Static Compression VS Dynamic compression

[johnef]

I'm putting a 1000cc late model bike engine together. And was thinking of running 18:1 on the static side, but would it be fair to speculate it could be 18:1 on the dynamic side?

Additionally do you guys consider all of intake cam duration as cylinder volume and deduce a percentage for backflow? Always wondered about this one.

[David Redszus]

I'm putting a 1000cc late model bike engine together. And was thinking of running 18:1 on the static side, but would it be fair to speculate it could be 18:1 on the dynamic side?

Additionally do you guys consider all of intake cam duration as cylinder volume and deduce a percentage for backflow? Always wondered about this one.

It is possible for static and dynamic compression ratios to be the same if, you close the inlet valve at BDC, and/or, the inlet pressure always stays above cylinder pressure.

Cylinder volume is not determined by cam duration but by piston downward displacement.

[Lizardracing]

Dynamic doesn't exist. Well, it actaully does but there are too many many variables to reasonably calculate the density of the A/F charge trapped in the cylinder.
Perhaps youre referencing Intake Closing Point?

[Roundybout]

18:1 is diesel territory. Is that a mis-type? Depending on the factors the dynamic ratio could be more/less/the same than the static. Cam timing has a significant role in the dynamic. Having 18:1 static I would assume some wild cam stats in order to be able to use a 18:1 static.

Kinda reminds of a NASCAR plate engine using very high static in order to get a decent dynamic ratio. Just don't ask it to do anything full throttle at low RPM or even part throttle until a high RPM is achieved.

[Stan Weiss]

What is your definition of Dynamic Compression?

If I have a 4 cylinder engine

Bore = 2.89"

Stroke = 2.323"

Rod Length = 2.323"

CR = 18:1

If my intake valve closes @ BDC I will have about 600 psi

If my intake valve closes 90 ABDC I will have about 290 psi

Now if I start and run the engine on 3 cylinders with a gauge in the 4 cylinder, and at some RPM I see 600 psi or more.

Stan

[johnef]

I understand that intake valve closing point determines trapped or dynamic compression.
Maybe it wasn't clear the question. If aiming for a net result of 18:1 compression, would it be figured on static or dynamic.

If it were dynamic, then it could be hypothetically 20:1 static and bleed off through increased intake duration and later intake closing.

We run 15:1 on pump gas regularly.

In this build were looking at 16,000 rpm levels, titanium rods, and increased intake cam duration of well over 300°. In order to effectively move power up along with all other supporting efforts.

We run VP Q16 for those levels of compression.

[Schurkey]

Maybe it wasn't clear the question. If aiming for a net result of 18:1 compression, would it be figured on static or dynamic.

If it were dynamic, then it could be hypothetically 20:1 static and bleed off through increased intake duration and later intake closing.

We run 15:1 on pump gas regularly.

What is your current dynamic compression, if you're at 15:1 static? Or are you at 15:1 dynamic, in which case what is your static compression? How much higher do you want to go?

[David Redszus]

I understand that intake valve closing point determines trapped or dynamic compression.
Maybe it wasn't clear the question. If aiming for a net result of 18:1 compression, would it be figured on static or dynamic.

If it were dynamic, then it could be hypothetically 20:1 static and bleed off through increased intake duration and later intake closing.

We run 15:1 on pump gas regularly.

In this build were looking at 16,000 rpm levels, titanium rods, and increased intake cam duration of well over 300°. In order to effectively move power up along with all other supporting efforts.

We run VP Q16 for those levels of compression.

There is no intake valve closure point that will increase trapped compression ratio above static. The only way to obtain a higher trapped CR is to raise the inlet pressure.

As Stan has illustrated, the TCR will depend on IVC (for a fixed chamber volume and inlet pressure). With any typical IVC, the TCR will be far below the SCR. A long duration inlet will lower the TCR.

What is your IVC?

[johnef]

Maybe it wasn't clear the question. If aiming for a net result of 18:1 compression, would it be figured on static or dynamic.

If it were dynamic, then it could be hypothetically 20:1 static and bleed off through increased intake duration and later intake closing.

We run 15:1 on pump gas regularly.

What is your current dynamic compression, if you're at 15:1 static? Or are you at 15:1 dynamic, in which case what is your static compression? How much higher do you want to go?

This particular motor is 13.5:1 factory OEM. Up until now we haven't been concerned with compression, because we maintained more or less factory Rev limiters and duration. Now building a lighter chassis with a lightweight rider we can move peak horsepower higher
with the increased benefit of horsepower.

This is why now we're looking at increased compression to offset the increased duration to support the higher RPM peak horsepower.

That motor was 15:1 static. Didn't measure dynamic.

[CamKing]

I understand that intake valve closing point determines trapped or dynamic compression.

You "understand" wrong.
In a running engine, the cylinder continues to be filled, long past BDC. As long as the intake valve is open, and the pressure above the valve is greater then the pressure below the valve, you will have positive flow into the cylinder. The air flow doesn't care where the piston is, it just flows towards the lower pressure. As the piston starts uo from BDC, it starts to increase the pressure in the cylinder, but if the cam is correct, the reduction in valve lift of the closing intake valve will also increase the pressure above the valve, and keep it above the pressure in the cylinder for an extended amount of time. This is truly the dynamic compression, because it changes thru-out the rpm range. Unfortunately, it's not something you can calculate with a simple formula.

[BradH]

I created a small sh!t-storm on another forum about a year ago by starting this thread: https://board.moparts.org/ubbthreads/ub ... rchpage/1/

[jsachs1]

Swept volume compression #. Factor in the intake closing, will give you ballpark corrected compression.
I will say that 18:1 static will loosen a lot of bolts, especially the ones that support the crankshaft in aluminum cases.
John

[johnef]

So if Dynamic compression is deemed 1 dimensional, is everyone saying static is good enough?

[David Redszus]

So if Dynamic compression is deemed 1 dimensional, is everyone saying static is good enough?

Dynamic compression is the target, but it can be elusive since it depends on IVC and pressure above the valve as Mike Jones indicated. Static is merely used to compare one engine to another but provides no useful info.

Actually, pressure ratios, either kind, don't mean much. We need to know actual compression pressure and temperature. And they are affected by inlet air temperature and pressure.

Look at Stan's data. Notice the very large difference in TCR when the IVC is at BDC vs at 90deg BTC.
What is your IVC angle BTC?

[johnef]

The intake valve closing is 82° ABDC.

What would be a manageable PsI reading?