However for once the laws physics are (somewhat) in our favor. While compressing a cylinder full of air takes more energy, a higher compression ratio allows a higher expansion ratio - how much a combustion chamber full of high-temperature exhaust can expand to do mechanical work. The increase in mechanical energy generated on the "power stroke" is bigger than the increase needed to compress the air-fuel mix on the "compression stroke". This isn't magic energy from no where - the mechanical energy produced is still less than the heat energy released burning fuel - but a bigger fraction of that heat is turned into work. In engineer-speak, a higher compression/expansion engine will have a higher thermal efficiency.
The greater the expansion ratio, the lower the pressure in the cylinder will be when the exhaust valve opens and the cylinder stops producing power, and the less energy is lost out the exhaust pipe as hot gasses. A high compression ratio, and high cylinder pressure, is simply a necessary evil in achieving that high expansion ratio, where the evil is that high cylinder pressures can lead to destructive detonation.
The great thing about increasing efficiency by increasing the compression/expansion ratio is that - all else being equal - it results in more power from burning the same amount of air-fuel mix. If all you changed was the CR the engine's torque and horsepower curves would just magically shift up at all RPMs. Increasing the CR is one of the few performance modification that generates more power at the low to mid RPMs where most streetdriving is done.
This sounds great until you start digging into things a bit more; the power gain from just increasing CR by 1 or 2 numbers is largely overblown. How much more you get depends on where you started: going from 8:1 to 9:1 will give a slightly bigger payoff than going from 9:1 to 10:1, but the rule of thumb is that for an engine around 10:1 CR, an increase of 1 CR number translates to about 3-4% more torque and power. That 3-4% is based on a simple model that assumes an ideal engine where the intake valve closes instantly at BDC and the exhaust valve opens instantly at BDC, and is very much an approximation; in reality the gain is even smaller. On a 200hp engine bumping the CR 1 number amounts to (at most) 8 horsepower. That might be significant for a race car, but not worth getting too excited about on the street. CR changes of .2 or .3 points are probably going to be un-noticed.
In addition to the increased efficiency, higher compression ratios have the secondary effect of making the engine a better air pump. We've all heard how engines are just air pumps - right? A higher CR creates a higher intake vacuum, improving cylinder filling - especially for engines with un-ported heads and small street-friendly carburetors - and so can improve mid-range torque and extend the useful power band a few hundred RPM. Likewise, a smaller combustion chamber reduces the "dead volume" that must be scavenged and increases pressure into the exhaust system, improving exhaust flow with a restrictive street exhaust system. This improvement in volumetric efficiency from increasing CR is small - again only a few percent - but it is on top of the thermal efficiency gain.
But there is no free lunch. As the CR goes up, cylinder pressures go up and detonation becomes a problem. Detonation occurs after the spark plug fires, when the burning air-fuel mix increases the temperature and pressure to the point that the still un-burned fuel in the chamber ignites spontaneously in a near explosion - typically as the piston approaches TDC. The result is a pressure spike that is like hitting the top of the piston with a hammer; it can break pistons, rings and connecting rods (after which things will go really badly).
In general, the idea is to use as high a CR as reasonable without risking detonation, but unless you're building a track motor there is no reason to get carried away. So how high can you go?
Lets rewind a bit and start at the beginning. When we talk about CR, we're usually talking about static compression ratio. That's the compression ratio based on just cylinder and combustion chamber volume; its just the ratio of the total cylinder volume at BDC divided by the total cylinder volume at TDC.
CR = (swept-volume + chamber-volume + clearance-volume) / (chamber-volume + clearance-volume)
or do a little algebra to get something easier to key into a calculator:
CR = (swept-volume/(chamber-volume + clearance-volume)) + 1
Some of the less technical articles you'll find on the internet define CR such that you'd leave out that '+1' part, and if you did that you'd compute a lower CR than you've got.
Let's start with the CR for a stock flat-top L28 like you'd find in a 81-83 280Zx. Finding the cylinder swept-volume is just geometry:
cylinder-volume = (bore x bore) x (pi/4) x stroke =
(86mm x 86mm) x 3.14159 x .25 x 79mm = 458,895 mm^3 = 458.9 cc
That's a big number because I used dimensions of millimeters, so the answer is in cubic millimeters, which are pretty tiny. A cubic centimeter is 1000 cubic millimeters, so our cylinder volume is 458.9 cc (remember, that's just 1 cylinder, multiply by 6 and we get 2753cc, the displacement of a stock L28).
