True, or False:

1. IVO and EVC go hand in hand with DCR. Static compression values are
used to determine how much IVO and EVC timing can be used.

2. Valve lift and duration is used to match head flow data. The difference
between advertised and 0.050" values can help determine the profilie of
the cam lobe. A wider duration @ 0.050" with similar advertised duration
values usually implies an aggressive lobe with more valve timing on the nose.

3. LSA tunes the resonant RPM range of a motor. Calulated exhaust pressure,
gas velocity and intake pressure within a window of RPM should be used to
select the lobe seperation angle.

4. Intake centerline helps to pin-point the peak HP RPM. Using a calulated piston speed at a cetain RPM will allow you to dial in the centerline for best
cylinder filling. Having the valve lift peak early in the piston sweep helps VE
at lower RPM (slower piston speeds); whereas peaking the max valve lift slightly
later helps VE at higher RPM (faster piston speeds).

 

False

True on the aggressiveness part......... As for the flow data being given for the set valve lift and duration it's only one part of the story.

LSA is byproduct of the cam events it means nothing other than to easily tell the cam grinder where to place the lobes on the cam.

IVC and Overlap have more to do with what you are talking about. Overlap can help the induction and scavenging effects of the motor or it can hurt them depending on the RPM range and the flow characteristics of the motor.

You could have a motor with the same intake and exhaust lengths and cross sections that would give you the same tuning and the two motors might need completely different LSA's for optimal results. LSA means dick when it comes to the "resonant RPM range of a motor"

If you are saying that a smaller ICL gives you more low end/mid range and a later ICL gives you less low end/mid range and better high end given the same duration...... True

IVO, IVC, EVO, EVC are the important events....... Not LSA and Duration.....

Bret

 

Interesting about #1. I would have thought that was more correct than the others. Maybe I worded the paragraph incorrectly.

Is it not fact that having more overlap will reduce cylinder pressure (DCR) with
an early opening intake and late closing exhaust?

Using that theory, that is why the static compression can be increased to
maintain some punch on the power stroke?

IVC closing late may also cause some charge to backup at certain RPM where
the intake pressure, and waves have less 'fight' than the pressure in the cylinder.

If not, what is the biggest factor which affects dynamic compressoin with respect to cam data.

Thanks for the clarification.

 

Zero,

No offense, but your "score" wasn't too good on your True or False quiz.

DCR is related to IVC and static CR (SCR) only. Think of it as how far up the cylinder the piston is on the compression stroke before the intake is completely closed and compression actually starts.

On another thread recently, a DCR calculator was linked. It gave DCR if you input the right numbers, but it also gave "Dynamic Stroke" which was how much below TDC the piston was when the intake finally closed. In an example with a 3.48 actual stroke, the "Dynamic Stroke" was 2.95. If you dig how static CR is calculated, this will be a very good visualization of what DCR really is.

Remember that the exhaust valve has been closed a LONG time before compression starts. Overlap affects cylinder filling (volumetric efficiency) which varies with rpm, throttle opening, etc. DCR is a calculated value which is fixed by SCR and IVC point and doesn't change unless you change the SCR or IVC point.

Clear as mud?

 

"No offense, but your "score" wasn't too good on your True or False quiz."

That's why they call me Zero!

Actually, it makes perfect sense. I always thought the DCR was calculated
with the intake stroke as well.

I can see how the overlap (IVO and EVC) is going to alter VE% over RPM with the changes
in cylinder pressure, exhaust pressure and intake pressure.


After your explanation, I can also understand how the piston reversing at BDC
with a late closing intake can be detrimental to DCR in certain cases.

The piston will push charge backward into the runner IF the momentum of the intake charge can't overcome the 'push' of the pressure building in the cylinder
at that moment.

 

Zero, I thought the old man had a good comment with the low score on the quiz but I think you beat him there!

Think about the overlap letting exhaust gasses back into the chamber at low RPM when the tuning events are crappy for that area. Overlap can be a killer there.

Bret

 

The harder I fall, the smarter I become!

How's that for an internet cliche?

I can picture what you mean about large overlap at low RPM where exhaust
gasses are slow and have high pressure.

