power increase from added compression

Good info on the tractor area of this site. Not much to gain after 9 to 1 compression.

First off I notice someone mentioned that thermodynamic efficiency is only one of multiple types of efficiency that all factor into the overall efficiency of the engine. That is true. The only one of these multiple "constituent efficiencies" that is positively affected by compression ratio increases is thermodynamic efficiency.

As for other factors, I am aware of only one reason that an increase in compression ratio would yeild more than the improvement in thermodynamic efficiency. If the volumetric efficiency is poor enough that pressures and temperatures in the cylinder aren't high enough to support thorough and rapid combustion an increase in compression ratio will improve the thoroughness of the burn. Which will improve power by more than the increase in thermodynamic efficiency. A good example is increasing the compression ratio of an engine with a big cam to improve the off-idle throttle response. Because of the big cam, the volumetric efficiency is poor off-idle and combustion tends to be poor off-idle because of low cylinder pressures/temperatures.

You have to remember, though, that the engine will make better torque off-idle with a smaller cam and less compression ratio (as is usually dictated by concerns about detonation now that the volumetric efficiency is up at off-idle) than it does with the big cam and the high compression ratio. And any gains that the big-cam engine sees in torque at the torque peak will be attributable to improvements in thermodynamic efficiency. Accordingly, these gains (at the torque peak) will fall in line roughly with the theoretical values in my other post.

Of course if your heads and cam are so poor/mismatched that you have poor volumetric efficiency at peak torque, an increase in compression ratio would have the dual effect of making combustion more complete and increasing thermodynamic efficiency. If this is the case, however, there are much bigger gains to be had by improving your volumetric efficiency than by increasing the compression ratio.

With the exception of making it more agreeable to run a bigger cam by crutching off-idle torque, I am unaware of any other aspect of an engine that can be improved because of an increase in compression ratio. Increased compression ratio will not increase the vacuum signal of the pistons - only a change in displacement (increase in bore or stroke) will do that.

Of course you're not prohibited from changing other things along with the compression ratio. You can improve the heads and put on a bigger carb, etc. but you can do those things without an increase in compression ratio. So, any increase in power from those improvements is not attributable to the increase in compression ratio.

Of course improvements do build on one another. For example, you could improve the torque of the engine by 50% with a 50% increase in displacement and then increase the thermodynamic efficiency of the engine by 5%. The 5% would be of the 150% of the original power. So, the compression ratio increase would improve the power by an amount that was 7.5% of the original power of the engine. But the fact remains that the compression ratio increase only created a 5% increase in power as compared to the same engine without the compression ratio increase.

It is also possible that the higher octane fuels that folks are forced to use with higher compression ratios have a higher BTU release and create more power as a result. Once again, however, if that is the case, using the higher octane/higher energy fuel in a low compression engine would also increase power. This increase in power for the lower compression engine would be equal to the increase in power of the high compression engine less the improvement in thermodynamic efficiency because of the increased compression ratio.

The only thing I know of is to change the camshaft. Less overlap and earlier intake valve closing will increase the vacuum signal created by the pistons in the intake manifold.

The overlap reduces the vacuum signal in the intake manifold because 1) the cylinder and the intake manifold are exposed to pressure from the exhuast port; and 2) when the exhaust valve is open and the piston is starting the intake stroke, the expansion in the cylinder is sort of shared between creating vacuum in the intake manifold and reducing pressure in the exhaust manifold.

Reducing intake duration increases the vacuum signal created in the intake manifold by the pistons because you decrease the amount of time that the intake valve is open during the compression stroke. During this time that the intake valve is open during the first part of the compression stroke the piston is actually "trying" to push air back into the intake manifold.

So a lot of overlap and late intake valve closing in a camshaft will tend to reduce the vacuum signal that the pistons create in the intake manifold.

At low RPMs, where the gases don't have much momentum, the strength of this vacuum signal in the intake manifold is the dominant factor in how much air flows into the cylinders (volumetric efficiency).

At higher RPMs, the air in the intake manifold is moving fast enough that its momentum can become a more prominent factor in filling the cylinders. Because the air in the intake tract is moving fast and has momentum at higher RPMs, it will actually continue to flow into the cylinder after the piston starts coming up on the compression stroke. Thus, while an earlier intake valve closing event will improve the vacuum signal (and thus the volumetric efficiency at lower engine speeds), it will reduce volumetric efficiency at higher speeds because it closes the intake valve before you take full advantage of the "ramming effect" that the momentum of the air has.

