static vs dynamic compression ratios

[hoffman900]

Is there a middle ground for those of us not at either extreme of either a) having a massive database of successful R&D and actual builds to leverage, or b) having the ability to model things using highly sophisticated programs requiring extensive inputs?

I think you either undershoot what you can get away with or overshoot it. Most people get away with things because certain combinations have been done thousands of times, so they can at least ballpark it by piggy backing off that experience, but they don't necessarily "know" why they can or can not do.

At the end of the day engines are complex systems, and I'd lean more cautious than not with just a general rule of thumb calculation.

[BradH]

I'd hate to think it boils down to this scale, but...

EXPERMENTAL/LIVE DATA -------------------------------S.W.A.G.----------------------------SOPHISTICATED SIMULATION

[David Redszus]

An engine with a SCR of 8.5 would typically have a DCR of about 6.5. A this should produce a cranking
pressure of about 186 psia...if there is no leakage. I might look for a valve and/or ring leak. What was the leakage?

Is there a mine field warning here I missed? Should there be? Popcorn?

How does a 8.5 SCR engine produce 186PSIG cranking pressure?

Mine has a SCR of 10.5 and cranking pressure is 180psig.

SCR does NOT determine compression pressure. And never has.
Compression pressure is determined by DCR, inlet pressure, and ratio of specific heats of the air.
SCR plays no role whatsoever.

When calculating compression pressure, we use ambient pressure lacking an accurate measure
of absolute inlet pressure. Actually, to be precise, we need to know cylinder pressure at IVC.

When pressure readings are reported, they may be presented as psia or psig. Psia (absolute) is
used for engine calculations. Psig (guage) is determined by subtracting ambient pressure from
absolute pressure. PSIA - 14.7 = PSIG. 186.7 psia - 14.7 = 172.0 psig.

The composition of the air will determine the compression pressure for a given DCR.
The fuel vapor present in the inducted air is represented by the Ratio of Specific Heats.
From pure air (1.40) to very rich mixture (1.30) the compression pressures are as follows.
SpH.....Pressure
1.40....201.1 psia
1.36....186.6
1.33....176.6
1.30....166.8

Of even greater importance is the determination of compression temperature.
Later.

[mt-engines]

So what you are saying is static compression ratio is meaningless?

How can you get a dynamic without a static? What if the static ratio is 1.0000000001:1 vs say 18:1 you mean to tell me a simple cam swap will make the two equal in power and brake specifics?

Atmospheric pressure can only do so much

[hoffman900]

So what you are saying is static compression ratio is meaningless?

How can you get a dynamic without a static? What if the static ratio is 1.0000000001:1 vs say 18:1 you mean to tell me a simple cam swap will make the two equal in power and brake specifics?

Atmospheric pressure can only do so much

I think compression ratio is such a misnomer. Ultimately, cylinder pressure is all that matters, it's just a number engine builders can reference and have an idea of how it works due to looking at other similar examples. I look at it like setting tire pressure on a road race car. 18psi cold doesn't mean you're going to get where you want hot pressure wise, and ultimately, tire temperature is what matters. It's something you can set, sure, and have an idea of where you're going to end up, but it all relies on so many other factors.

Like I said, look at a modern NA Porsche GT3 engine or the modern Superbikes. 13:1 compression ratios, some still only have port injection, and they run all day on 93 octane. Everything I listed before effects it, and they're not doing it with late IVC timing.

Another example, NASCAR pre-compression rules, builders used static compression ratios in excess of 16:1 in restrictor plate engines. On pit road, and pace laps, engine speeds were low enough so that the cylinder saw the full weight of the atmosphere and they had a propensity to detonate and blow up if the throttle opening was too much. At higher rpm's, when the restrictor was in play, the engine didn't see the full weight of the atmosphere, so the higher static compression ratio was about trying to build back more cylinder pressure, by squeezing it tighter, to make up for having less "atmospheres".

In that case, compression ratio is pretty much meaningless as the dynamic compression ratio, as most understand it from magazines, changes due to the effects of the restrictor (then there is the whole wave dynamics aspect).

