Why does a tighter lsa need more compression?

[Belgian1979]

There's nothing Dynamic about DCR, and it has no mathematical significance to what's going on in a running engine.
DCR is based on the misconception that you need the intake valve to be closed, to start compressing the mass in the cylinder, and that's far from the truth. As long as the pressure above the intake valve is higher then the pressure below the intake valve, the open valve will not bleed off any compression.

Is the pressure above the intake valve higher than below the valve when the piston is at IVC?
I mean, in my itb engine, when connecting a vacuum gauge to one cyl, I see the needle fluctuating wildly at idle. Where the minimum vacuum measured is equal to the vacuum when measured from a common source. This makes me think that the air reacts in fact quite slow to the piston movement or in other words is lagging behind the motion of the piston. So I'm inclined to think that above the intake valve there would always be a certain amount of vacuum.
Whether or not the piston would push out air through a valve that is a little open, would depend on the vacuum above the piston and if it would be higher, equal or lower than the vacuum above the intake valve. I think this is difficult to know, but if the air moves slower than the piston, I could see that it would create a situation where the air would still be rushing in even though the piston moves up. That would then create a higher pressure above the piston than what would be above the valve hence the propsensity to reverse flow if the valve is still open at a certain number of degrees.

Wouldn't this create less compression pressure ?

[David Redszus]

There's nothing Dynamic about DCR, and it has no mathematical significance to what's going on in a running engine.
DCR is based on the misconception that you need the intake valve to be closed, to start compressing the mass in the cylinder, and that's far from the truth. As long as the pressure above the intake valve is higher then the pressure below the intake valve, the open valve will not bleed off any compression.

Mike, I agree with you that DCR has no mathematical significance in a running engine or to any engine simulation program.

But the problem is not with the term "Dynamic" it is with the term "Ratio". No engine cares a wit about ratios, of any kind.

As you indicate, as long as pressure above the valve is higher than the cylinder pressure, the cylinder pressure will continue to rise with upward piston motion. But when cylinder pressure is equal or greater than inlet pressure, flow reversal can and often does occur.

The key factor is cylinder peak pressure, and even that is determined by DCR and the actual inlet tract pressure prior to IVC.

But pressure has only a limited impact on combustion characteristics. DCR combined with inlet air temperature will determine
chamber temperature at ignition which combined with compression pressure will affect ignition delay angle.

At this point in the combustion process, temperature and squish velocity are the principal determinants of combustion burn angle and combustion peak pressure location.

Assuming a conventional engine we find that compression pressure at:

110 IVC (BTDC) = 236 psi (TDC), 105 psi @ 30 BTC
114 IVC (BTDC) = 246 psi (TDC), 110 psi @ 30 BTC
118 IVC (BTDC) = 256 psi (TDC), 115 psi @ 30 BTC

But combustion properties, being a principal function of inlet temperature, are affected by ambient temperature,
DCR and fuel evaporation. Now we have a bit of a problem in that inlet air temperature above the valve is very difficult
to measure. Forget about in-cylinder temperature measurement.

Our recommendation is to insert an electronic pressure sensor in the spark plug hole and measure cylinder pressure of
a running or motored engine. The pressure variation in fifty measured cycles should be some cause for concern.

[maxracesoftware]

There's nothing Dynamic about DCR, and it has no mathematical significance to what's going on in a running engine.
DCR is based on the misconception that you need the intake valve to be closed, to start compressing the mass in the cylinder, and that's far from the truth. As long as the pressure above the intake valve is higher then the pressure below the intake valve, the open valve will not bleed off any compression.

Mike you're exactly correct ... nothing "dynamic" in the Internet's DCR definition


the Internet's popular inaccurate DCR definition : Dynamic Compression Ratio

there is nothing "Dynamic" in the DCR or Dynamic Compression Ratio ... as defined by popular Internet DCR calculators .

Definition of Dynamic : "marked by usually continuous and productive activity or change"

there's NOTHING changing in the DCR calculation .... its just as STATIC a calculation as the Static Compression Ratio !

the Internet's DCR should really have been termed or named something like IVC Compression Ratio ... instead of Dynamic Compression Ratio

BEFORE the Internet existed ... Dynamic Compression Ratio as SAE ( Society of Automotive Engineers ) defined it as also including effect of "Trapped" Volumetric Efficiency
and no degreed Engineer before the Internet , would use the term "Dynamic Compression Ratio" without referring to "Trapped" Volumetric Efficiency

Trapped VE% changes with things like RPM and Throttle CFM, and affects the real "dynamic" DCR of an engine.

