Type 4 Connecting Rods

Stock | Lightening | Shotpeening | Length | Rod ratio | Your choices

This page was last updated Friday, May 18, 2007

Very few people realize that the connecting rod does more than connect the piston to the crankshaft. Connecting rod, or con-rod for short, can affect the overall nature of the engine, much like the camshaft. Let's start by looking at the stock con-rods.

As with all of the air cooled Volkswagen engines, forged con-rods were standard issue. The stock Type 4 connecting rods can be grouped into two different rods. The first style was used in the 1700cc(1.7L) and 1800cc(1.8L) engines and had a 55mm big end, 127mm center-to-center length, and 24mm wrist pin bushing. The big end diameter is the same as the Type 1 "311B" (1600cc) con-rod, but it's much wider, making the use of the 311B difficult. The wrist pin bushing is 2mm bigger in diameter compared to the 311B.

With the introduction of the 2000cc(2.0L) Type 4, another connecting rod was introduced. This new con-rod was needed, as the 2.0 crank was offset ground 5mm to gain an additional 5mm of stroke. As a result, the new rod had 50mm big end diameter, 131mm center-to-center length, and the same 24mm wrist pin bushing.

Here's a quick table with the Type 4 con-rod dimensions and the 311B dimensions:

Both of these rods are robust connecting rods are up to the stress of most higher performance engines. The weakness of the Type 4 are the bolts, just like it's Type 1 sibling. Installing heavy duty rod bolts make the stock con-rod strong and a fiscally sound decision and one that won't leave you stranded on the roadside.

This is all and great, but the stock rod isn't ideal. The Type 4 stock con-rod extreme robustness is evident in the weight of these rods. This is extra weight robs the engine of horsepower, power that could be used in other areas. This extra weight is not ideal if you intend on racing.

For most mild street engines, this extra weight isn't noticable, and as a result, the stock rods work quite well with 66-71mm stroke crankshafts. It provides an economical and simple solution when building a budget minded Type 4.

Lightening

There are a couple of tricks that you can try to make the stock rod better suited for performance applications. I've been told that BAS Ahnendorp of Germany offers lightened 2.0 con-rods. I don't know how much they take off or where they grind it from, but I was told that these rods have regularly been run up to 7500 rpm on a 71mm crank.

Shotpeening

During my watercooled VW days (yeah, I was into waterpumpers for a while), I recall reading a book about getting more power out of a VW waterpumper. The book detailed how a machine shop can lighten, polish and shotpeen rods to make them much stronger than they are from the factory. They were quick to point out that if you lighten or polish a connecting rod, you MUST shotpeen it, else the rod will be weak. I'm not 100% clear on the reason for this, but it's something that I'm looking into for my engines. I'll share my experiences later.

Rod Length

Besides weight, there are other factors to take in consideration when trying to get the most out of your Type 4. The center to center length has the most influence over the engine. Numerous builders and tuners have found that the longer the connecting rod is, the farther the resulting RPM band is. This is just like increasing the duration on the camshaft.

So now that you have ideas of what the stock rods are like and their characteristics, we'll now discuss where you want short rods and where you want long rods. We know the a short rod length moves the RPM band down, so it would be ideal in situations where top RPM is of little concern. This type of application could be a Bus engine or a daily driver with plenty of power to pull away from stop signs.

If your project requires increased RPM to make the most horsepower, like a hot street car or a off road cruiser, the power is best utilized up top. A longer rod will help to move the power up top. This is best done with a longer stroke crank and long stroke crank.

Con rod length to stroke ratio

The subject of rod ratios will inevitably turn up whenever someone is talking about getting more performance from an engine. It's a measurement that a lot of the hot rods like to use.

An engine's rod ratio is just the ratio of the con rod's center to center length compared to it's crank's stroke. For example, the Type 4 2.0L features a 1.84:1 rod ratio. How did I get that number? Easy. Divide the con rod length by the stroke, in this case 131mm ÷ 71mm = 1.84. If you increase the rod length, you increase the rod ratio. On the other end, if you decrease the stroke, you will also increase the rod ratio. Do the opposite and you decrease the rod ratio.

So why did I go to the effort of explaining the rod ratio? Well, because it's another way of determining what's the best rod for your engine. The lower the rod ratio has found to exert a lot of pressure on the side of the piston and cylinder. Too low a rod ratio will create wear problems and cause excessive blowby and broken parts.

If you look at the factory numbers in the above table, you'll notice that as VW set to gain more bottom end torque, they used lower rod ratios. This helped the Type 4 to make a lot of bottom end power to push a heavy 411 wagon or Bus. With a lightweight car like a 914 or a Bug, a higher rod ratio would have been better suited.

It's usually acceptable to find rod ratios as low as 1.7:1 or as high as 2.1:1, but beyond that can lead to problems. Take a look at the table above and see what rod ratios were used by the factory. For a street engine, you might want to keep the rod ratio somewhere near the factory numbers.

Con Rod choices

One option for a con-rod for your Type 4 project is a properly modified 311B. Rolf Christensen states that a properly modified Type 1 "311B" rod can be used with a specially prepared crank. This makes the rod price reasonable and rod selection numerous. The Type 1 rod is also longer at 136mm center-to-center. At the small end, the rod must be modified for 24mm wrist pins or use pistons that use 22mm wrist pins.

If your needs aren't satisfied by the VW offerings, there is a large selection of aftermarket con-rods. These con-rods are available from standbys such as Carrillo, Eagle, Bugpack, Pauter, Scat, CB Performance, and other such manufacturers. These will give a larger range of choices in dimensions of the big end, small end, and center-to-center length.

These aftermarket con-rods are much stronger than the stock offerings and they weigh a fraction of the stock weight. Of course all of these plusses make for an expensive pricetag. Unless you are looking to make the next import killer or V8 eater, stick with the VW forgings.