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[tjkj2002]

The duration changes to keep the valves open longer for better intake, and exhaust; of course. Lift? That puts more strain on it more than duration does.

Lift is the linear measurement of how far off-seat a valve extends at its farthest point, which should be a direct relation to the measurement of the cam's lobe at its apex, or highest point out of round from its base circle. If placing a cam with more lift, into a stock head, the springs (compressing) may not be able to handle it. The only thing that was changed in cam profile, was duration.

Duration is the amount time, measured in degrees of crankshaft rotation; the valve remains past a certain distance off-seat and likewise, should proportionately reflect the amount of degrees of cam rotation the cam's lobe remains above a certain amount out of round.

With interference engines, duration has to be precise, You can't have to much lift om valve overlap with an interference engine, because both valves would get hit by the piston when it reaches TDC on that stroke. So, that means, say on the exhaust stroke, the exhaust valve can't remain open too far for just a tiny part of the intake stroke, the exhaust valve would get smashed by the piston at TDC on the exhaust stroke. There would never be intake overlap on the compression stroke, as that air that escapes would back out through the intake. Exhaust Valve overlap, can be used to increase low end torque, with non interference engines. With interference engines, BACK PRESSURE is tuned in the exhaust system for the torque curve. MORE back pressure means lower RPM range for max torque curve. The less back-pressure, the higher the RPM range shifts upward for max torque.

So, at each stroke, the both valves have to be closed at TDC for that stroke, or wham...

Glad you can copy and paste from other places.


Oh and yes you can valve overlap on the compression stroke and it has it's benefits for increased power.Both valve do not need to be closed at TDC since 99.99999999999% of engines do not fire(spark) at TDC.Oh and your back pressure statement is all wrong and false.

 

[ShafferNY]

I hear a lot of people use the term 'back pressure', often followed by "it gives you more low end torque". This isn't accurate.

Back pressure doesn't give you more low end torque. It's the scavengering effect of the exhaust that helps with low end torque. This is the same reason there is a performance difference between short headers and long headers.

 

[tjkj2002]

IMO: Valve overlapping (both valves open at the same time) is invented to 'flush' out the exhaust gasses. More backpressure from an exhaust system would make that process go slower...



Eeeeeh...(I admit, I am not a NASCAR mechanic)....wouldn't I then pump valuable clean air back into the intake, losing compression ....??

You answered your own question.

ShafferNY I hear a lot of people use the term 'back pressure', often followed by "it gives you more low end torque". This isn't accurate.

Back pressure doesn't give you more low end torque. It's the scavengering effect of the exhaust that helps with low end torque. This is the same reason there is a performance difference between short headers and long headers.

The above is a more technical term for it.Scavenging suckes out more of the exhaust gases better and faster if the intake valve is open a little with the exhaust valve.This is more used in high rpm race engines and very popular in forced induction engines to cram more fresh O2 in the cylinder for more power.


Less back pressure can help and can hurt,it depends on how the engine was built.

 

[04Liberty]

All above is another reason that newer 3.7's have more issues then the older 3.7's.Longer duration equals more stress on the valve train.

How much more stress can there really be? It's a SOHC engine; there are no pushrods or rocker arms, just straight cam followers and valves.

 

[Powerslave]

Glad you can copy and paste from other places..

Only those two SHORT precise paragraphs were, but I added "If placing a cam with more lift, into a stock head, the springs (compressing) may not be able to handle it. The only thing that was changed in cam profile, was duration." to that one.

As TonyCRD said, as I said, you can't have both valves open on the intake stroke, not with this engine, as it is not a Forumula 1 with what he said... I know you are in such a hurry to FALSIFY my information, but this time you stuck your foot in your OWN mouth.

Well, think what you want with the back pressure issue, I won't change what I have experienced with it.

Tony, just as a note: The Valve Spring compressed thing, I wasn't talking duration, I know they stay compressed longer, I was talking LIFT. There was no lift change in 2005+, just duration. I was pointing out the valve springs would compress MORE with more lift, and I don't think the springs in these heads can handle it.