A stock p79 cylinder head is spec'd at 53.6 cc, but you also have to add in the volume of the head-gasket between the piston and head at TDC, plus any piston dish. Since we're assuming flat-top pistons, the dish-volume is 0cc and the clearance-volume is just the volume of a cylinder with the diameter of the head-gasket bore and height equal to the piston-to-head clearance.
clearance-volume = (88.5mm x 88.5mm) x 3.14159 x .25 x .75mm = 4,613 mm^3 = 4.6cc
CR = 1 + (458.9 / (53.6 + 4.6)) = 8.9:1 (published value is 8.8:1)
A typical L28 mod is to swap a stock N42 head with chamber-volume 44.6cc onto a flat-top block (or swap the flat-top pistons into an 280Z engine with a N42 head). Recomputing the CR with the smaller chamber-volume gives:
CR = 1 + (458.9 / (44.6 + 4.6)) = 10.3:1
CRs are very sensitive to changes in chamber volume. If the chamber was just 3cc smaller in this example, corresponding to a 0.5 mm (.020 inch) head shave, the CR would go up to 11:1.
A 10.3:1 CR is not a lot in a modern computer controlled engine where feedback systems constantly adjust air-fuel ratios and ignition timing, but its borderline safe for an L6 with 40 year old ignition technology and chamber design. Many people do this mod and report great results, while others report moderate to extreme detonation. I'll touch on some of the reasons an engine might experience detonation, but limiting the static CR to 10:1 is a good idea for a conservative build; there are safer ways to pick up a few percent more torque than pushing the CR past the 10:1 mark.
OK, that was the static compression ratio that is listed in all the manuals. A big problem with the static CR is that it ignores the effect of cam timing on cylinder pressure. In real engines the intake valve stays open as the piston moves up past BDC. This is because the piston is barely moving at BDC so leaving the intake open a few degrees past BDC gives more time to fill the cylinder with air-fuel mix, especially at high RPM where the inertia of the in-rushing air can maintain flow into the cylinder even as the piston starts moving back up the bore at the start of the compression stroke.
However at mid-range RPMs (where detonation is a problem) a "big" (long duration) camshaft will close the intake valve so late that some air-fuel will be pushed back into the intake manifold as the piston moves up the bore. This reduces the amount of air-fuel being compressed (and burned) in the cylinder, and effectively reduces the cylinder pressure, reducing the chance of detonation. The so called dynamic compression ratio is computed using the cylinder volume when the intake valve closes to approximate the effect of cam-timing on cylinder pressure. Since the intake valve always closes after BDC, the cylinder volume at that point will be less than the full volume and the computed dynamic CR will be less than the static CR.
There are two complications: figuring out when the intake valve is truly closed, and determining the cylinder volume at that point. Although cam grinders all list the crankshaft angle when the intake closes, that number is often a little approximate. Most cam grinders assume less than .004 inches of lift at the valve is "closed", but the cam profiles generally aren't well defined at that point. There can be several degrees of variation across lobes on the same cam, and any lash in the valve train will cause the valve to close a little earlier than spec. Most shade tree engine builders just use the advertised closing point to compute dynamic CR and hope for the best. Despite the more complex math, dynamic CR is still an approximation of a fairly complex system - basing it on yet another approximation still gives a meaningful - although approximate - result. Once you've determined the intake-closing angle, you need to determine the cylinder volume at that crank angle. This is just trigonometry (it depends on bore, stroke, and rod length) but unless you like doing trigonometry you'll want some kind of computer program to figure this out. I've got an excel spreadsheet that does it in a brute force kind of way, but I'm not going into the details here; you'll have to trust me on this.
