- Mar 31, 2003
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There is a lot you have to learn before you should be asking big questions. First try to understand the principle of an air pump and the basics of the conversion of mechanical energy before you jump into differentials.
From all the responses, you could tell there is a lack of knowledge across the board!
No. "HEMI" refers to the shape of the combustion chamber and placement of the sparkplug. Hemi is short for hemispherical. The true Hemis' were in the Dodge motors in the late 60s-70s. The new Chrysler Hemi is not a true hemi, but similar. One of the advantages is since the spark plug is in the center of the combustion chamber, the air/fuel mixture can burn quicker (less distance for the flame front to travel) and more completely to produce a bit more power.
VTECH varies the cam timing, allowing through adjustments, more air velocity in the intake runners = more power.
Overall, an engine is nothing more than an air pump. More air in and out = more power. That's why turbochargers, superchargers and large displacement motors make so much more power.
Too many disadvantages, weight distribution, service access and more.
Basically, our society and most automotive support systems are set up for front-engine cars. They also assist in frontal crash ratings.
iamwiz82
Lifer
- Jan 10, 2001
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Yes. H1's have front and rear, IIRC. Being locked is not the same as an LSD. A locked differential will never allow the tires to spin at different speeds. On turns, the outer will drag a bit as it's spinning at the same speed as the inner, which isn't fast enough.
An LSD still allows some slipping.
Now, for subsets of lockers:
1. There are lockers out there that only engage while powering the axle, and when there is no power, it unlocks. Examples are Detroit Lockers.
2. There are lockers that can be turned on and off automatically, so it's open when you want and locked when you want. Examples are ARB lockers, OEM Jeep Wrangler Rubicon, Hummers
3. There are full time lockers, meaning it never unlocks, ever. I don't believe there are any manufacturers out there that make these, but I know many people who have welded the spider gears together to create a locker. I had an automatic locker (see #1) that broke and ended up turning into a fully locked differential. turning is a chore and it puts TREMENDOUS strain on the whole axle.
Zenmervolt
Elite member
- Oct 22, 2000
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No, "Hemi" is a marketing term describing an engine with hemispherical combustion chambers. Many, many cars have hemospherical combustion chambers today.
VTEC (not "vtech") is for "Variable valve Timing and lift Electronic Control" and is a method of employing two (or more) dinstinct camshaft profiles and the ability to alter camshaft timing to allow greater flexibility from an engine. A camshaft that works great at high RPM will make the engine soft at low RPM and vice-versa. Lowering the VTEC transition threshold simply makes the high-rpm cam profile and timing come on sooner. Set it too low and you will actually lose performance. A camshaft is one of many variable in an engine's design and the optimum transition point will vary depending on what other work as been done to the engine.
The disadvantage of a rear engine is horrible handling (with rear drive) and lower traction (if rear engine and front drive). Having all the weight in the front (front engine/front drive) causes understeer which is bad, but predictable and safer for an inexperienced driver. Having all the weight in the rear (rear engine/rear drive) results in severe oversteer, an unpredictable habit that is very dangerous for inexperienced drivers. Additionally, the routing necessary for exhaust because of the required mufflers is actually _harder_ than for a front engine car since a rear engine requires many twists and turns in the piping while a front engine has basically a straight exhaust, certainly far fewer bends and zero 180 degree turn-arounds.
Regarding differentials, an standard ("open") differential sends power to the wheel with the least resistance. When travelling in a straight line, both wheels will recieve power because they will both have the same resistance against turning. When cornering, the outside wheel has to spin faster and an open differential allows this. If you spin the tires though, only one wheel needs to break loose for the car to lose all traction.
For a limited-slip differential, there is a percentage listed (30%, 40%, something like that) and that is the maximum difference in speed between the wheels that it will allow before the clutches start to have enough friction to send power to the other wheel. This allows cornering without scrubbing tires but gives better traction during acceleration because if a wheel starts to spin it will lock and start sending power to the other wheel as well.
A locked differential does not allow any difference in wheelspeed between the two sides and is really only useful in off-road or severe low-traction situations as it makes the outside wheel scrub during cornering and causes rapid tire wear if locked in normal situations. It also makes the vehicle very difficult to steer.
A TORSEN (TORque SENsing) differential is kind of like a limited slip but not really. A TORSEN is rated by multiplication factor (4x, 5x, something like that) meaning that it will send 4 or 5 times the torque of the wheel with the least traction to the wheel with the most traction. (I.e. if the wheel with the lowest traction can handle 1 ft/lb and the differential is getting 6 ft/lb of torque, it can send the other 5 ft/lb to the wheel with more traction.) However, is one wheel spins completely with a TORSEN unit, you're still stuck since the spinning wheel can handle 0 ft/lb and 5 times 0 is still 0, meaning no torque will go to the wheel with traction. This is a disadvantage. However, since a TORSEN uses gears instead of clutches (LSD's use clutches) it won't wear with time. An LSD typically needs to have the clutch packs renewed every 80,000 to 100,000 miles for it to maintain its ability to lock.