ShafferNY: You can't read obviously; I did not say back-pressure ADDED MORE TORQUE, I said that it CHANGED the torque curve to a different RPM range. If I said ADDED I mistyped, but look at what I typed:

"BACK PRESSURE is tuned in the exhaust system for the torque curve. MORE back pressure means lower RPM range for max torque curve. The less back-pressure, the higher the RPM range shifts upward for max torque."

Nothing there about ADDING anything, it changes the RPM where the torque curves maxes out to. You can experiment with that ALL BY YOURSELF. Put a straight pipe exhaust on, and see where your pickup is. Put your stock one back on, and see where it is...

 

[tjkj2002]

http://static.howstuffworks.com/flash/engine.swf



You may want to re-read your posts TJKJ. Unless you are talking about a Formula 1 engine with pneumatic valve control it is technicaly impossible to open both valves on a compression stroke using a conventional camshaft.

Only on the exhaust stroke.

You can keep the intake valve open longer, but not until after TDC.

(Your TDC valve lapping engine would run like crap, have no power and would use more fuel then a space shuttle...)

So I never answered my own question.

I admire your knwowledge and take a deep bow to how you setup your SFA. But this is incorrect.

Hey you never classified it to just the 3.7 so yes I was correct by saying you can have both valves open.You can also get custom ground cams to have any overlap,opening,duration,and lift you want.Oh and then don't get me started on 2 strokes where alot of them always have both the intake and exhuast ports and/or valves open at the same time.Thedn you got Jake brake equipped engines,holds the exhaust valves open all the time when your foot is not on the throttle(cuts fuel also).

"BACK PRESSURE is tuned in the exhaust system for the torque curve. MORE back pressure means lower RPM range for max torque curve. The less back-pressure, the higher the RPM range shifts upward for max torque."

Nothing there about ADDING anything, it changes the RPM where the torque curves maxes out to. You can experiment with that ALL BY YOURSELF. Put a straight pipe exhaust on, and see where your pickup is. Put your stock one back on, and see where it is...

That still is false,the exhaust is done after the engine is tuned,the cam(s) and cylinder head design is what determines torque and where it's most or not.Now back pressure can play a role in this but usually never effects your torque by about 5% at max.You can build a engine that needs alot of back pressure and you can build a engine that needs no back pressure,it's all in the cylinder head design and cam.Our engines where designed for back pressure since cats and a muffler where required to be legal and thus if you reduce the back pressure to much you can loose some low end torque.

 

[Powerslave]

"Hey you never classified it to just the 3.7"

Umm, yeah, but we are talking about the 3.7, no one mentioned any other engine, YOU did, as usual. You change the subject to fit YOUR data, you don't stay with the subject at hand.

2-stroke engines; they are REED valves, which works and moves more like a GATE than a valve; totally different monster.

Back-pressure thing; Keep thinking that, let me know how that all works out for you...

 

[04Liberty]

"Hey you never classified it to just the 3.7"

Umm, yeah, but we are talking about the 3.7, no one mentioned any other engine, YOU did, as usual. You change the subject to fit YOUR data, you don't stay with the subject at hand.

2-stroke engines; they are REED valves, which works and moves more like a GATE than a valve; totally different monster.

Back-pressure thing; Keep thinking that, let me know how that all works out for you...

I have to step in here, other than small gas engines (outboards and outdoor stuff like lawnmowers and chainsaws), most 2 strokes are diesels, and as such have a combination of reed type valve/ports (usually intake) and regular tulip valves for exhaust. Prime example would be a Fairbanks-Morse opposed piston engine.

 

[Powerslave]

I have to step in here, other than small gas engines (outboards and outdoor stuff like lawnmowers and chainsaws), most 2 strokes are diesels, and as such have a combination of reed type valve/ports (usually intake) and regular tulip valves for exhaust. Prime example would be a Fairbanks-Morse opposed piston engine.

Heh, Diesels don't have spark plugs, 2-strokes do... In warm weather, a diesel will start without a working glow plug, 2-strokes won't... You are also talking a huge industrial motor -vs- a small engine for a car or a weed-whacker... They also have two cranks as there are two pistons per cylinder; for an intake piston, and exhaust piston in the same chamber, eliminates valves, heads; for less than half the moving parts, bla bla bla...

Will we see these shrink to car size? Would be interesting...

