How High Compression Does What It Does
I have answered so many emails about the mythological dangers of high compression, I figured it would be a good idea to just write a short essay on the topic and be done with it. So here goes:
In no particular syntax, here's the scoop:
If there is truly ONE magic powerband bullet for a 4-stroke, high compression is it,.no question.
4-strokes love, love, LOVE high compression!
High compression does not cause a sharp or dramatic top end power loss in 4-strokes the way it can add a pumping load to a 2-stroke engine; that is a very persistent held-over myth to beat down. There are limits to everything, but 2-strokes are far more sensitive to over-compression as far as top end power limiting goes.
With high compression, your engine performs like it has a bigger displacement at lower RPMs, and it will perform like it has more camshaft at high RPMs - more bottom, more top, and better throttle response across the board. A beautiful thing, and very hard trick to beat, short of forced induction (more on that another time).
So as to not perpetuate any sort of mythological fog here, it needs to be explained exactly how high compression does all of that.
High compression is not just a high dome that squeezes the A/F mix so tight is goes off like an atomic bomb --. the tighter pressure squeeze does indeed help the power output, but it isn't all the magic.
It's tough to paint an analogy in layman's terms with words alone, so I will use exaggerated illustrations for the purpose of clarity:
The piston and cylinder arrangement has now become a GIANT syringe. The piston is the rubber plunger, and the clear tube is your cylinder. And while we are at it, let's give it 2 needle outlets on top too, one for intake and one for exhaust.
In our low compression model, we will exaggerate and say that the piston/plunger only goes half way up the tube at the top of its stroke. The HIGH compression model’s piston/plunger goes very close to the tip of the tube at the top of its stroke. That exaggeration will help with understanding all the other dynamics besides how tight the mixture gets squeezed alone.
So, besides being used to squeeze the A/F charge before ignition, the piston/plunger is also important to how much vacuum occurs during the intake/suction stroke.
Let's say you could put your finger over the intake side of the LOW compression syringe, and then feel the amount of vacuum generated as you pull the piston/plunger to the bottom of the stroke. You will notice that the vacuum builds slowly, and doesn't become very strong until the bottom of the stroke.
Doing the same test with the HIGH compression piston/ plunger -- where the piston has a much smaller volume of air trapped above it to begin with -- you will see a very fast, very sharp rise in the vacuum it generates, since it has less trapped volume to dampen the vacuum in the first place.
So what does that do for a running engine? Some Magic, all good!
The higher compression version provides a STRONGER and EARLIER vacuum pulse into the intake tract, which makes for better/sharper throttle response by delivering a stronger signal to the carb's metering circuits.
Also the sharper vacuum drop makes the incoming fuel droplets break up/atomize into a finer air + fuel fog -- the smaller the fuel droplets, the better the combustion. The only part that can burn is the part that comes in contact with oxygen. Big droplets only have the "skin" of the drop burn away during combustion; the remainder of the drop not only doesn't burn, it adds unburned hydrocarbon emissions to the atmosphere. And it serves to dampen the combustion process by absorbing latent heat energy from the part that does combust.
The other thing that the stronger vacuum signal from the higher compression piston does is also wonderful -- it creates a higher velocity incoming intake charge. What does that do? One thing that higher velocity does is keeps atomized fuel droplets in suspension better than a lower velocity charge does, and we know that is a good thing.
And we sort of know that higher compression gives back a lot of the torque that a BIG duration cam loses, but most people think that the tighter squeeze of the A/F mix prior to ignition is what does this, and of course, that's part of it.
First we need to know why a big cam actually loses bottom end power and response in the first place. A modern performance cam opens the intake some 20 to 30 degrees before the piston is all the way to the top of the exhaust stroke -- just prior to the beginning of the downward intake stroke -- and it doesn't close the intake valve until somewhere from 50 to 70 degrees AFTER the piston has reached the bottom of the intake stroke and has started back up on the compression stroke.
At high speeds you need to have the intake valve open those long periods of time to simply have enough time at high RPM to get any sort of decent cylinder fill. And at high piston speeds at high RPM you will get a stronger vacuum pull into the intake port, and the velocity generated in the port can sort of "ram charge" the incoming mix into the cylinder even though the intake valve is still open as the piston is traveling upwards for as much as 70 degrees of rotation.
But at lower speeds, you not only don't get as much piston speed generated vacuum signal (with a big cam you are still leaving the intake open long enough after bottom) that the piston is able to push charge that has already entered the cylinder back up through the open intake valve. I've said many times that you can't compress a charge in a cylinder that isn't sealed.
So, as we have already discussed, the high compression piston imparts more vacuum, more signal, and more velocity into the intake tract in a big-cammed engine. That added intake velocity helps to give enough inertia to the incoming charge that it helps to counter act the low speed reversion of the intake flow.
High compression one-two punch to help with low end loss on big cams, tighter squeeze is always bigger boom, PLUS higher velocity/earlier acceleration of the intake charge making for more cylinder fill and less reversion loss of that charger by virtue of that greater velocity.
So could high compression possibly do anything else beyond the wonderful stuff outlined already? Definitely!
On the exhaust stroke, it is more effective at getting more of the burned charge out of the cylinder. Think of the 2 different piston/plunger/ syringes again -- the one that leaves the least space at the top of the cylinder is the one that pushed the most spent charge out the exhaust. And it did it with higher velocity, too. And since higher exhaust velocity has more inertia heading in the OUT direction, it creates a stronger vacuum in its wake.
Which brings us to another good thing. At top dead center (piston at its highest point, the end of the exhaust stroke and beginning of the intake stroke) during that period known as "cam overlap", for a brief segment of time, just before and just after the top, the intake AND exhaust valves are open just a little bit, and for very good reason. The exiting high velocity exhaust, and subsequent vacuum tail it leaves in its wake, will pull the last bit of spent charge out of the cylinder and use its energy to begin pulling the intake charge into the cylinder, even BEFORE the piston begins its downward intake stroke. It couldn't vacuum the rest of the combustion chamber out completely, or begin the movement of the fresh charge inward from the intake tract unless both intake and exhaust valves were open simultaneously at TDC, which is exactly why there is overlap timing in high performance cams in the first place.
Now, which would take better advantage of a strong exhaust vacuum signal, and both clean out the combustion chamber and transfer some of that vacuum energy effectively to the intake port? The large combustion chamber volume of low compression OR the small/efficient combustion chamber volume of the high compression piston?
Once again, advantage high compression!.
I hope this was an effective at illustration of the many unseen, and largely unknown, advantages of how a high compression setup works, well beyond the simple "tighter squeeze of the charge".
Reliability vs High Compression
Now, to debunk the Reliability vs High Compression myth -- hopefully for the last time. Horsepower and torque are a direct reflection of the combustion pressures seen inside an engine. ANYTHING that makes your engine have a higher output is a result of it creating more combustion pressure within your engine, whether the power came from a jet kit, pipe, cam, special fuel, etc.
As far as the stress on your engine components, they have not the slightest idea where the pressure comes from, and they wouldn't really care either. More pressure = more power = more stress on everything.
A 50-hp pump gas setup is putting out more stress on the engine components than a high compression engine delivering 47hp. The compression isn't what is the stress. The actual pressure from combustion is, and combustion pressure is MANY times greater than cranking compression in any event.
Increased power = stress and accelerated wear. That is the bottom line, and it doesn't have anything to do with what compression you have aside from the actual power it adds to the engine.
And by the way, on the piston reliability thing -- compression notwithstanding -- there are design and material components that will make one piston/ring setup better for reliability and longevity.
Venom Performance
www.venomperformance.net
517.655.2870