TDC question

[taz3]

How many degrees will the piston be at TDC?My case 396/402

I guess,the more stroke of the crank and a longre rod will influence this too,correct?

[ALLT4]

Don't understand the question, TDC means 0 degrees.

[veno]

Theoretically none.... 0 is tdc. 259 is 1 degree for for tdc.... 1 degree is 1 after tdc.. then you have to breakdown the degrees in to 60 minuets, the for every minuet 60 seconds is marked...

so in theory. TDC is.. 359 degrees, 59 minuets, 59 seconds before TDC and 59 seconds, 59 minuets 0 degrees after TDC....

dwell time is linear.. the higher the RPM the shorter the dwell time... and the dwell time is so small, it is measurable but most dont even deal with it..

Long rods Many arguments present them selves, like less prone to detonation, slower this, faster that, less side loading here more load there... many arguments.... In truth.... If your racing... worth it. street.... if it makes you feel better then do it...

[taz3]

The crank should have some movement either way while the piston sits at TDC,I want to know what that window is.:)

[veno]

2 degree seconds... 1 second before tdc and 1 second after tdc

[taz3]

I wanted to know more just for the sake of knowing,while I bolt up my engine.I've borrowed some guages to measure various things such as a dial indicator to find true TDC to set my timming tab,as well I'm trying to calculate the compression ratio,so I know if I should use a 030 or 050 head gasket.Fuel is the real problem,it's not readily available here.

Thanks I appreciate it,Paul

[veno]

on any 180 degree arc line... there is always movement.... till 90 degrees is archived or perpendicular to the exact center of the 180 degree plane... even tho the movement is so small it takes very accurate measuring tools to see it.. the piston really never stops moving... you need a degree wheel to find the momentary stopping point.... the piston only stops moving when the rod is parallel to the bore centerline... and its a NANO second. or less.. depending on the rpm

[The Big Al]

A dial indicator is not the true way to find top dead center.

There are many variables. Crank alignment, bore location in relation to crank center line, rod journal clearance,

Go to this link and follow instructions. This is the true way to find TDC!

http://lunatipower.com/Tech/Cams/HowToDegreeACam.aspx

Note: You can use your harmonic balancer if it is degreed.

[Scooter]

In my mind, the crank turns in a circle. The piston follows the circle. Therefore the piston is either going up or going down. There's no flat spot.

[veno]

In my mind, the crank turns in a circle. The piston follows the circle. Therefore the piston is either going up or going down. There's no flat spot.


welllllllll all most... BDC and TDC... the piston stops at these locations... albeit a small amount of time.. miniscule at best.... it still takes time for the piston to stop and change direction...

[ALLT4]

Oh you're talking about piston dwell time. Sure you could measure how many degrees that is easily with a dial indicator. Just slap your indicator on top and turn the crank to TDC. (This is after you've already found TRUE TDC) Tun one way from TDC until the needle starts moving, note the degrees, turn the other way from TDC and note where the needle starts moving. Calculate the degrees.

Actually there's several ways I think you could it, if I was standing at the motor with dial indicator in hand I would probably check it a couple different ways to eliminate error.

I'm curious why you need this measurement though, what supporting parts are you picking that need this measurement? Or is it just a "want to know" thing?

[rhorne12]

True TDC is only TDC, not before or after. We are splitting hairs here and the Lunati method from above is perfect for that.
As for the head gasket, you should read the info available from Keith Black Pistons. I have done a lot of testing and research on this for high loads and performance in the boating end, and the less quench distance you can tolerate, the better the octane toleration will be. Better quench and a little more compression will win over excessive quench clearance every time.
Jim Fueling wrote some very interesting things on thisw subject. You might google him for some good info.
You MUST get your compression where you need it with the combustion chamber and piston tops, NOT THE DECK OR THE HEAD GASKET!!!!

[Nova_Guy]

In my mind, the crank turns in a circle. The piston follows the circle. Therefore the piston is either going up or going down. There's no flat spot.