Trapped VE% was always part of Dynamic Compression Ratio definitions "before" there was an InterNet !

Basically,
InterNet DCR version is analogous to Static Balancing your Car Tires
SAE ( Society of Automotive Engineers ) 's DCR version is analogous to Dynamically Spin Balancing Engine Rotating Assembly ,
that is , imbalance or balance will "change" with changes in RPM ( Cylinder pressure will change with changes in RPM )


a little history on the Internet's popular DCR calculation and inaccurate definition :

many years ago , Pat Kelley along with a VB6.0 Programmer , created the DCR Calculator
Pat chose 0.006 or Seat-to-Seat Durations , and his Calculator became widely accepted as true DCR defintion, but it was actually a wrong definition !

http://cochise.uia.net/pkelley2/DynamicCR.html

way back then, around same time as beginning of Pat's DCR , a very popular real lead-Engineer at Edelbrock , Jim McFarland started his own Website/Forum,
which i visited and saw a few Internet Kiddies/Newbies harrasing and telling Jim McFarland how wrong his defintion of Dynamic Compression Ratio was
and how Pat Kelley's DCR defintion was correct one

however, if you went to Pat Kelley's DCR website , even Pat Kelley stated this

a Quote from Pat Kelly : Two important points to remember:

The DCR is always lower than the SCR
The DCR does not change at any time during the operation of the engine

as Pat Kelley knew or realized that his DCR definition does not really CHANGE ... that its actually STATIC
Pat still named his Calculator a DCR Calculator ... which is a wrong definition !
and sadly his DCR definition became super popular across the Internet
and replaced the real Dynamic Compression Ratio defintions SAE Engineers had used way before the Internet existed !

Many years ago , corresponding by Emails with Pat Kelly , he admitted to me that he was wrong in naming his calculator "DCR" ,
but at that time could not come up with a better name
Pat also asked me if it was OK to place my Spark Plug Reading Pictures in an article on his website here :
http://members.uia.net/pkelley2/sparkplugreading.html

[CamKing]

Is the pressure above the intake valve higher than below the valve when the piston is at IVC?

At a given RPM range, yes.

Look at it like this.
Even though the velocity in the port is slowing down, the valve is closing, reducing the area between the valve and the seat for the air to flow through. This creates a high pressure zone above the valve. It's a lot like squeezing the end of a water hose.

[MadBill]

Is the pressure above the intake valve higher than below the valve when the piston is at IVC?

At a given RPM range, yes.

Look at it like this.
Even though the velocity in the port is slowing down, the valve is closing, reducing the area between the valve and the seat for the air to flow through. This creates a high pressure zone above the valve. It's a lot like squeezing the end of a water hose.

I believe the water hose analogy is correct but although the pressure assumption is widely accepted and apparently logical, precision-calibrated pressure traces show, as do mid range simulation packages such as Dynomation and other higher end programs, that in a well-tuned engine running in its designed operating window the port pressure behind the intake valve drops below cylinder pressure well before IVC, but that charge inertia through the shrinking curtain area sustains inflow for as much as 10-15° more.
In fact with an IVC correctly chosen for high RPM, simulations show significant reverse flow up to perhaps 1,000 RPM below peak power; so much so that if such flow is absent in lower RPM simulations, a later IVC invariably predicts more power.
I don't know why (perhaps to slow the power drop off beyond peak), but I've heard that even at peak power, some slight degree of reverse flow can be desirable.

[n2omike]

Mike you're exactly correct ... nothing "dynamic" in the Internet's DCR definition


the Internet's popular inaccurate DCR definition : Dynamic Compression Ratio

there is nothing "Dynamic" in the DCR or Dynamic Compression Ratio ... as defined by popular Internet DCR calculators .

True. There is nothing 'dynamic' about it. DCR just gives us an idea of how changing the LSA and intake duration (intake closing point) affects how much air is trapped in the cylinder for the compression stroke. Again, the ONLY thing DCR does for us, it it takes the intake closing point into consideration for calculating a more ideal compression ratio. It's not perfect, but works better than using compression ratio alone.