The overlap of the intake and exhaust valves can also have a positive effect on volumetric efficiency at higher engine speeds for reasons that I don't understand as well.

Another way to improve volumetric efficiency at a particular engine speed is to "tune" the length of the intake and exhaust tracts. This is probably best explained by talking through what happens inside the exhaust track when the exhaust valve opens.

When the exhaust valve opens, a pressure starts at the exhaust valve and travels through the exhaust track at the speed of sound. When that pressure wave reaches the end of the exhaust track (the opening on your muffler or straight pipe) it is reflected back and tends to resist the exhaust gases flowing out as it travels back toward the exhaust valve. When that reflected pressure wave reaches the exhaust valve it is reflected back again and tends to give the exhaust gases a renewed surge out of the exhaust pipe as it travels back toward the outer end of the exhaust track.

If the reflected pressure wave hits the exhaust valve in the time shortly before the exhaust valve is closing it will bounce back toward the muffler just in time to help scavenge just a little more exhaust gases out of the cylinder.

Because these pressure pulses travel at the speed of sound, you can control how long it takes for them to travel to the end of the exhaust pipe and back by controlling the length of the exhaust track.

Additionally, for a given engine speed you can calculate the time (in fractions of a second) from the exhaust valve opening to the exhaust valve closing.

So, if you want to maximize exhaust scavenging for a given RPM, you make your exhaust length such that the time it will take for the pressure pulse to travel to the end and back is slightly less than the time from the exhaust valve opening to the exhaust valve closing.

I guess I should say that it is actually the second reflection that is the strongest. Which means that you actually want to set the exhaust length so that the pressure pulse has time to travel to the end and back twice during the time that the exhaust valve is open. Of course you'll still get the tuning effect when the engine is running at twice the speed and the pressure pulse only has time to travel to the end and back once, it just won't be as strong a pulse.

The same concept is applied to the intake manifold. I.E. when the exhaust valve opens a ("negative") pressure wave travels to the outermost point of the air intake tract and is reflected back. As with the exhaust side you want to set the intake tract length such that this pressure wave has time to travel to the end (air cleaner) and back to the intake valve twice during the period the intake valve is open (for whatever engine speed you're trying to optimize).

Of course an intake tract and an exhaust tract tuned for the same RPM will be of different lengths becuase the speed of sound (the speed of the pressure pulses) is different in the cool intake air than it is in the hot exhaust gases. Additionally, it can be difficult to know just exactly what the temperature of the exhaust gases is.

And speaking of hot air, I've met my quota.

Yes, manifold vacuum at idle (or starter cranking) and cranking compression will be affected significantly by valve timing. It's actually opposite what you said, though. To increase manifold vacuum and cranking compression you want to open the intake valve later (which decreases the overlap with the exhaust valve being open) and close the intake valve sooner.

Also, yes, by changing the lash, what you are doing is affecting the timing of the valve opening and closing events as well as the lift of the valves. As opposed to changing the lash from the factory spec., changing the cam is a better way to change valve events. But that's a side topic.

Now, if I understand correctly, what you did was reduce the valve lash from the factory spec. This would open the valve sooner and close it later.

According to every authority I've read, that should cause a decrease in manifold vacuum and compression. So, I don't know how it would be that you observed the opposite.

As I said in the last post, if you want to increase vacuum and cranking compression you want a "smaller" cam (open the valves later and close the valves sooner).

As I also mentioned in the previous post, however, volumetric efficiency is affected by both the vacuum signal from the pistons and the "ramming effect" caused by momentum of the intake charge. At "low" RPMs vacuum is the dominant factor because the intake charge doesn't have much momentum. At "high" RPMs intake charge momentum becomes the dominant factor.

In order to maximize volumetric efficiency at a given RPM you need to strike the right compromise between vacuum signal and sufficient camshaft duration to take advantage of the available "ramming effect" of the intake charge at that RPM.

I certainly don't have the knowledge to tell you what valve timing is best for 2500 RPM. For that you really need to talk to someone who really knows cams.

There are a lot of other topics in your post such as fuel atomization, intake charge velocity, and correlation between manifold vacuum and volumetric efficiency (which there is no direct correlation between these two - high manifold vacuum doesn't necessarily mean you will have high or low volumetric efficiency). But I'm wearing down for the day, and I don't know enough about some of that to give you a really good explanation. Besides which, for the most part I usually only share what knowledge I have when someone is perpetuating a myth that I find particularly troublesome.

In any event, discussing this on this forum is clearly a very inefficient way for you to learn about this stuff. If you really want to get a better understanding of the science of engine operation it would be a lot less confusing for you to get a good book.

