/2002
01:51:52
| RE: Hemi Engine Question
IP: Logged
Message:
Better look again. What looks good on paper doesn't always work in the field. First of all compare apples to apples. Engines that have 4 valves per cylinder makes less torque (especially down low in the rpm range) while providing the approximate same h.p. as their counterpart that has 2 valves per cylinder. The theory that you speak of was used for years until the G.M. engineers got mad when Chevy stuck the ZR1 engines in the Vettes. They set out to prove that you could have the hp and more torque from a 2 valve/cylinder engine. First the LT1/LT4 was born and that research lead to the LS1/LS6. Meanwhile in the Ford camp they were busy trying to get their DOHC motor to move a Cobra as fast as the LS1s. Hp ratings were their but torque was down. 1/4 mile times showed it. They even stopped production of the Cobra for 1 year to correct the previous year's slow ones. Finally in 2002 they got one to run faster than the everyday Camaro but look at how much hp they had to develop to get there. During this time the 2 valve SOHC motor was quickly catching up to their DOHC. Their '02 SOHC is as fast as their earlier DOHC Cobras with a lower hp rating. BUT check out their torque rating.
|
Steve
Dodge Dakota
JOIN HERE
11/22/2002
09:11:29
| RE: Hemi Engine Question
IP: Logged
Message:
In the hemispherical chamber, the valves are angled toward the center of the cylinder. This eliminates valve "shrouding" which is common in wedge engines and lowers the volumetric efficiency in wedge engines. The angle of the valves also allows for cleaner, more streamlined ports and also you can install larger valves in a given cylinder size than you can in the same size wedge engine. It also allows a centrally located spark plug which encourages more efficient combustion.
|
Texas Todd
Dodge Dakota
JOIN HERE
11/22/2002
10:03:18
| RE: Hemi Engine Question
IP: Logged
Message:
Ever thought about this?
How do they get a hemi, opposing valve design, with a single cam, pushrod engine?
I guess the valves are in line front to rear, so the pushrods would be the same length?
Anyone actually seen how it's set up on the new 5.7?
|
brianman
Dodge Dakota
JOIN HERE
11/22/2002
13:50:45
| RE: Hemi Engine Question
IP: Logged
Message:
air velocity is how you fill cylinders. two valve engines have an advantage. the intake valve has a very small surface area in reference to the combustion chamber and cylinder size. look at the chamber of a 4 valve the valves take up most of the area, which can kill air velocity. you see these setups in small displacement engines because the only way to make power is to rev it which is why they need all that surface area, to help fill teh cylinder at high rpm. four or five valves will work fine in torque motors by minimizing pumping losses, as long as port cross section is small and consistent and port length is long. how come a cummins 24 valve made more power at a lower rpm than the 12 valve head. just look at the physics.
|
50
Dodge Dakota
JOIN HERE
11/22/2002
17:50:25
| RE: Hemi Engine Question
IP: Logged
Message:
O.K. I see what you are saying now. We are on the same page. You are looking at the technical aspect while I'm looking at the practical. It rang a little bell in my head when I read the part about small engines.
Sorry about the attitude. I reread my post. It wasn't worded right. Look at the post time. I wasn't really awake.
|
brianman
Dodge Dakota
JOIN HERE
11/22/2002
20:01:26
| RE: Hemi Engine Question
IP: Logged
Message:
no worries. the big three have so much invested in two valve technology, that they are not that willing to try anything else.two valves work great for what they are but they are dated. 4 or 5 valve engines would probably give a good leap in performance and effiency, but are very costly to manufacture. so until this changes i will always want a hemi.
mopar or no car
|
ruha
Dodge Dakota
JOIN HERE
11/25/2002
02:21:02
| RE: Hemi Engine Question
IP: Logged
Message:
Ok, new question for ya, what does the second spark plug do in the new hemi. I have two theories.
A) it is for the exhaust stroke, burning the excess fuel and a lil bit more umph.