Pistons dwell at TDC and BDC. The longer the stroke of the engine the more dwell it has. I have always heard that a longer stroke engine makes more torque because of it. The air/fuel mixture is compressed longer and has more time to burn completely. That dwell time is why you don't use real TDC to degree a camshaft because the dwell time will throw the measurment off.

http://www.iskycams.com/degreeing.html
It is a common error to miss T.D.C. by a few degrees due to the piston dwell at top center. Inasmuch as this inaccuracy will substantially affect subsequent timing, the following procedure is suggested to correct this error.

http://victorylibrary.com/mopar/rod-tech-c.htm
Effects of Long Rods
Pro: Provides longer piston dwell time at & near TDC, which maintains a longer state of compression by keeping the chamber volume small. This has obvious benefits: better combustion, higher cylinder pressure after the first few degrees of rotation past TDC, and higher temperatures within the combustion chamber. This type of rod will produce very good mid to upper RPM torque.

http://www.patentstorm.us/patents/5875755-description.html
A particular advantage arises where the piston dwell time is extended at its top dead center position to facilitate combustion of the fuel charge while the piston remains at that location to enable maximum energy transmission to the piston and thereby achieve the best possible conversion of combustion energy to engine output power and torque

http://www.team-integra.net/sections/articles/showArticle.asp?ArticleID=19

With short piston dwell time, there is less force compressing the air:fuel mix on the compression stroke and the piston quickly changes direction downward as the mix is being ignited. The air fuel mix, with less compression force and rapid change in piston direction downward, does not combust as completely.

[veno]

There is no dwell time only the resemblant of the piston stopping... as the crank approaches the apex of its revolution where the rod becomes parrallel with the Center line of the piston bore.. the piston slows and comes to a complete stop then reverses course in the same manner and speed as it arrived at TDC... the Piston does not dwell for degrees.. it appears to because the crank in its apex has little rise between 3 degrees before and after TDC.. so it apears the piston dwells in time while the crank apears to rotate...

in fact the piston is still moving up till TDC is found and down in its perspective relationship with TDC... and BDC... the piston does stop HO so briefly.

At idle of 1000.. it takes #1 piston .6 seconds to travel from tdc to bdc and back to tdc again...

At 5000rpm #1 piston makes 1 cycle from tdc to bdc to tdc in .012 seconds
just how much dwell time do ya think ya need?

[D-Man]

Pistons dwell at TDC and BDC. The longer the stroke of the engine the more dwell it has. I have always heard that a longer stroke engine makes more torque because of it. The air/fuel mixture is compressed longer and has more time to burn completely.

Sorry, I gotta raise the B.S. Flag on this one.:rolleyes: "Dwell Time" has NOTHING to do with why a longer stroke engine generally makes more torque!! The longer stroke stroke acts like a longer lever on the crankshaft centerline thus producing more torque. Kinda like using a longer handle ratchet for those stubborn bolts. More leverage = more torque output for the same amount of applied force. "Gimme a long enough lever and I'll move the world" ring a bell?

[Scooter]

Oh you're talking about piston dwell time. Sure you could measure how many degrees that is easily with a dial indicator. Just slap your indicator on top and turn the crank to TDC. (This is after you've already found TRUE TDC) Tun one way from TDC until the needle starts moving, note the degrees, turn the other way from TDC and note where the needle starts moving. Calculate the degrees.

Have you ever tried this? I did and it was impossible. ANY turning of the crank at all would move the piston up or down ever so slightly.


I agree with Veno on the issue so far.

[Nova_Guy]

Sorry, I gotta raise the B.S. Flag on this one.:rolleyes: "Dwell Time" has NOTHING to do with why a longer stroke engine generally makes more torque!! ?There are about 500 sites that say just the opposite. You'll need to go educate them as well.

http://www.circletrack.com/techarticles/crankshaft_tech_terminology/index.html
Crank’s Influence on Parts

For example, let’s say you decide to increase crankshaft stroke. This implies an increase in low- and mid-rpm torque is desirable. Therefore, so would the selection of an intake manifold, camshaft and header system that favors torque output in the same range of engine speed. Similar to events from lengthening connecting rods (all else being equal), a stroke increase changes both the rate at which intake flow velocities are created (vs. crank angle). It also affects piston dwell around TDC and BDC. This suggests a re-think/adjustment of sparking timing (at least initial spark) when comparing engine applications of a stroked and un-stroked crank.

http://www.knizefamily.net/minimopar/perf/longrodengine.html
Another advantage of the increased rod ratio is the amount of dwell time of the piston at TDC. While technically, the pistons are at TDC and BDC for an infinately small amount of time on both engines, the effective amount of time they stay at TDC is increased on a long rod engine. For example, if you consider 5 degrees before and after the actual TDC on the crankshaft to be the effective TDC, then in a long rod engine the piston will move less during this 10 degree sweep than in a short rod engine. This is again because the long rods have a lower change in angle for any given change in angle of the crankshaft. At BDC however, the dwell time is actually decreased with longer rods, though this is not as big of a concern on forced-induction engines. So, there is a "happy medium" for the rod ratio
here, which seems to be about 1.80. Increasing dwell time increases the amount of time that the valves can stay open, which increases the volumetric efficiency of the engine (the effectiveness of the engine to move air in and out of the cylinder).