ZV
I can barely feel VTEC in my '99 Civic EX (but that's their tiny SOHC)...I noticed the engine revs quicker as it revs higher. I've only revved high enough maybe twice anyway...
HEMI has nothing to do with VTEC.
HEMI means that the combustion chambers are HEMIspherical, nothing more, nothing less. A lot of engines are HEMIs by definition. I believe some Hondas are.
Torsen diff do just that and it's a simple mechanical design. Torsen means TORque SENsing. I can buy those for my 4WD 1/8 scale radio controlled buggy.... which leads us to the question why not always use torsen diffs?
Well, in the buggy the Torsen diffs work great on a smooth track with just minor bumbs and an open diff works better on a very bumpy track. The reason is that a torsen diff always sends the power to the wheel with the MOST traction. When running on a bumby track this will make the car very unpredictable to drive. An open diff calms down the car and makes it easy to drive.
Zenmervolt
Elite member
- Oct 22, 2000
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Because TORSEN units, as I pointed out earlier, are useless if one wheel breaks free. They are great as long as one wheel has at least a little bit of traction but if one wheel can spin freely it will not send power to the other wheel.
ZV
KentState
Diamond Member
- Oct 19, 2001
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The one thing I always wondered about is what makes a 5.7L HEMI that much better than other pushrods V8s from this era. GM's LS1/LS6 seem to match it pretty straight forward with torque, hp, power curve, and fuel economy.
To the OP, mixing VTEC with a HEMI would be difficult as changing cam lobes on a pushrod engine presents many challenges. Also, VVT on a single cam engine would also be difficult since most engines like the iVTEC or Toyota's VVT-L engines only advance/retard the intake timing and do not affect the exhaust cam.
I can say that Toyota's VVT-L engine has a significant kick at 6500rpm when the lobes switch over. It feels like an on/off switch which is very audible if you add an air-intake.
There was also the point of large exhuast sytems making more power. Smaller engines require a certain amount of back pressure on NA models to produce power. Adding a 4" exhaust to a Honda Civic would not be a smart idea. On the flip-side, a turbo engine benefits from a larger exhaust since the decrease in back pressure allows the turbo spoll quicker.
To the OP, mixing VTEC with a HEMI would be difficult as changing cam lobes on a pushrod engine presents many challenges. Also, VVT on a single cam engine would also be difficult since most engines like the iVTEC or Toyota's VVT-L engines only advance/retard the intake timing and do not affect the exhaust cam.
I can say that Toyota's VVT-L engine has a significant kick at 6500rpm when the lobes switch over. It feels like an on/off switch which is very audible if you add an air-intake.
There was also the point of large exhuast sytems making more power. Smaller engines require a certain amount of back pressure on NA models to produce power. Adding a 4" exhaust to a Honda Civic would not be a smart idea. On the flip-side, a turbo engine benefits from a larger exhaust since the decrease in back pressure allows the turbo spoll quicker.
No one has really commented on the exhaust issue. Long exhaust plumbing actually allows tuning of the exhaust pulses, so that the low pressure trail of one pulse can be used to pull the leading high-pressure edge of the next pulse (which is the basic function of a header). Furthermore, a longer exhaust will move the turbulence caused by the change of pressure from the exhaust port pressure zone to atmospheric pressure away from the port, causing less turbulence, so the engine (pistons on the power stroke at any point in time) are not trying too hard to push the exhaust out.
The biggest issue is that the exhaust needs to be too long to be side-exhausted in a (typical) street car, but not long enough to vent rearward of the vehicle. It's better to extend it and make good use of headers and such than to shorten it and narrow the rpm range in which the long pipes increase the efficiency of the motor. This extra room also gives space for a muffler...
High-RPM racing vehicles have other consideration, most notably that the exhaust pulse is so fast that a header cannot be made of any practically sized pipe to time the pulses. But you'll notice that straight pipes on the big Nitro-Methane cars still are several feet long, even though they are made for higher rpm (Higher RPM = shorter tubes). Making a streetable engine requires many compromises against what is theoretically most efficient.
The biggest issue is that the exhaust needs to be too long to be side-exhausted in a (typical) street car, but not long enough to vent rearward of the vehicle. It's better to extend it and make good use of headers and such than to shorten it and narrow the rpm range in which the long pipes increase the efficiency of the motor. This extra room also gives space for a muffler...
High-RPM racing vehicles have other consideration, most notably that the exhaust pulse is so fast that a header cannot be made of any practically sized pipe to time the pulses. But you'll notice that straight pipes on the big Nitro-Methane cars still are several feet long, even though they are made for higher rpm (Higher RPM = shorter tubes). Making a streetable engine requires many compromises against what is theoretically most efficient.
