[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [New Search]
<x-charset iso-8859-1>Hello- Just a few more notes about compression ratio and "semi-hemi" heads... Actually, not a few. This post is kinda long, and I apologize for that. I think there's a lot of useful technical info in here though... hopefully it will begin explain how the physics of combustion relate to the combustion chamber and the CR of your aircooled VW... This is one of the most volatile subjects (along with synthetic oil :-) that one can discuss in an aircooled technical arena. The problem is that both of these topics were given very strong opinions by a man named Gene Berg. Unfortunately, he didn't really have the knowledge to be able to adequately assess either of these. His results on the "need" for ludicrously low CR and "benefit" of semi-hemi heads have led to the saying "Gene Berg invented carbon." :-) I'm sorry to say it, but Berg had no idea what he was talking about with regard to combustion dynamics. It's all about THERMAL EFFICIENCY. What the heck is thermal efficiency? Well, when you burn gasoline, it converts chemical energy into two things. One is work. We like work - work is good. Work is what pushes the piston down and makes the car move. The other is heat. We don't like heat. Heat is waste - it doesn't help the car move. Thermal efficiency is defined as the amount of work created divided by the amount of chemical energy released. Let's look at extremes. If we had a 100% thermally-efficienct engine, the exhaust gases would be ambient temperature. Obviously, this case is purely theoretical - because of various reasons (like the second law of thermodynamics), this is impossible. If we had a 0% thermally-efficient engine, we'd make no work but have lots of heat. This case is not theoretical. In fact, it's how a furnace operates. What does a high thermal efficiency do for us? - MORE POWER. For the same amount of air/fuel mix burned at WOT, more power is produced. - BETTER FUEL ECONOMY. For the same amount of power needed to overcome drag forces at a 60mph cruise, less fuel is needed. - LESS HEAT GENERATED. For the same amount of power made, less heat is made, so things run cooler! In short, we love high thermal efficiency! What does CR have to do with anything? Well, first, let's look at extremes. Diesel engines run around 25:1 CR. Their fuel economy is fantastic. A gas furnace runs a compression ratio of 1:1. Their heat-creation ability is fantastic. Neither of these use the Otto cycle our aircooled engines use, but noting them is useful nevertheless. What about CR in the Otto cycle? Well, in fact, if you go through an idealized analysis of the Otto cycle, you'll find that CR is the PRIMARY factor responsible for making higher thermal efficiency! Now, so far this post has been primarily theoretical. While all the points are very valid, their exact context is not certain. I won't attempt to get into the absolute nitty-gritty here of how combustion works... no one would feel like reading what I'd write... but we should delve in a little bit further for an even better understanding. When discussing the thermal efficiency of Otto-cycle engines, one of the first things you'll note (after CR) is a thing called "burn efficiency." Basically speaking, the faster the air/fuel mix can burn, the more thermally efficient the combustion is. This gives rise to terms like "fast burn chambers." As a first-order approximation, we can consider the length of time necessary to burn the fuel as double the amount of advance the engine needs. In other words, an engine with 28 degrees of advance takes 56 degrees to burn the fuel - 28 before and 28 after TDC. There are no simple equations or formulae that can be used to get the amount of advance required for an engine. Combustion dynamics is a complex subject! But, we do have some simple observations that are quite useful to us. First and foremost, cylinder pressure changes the burn rate. The higher the pressure, the faster the burn. High CR obviously leads to this. Turbos also lead to it (one reason that they need retarded timing...). Lean mixtures lead to a fast burn, too (this is why modern ultra-efficient engines are lean-burn engines). What leads away from it? Well, low CR is one thing. Also, running at a very low volumetric efficiency does it too. Remember that running at part throttle drops volumetric efficiency... this is why we need vacuum advance in the first place - the fuel doesn't burn as quickly at part throttle! This is also one reason why a big, huge V8 at a very low throttle gets crappier gas mileage than a small engine at a higher throttle. Rich mixtures also lead to a slow burn, and this is why running too rich will kill gas mileage. One more thing to think about is combustion chamber design. Again, there are no simple equations or formulae here we can plug a couple numbers into and get a result. However, combustion chambers that tend to promote a high amount of turbulence during compression also tend to have a higher thermal efficiency. Let's limit our discussion to that of "wedge" combustion chambers. A couple of examples of this type of combustion chamber are aircooled VWs, small block Chevy V8s, Corvairs, etc. They have two valves parallel (or almost parallel) to one another but inclined to the chamber with one plug between them. The side-view of the chamber looks like a wedge. One of the absolute oldest tricks in the book to getting a fast burn in a wedge is to have a "quench area." If you look in a stock VW cylinder head, you'll first notice the depressed area with the valves and spark plug (the wedge). Then you'll notice a lot of area that is flat and parallel to the piston crown. This area is the quench area. If you run a tight deck height, then during compression the mixture at the outer edges of the cylinder will be forced past the quench area inward into the wedge and will create a lot of turbulence. This is VERY good! What is a "tight deck height?" The rule-of-thumb says that beyond 0.060", the aid of quench areas dies away quickly. The same rule-of-thumb says that tighter than 0.040" may cause piston/head interference unless the engine is assembled very carefully with high quality parts (I've personally run a well-assembled bone stock 2.0 T4 bottom end to 6800rpm repeatedly with 0.035" deck, but that is kinda pushing the limits). Now you see the reasoning behind the recommendation I made to the original post - if the original poster set CR at 7.8:1 to 8.2:1 with his engine specs, it corresponds to a 0.040"-0.060" deck height :-). Also note that Gene Berg's famed "semi-hemi" heads have NO quench area. It now should be obvious why this is such a bad idea. This is neat "sciencey" stuff, but does all this deck height and quench area rubbish have a real effect on my engine? YES. BIG TIME. A good friend of mine owns a dyno and has logged many, many hours determining the optimal advance needed for various aircooled VW engine combinations. The effects of having a large deck height and/or no quench area changes the needed ignition advance from a typical 26-30 to 36-40. Obviously, the burn is MUCH slower. Remember that to the air/fuel mix, the piston is just as much a part of the combustion chamber as the head. Large domes tend to really screw up the distribution of the mixture. Shallow domes (like those found on some stock higher CR VW engines) tend to be fine. And, some piston designs can actually increase turbulence themselves. One tried-and-true piston design for a wedge combustion chamber that helps is that of a half-dome - the dome is very carefully sized to perfectly match the combustion chamber, then given the requisite 0.040" overall clearance or so to keep things from hitting. But here's the catch: you only match the intake-half of the combustion chamber. At TDC, all of the mixture is forced over to the exhaust-half of the combustion chamber. This makes even more turbulence! There is one problem with this design, though. For a wedge, it tends to inhibit the flow ability of the head, leading to less power, so it's not often used. There are some newer more experimental piston designs out there, too. One is designed specifically for the aircooled VW - John Connolly's Supersquish pistons. These are designed to create a faster burn without the funny half-dome of the above design. Preliminary results show the necessary advance has dropped to 22-24 degrees or so (IIRC... correct me here if I'm wrong, John). Is there a practical limit to what we can do with CR on a gasoline engine? Yes. There are two problems. One is structural: can the parts in your engine structurally handle everything? A 13:1 CR is not adviseable for a regular engine since the pressures are higher and parts are more mechanically stressed. And the other we should all be familiar with: detonation. For our cases, there are four primary things that lead to detonation. - The first is ultra high cylinder pressures. 20:1 CR on a street engine is a bad idea :-). Likewise, 50 psi of turbo boost is also a bad idea. 9:1 CR under normal aspiration should be nothing to care about. - The second is a poor mixture distribution in the combustion chamber. Lack of quench (semi-hemi!) or large, funky domes create this problem. Lots of quench removes it. Adding a very special piston design also helps remove it. - The third is INCORRECT ignition timing. The combustion chamber and other criteria determine how fast the mixture burns. From this information, you then can figure out how much ignition timing is NEEDED for that engine. If you run more advance than is needed, be ready for detonation. If you run less advance than is needed, your thermal efficiency drops. (as a sidenote, go back to see what three things a high thermal efficiency does to your engine and you'll again see why vacuum advance is a good thing :-). - The fourth is piston speed near TDC. Short rod engines are advantageous in this respect. Also, note that detonation is most likely to occur at a lower rpm... now you know why. Note that ignition timing is NOT an absolute scale - what's needed varies from engine combo to engine combo. If you are running an engine with healthy CR (9:1 or so), excellent quench, and perhaps an excellent piston design, then very likely 32 DEGREES IS TOO MUCH!!! This is a very misunderstood point... people think that because when they set their engine to the blanketed 32 degree recommendation it runs too hot, that CR is to blame. It's not!!! They need to back off their timing back to what is REQUIRED. They are running too advanced for what their engine needs. My "experimental" personal daily driver aircooled VW engine (6500rpm redline, peak power at 5000-5200rpm) only NEEDS 28 degrees of total advance. My testing shows that 32 degrees actually drops power output and increases head temperatures. My testing involved using a knock sensor, a knock computer, several gauges, a laptop to control ignition timing on-the-fly, a stopwatch, and several miles of open road. This is not the fault of my 9.2:1 CR. My gas mileage is excellent, my engine temperatures are excellent, etc. All it means is that my burn is faster, which is good! As a testament to the gas mileage, how many Type 3 cars are there out there that can get 26mpg at 90mph? Sure, getting 26mpg at 60mph is a piece of cake (I'm WAY over 30mpg at that speed), but at 90mph... that's tricky. Make sure you are as friendly with your local law enforcement as we are before testing these ideas, though :-). Now, if your mixture gets too lean, then the amount of required advance goes down, right? That means that if the ignition timing didn't change, the engine is now running too advanced for that mixture... detonation will ensue. Obviously, ignition timing is the last thing to be set after CR, combustion chamber design, piston design, etc. So, what kind of "limits" should we follow here to keep away from detonation? Again, there is no simple equation or formulae to use. But, we can see that it is generally a game of give-and-take. If you have an engine that doesn't breathe very well at low rpm (low cylinder pressure when piston speeds are low), then you can afford a higher CR. This is a well-known fact. If you have excellent quench and/or a good piston design, you can run a higher CR than you would be able to otherwise. If you have no quench area, then you have to run a LOWER CR to keep from detonating. "Hang on here," you say "this is exactly opposite of what Gene Berg says!" No kidding. Physics says that having better quench allows a higher CR than otherwise. But, Gene Berg says that 6.6:1 is the so-called maximum for a normal combustion chamber (quench!) and 6.9:1 is the so-called maximum for a semi-hemi combustion chamber (no quench!). Like I said, Gene Berg had no idea what he was talking about. Sorry for being so long... but I hope this helps to explain a thing or two... Take care, Shad Laws LN Engineering - Aircooled Precision Performance http://www.LNengineering.com ------------------------------------------------ To unsubscribe, E-mail to: <type3-off@vwtype3.org> For more help, see http://vwtype3.org/list/ </x-charset>