http://www.dogpile.com/dogpile/ws/results/Web/piston%20dwell%20time%20applied%20to%20torque/1/417/TopNavigation/Relevance/iq=true/zoom=off/_iceUrlFlag=7?_IceUrl=true

http://www.valvolinecup.com/Newsletter/rtstory.asp?NewsItemID=19276
Crank’s Influence on Parts

For example, let’s say you decide to increase crankshaft stroke. This implies an increase in low- and mid-rpm torque is desirable. Therefore, so would the selection of an intake manifold, camshaft and header system that favors torque output in the same range of engine speed. Similar to events from lengthening connecting rods (all else being equal), a stroke increase changes the rate at which intake flow velocities are created (vs. crank angle). It also affects piston dwell around TDC and BDC. This suggests a re-think/adjustment of sparking timing (at least initial spark) when comparing engine applications of a stroked and un-stroked crank.

http://www.vintagemg.com/ArticlePDFs/Tech108.pdf
The lower velocity also affects the combustion process because the piston dwells near TDC longer. When the piston dwells longer at TDC, combustion gases have more time to act upon the piston before the piston begins to lower in the bore.

[veno]

There are about 500 sites that say just the opposite. You'll need to go educate them as well.

http://www.circletrack.com/techarticles/crankshaft_tech_terminology/index.html
Crank’s Influence on Parts

For example, let’s say you decide to increase crankshaft stroke. This implies an increase in low- and mid-rpm torque is desirable. Therefore, so would the selection of an intake manifold, camshaft and header system that favors torque output in the same range of engine speed. Similar to events from lengthening connecting rods (all else being equal), a stroke increase changes both the rate at which intake flow velocities are created (vs. crank angle). It also affects piston dwell around TDC and BDC. This suggests a re-think/adjustment of sparking timing (at least initial spark) when comparing engine applications of a stroked and un-stroked crank.

http://www.knizefamily.net/minimopar/perf/longrodengine.html
Another advantage of the increased rod ratio is the amount of dwell time of the piston at TDC. While technically, the pistons are at TDC and BDC for an infinately small amount of time on both engines, the effective amount of time they stay at TDC is increased on a long rod engine. For example, if you consider 5 degrees before and after the actual TDC on the crankshaft to be the effective TDC, then in a long rod engine the piston will move less during this 10 degree sweep than in a short rod engine. This is again because the long rods have a lower change in angle for any given change in angle of the crankshaft. At BDC however, the dwell time is actually decreased with longer rods, though this is not as big of a concern on forced-induction engines. So, there is a "happy medium" for the rod ratio
here, which seems to be about 1.80. Increasing dwell time increases the amount of time that the valves can stay open, which increases the volumetric efficiency of the engine (the effectiveness of the engine to move air in and out of the cylinder).

http://www.dogpile.com/dogpile/ws/results/Web/piston%20dwell%20time%20applied%20to%20torque/1/417/TopNavigation/Relevance/iq=true/zoom=off/_iceUrlFlag=7?_IceUrl=true

http://www.valvolinecup.com/Newsletter/rtstory.asp?NewsItemID=19276
Crank’s Influence on Parts

For example, let’s say you decide to increase crankshaft stroke. This implies an increase in low- and mid-rpm torque is desirable. Therefore, so would the selection of an intake manifold, camshaft and header system that favors torque output in the same range of engine speed. Similar to events from lengthening connecting rods (all else being equal), a stroke increase changes the rate at which intake flow velocities are created (vs. crank angle). It also affects piston dwell around TDC and BDC. This suggests a re-think/adjustment of sparking timing (at least initial spark) when comparing engine applications of a stroked and un-stroked crank.

http://www.vintagemg.com/ArticlePDFs/Tech108.pdf
The lower velocity also affects the combustion process because the piston dwells near TDC longer. When the piston dwells longer at TDC, combustion gases have more time to act upon the piston before the piston begins to lower in the bore.


OK so @ 1000 rpm how long does the piston spend at rest @ TDC? or BDC and are they equal?:rolleyes: Then how long does the piston spend at rest which is what your refering to as dwell at 5000rpm....