 

[ThunderbirdJunkie]

Heh, Diesels don't have spark plugs, 2-strokes do... In warm weather, a diesel will start without a working glow plug, 2-strokes won't... You are also talking a huge industrial motor -vs- a small engine for a car or a weed-whacker...

Hmm.

Out of the 20+ 2 stroke engines ThunderbirdJunkie owns
Only two use spark plugs.

Go figure, Powerslave is right again.

 

[boebr1]

"BACK PRESSURE is tuned in the exhaust system for the torque curve. MORE back pressure means lower RPM range for max torque curve. The less back-pressure, the higher the RPM range shifts upward for max torque."

Nothing there about ADDING anything, it changes the RPM where the torque curves maxes out to. You can experiment with that ALL BY YOURSELF. Put a straight pipe exhaust on, and see where your pickup is. Put your stock one back on, and see where it is...

Powerslave is right!
okay, yes, i am gonna give an example on a different engine, it still applies.
Stock 1979 dodge aspen 2dr car, 360 police interceptor engine with 850 cfm 4bbl carb (stock), tf727 a/t. idk what the rear end gears were, thinking it was around 3.73. stock exhaust was as follows: dual headpipes, dual cat converters, dual midpipes, dual mufflers, into a cross-over above the axle, into a single resonator, out a single tail pipe. this exhaust system was complete, except it was just rusty enough that Utah made me replace mufflers. in stock form, i could turn the steering wheel to lock one direction, and without holding the brake, i could floor it and burn a 360 with both tires smoking up a storm.

first repair: had the stock mufflers, crossover, resonator and tail pipe removed. installed dual turbo mufflers, and dual tail pipes, no cross over... performance: had to apply a little brake pressure to get the tires to light up, couldn't burn a 360 any more.

second change: turbos kept hitting driveway berms, had turbos and tail pipes removed. installed dual glass packs, and turn downs right before the rear axle.. perf changes: sounded cool. went down the freeway like a bat out of hell (135 was no problem, wrapped speedo back around to 0) but, now, i could only spin one rear tire, and i had to be leaning the car into a corner.

final change before i sold car... right hand cat converter rusted through at back end, mid pipe, glass pack, and turn down fell off. left hand cat converter rusted off at front, cat, mid, glass pack, and turn down fell off (on freeway no less). lots of back firing. puddled gas in existing cat converter. ignited, melted core, blew core apart, cat was hollow now. so in this version, i had roughly 18 inches of head pipe on engine, with a cat shell on end of one. performance... couldn't light up tires at all anymore, no matter what. sluggish pulling away from a stop. sounded bad as hell, could still do good speed going down the freeway though

what's this mean? it lost all it's low end torque with less back pressure. gained top end horse power... but it had an a/t so not very useful unless wide open on freeway.
I have done reverse of this process going from crappy dual exhaust to large bore single exhaust, or adding crossovers, and tailpipes and getting rid of free flowing mufflers like glass packs and have always noticed an increase in off-the-line performance

 

[boebr1]

this is why you will never see a "true" dual exhaust on my jeep. maybe if i built it up like a stock car, naw, i'd still run a crossover cuz they do too, you need it to help scavenge your exhaust. you will also never see a straight thru turbo or glass pack on my jeep, cuz you pretty much need to be running a huge amount of exhaust, like maybe from a blown big block, for it to not cut your low end torque out. i will put a turbo of some sort on my jeep, but i will not alter the rest of the exhaust, it was designed to work with a stock engine, and putting a cold air intake on it doesn't change your engine enough to warrant changing the exhaust. If you can super charge this engine, change your cam profiles, maybe increase your compression ratio, make your valves bigger, run some serious boost, and also run lower gears so that this monster is running at high rpms... that might warrant a dual exhaust, but you still won't flow enough to use a dual 2 1/2 inch, just upsize, and run a 3 or 3 1/2" single and have waayy more torque where you want it, unless your idea of wheeling is to run with the ricers down the freeway wide out, then have a ball. i for one, like to go off-roading, i need low end torque

 

[boebr1]

Thedn you got Jake brake equipped engines,holds the exhaust valves open all the time when your foot is not on the throttle(cuts fuel also).

uh... i drive semi's for a living, and i have a degree in repairing them... if you kept both valves open by letting off the throttle, you would have a totally free spinning engine with no braking effect, and your truck would reach speeds in excess of 120mph as it goes down some hills... so you would have to step on the brakes, and then you would burn them out and go just as fast... or you would turn off your jake brakes so your valves would close and give you a little engine slowing, but still not enough to make a difference, that's why they invented jake brakes... they will hold both valves open to release compressed air, but they close again for the compression to build again, then they pop open and close again.