Let's compare the dynamic CR of a stock L28 cam and an Isky stage I, II and III cam in our hypothetical flat-top N42 engine with 459cc cylinders and 10.3:1 static CR:
stock - intake closes at 52 ABDC, cyl vol = 393cc, dynamic CR = 8.9:1
isky stage I - intake closes at 64 ABDC, cyl vol = 359cc, dynamic CR = 8.3:1
isky stage II - intake closes at 69 ABDC, cyl vol = 343cc, dynamic CR = 7.9:1
isky stage III - intake closes at 74 ABDC, cyl vol = 325cc, dynamic CR = 7.6:1
Remember that you can't compare dynamic and static CRs; where 10:1 is considered a reasonably "safe" static CR, for dynamic CRs the safe point is generally accepted to be around 8.5:1 with modern premium gas. So we see the flat-top N42 is marginal with the stock cam, whereas the Isky stage I (advertised as a mild street cam) will lower the dynamic CR about 0.4 compared to stock and gets us to the "safe" mark (just barely) and the stage II cam is well into the "safe" range.
A bit of an aside: shade-tree engine builders often think in terms of using a big cam to deal with the detonation problems of a high static CR. This is really sort of backwards; the better approach is to pick a cam with the power band you're looking for, and then pick a static CR to get a "good" dynamic CR. The Isky stage III cam would give a "safe" dynamic CR of 8.5 with a static CR of 11.4 to one, but that is about as big a cam as you probably want for the street (it doesn't make a lot of power below 3000 RPM).
The dynamic CR just looks at the compression part of the cycle; the point is to determine if a cam has a late enough intake valve closing point to make a particular static CR safe from detonation. We could compute a similar dynamic expansion ratio based on when the exhaust valve opens to see how the cam affects the efficiency of the power stroke. However it turns out for many street cams the exhaust valve opening is (nearly) symmetric with the intake valve closing, e.g. for the stock N42 cam the intake closes at 52 degrees ABDC and the exhaust opens at 54 degrees BBDC. That means the dynamic ER will be (nearly) the same as the dynamic CR, and in our example the Stage I cam will give a dynamic ER about 0.6 lower than stock, which means it is giving up about 2% thermal efficiency to the stock cam. By the way, a stock late-model L28 with 8.8:1 static CR and stock cam has a dynamic CR and expansion ratio of 7.6:1, which is why the stock engine will run on less than premium gas.
Note that compared to stock, the Stage I cam also reduces the effective cylinder volume by about 9% (and the stage III cam a whopping 17%) - that reduces the volumetric efficiency and torque at low to mid RPMs by a similar amount on top of the decreased thermal efficiency from opening the exhaust valve earlier. Even this "mild" street cam will give up (approx) 8% of torque and power at low RPM; a big cam is only a win at high RPM where the increased cam duration combined with ram-charging effects actually improves volumetric efficiency.
Another aside: the secondary improvement in air pumping efficiency that comes with a higher CR are not affected by camshaft duration or valve opening/closing points, so the benefits of a higher CR in a street driven car is not completely washed out by a big cam.
If you're reading about dynamic compression and thinking CR and volumetric efficiency are somehow related, you're right. Typical stock volumetric efficiency is around 80% at the RPM where max-torque occurs, and that value is baked into the rules of thumb for what is a "safe" CR for avoiding detonation. If you do all the hot-rod mods to improve air flow and cylinder filling (big cam, port the head, header, etc) you might get the volumetric efficiency up to 90% - which will increase pre-ignition cylinder pressure about as much as increasing the CR one number and potentially cause detonation problems even without increasing the static or dynamic CR.
So you might ask - which is more important: volumetric efficiency or CR? As with most things, it depends... Increasing volumetric efficiency by 10% will produce 10% more torque, while increasing the CR by 1 only yields about 4% more torque. The twist is that for an NA engine you can only increase volumetric efficiency to 100-105%, a 15-20% increase over stock, while you can potentially increase the CR to 13:1 (or more) - if (big if) you're willing to make the mods needed to avoid detonation - which will become increasingly hard to do as the CR goes into the stratosphere.
A better way to look at it is that for a simple bolt-on build, with no head porting and stock intake and exhaust, a little extra CR (low 10:1 range) with a slightly longer-than-stock duration camshaft (270 degrees advertised) is a good step up from stock. If you're looking for bigger horsepower numbers, head work and better flowing intake and exhaust systems are both necessary and somewhat mitigate the need for a big CR and lumpy cam. An all out performance build - where all you care about is maximizing high-RPM horsepower for the track - will still require a big cam and 13:1+ CRs, but there are better ways to approach a middlin-hot street build. Stay tuned.
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