At what point does the crank stop to let the piston rest? Is there a phenomena that allows the piston to completely come to a rest while the crank keeps turning... If so when does the piston stop in relation to crank degrees or rotation and just how many degrees of crank rotation does the piston rest? and where does it start and end? how many before tdc and after tdc?...

[D-Man]

You're absolutely correct. The dwell time has everything to do with why the longer stroke makes more torque and the actual added stroke is just there for conjecture.
For example, we can build a 355 with 6.125" rods and a 383 with 5.7" rods. Both engines will have similar piston speeds and "Dwell times" so they both should make comparable torque output?? I don't think so!!
I guess ReherMorrison and all of the other ProStock engine builders need to re-think their entire engine programs because they are using shorter rods!!
Quote by David Reher:
"We also wanted to point out some of the common myths and misconceptions about high-performance motors. For example, I've seen dozens of magazine articles on supposedly "magic" connecting rod ratios. If you believe these stories, you would think that the ratio of the connecting rod length to the crankshaft stroke is vitally important to performance. Well, in my view, the most important thing about a connecting rod is whether or not the bolts are torqued!

If I had to make a list of the ten most important specifications in a racing engine, connecting rod length would rank about fiftieth. Back in the days when Buddy Morrison and I built dozens of small-block Modified motors, we earnestly believed that an engine needed a 1.9:1 rod/stroke ratio. Today every Pro Stock team uses blocks with super-short deck heights, and we couldn't care less about the rod ratio. A short deck height improves the alignment between the intake manifold runners and the cylinder head intake ports, and helps to stabilize the valvetrain. These are much more important considerations than the rod-to-stroke ratio. There's no magic - a rod's function is to connect the piston to the crankshaft. Period."

When I have conversations with record setting engine builders that all concur, I could care less what the rag sheets are saying. Especially when they are basically repeating something that was said over 25 yrs ago and has long since been disproven. I'm sorry you didn't get the memo.

[ALLT4]

Have you ever tried this? I did and it was impossible. ANY turning of the crank at all would move the piston up or down ever so slightly.


I agree with Veno on the issue so far.

Well I guess I would have to ask if you were using a dial indicator to watch it or just your vision.

There has to be some amount of dwell however small. When the crank throw reaches the very top of it's travel there is a transition point when it stops moving up changes direction and starts moving back down the other side. A longer rod and throw magnifies this at the rod end or wrist pin in the piston. I'm not a physics major by any means but when you think about it nothing is capable of changing directions instantaneously, there HAS to be a point in time where it stops moving one direction and starts moving the other direction. Stops being the key word in that last sentence.

This is one of those things you're just going to get or not, if we take the connecting rod out of the equation I'll agree fully that there is no dwell time because the crank throw is a circle.

[Lgian]

Great post guys and very deep really gets you thinking. As for me I dont want any of my pistons loitering and cavorting at TDC. I want those boyz going down, isnt that what its all about in the end? Just one knuckle heads opinion.

[veno]

Well I guess I would have to ask if you were using a dial indicator to watch it or just your vision.

There has to be some amount of dwell however small. When the crank throw reaches the very top of it's travel there is a transition point when it stops moving up changes direction and starts moving back down the other side. A longer rod and throw magnifies this at the rod end or wrist pin in the piston. I'm not a physics major by any means but when you think about it nothing is capable of changing directions instantaneously, there HAS to be a point in time where it stops moving one direction and starts moving the other direction. Stops being the key word in that last sentence.

This is one of those things you're just going to get or not, if we take the connecting rod out of the equation I'll agree fully that there is no dwell time because the crank throw is a circle.

So again I have ask... now many degrees, or minutes, or seconds does the piston rest @ tdc? is it 6 minuets? 30 seconds? 1 degree? how long?

When I refer to minuets and seconds... I an referring to Degree minuets and seconds.. very different from clock time... If your familiar with map coordanets of degrees, minuets, and seconds.. or if you have ever done any surveying this is common knowledge. The same applies to the crank shaft degrees... but you never see it broken down like that... till some one says dwell or rest time of a piston

The rod where it connects to the piston, at the piston pin, is the top of a triangular fulcrum.. the crank is a lever, the rod an extension of that lever but only half of it..