 

[boebr1]

here is how a jake really works, if anyone is interested: (gotta love copy and paste)
we turn on our Jake this energizes a solenoid within the Jake head preventing engine oil from draining back to the oil pan the Jake head is now charged. Now as an engine push rod comes up to open the exhaust valve it contacts a master piston which has its rod extended due to engine oil pressure (ordinarily the master piston is retracted and can not contact the exhaust valve bridge) as the push rod continues to rise oil is pushed out of the master piston to a slave piston, the master piston and slave piston are joined by oil passages within the Jake head and open the exhaust valve (only one exhaust vale is opened usually, though most four stroke diesels have two valves actuated by a single bridge) in a corresponding firing order i.e. if #1 exhaust push rod was coming up on exhaust the master piston for #1 would activate slave piston on #5 (typical firing order for a six cylinder 153624) which is coming up on compression, all the work that #5 cylinder performed in compressing the air is wasted to atmosphere, we have engine braking. As #1 exhaust valve closes oil from the master piston is retracted, slave piston on # 5 retracts and # 5 exhaust valve closes (no bent valves).

 

[tjkj2002]

here is how a jake really works, if anyone is interested: (gotta love copy and paste)
we turn on our Jake this energizes a solenoid within the Jake head preventing engine oil from draining back to the oil pan the Jake head is now charged. Now as an engine push rod comes up to open the exhaust valve it contacts a master piston which has its rod extended due to engine oil pressure (ordinarily the master piston is retracted and can not contact the exhaust valve bridge) as the push rod continues to rise oil is pushed out of the master piston to a slave piston, the master piston and slave piston are joined by oil passages within the Jake head and open the exhaust valve (only one exhaust vale is opened usually, though most four stroke diesels have two valves actuated by a single bridge) in a corresponding firing order i.e. if #1 exhaust push rod was coming up on exhaust the master piston for #1 would activate slave piston on #5 (typical firing order for a six cylinder 153624) which is coming up on compression, all the work that #5 cylinder performed in compressing the air is wasted to atmosphere, we have engine braking. As #1 exhaust valve closes oil from the master piston is retracted, slave piston on # 5 retracts and # 5 exhaust valve closes (no bent valves).

Not 100% true,I worked on Jake braked equipped Detroit 8V92TA's,they are a 2 stroke diesel with 4 exhaust valves.When the Jake brake is on low it holds 4 of the cylinders exhaust valves open(all the time) for a total of 16 valves(4 master actuators).With the Jake brake on high all 8 cylinders have there exhaust valves held open(all 4 master actuators and the 4 slave actuators).That is also verified by Detroit and OshKosh engineers we worked with also.

Powerslave is right!
okay, yes, i am gonna give an example on a different engine, it still applies.
Stock 1979 dodge aspen 2dr car, 360 police interceptor engine with 850 cfm 4bbl carb (stock), tf727 a/t. idk what the rear end gears were, thinking it was around 3.73. stock exhaust was as follows: dual headpipes, dual cat converters, dual midpipes, dual mufflers, into a cross-over above the axle, into a single resonator, out a single tail pipe. this exhaust system was complete, except it was just rusty enough that Utah made me replace mufflers. in stock form, i could turn the steering wheel to lock one direction, and without holding the brake, i could floor it and burn a 360 with both tires smoking up a storm.

first repair: had the stock mufflers, crossover, resonator and tail pipe removed. installed dual turbo mufflers, and dual tail pipes, no cross over... performance: had to apply a little brake pressure to get the tires to light up, couldn't burn a 360 any more.

second change: turbos kept hitting driveway berms, had turbos and tail pipes removed. installed dual glass packs, and turn downs right before the rear axle.. perf changes: sounded cool. went down the freeway like a bat out of hell (135 was no problem, wrapped speedo back around to 0) but, now, i could only spin one rear tire, and i had to be leaning the car into a corner.