When I think about air flow through the engine, I imagine a liquid getting rocked
back and forth:

Intake waves crashing into valves and reflecting back to the plenum

Exhaust waves hitting a "wall" and bouncing back toward the exhaust port
fighting with intake waves in the chamber (during overlap)

When the waves bounce and mix with new incoming waves, they help carry
more water to the chamber (in phase)

or

waves bounce and mix with new incoming waves, they might cancel out
to carry less water to the chamber (out of phase)


When exhaust is flowing well, creating a nice stream of water in the header
primary,
it sets up a low pressure area in the chamber (to scavenge, and suck in new
intake charge).

I know we can't equate a liquid to a gas, but using water as a visual helps to
paint a better picture of how gas might behave in the same environment.

Thanks for the replies Jon and Bret,
Tino
(I should change my sig...getting a complex with the label 'Zero' )

 

LOL!

I won't ask about the 69 part.

When I first was learning about the Otto cycle (in the 1950s...time flies when you're having fun ), the explanation was so basic that all valve events were assumed to occur at TDC and BDC. On very early engines, like the 1905-1910 2 cylinder stuff some of my buddies restore, the valve events are pretty close to T & BDC. These engines don't twist very high, as you can imagine. 1500 is fast, as is a 28 mph top speed, unless it's a down grade.

Sometimes it helps to visualize engines like this, and what happens as rpm increases. Think of the early engine designers who exnperimented with stuff like a valve opening before TDC and found more power. Hey on a 10 hp engine, every 1/10 hp helps!

FWIW, don't confuse DCR, a hard, calculated value with how much compression the engine achieves when running at various rpms. At the risk of more confusion (I'm good at that, according to at least one of my kids), what you describe might be called something like "effective compression" which depends on how much air/fuel actually got into the cylinder.

To carry this farther, imagine a good, strong engine with 100% volumetric efficiency at wide open throttle (WOT) and 5000 rpm, it's torque peak. Obviously the engine would need to be under load to hold it there at WOT. Now imagine the same engine, but with the clutch disengaged at 5000 rpm ready to launch. To hold 5000 the throtttle will barely be open, and very little air will be getting into the engine because it's only producing enough power to overcome its own friction and pumping losses. Now SCR and DCR are still the same, but VE and "effective compression" are vastly different. VE might be less than 20% or maybe less than 10%.

Is the mud up to your knees yet?

 

"Is the mud up to your knees yet? "

Not a chance! I always slap on the stilts after getting a reply from you.
We're just getting the toes wet at this point.

"FWIW, don't confuse DCR, a hard, calculated value with how much compression the engine achieves when running at various rpms."

So, DCR (Dynamic Compression Ratio) is a fixed value based on IVC.

The amount of "punch" upon the power stroke is affected by the amount
of charge that stayed in once the intake valve closed (< Effective compression).

Up until this very second, I have always believed DCR and Effective Compression
to mean the same thing!

"To hold 5000 the throtttle will barely be open, and very little air will be getting into the engine because it's only producing enough power to overcome its own friction and pumping losses. "

Understood. A weight lifter is only going to exert enough force to lift a 100 lb.
weight and nothing more.

VE is dynamic.
Effective Compression is dynamic
DCR is STATIC

Am I getting back in your good books yet?

 

He got it! I think he f%#@*&g got it!

Now do a retake on your own Quiz. Well, at least on the DCR part.

You owe me a brew when we meet, OK?

 

It's the least I could do for all of the lessons.

I might take you up on that one day. I have cousins in Mahopac, Bronx and
White Plains.

Where would "upstate" locate? I would probably pass your area on my trip.

We could sit and talk about life, engines and my pet rock.

Come on, it will be fun! You might even learn a thing or two about engines

(P.S. Mods and members, sorry for the personal, and non-car related reply.
Jon says, "bite me".)

 

Upstate.... lol If you can take a train in to the city you ain't in upstate! Are minorities around here are freakin cows, it's a little different from downstate. lol

I had a EX who lived down there it's a 3 hour trip EVEN if you drive good ole 17/86 like a *** like I do.

Bret