Volkswagen 4 Cylinder Engine Performance
If you're driving a Rabbit, chances are you are about due for some engine work. This section of the site deals with testing the VW four cylinder engine, as well as performance modifications that go beyond the typical bolt ons.
Is my engine ready for an overhaul?
Performance modifactions performed on a worn-out engine are a waste of time and money. The first thing you need to do is determine if your engine is in good shape! To start, there are the usual sensory checks to determine if something is wrong with your engine. First, there's the traditional mosquito killing cloud of blue smoke that eminates from the tailpipe, or the good old rod knock that are obvious indications that something isn't quite right. But, most of us have maintained our cars pretty well over the years, and none of these types of things are going on. So, if you're ready to soup up your engine, how can you tell if you're in good shape or if you're going to throw a rod?
Leakdown testing: The leakdown test helps you determine if there is any wear in the components around the combustion chamber, such as the rings, valves, cylinder head, etc. A low leakdown percentage means that everything is still sealed up properly, and that only a very small amount of combustion gas is leaking into areas it shouldn't. To perform this test, you will need the tester itself and compressed air. The engine should be at TDC. Keep in mind that when you pressurize the cylinder, the engine WILL rotate if you don't have the car in gear and firmly planted to the ground - watch out! Any engine that has a leakdown percentage less than 10% is okay. Beyond that it's time for a teardown to correct the problem. You will be able to hear any air leaks, so if your engine fails this test, listen in different areas of the engine to determine what the problem is. Intake valve leaks can be heard in the intake, exhaust valve leaks can be heard at the exhaust. Leaks in the head gasket or a cracked head will usually result in bubbles in the coolant, while poorly sealing rings will make noise at the oil filler cap on the cylinder head cover.
Compression Testing: Compression testing an engine is much easier than a leakdown test, but doesn't tell you as much. It is, however, an easy way to spot major problems with the engine. The results you get will be dependent on how good your battery is, since the faster your engine spins, the higher your readings will be. When performing this test you'll need two people (one to crank the engine, one to watch the pressure gauge), the coil wire should be disconnected from the distributor cap and grounded, and the engine should be warm. Once you're ready, crank over the engine. The first stroke should bump the gauge up to about 80 psi or more, with subsequent strokes building up even more. Any less than that and you've got a compression leak somewhere and it's time for a teardown.
Overhauling a VW engine
The intent here is not to go over step-by-step how to overhaul your engine - for that you can consult the Bentley or Haynes manuals. Here we're going to talk about some of the nuances of VW engines, as well as some performance enhancements you can make while performing an overhaul. These are just things I've learned and read over the years, and are therefore in no particular order, so read the entire section.
It sounds like common sense, but the cleaner you keep everything, the longer your engine is going to last once you have it put back together. The easiest way to do this is to just clean everything at a car wash. Some will say that your block needs to be hot tanked, which is fine. If you have this done, keep in mind that you will need to replace the elusive intermediate shaft bearings. These bearings are difficult to find locally (Techtonics sells them), and they come both in finished and unfinished form. Make life easy on yourself and purchase the finished variety.
While you have everything scattered about, take a bit of time and clean the block of all the remains of the casting process. This means taking a grinder and cleaning up all casting flash, as well as (believe it or not) areas of sand that are left. Don't get carried away and think you need to polish the inside of the block. Just clean it up as best you can.
Someone is bound to tell you that the freeze plugs should be replaced during the overhaul. This isn't the case, since the factory plugs are by far better than anything you'll find at your local parts store. Of couse, if yours have rusted out, you'll have to replace them, but use brass ones instead of steel.
CLEAN THE INSIDE OF THE CRANK! This means taking out the oil galley plugs and flushing the inside of the crank. Run cleaning pads through the oil galleys until they come out clean. When you're satisified that everything is clean, you can put hammer-in plugs back in place, or tap the holes and use 9/16" pipe plugs. Make sure to use thread locking compound, since if you don't, the plugs will work their way out.
It sounds odd, but buy the Brazilian made bearings for your engine. The German ones are oversized a bit, and require a bit of sanding to get the proper fit.
While the engine is apart, something to consider is getting all the parts balanced. This is quite a simple concept, as it means making sure that all the reciprocating parts of the engine weigh the same amount. This makes the engine run smoothly even a high RPMs. Taking this concept a step farther is called blueprinting, which means taking the origial specifications of the engine and making your engine match them exactly. This takes out the manufacturing variances that are common in mass produced engines. If any of the parts are ground on during this process, such as the piston rods, it may be a good idea to have them shot peened. In this process, the part is blasted with steel shot, which in turn relieves stresses in the part.