TDC and BDC are the Apex of the pivot.. Like wise it could also be said that @ 90 degrees for TDC is also a Apex for transition

Your right the piston does stop, well acculay its yanked and pushed... Its yanked from tdc stretching the rod and trying to pull the pin out of the bottom of the piston. then the rod snaps back to its center to center size, then the piston reaches MAX Q then slows as the crank start to reach 180 degrees from TDC and arrives @ BDC then the rod is compressed and the piston tries to push the rod out of the bottom of the engine and self destruct the pin boss from inertia, then the rod begins the rise and push the piston pin through the top of the piston compressing the piston and reaching maxq the rod relaxes on its way to tdc only to have its self stretched again from the piston trying to exit the top of engine via the cylinder head...

so again I have to ask.. when does the piston rest?... OOOO OOOO I Know Mister Carter... what Horshack..... when the engine is off... the piston is at rest!:devil:

[ALLT4]

so again I have to ask.. when does the piston rest?... OOOO OOOO I Know Mister Carter... what Horshack..... when the engine is off... the piston is at rest!

Agreed true dwell is so infinitesimally small that we don't have instruments that can accurately measure it.

However in the context of the way a piston engine works the term is more used a reference as to how long the piston "hangs around" up there. To your eyes and even with a dial indicator it may seem like some amount of time or crank degrees. I don't know, pick a measurement say .005 from the top and .005 past the top on a typical 350 it would be some amount of crank degrees albeit maybe small. Then check a stroker and I bet you would see more degrees given the same distance from the top and past the top. We could then say the stroker must have more dwell because we see more degrees of rotation to get the same .005 readings on both sides of the apex when the rod is fully extended.

To answer your question the piston rests at the apex of the throw. When the rod is completely extended from the throw during the transition from the throw moving the rod up then down. Same for BDC. Time is really irrelevant because it changes with RPM, but the distances and degrees we measure it to will never change. Again the time it takes for absolute dwell is infinitely small and kind of out context in motor building and really not worth talking about as far as generating anymore power. I'm not even so sure talking about how many degrees the piston "hangs around" up there is either. But it does get the old brain in gear huh?

[veno]

Agreed true dwell is so infinitesimally small that we don't have instruments that can accurately measure it.

a dial indicator accutrate to .00005 can see it..

To your eyes and even with a dial indicator it may seem like some amount of time or crank degrees. I don't know, pick a measurement say .005 from the top and .005 past the top on a typical 350 it would be some amount of crank degrees albeit maybe small. you have to account for piston rock...

Then check a stroker and I bet you would see more degrees given the same distance from the top and past the top. We could then say the stroker must have more dwell because we see more degrees of rotation to get the same .005 readings on both sides of the apex when the rod is fully extended.

Ever own a record? a 33.5 or a 45 rpm record? ever watch it spin? the out side spins at the same rate as the inside.... the out side has more MPH then the inside because they are connected. but each are on a different arc. but maintain RPM.. and what ever the inside does... the out side must compensate for...

To answer your question the piston rests at the apex of the throw. When the rod is completely extended from the throw during the transition from the throw moving the rod up then down. well if the rod is at the apex and the piston is at rest when does the crank stop to allow the rod to rest? since all are connected.... the top of the arc rises and descends much more slowly at 280 btdc and 40 atdc so between 2btdc an 2atdc the piston apears to dwell because the amout of rise is only 2 degrees for crank rotation or 0.01933" in rise on the typical 3.48" stroke.. at 1 degree the rise is 0.00966666" piston rock accounts for more than three quarters of that

Time is really irrelevant because it changes with RPM so true

but the distances and degrees we measure it to will never change.
absolutely

Again the time it takes for absolute dwell is infinitely small and kind of out context in motor building and really not worth talking about as far as generating anymore power. :notworthy::clap::horse:

I'm not even so sure talking about how many degrees the piston "hangs around" up there is either. its not degrees.... its seconds of a degree

But it does get the old brain in gear huh? made me get the calculator out:rolleyes::thumbsup:

[taz3]

I'm curious why you need this measurement though, what supporting parts are you picking that need this measurement? Or is it just a "want to know" thing?

It's mostly,something I want to learn.Just look at the info that is posted here:).I'm not always certain on what else maybe directly related and the posts are very educational with an expanded view of what else maybe affected.:D

Thanks,I'm reading all the links!:yes:

[Scooter]

Well I guess I would have to ask if you were using a dial indicator to watch it or just your vision.

I used a dial indicator but it only measured to .001. I guess the dwell you mentioned is just too small to measure with .001 accuracy. After reading recent posts, I agree that there is a dwell time and also that it is too small that it really doesn't matter to most engine builders.

[ALLT4]

Even if we all knew how long our pistons "hung around" TDC compared with other motors, strokes, rod lengths and so forth would we know how to exploit that information into extra power? I know I don't.