Replace restrictor with some other "choke" in the system, and the same thing is happening.

I think dynamic cylinder pressure is a way better way of thinking about it.

[David Redszus]

So what you are saying is static compression ratio is meaningless?

How can you get a dynamic without a static? What if the static ratio is 1.0000000001:1 vs say 18:1 you mean to tell me a simple cam swap will make the two equal in power and brake specifics?

Atmospheric pressure can only do so much

Do not confuse compression RATIO with compression PRESSURE.

A compression RATIO is simply a comparison of two volumes.
SCR uses the full stroke cylinder volume while DCR uses cylinder volume counting from IVC.

In order to cause a change in SCR, it is necessary (for a given displacement) to change the combustion
chamber volume. When the chamber volume is modified, the DCR also changes, as does the squish area.
But for any given engine, whatever the SCR, there is some corresponding DCR which determines the
compression PRESSURE and engine performance.

For an engine with determined SCR and DCR, changes in inlet pressure and temperature will not change
the SCR and DCR RATIOS. They will however, have an effect on the resulting compression PRESSURE and
TEMPERATURE.

[ClassAct]

So what you are saying is static compression ratio is meaningless?

How can you get a dynamic without a static? What if the static ratio is 1.0000000001:1 vs say 18:1 you mean to tell me a simple cam swap will make the two equal in power and brake specifics?

Atmospheric pressure can only do so much

Do not confuse compression RATIO with compression PRESSURE.

A compression RATIO is simply a comparison of two volumes.
SCR uses the full stroke cylinder volume while DCR uses cylinder volume counting from IVC.

In order to cause a change in SCR, it is necessary (for a given displacement) to change the combustion
chamber volume. When the chamber volume is modified, the DCR also changes, as does the squish area.
But for any given engine, whatever the SCR, there is some corresponding DCR which determines the
compression PRESSURE and engine performance.

For an engine with determined SCR and DCR, changes in inlet pressure and temperature will not change
the SCR and DCR RATIOS. They will however, have an effect on the resulting compression PRESSURE and
TEMPERATURE.

So David, to follow up with where you’re going is it a fair statement to say that even though compression ratio doesn’t equal compression pressure and increase in compression ratio will increase the compression pressure all things being equal? Is that a fair statement?

[David Redszus]

So what you are saying is static compression ratio is meaningless?

How can you get a dynamic without a static? What if the static ratio is 1.0000000001:1 vs say 18:1 you mean to tell me a simple cam swap will make the two equal in power and brake specifics?

Atmospheric pressure can only do so much

Do not confuse compression RATIO with compression PRESSURE.

A compression RATIO is simply a comparison of two volumes.
SCR uses the full stroke cylinder volume while DCR uses cylinder volume counting from IVC.

In order to cause a change in SCR, it is necessary (for a given displacement) to change the combustion
chamber volume. When the chamber volume is modified, the DCR also changes, as does the squish area.
But for any given engine, whatever the SCR, there is some corresponding DCR which determines the
compression PRESSURE and engine performance.

For an engine with determined SCR and DCR, changes in inlet pressure and temperature will not change
the SCR and DCR RATIOS. They will however, have an effect on the resulting compression PRESSURE and
TEMPERATURE.

So David, to follow up with where you’re going is it a fair statement to say that even though compression ratio doesn’t equal compression pressure and increase in compression ratio will increase the compression pressure all things being equal? Is that a fair statement?

Yes, it is.

To be even more specific assume an engine with a SCR of 9.0 and a DCR of 7.08.
It will produce a compression pressure of 198 psia, a thermal efficiency of 54.3%.

Raising the SCR to 10, with a commensurate DCR of 7.8, due only to a change in chamber volume,
it will produce a compression pressure of 226 psia, and a thermal efficiency of 56.08%.

If the same SCR of 10, yielded a DCR of only 7.08, due to a cam change, the engine
will produce the same compression pressure of 198 psia, and a TE of 54.3%, as does the SCR of 9.0.