final change before i sold car... right hand cat converter rusted through at back end, mid pipe, glass pack, and turn down fell off. left hand cat converter rusted off at front, cat, mid, glass pack, and turn down fell off (on freeway no less). lots of back firing. puddled gas in existing cat converter. ignited, melted core, blew core apart, cat was hollow now. so in this version, i had roughly 18 inches of head pipe on engine, with a cat shell on end of one. performance... couldn't light up tires at all anymore, no matter what. sluggish pulling away from a stop. sounded bad as hell, could still do good speed going down the freeway though

what's this mean? it lost all it's low end torque with less back pressure. gained top end horse power... but it had an a/t so not very useful unless wide open on freeway.
I have done reverse of this process going from crappy dual exhaust to large bore single exhaust, or adding crossovers, and tailpipes and getting rid of free flowing mufflers like glass packs and have always noticed an increase in off-the-line performance

Not so much,you did not adjust the carb and timing.On carbed engines you need to tune the carb and timing which is automatically done on a FI vehicle.

 

[tjkj2002]

Powerslave "Hey you never classified it to just the 3.7"

Umm, yeah, but we are talking about the 3.7, no one mentioned any other engine, YOU did, as usual. You change the subject to fit YOUR data, you don't stay with the subject at hand.

2-stroke engines; they are REED valves, which works and moves more like a GATE than a valve; totally different monster.

Back-pressure thing; Keep thinking that, let me know how that all works out for you...

Actually you did from the get go................

From one of your posts.

With interference engines, duration has to be precise, You can't have to much lift om valve overlap with an interference engine, because both valves would get hit by the piston when it reaches TDC on that stroke. So, that means, say on the exhaust stroke, the exhaust valve can't remain open too far for just a tiny part of the intake stroke, the exhaust valve would get smashed by the piston at TDC on the exhaust stroke. There would never be intake overlap on the compression stroke, as that air that escapes would back out through the intake. Exhaust Valve overlap, can be used to increase low end torque, with non interference engines. With interference engines, BACK PRESSURE is tuned in the exhaust system for the torque curve. MORE back pressure means lower RPM range for max torque curve. The less back-pressure, the higher the RPM range shifts upward for max torque.

So, at each stroke, the both valves have to be closed at TDC for that stroke, or wham... __________________

Well you stated a whole different engine since the 3.7 is not a interference engine the_finger.gif

 

[04Liberty]

Heh, Diesels don't have spark plugs, 2-strokes do... In warm weather, a diesel will start without a working glow plug, 2-strokes won't... You are also talking a huge industrial motor -vs- a small engine for a car or a weed-whacker... They also have two cranks as there are two pistons per cylinder; for an intake piston, and exhaust piston in the same chamber, eliminates valves, heads; for less than half the moving parts, bla bla bla...

Will we see these shrink to car size? Would be interesting...

That was just an example, would a smaller displacement V-6 (with only one crankshaft) make you feel better? A 2 stroke engine is a 2 stroke engine, regardless if it has spark plugs or not. Do you actually know what the difference between 4 stroke and 2 stroke? Here, I'll help you, check out these 2 links:
http://en.wikipedia.org/wiki/4_stroke
http://en.wikipedia.org/wiki/2_stroke

 

[ridenby]

When I think I can't get any stupider.....

 

[boebr1]

yeah, then there is volvo with their new jake system that decides what setting you want for you... i'm sure that one is a little more complicated to, and i did adjust carb as much as could b, and adjusted timing as well. i even put a different carb on it. i was never able to get the power out of it that it had when it was stock set up. the engine just wasn't built to breath that easy

 

[Powerslave]

Actually you did from the get go................

From one of your posts.
Well you stated a whole different engine since the 3.7 is not a interference engine the_finger.gif

I was told in another thread in these forums, that the 3.7 WAS an interference engine. I said that it wasn't, then I was corrected, wrong then? People need to get ALL the facts straight then. I originally said it wasn't, I was told it WAS.

Either way, there is no valve overlap on intake stroke.

SO, you can do what your little graphic suggested...