Any parts that are suspect, such as the crankshaft, block, rods, etc. should be checked before much time is spent on them. A good way to accomplish this is to have the parts magnafluxed. In this process, the parts are placed in a magnetic fiield, and a solution that glows under a black light and collects in even the smallest cracks is observed. Don't waste time on a cracked part!
Unless you're building a turbo-nitrous racing engine, stick with cast pistons instead of forged ones. Forged pistons are tougher, but they also expand more and therefore require more clearance between the piston and the cylinder wall. You'll burn more oil, and the engine will be noisy due to the pistons rocking in the bores.
There is sometimes some confusion between honing the bores and simple deglazing. Honing is done when the cylinder bores are out of round, and should only be done by a good machine shop. Deglazing should be done any time the piston rings are replaced, and only requires a hand drill and a deglazing tool. Match the pattern of the factory finish pattern in the bores - if you can't see the pattern, you need more than just deglazing.
When torquing bolts on the engine, make sure that the threads are clean. This is especially true with the head bolts. Remember that fluids don't compress. Make sure there is no fluid in the headbolt holes, becuase when you torque the bolts you run the risk of cracking the block!
What to do next?
Once you've determined that your engine is ready for replacement, or that you're just ready for some more ponies under the hood, you'll have to decide how you want to go about getting to your destination.
Increased Displacement
There's no substitue for cubic inches as the saying goes. Bigger displacement engines are capable of producing more usable torque and horespower than buzzy high-revving small engines such as the Honda VTEC. The easiest way to get more displacement is to purchase an off the shelf 2.0 VW/Audi block. The block from the Audi 80 is almost a direct fit, and the later model 115hp 2.0 VW engine will fit quite nicely in an A1 chassis. There are a few minor tricks that must be performed when making this swap (as with any other major project), such as blocking off the crankcase breather, or fitting a longer downpipe on the newer 2.0 VW engine. None are unsurmountable, and have been performed by a variety of VW enthusiasts. Techtonics sells kits that will make these swaps very straitforward. One thing to consider, however, that a used block in some ways is BETTER than a new one. An older block has been properly "seasoned," meaning that everything has settled into place and your machine work will be much more accurate over time.
If you are planning on keeping your existing block, another easy way to increase displacement is to purchase one of the off the shelf stroker kits that are sold by a number of VW performance shops such as Autotech or Techtonics. These kits come with instructions on how to make the retrofit, including where the machine shop should grind away. Yes, there will be some minor machine work involved. For one, the intermediate shaft will require milling to eliminate interference with the crankshaft. Also, if you're using a 1.8 liter engine, some of the block material will require removal so the crank counterweights won't whack the block itself. If you're the adventurous type, there are literally an infinite number of possibilities for bore and stroke increases, although boring out the engine is easier and often more productive than increasing the stroke. There are several typical combinations that are used, however, which is why you see standard displacements such as 2008, 2033, 2057, & 2082cc fairly regularly.
Increased Compression Ratio
This is another relatively easy way to increase horsepower. Increasing the compression ratio increases the thermal efficency of the engine, and can also sometimes increase fuel economy. The downside is you have to worry about detonation, and will most likely require premium fuel for the engine to operate properly. In places where there are tailpipe tests to check for proper emissions control, increasing the compression ratio too much will result in failed tests due to increased nitrogen oxide generation.
CR can be increased as easily as fitting a thinner head gasket. Other methods include milling the head (effectively reducing the size of the combustion chamber), or fitting pistons that are made for higher compression engines such as from the 85 and up 1.8 GTI engine. Shops like Techtonics sell pistons for a wide variety of compression ratios.
Better Breathing
If you don't have a GTI engine, you can gain a bit more horsepower by fitting a GTI head to your engine. The GTI head has larger valves, and therefore allows more air to flow into the engine. Beyond this, there are many folks out there that have had their head ported and polished.
There are many advocates of porting and polishing. This entails polishing and cleaning out the intake and cylinder head ports. This should only be done by someone who knows what they are doing, since once a mistake is made, there is no going back. Taking out too much material and making the runners too large will reduce low end torque, although it will provide benefit at extremely high RPMs. (this is one of the reasons that US 16v motors have smaller intake runners than European 16v engines) There is also the possibility of taking out too much material and getting into the head's water jackets. Techtonics used to sell ported heads, but alas, no longer offers them. Whatever shop attempts this for you should have a flow bench, which will allow them to measure the affects of what they are doing. With a flow bench, they can find restrictions in the head and clear them out properly.
Beware, however, as I've seen many horror stories out there of people sending their head off to someone who claims to know what they are doing, and in return they get a useless, damaged head. Only use a reputable shop for this procedure.