AMCRambler
Diamond Member
- Jan 23, 2001
- 7,715
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Lol at CO2 poisioning. You mean CO poisoning. MON-oxide, not DI-oxide. Can you even get CO2 poisioning? Having too much CO2 and not enough oxygen in your lungs is called suffocation I think. Although that is effectively what carbon monoxide poisoning is. The CO bonds with the oxygen in your lungs and makes CO2 and you starve for oxygen. Poisoing is really a stupid name for it altogether!
- Mar 31, 2003
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I can see that i have poorly worded one of these questions. I know that HEMI refers to the combustion chamber, but someone said that most HEMI's are capable of the same thing that a VTEC engine is. That is more what i was getting at, sorry for the mixup.
Since we are on the topic of differentials and whatnot, when i lock the hubs in my car for 4x4 operation is that essentially creating a locking differential from the front and rear tires? Or am i referring to a completely different concept?
Gosh, i am learning a LOT
!!!
-Kevin
Since we are on the topic of differentials and whatnot, when i lock the hubs in my car for 4x4 operation is that essentially creating a locking differential from the front and rear tires? Or am i referring to a completely different concept?
Gosh, i am learning a LOT
!!!-Kevin
I am writing a paper on VTEC engines for my engineering analysis class. When I get the final product, I can send it to you if you wish
iamwiz82
Lifer
- Jan 10, 2001
- 30,772
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Originally posted by: Gamingphreek
I can see that i have poorly worded one of these questions. I know that HEMI refers to the combustion chamber, but someone said that most HEMI's are capable of the same thing that a VTEC engine is. That is more what i was getting at, sorry for the mixup.
Since we are on the topic of differentials and whatnot, when i lock the hubs in my car for 4x4 operation is that essentially creating a locking differential from the front and rear tires? Or am i referring to a completely different concept?
Gosh, i am learning a LOT
-Kevin
When you locking manual hubs you are just connecting the axles to the wheels, which then allows power from the transfer case to go to the axle, and then to the wheels. The differential can still be open or locked, hubs do not play a part in that.
The advantage of manual hubs is if you break something upfront (driveshaft, axle shaft, ring, pinion, etc), you can just disco the hubs and you can drive home.
Zenmervolt
Elite member
- Oct 22, 2000
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No, locking the hubs is not the same as locking the differential.Originally posted by: Gamingphreek
I can see that i have poorly worded one of these questions. I know that HEMI refers to the combustion chamber, but someone said that most HEMI's are capable of the same thing that a VTEC engine is. That is more what i was getting at, sorry for the mixup.
Since we are on the topic of differentials and whatnot, when i lock the hubs in my car for 4x4 operation is that essentially creating a locking differential from the front and rear tires? Or am i referring to a completely different concept?
Gosh, i am learning a LOT
-Kevin
A 4wd vehicle has 3 differentials: One front, one rear, and one center. Just like the front and rear differentials split power side to side, the center differential splits power front to back (as with the right and left sides, the front and rear axles scribe slightly different circles when turning so there needs to be some provision for a speed differential (hence the name "differential") between the front and rear axles).
Front hubs on a 4wd vehicle are normally not connected to the axle (unlocked) and are allowed to freewheel. This means that in 2wd the front drivetrain isn't spun by the engine and isn't spun by the front wheels. This reduces rotating mass and reduces friction and is typically good for 1-2 mpg as a result. As long as the transfer case isn't engaged you can leave the front hubs locked in 2wd and not notice an issue. In fact, some less expensive "4wd" setups omit the ability to unlock the front hubs altogether. Locking the hubs merely re-connects them to the front axle. In order to get 4wd, the hubs have to be locked _and_ the transfer case has to be engaged.
ZV
EDIT: Excellent and practical questions too. I like how you're connecting what's being said about one area and letting it lead you to questions in another area. You'll do well if you're even half decent at remembering what you hear.
Zenmervolt
Elite member
- Oct 22, 2000
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No, the pistons are not. The roof of the combustion chamber is.
This was a harder thing to do with pushrod engines as it's difficult to route pushrods such that they can actuate the valves with that sort of combustion roof design. IIRC the early hemis earned the nickname "Porcupine Head" because of the way the valvetrain had to be arranged.
ZV
Sorry this isnt spell checked, I have a paper on Russian Politics to finish.
Thats because you bought the wrong year
You can definately feel my '02, but its still a reletive thing, it is NOT nearly as noticeable as a turbocharged vehicle with a narrow powerband (thinkWRX), but much more noticeable than changes in the powerband of most naturally aspirated cars (think pre-03 mustang cobra).