Good luck to you.

Neil

As one rule of thumb, not all engines are equal, no where near the same conditions from one engine to the next, even if the same make and model.. Also, when compression is increased, air flow, speeds up, vacuum pressure also increases, which means, it is using more of the volume in the manifold, and head, it can take a bigger breath on the intake stroke,, you cant just change the compression, and not change air flow in the manifold, it is not possible for it not to, if it does drop in vacuum pressure, you have a leak. Low rpm engine cannot be compared to high rpm race engines, the amount of time, that the air flows through the cyl on the high rpm engine is much much faster than a low rpm engine,, Usually, when you have a high compression, low rpm engine,, the first thing pullers do, is go get a big carb, in my opinion, wrong thing to do. Cause the key is, to speed up air flow for a better mixture in the intake, when you add the big carb, vacuum drops too soon, making it erratic, low rpm engines have a higher vacuum reading than a high rpm engine,, cause, there is more time to feed it, less rpm to neutralize vacuum. it is backwards from race engines. Chad

Correct on adding the bigger carb with just extra compression causes bogging and poor air/fuel mixture control. The airflow was never increased. All the extra compression does is convert more of the combustion chamber power stroke pressure into more and longer duration thrust pressure upon the piston. Also as you say stuck timing advance, wrong timing settings and erratic timing from worn parts wreaks absolute havoc with power, efficiency and reliability.

Quote

"when compression is increased, air flow, speeds up, vacuum pressure also increases, which means, it is using more of the volume in the manifold, and head, it can take a bigger breath on the intake stroke,, you cant just change the compression, and not change air flow in the manifold, it is not possible for it not to, if it does drop in vacuum pressure, you have a leak. Low rpm engine cannot be compared to high rpm race engines, the amount of time, that the air flows through the cylinder on the high rpm engine is much much faster than a low rpm engine,, Usually, when you have a high compression, low rpm engine,, the first thing pullers do, is go get a big carb, in my opinion, wrong thing to do. Cause the key is, to speed up air flow for a better mixture in the intake, when you add the big carb, vacuum drops too soon, making it erratic, low rpm engines have a higher vacuum reading than a high rpm engine,, cause, there is more time to feed it, less rpm to neutralize vacuum."

Unquote.

A lump on top of the piston does not increase and engine airflow. Larger bore and/or stroke does if the ports can supply the flow. Higher compression allows longer cam duration to be used which can improve breathing with extra valve open time. Since there is more mechanical expansion ratio on the power stroke the exhaust valve can be opened earlier. This extra open improves cylinder savaging to make room for more intake mixture. Since the extra expansion ratio(compression ratio) is there. Power isn't lost from opening the exhaust valve as the useable pressure to make "torque" has already dropped. There is no such thing as “vacuum pressure” and no vacuum below 0.0psi absolute. 14.7psi atmospheric is all the push there is unless forced induction is used. Contrary to popular belief the engine does not suck air in. The retreating piston leaves a low pressure area that is filled with atmospheric pressure which is now higher than combustion chamber pressure. Even at idle with a short cammed engine the best an engine will do is about 22-25" of mercury or 12.25 psi below atmospheric or 2.2psi absolute. At higher rpms delta P increases through the aircleaner restriction, port wall friction and turbulence at turns. The engine will do well to have 10psi to push air into the combustion chamber no matter what is done. Ramming air into a combustion chamber from inertia due to the energy stored in a column of moving air through a tuned intake/exhaust system is the only way to get more than 10psi acting across that combustion chamber. Even then about 14-15 psi delta P is max.

AND HOW IS IT MEASURED CORRECTLY. WHEN INCREASING CUBIC INCH BY A PERCENTAGE, SAY (10) PERCENT 281CI TO 309CI, FROM THE AIR CLEANER, THRU THE CARB AND THE VALVE IN THE HEAD , WHAT ARE YOU LOOKING FOR THERE TO CREATE EFFIENCY!

DOES A DEGREE WHEEL COME INTO PLAY ? DOES OPENING THE EXHAUST VALVE SOONER (BY HOW MANY DEGREES) INHANCE EFFICIENCY. DOES THAT TAKE AWAY FROM THE POWER STROKE.
WITH CAM LIMITATION, HOW CAN YOU MOVE LOBE SEPERATION CLOSER TOGETHER, WHEN ADDING LIFT TO THE INTAKE LOBE, SHOULD THE EXHAUST LOBE BE LEFT THE SAME