B)it has two spark plugs that fire at the exact same time on the power stroke for more umph.
|
rocco
Dodge Dakota
JOIN HERE
11/25/2002
20:08:00
| RE: Hemi Engine Question
IP: Logged
Message:
From what I understand is the plugs do not fire at the same time, only slightly off. It seems they are needed for the large volume created by the hemispherical dome.
|
Lab Rat
GenIII
11/25/2002
20:17:24
| RE: Hemi Engine Question
IP: Logged
Message:
From what I've heard it's for improving emmisions.
LR
Gibson Dual Sport, AirForce One air intake, Jet II, Jet TBS, 180 stat, IAT, TPS, no viscous fan, Lakewood traction bars, "Wilson" antenna ball...when are we gonna see some real parts??!!
|
50
Dodge Dakota
JOIN HERE
11/25/2002
22:21:17
| RE: Hemi Engine Question
IP: Logged
Message:
One mag said it fires a plug in cylinder it normally would while firing in a second cylinder across from it to burn off unburned gases from previous firing. As the LabRat said, it cleans up emissions.
|
blued
Dodge Dakota
JOIN HERE
11/26/2002
23:47:20
|  RE: Hemi Engine Question
IP: Logged
Message:
Dont know if this helps or not but most piston driven aircraft have a twin magneto (sprak plug for all intents and purposes) setup. They fire at almost exactly the same time. they are their for redundancy and for efficiency. you can turn them off independantly and feel the drop in power. It results in about 12 - 15 hp increase in a 150 hp motor, plus ensures better burn for lesser chance of plug failure and misfire... bad at 10,000 feet.
|
CW
GenIII
11/27/2002
09:37:05
| RE: Hemi Engine Question
IP: Logged
Message:
One plug fire on power stroke the other fires on exhaust to help get rid of any unburnt fuel.
4.7 HO, CW style
2001 4.7 5sp 3.92 LSD
Ported 68mm TB, IAT adjuster, 3" flowmaster cat back, TPS @ .76V, 4" Z-tube, Roadmaster active suspention, Removed Third cat, Electric fan Convertion, HO cams, Robert shaw 180 therm, HO intake manifold.
|
Dr.Dakota
Dodge Dakota
JOIN HERE
11/27/2002
13:29:42
| RE: Hemi Engine Question
IP: Logged
Message:
CW is right, Ford has a 2.3L that does the same thing...but after a couple thousand miles they fire together, something to do with EMI and the 1 ciol per sparkplug located really close to each other.
The 4.7L has opposed valves and a spark plug in between...I got pictures if anyone doesn't beleive it. The combustion chamber is semi-hemi, the piston is even closer to a hemi. Its called a quenching chambered engine, a lot like a hemi but with a flat area. The fuel/air mixture travels into the combustion chamber like a hemi (opposed valves etc) but just before combustion its squeezed into the qhenching area where the sparkplug is located, kinda cheating the compression formulas. This is why some people beleive its good to index your plugs.
|
big head boy
Dodge Dakota
JOIN HERE
11/27/2002
21:20:48
| RE: Hemi Engine Question
IP: Logged
Message:
good answers everybody!
i have some more to add as well...
hemis with canted valves have mondo volumetric efficiency qualities , in that they get air out just as fast as they can get it in. providing they're cammed right. therefore you can have straighter ports like on the 426, as opposed to the swirl-port types which almost always have a low side roof to create a pressure drop to fill the cylinder better.
if youll notice , the new hemis arent exactly the same as the old hemi's in that they are more of a "semi-hemi". meaning the bowl shape isnt as recessed into the head as a 392, 426, etc. some folks call polyspherical heads semi-hemi by mistake in that they think seeing it has a flat side off of the radius, its semi-hemispherical. but really all that is semantics.