As mentioned in previous posts, there's a LOT more going on, but DCR is a more accurate tool than simple compression ratio, and gets us in the ballpark when putting a combination together.

[MadBill]

... There is nothing 'dynamic' about it. DCR just gives us an idea of how changing the LSA and intake duration (intake closing point) affects how much air is trapped in the cylinder for the compression stroke...

That would be a far less misleading term: "trapped compression ratio", or maybe "cranking CR".

[Roundybout]

We are talking about static cam timing and how the valve events effect trapped pressure in the cylinder, how does variable valve timing play into this? You’re only going to get a certain amount of trapped cylinder pressure based on a static cam profile at any RPM. Now with VVT I can manipulate that trapped pressure.

[MadBill]

That caveat would also apply to the Nissan variable compression ratio engine, but I think both are well outside the scope of this discussion...

[GARY C]

... There is nothing 'dynamic' about it. DCR just gives us an idea of how changing the LSA and intake duration (intake closing point) affects how much air is trapped in the cylinder for the compression stroke...

That would be a far less misleading term: "trapped compression ratio", or maybe "cranking CR".

I had always known it as cranking cylinder pressure until recently.

I found it interesting that in this interview with Don the builder was more concerned with cranking cylinder pressure than with compression ratio.

Bill has built engines at the highest levels of both stock car and drag racing. In '85, '87 and '93 Bill was the nhra world champ in competition eliminator - a series of racing that clearly shows who makes the most power. Today he's focusing on building engines for the nascar west series and looks to be having the same level of success. viewtopic.php?f=47&t=57586

[MadBill]

Cranking pressure is certainly a useful metric, but although closely related, it is of course not the same as a cranking ratio.

BTW, to be clear for anyone that doesn't click on the above link, I'm not the referred-to Bill.

[maxracesoftware]

"Cranking pressure" ? , but at what Starter RPM ??

Starter RPM can change Cranking pressure a lot , one way or the other !!!

the various Tests i've made with Starter RPM on my Dyno prove this .

Likewise , whats so special about Seat-to-Seat Duration in so called DCR ?
Seat-to-Seat is still not starting off at ZERO .
Pat chose "Seat-to-Seat" in his DCR Calculator
but 0.050' Durations are much more accurate to use than Seat-to-Seat durations.

Most probably using an even larger than 0.050" would be a better correlation .

[n2omike]

Thing is... Most of us here can understand what is commonly referred to as DCR. We know that actual air/fuel flow changes throughout the rpm range in a running engine... and that more/less air actually enters/gets trapped in the cylinder under these 'dynamic' conditions through the rev range.

BUT... Can we actually show how much, and predict it??? NO. We cannot.

What is commonly known as DCR is about as technical as we can get in the real world. We just have to keep in mind that it is an approximation, and that the better the VE, etc... the greater the cylinder pressure may get in an actual running situation. Outside of that, we can't make valid predictions.

This being said, the following basic guidelines hold true:
All else being equal, a tighter LSA closes the intake valve sooner, trapping more air, and raises cylinder pressure.
All else being equal, longer intake duration closes the intake valve later, trapping less air, and reduces cylinder pressure.

Longer duration cams require a higher compression ratio to maintain cylinder pressure.
Tighter LSA's can help a combination that is low on compression ratio to build cylinder pressure.

[GARY C]

"Cranking pressure" ? , but at what Starter RPM ??

Starter RPM can change Cranking pressure a lot , one way or the other !!!

the various Tests i've made with Starter RPM on my Dyno prove this .

Likewise , whats so special about Seat-to-Seat Duration in so called DCR ?
Seat-to-Seat is still not starting off at ZERO .
Pat chose "Seat-to-Seat" in his DCR Calculator
but 0.050' Durations are much more accurate to use than Seat-to-Seat durations.

Most probably using an even larger than 0.050" would be a better correlation .

Thats why I said a cpl pages back that it can change with RPM (cranking pressure)

I always figures seat was used because you can't trap pressure with the valve opened. but I gots know Idea... I have found Pat's calculator handy for street engines when wanting my cranking compression in the 200 range for safe pump gas use.

[Firedome8]

Is the term VE also a misnomer ? Doug Roe refers to DE density efficiency..