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Re: [T3] Compression Ratio Recommendations

<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.


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

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

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
- 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

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

Take care,
Shad Laws
LN Engineering - Aircooled Precision Performance

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