Mopar Action did some good research on the aspects of the new motor, and pointed at the fact it now has eight frickin coils to drive sixteen plugs. to me this is truly overkill (but then again, i dont make a fortune selling chrysler parts) to eliminate a potential misfire.
you see: as simple and effective as the hemi design is, they will always have a problem with flame travel around the dome of the piston. this is why fuelers have been running dual plug heads for years. it creates incomplete combustion scenarios, unless you run flat top or dished pistons. when you do that you kill compression and henceforth you rob power. this is why my stock 241 redram hemi from 1954 only sported from the factory with a sorry 145 ponies.
well you might say, "just build a slightly domed piston to fill the combustion chamber better." well now what you have done is add tons of weight to an already beefy piston. so the engine loses power that way too in that it cant get up to a good piston speed fast enough. and we already know how the buying public loves jack rabbit starts.
so lets look at another way. make the dome shape a little "less" deep. then make make the piston have very little dome. ah-ha! that'll do it. you couple these ideas with the dual plug idea, and you might just have a winner. might as well throw in a couple of knock sensors to the mix to make absolutely sure of no detonation and premature ignition problems and buddy it should roll. now if they can just build a transmission to handle decent power again. hopefully the best engine they have wont be one they buy from Cummins anymore.
P.S.:to add to the multi-valve, overhead-cam fray... a company called McGee had that same big idea back in the late eighties with 4 valves per head, dual cams per head, dual plugs per head and a frickin blower that could give both my wife and my mistress a good lesson and it just didnt have the bottom end power to keep up with Ma-Mopars tested design in an all aluminum form. basic fact is pushrod engines make better bottom end power, unless you do some real magic with the ports and cam. just ask Kenny Bernstein and Don Prudhome. they tried it and ditched it within five events. if you want top end, stick with the OHC, but then again who wants to twist 8500 revs in order to keep up? oh yea, ford built its blue cresent motors in a similar fashion as McGee and they didnt do so hot either. even with Holman-Moody doing the work for them back in the early seventies, but it sure did scream in indy cars. til they decided 16 feet of timing chain and eight gears is just too much over budget.
|
big head boy
Dodge Dakota
JOIN HERE
11/27/2002
22:11:43
| RE: Hemi Engine Question
IP: Logged
Message:
oh yea, answer to texas todd:
you can use the same length pushrods by simply making the rocker arms offset. what i mean is,
imagine the pushrod rod running a perpendicular line to the deck surface or head surface of the block.. straight up and down from the lifter if you will.
well then all you need to do is create a rocker arm that is either kicked over on one end and mates the two surfaces of the valve tip and pushrod tip, or has the the two mating surfaces of the valve tip and the pushrod tip spread apart the required distance, creating a somewhat "z" shaped rocker arm.
all of chrysler's previous hemis and polys used the latter set-up and the made the pivot point be the rocker shaft that ran through a hole in all of the rocker arms. they even continued this design on the pre magnum small and big block designs. the oil pump pushed oil up the passages in the block to the center main bearing.from there it pushed up through the block to the center cam bearing. once the oil made it to the center cam bearing it "y"'d . one leg of that y fed one rocker shaft oil and so on with the other bank.the pushrods never fed oil to any part of the valvetrain. this would make them like a magnum with its chevrolet-esque oiling system and ball stud rocker assemblies.
im certain mopar used a different setup on the new hemi in that they used a cast rocker on a shortened rocker shaft. that way, if one valve or rocker is bad you dont need to remove all the rocker assembly to get to it.
hemis had two shafts per head and the biggest bitch of any early rocker shaft motor was pulling the whole damn assembly just to swap out one bad lifter or whatever component. plus long shafts are way more expensive than individual shafts.not to mention the carriage assembly that held all this stuff together used the head bolts to hold it all down. im just glad they went back to investment cast type rockers as opposed to stamped steel.
hope this maybe helps out.
|
brianman
Dodge Dakota
JOIN HERE
12/21/2002
12:28:07
| RE: Hemi Engine Question
IP: Logged
Message:
the hemi valve train may be complex but shes hard like a boiled owl. using shafts for the rockers makes them extemely tough, it positively locates the rockers through their motion. its complex but damn does it work.
|
| | P 1 |
|
Post a reply to this message:
Username Registration: Optional
All visitors are allowed to post messages
|
