Type 4 Compression Ratios

This page was last updated Monday, April 13, 2009

What is CR? | How it affects your engine | Octane Ratings | Leaded vs. Unleaded | Recommendations

If there is a hot debatable topic (no pun intended) about VW engines, compression ratios is that subject. Many people have run higher compression ratios (CR) than stock and have never had a lick of trouble. Others, like myself, believe that with today's low octane gasoline, you should run more conservative compression ratios and stick with the stock ratio.

Before you can decide what CR your Type 4 will need, you first need to understand what the CR is measuring, how it affects your engine, and the octane ratings of fuel. Only then can you make a informed decision on what compression ratio your fresh new Type 4 will run it's best.

What is CR?

Compression ratio is the measurement of how much the fuel/air mixture is compressed when the piston reaches the top of it's travel. For example, a compression ratio of 8.0:1 means that the volume of the fuel/air mixture is compressed to 1/8th of it's volume when the piston reaches the top of it's travel.

A higher compression ratio, like 9.0:1 for example, will compress the mixture to 1/9th of it's original volume. A lower compression, like 7.0:1 will only compress it to 1/7th of it's volume. So, the higher the compression ratio, the more the pistons compact the fuel/air mixture as they reach the top of their path. The denser the fuel/air mixture is, the more potential power is available and a hotter burn results.

How it affects your engine

The compression ratio is a major factor in determing the lifespan, powerband, and tunability of an engine. First we'll looking at the effects of the compression ratio.

Compression plays a large factor in the resulting combustion chamber temperature. A really low compression ratio, like 7.0:1 for a Type 4 engine, will generate less heat in the combustion chamber and accordingly in the head and oil. Other factors also determine how much heat is produced, but with everything else being identical, the higher the compression ratio, the higher the resulting temperature at the head.

A highly desirable side affect of increasing the compression ratio is the increase in engine power. 9.0:1 has the potential to make more power then an engine with 7.0:1. This results from the air/fuel mixture being more thoroughly burned.

Octane Ratings

From what you've learned so far, you realize that to make the most power and to burn the mixture evenly, you need to increase the compression. Now, you maybe thinking "Why didn't VW set the CR higher if it made more power and burned cleaner?" That my friend is where we become familiar with our friend, gasoline and his measurement, octane.

As the combustion chamber gets hotter, a little devil known as detonation, or pre-ignition, can occur. Basically the engine gets so hot that before the spark plug fires, the fuel combusts on it's own. The octane rating of gasoline is a determing factor of how resistant it is to detonation. The higher the octane, the higher the tolerance to detonation.

Octane numbers are measured two ways. The 'research' method of measuring the octane number uses a constant speed (1500 rpm) engine in laboratory conditions. This is the RON - Research Octane Number. The other method is the MON - Motor Octane Number, which uses a harsher test regime more closely related to road conditions. So the MON is usually lower than the RON for the same fuel.

Here in the US, we don't use either RON or MON at the gas pump. We use the average of both of these numbers, commonly referred to as (R+M)/2. To get this number, the RON number is added to the MON number and the resulting number is divided by two. This is the number that American drivers see at the pump.

Leaded vs. Unleaded

For many decades, lead was added to gasoline. The reason for this was primarily to provide lubricant to the valve seats. Without the lead, the valve seats and valves would live a short life. As technology progressed, the process for manufacturing valve seats that didn't require lead was perfected. By the time VW was designing the Type 4, it was using this new material. This means that the lubricating properties of lead are not necessary for the Type 4.

An added side effect of lead in gasoline is that it acts as a retardant to detonation. Without going into too much detail, the lead in the fuel kept a hot running engine from detonating. This meant that you could run a higher compression ratio and not worry about detonation.

As you read this, you're wondering why they stopped selling leaded fuel. Well it has to do with pollution. The burning of lead cause a lot of now unneeded pollutants in the air. Here in California (USA), lead fuel was last offered in the late 1980s and is illegal to sell for vehicles intended for use on public roads. You can only find leaded fuel as racing gas, which is quite expensive.

Recommendations

For most cars run on the street, I recommend running the stock compression for the quality of fuel available. This table has my recemmondations for the commonly available octane rating unleaded gasoline here in the US. This CR should not cause detonation under 99.9% of the driving conditions. The Transporter owners should run a slightly lower C.R. in relation to their fuel. My '74 Westy Camper has a compression of 7.3:1 and I run 91 octane (R+M/2) and it runs strong and cool. As with any air cooled engine, it is important to never "lug" the engine, or drive it at a low engine speed. This turns the cooling fan slowly and creates excessive heat. Shift down and the engine will run cooler.

If the car is a limited use vehicle, like a sand rail or a wild Friday night cruiser, you can safely run the 8.3-8.6:1 compression. It's imperative that you always run the best quality with this fuel and that it's of the highest octane rating. On cars like this, chances are you'll breaking something else before the excessive temps get to the engine. You may want to run an even higher CR with a quality racing gasoline, such as VP. Since the vehicle will only be used for short periods of time, the expense of the racing gas won't be as large a factor.

There is a way to run higher CRs on the street, but it comes with a large price tag. Many modern cars are using an ignition device called a knock sensor or Hall sensor. The way I understand it, the sensor is placed as close as possible to the combustion chamber on the outside of the engine. It turns out that a moment before an engine experiences detonation, it emits a certain frequency. Well the knock sensor is tuned to this frequency and when it detects the frequency's presence, it instantly retards the timing until the frequency ceases. Thus the ignition is constantly tuning itself to right below the detonation point. With this setup, I expect being able to run 9:1 or 9.5:1 on the street with a good quality 91 octane gasoline and progammable electronic fuel injection. I read in European Car magazine about a company that did such a conversion, but I don't have it here with me at this writing. I'll include it at a later date.

Please note again . . . .

I must strongly reiterate that this article is strictly my opinion and is the result of my years of Type 4 research and experience. I suggest that you use my information, along with your own experience and information provided by others to form your own opinion.

Thanks to Dave Darling of Pelican Parts and Rolf Christensen for their assistance on this article.