It is only the DCR that produces a pressure change, a change in TE, and a resulting change in power output.

Remember there are also squish and temperature effects on combustion to consider.

[RT Machine]

I worked with a Ford Engineer in Australia who used to be in charge of prototype engine calibration in the UK.
He told me many things that went on in the dyno test cells etc
They always measured DCR and combustion pressures etc with transducers in the combustion chamber.
The OEM,s monitor every parameter possible to get that engine as perfect as possible.
If DCR is important to the OEM,s then its important to the rest of us.

[CamKing]

I worked with a Ford Engineer in Australia who used to be in charge of prototype engine calibration in the UK.
He told me many things that went on in the dyno test cells etc
They always measured DCR and combustion pressures etc with transducers in the combustion chamber.
The OEM,s monitor every parameter possible to get that engine as perfect as possible.
If DCR is important to the OEM,s then its important to the rest of us.

If you could accurately calculate DCR, he wouldn't need the transducers.
You can not accurately calculate the Dynamic Compression Ratio in a running engine.
I do valvetrain development for one of the largest OEM engine manufacturers in the world, and not once have any of their engineers brought up calculating the Dynamic Compression Ratio via the Exhaust closing point..

[Nut124]

Yes, it is.

To be even more specific assume an engine with a SCR of 9.0 and a DCR of 7.08.
It will produce a compression pressure of 198 psia, a thermal efficiency of 54.3%.

Raising the SCR to 10, with a commensurate DCR of 7.8, due only to a change in chamber volume,
it will produce a compression pressure of 226 psia, and a thermal efficiency of 56.08%.

If the same SCR of 10, yielded a DCR of only 7.08, due to a cam change, the engine
will produce the same compression pressure of 198 psia, and a TE of 54.3%, as does the SCR of 9.0.

It is only the DCR that produces a pressure change, a change in TE, and a resulting change in power output.

Remember there are also squish and temperature effects on combustion to consider.

Talking about changes in DCR and resulting changes in thermal efficiency and power output are confusing me here.

Are you saying that DCR and power output correlate?

[hoffman900]

I worked with a Ford Engineer in Australia who used to be in charge of prototype engine calibration in the UK.
He told me many things that went on in the dyno test cells etc
They always measured DCR and combustion pressures etc with transducers in the combustion chamber.
The OEM,s monitor every parameter possible to get that engine as perfect as possible.
If DCR is important to the OEM,s then its important to the rest of us.

Did you ever ask him how he defined DCR? It's probably not what most think.

[CamKing]

Did you ever ask him how he defined DCR? It's probably not what most think.

Exactly.
There's a lot of good information you can compile, looking at pressure waves in a running engine.
None of it has any relation to the output you receive from a "DCR" calculator.
How can you input static parameters, and get a Dynamic result ?

[PackardV8]

Just yesterday a guy building an el cheapo BBM told us of a new cam grind he was going to use, marketed as a crutch for the low SCR OEM pistons and heads; supposedly, it closes the intake fast and early to trap more DCR while still having overlap to give the required fairgrounds idle lope. Anyone know of this deal?

[CamKing]

Just yesterday a guy building an el cheapo BBM told us of a new cam grind he was going to use, marketed as a crutch for the low SCR OEM pistons and heads; supposedly, it closes the intake fast and early to trap more DCR while still having overlap to give the required fairgrounds idle lope. Anyone know of this deal?

That's the Comp Thumpr series.

Thumpr™
Benefits:

Great sound (early exhaust opening and more overlap)
Great power (new lobe designs give great area and air flow)
Great mid-range response (early intake closing traps more air)

Important Considerations:

Minimal engine vacuum – requires mods for any vacuum accessories such as power brakes
Designed for use only with valve train upgrades including COMP Cams® Beehive™ Valve Springs and roller rockers
Works best with looser converters, more rear end gear and other performance engine modifications

They idle like crap, and make no top-end power.
I'm sure some people love them.