OP:
1 - HEMI and VTEC have basically no common elements. HEMI is a marketing tool these days, in the 60s/early 70s it was a higher performing v8, like others have said, due to the design of the cylinder head.
2 - As others have said, short exhaust may be good in some extrememe performance situations, but on the street the back-pressure created by exhaust pipes is desireable.
3 - VTEC (besides the technical side which has been explained) basically allows the engine to be tuned for BOTH high torque output (this is reletive, its still a 2.0 liter in the case of my car) and high horsepower. The benefit here can best be demonstrated by going to a drag strip and listening to how terribly many full on drag cars run at idle. Their engines are designed to operate really well high up in the power band, but they sacrifice low end power (actually, as an aside, the original HEMI engines were notorious for this). The VTEC engine in my s2000 basically allows the car to act like it has both a low revving (say 6k RPM) engine that produces a lot of easy to access low end power (for the size) AND a peaky high output motor good from perhaps 6k onward. The tradeoff of this is $$$ and complexity.
Advantages of rear engine vehicles (and mid engine, since the engine is still behind the driver and no vehicle other than Porsche 911 is actually read-engined)
-better breaking
-less power loss
-better at the limit handling
-easier to have a neutral to rearward weight bias
Disadvantages
-most drivers don't want a hood off the back of the car, so the engine is normally less accesable
-less crashworthy in a forntal impact
-poor rearward visibility
-certain parts still need to be in the front (in most cases) such as radiator, AC condenser, etc) so front storage space is oftentimes limited
Thats because you bought the wrong year
You can definately feel my '02, but its still a reletive thing, it is NOT nearly as noticeable as a turbocharged vehicle with a narrow powerband (thinkWRX), but much more noticeable than changes in the powerband of most naturally aspirated cars (think pre-03 mustang cobra).
OP:
1 - HEMI and VTEC have basically no common elements. HEMI is a marketing tool these days, in the 60s/early 70s it was a higher performing v8, like others have said, due to the design of the cylinder head.
2 - As others have said, short exhaust may be good in some extrememe performance situations, but on the street the back-pressure created by exhaust pipes is desireable.
3 - VTEC (besides the technical side which has been explained) basically allows the engine to be tuned for BOTH high torque output (this is reletive, its still a 2.0 liter in the case of my car) and high horsepower. The benefit here can best be demonstrated by going to a drag strip and listening to how terribly many full on drag cars run at idle. Their engines are designed to operate really well high up in the power band, but they sacrifice low end power (actually, as an aside, the original HEMI engines were notorious for this). The VTEC engine in my s2000 basically allows the car to act like it has both a low revving (say 6k RPM) engine that produces a lot of easy to access low end power (for the size) AND a peaky high output motor good from perhaps 6k onward. The tradeoff of this is $$$ and complexity.
Advantages of rear engine vehicles (and mid engine, since the engine is still behind the driver and no vehicle other than Porsche 911 is actually read-engined)
-better breaking
-less power loss
-better at the limit handling
-easier to have a neutral to rearward weight bias
Disadvantages
-most drivers don't want a hood off the back of the car, so the engine is normally less accesable
-less crashworthy in a forntal impact
-poor rearward visibility
-certain parts still need to be in the front (in most cases) such as radiator, AC condenser, etc) so front storage space is oftentimes limited
Zenmervolt
Elite member
- Oct 22, 2000
- 24,514
- 44
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Whoa, whoa, whoa there.
Rear engine cars do have better braking (note the spelling).
They do not have noticeably less power loss than other cars, this is more dependant on transmission internal design, not so much on engine and drivewheel layout. Losses on my 951 are less than 10% and it's front engine/rear drive. That low of a driveline loss is less than most front engine/front drive cars.
They most DEFINITELY DO NOT have better at-the-limit handling. RWD cars are EXTREMELY prone to sudden snap oversteer and are notoriously twitchy at the limit. Mid-engine cars a also very twitchy at the limit because there is a very low polar moment of inertia and thus very little keeping the car from snap-spinning. The safest, easiest, most predictable, and best setup for at-the-limit handling is front engine and rear transaxle, this distributes the weight evenly in both ends of the car making it very difficult to spin.
It is NOT easier to have a neutral weight bias with a rear engine. In fact, it is easier to have a neutral weight bias with a front engine (by relocating the transmission to the rear) than it is with a rear engine vehicle. You are correct regarding mid-engine setups though.
ZV
- Mar 31, 2003
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Wow, i cant begin to say how much i have learned. If i have any more questions, ill definitely post back, it is early right now, so i probably jus cant think of any off the top of my head.
Happy Thanksgiving,
-Kevin
Happy Thanksgiving,
-Kevin
They aren't any better, it's mostly all marketing. The ORIGINAL HEMIs were awesome, but the combustion chamber design caused horrible gas mileage and high emissions.
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