Building a ‘Big’ Aircooled VW Engine (Type 1)
This article is a guide to the theory and choices involved in building performance engines for Aircooled VWs (Type 1). There are careful decisions to be made with regard to the engine size and components that are “right” for what you have in mind. The Beetle (and other Type 1 VWs) are fairly light cars. Relatively speaking, it doesn’t take a lot of power for you to really get moving! 140 HP will keep even the Mustang GT’s in your rear view mirror on the street, and 180 HP will eat Corvette’s and Vipers for lunch all day.
written by: John Connolly, Aircooled.Net
The very first thing you must do is determine exactly what it is you would like to achieve, and realistically assess the budget that you have to work with! Engine selection is a series of compromises – you can’t have everything! You have to decide which aspects of engine building and performance are most important to you, and then quantify and assign them each a priority. Ask yourself the following questions regarding:
- What is my budget? What can I afford? (aka: How much will my wife let me spend? LOL!)
- Am I prepared to put more money into a car than I can reasonably expect to get back out of it?
- How will I be using the vehicle? (daily driver vs. race vs. weekend warrior vs. sole source of transportation, etc).
- Highway or city driving – or both? If you’ll be doing any highway driving you’ll need a reasonable RPM for your cruising speed.
- How reliable does the engine need to be? Are you prepared to tinker with it every other weekend? Are you prepared to pull the engine for maintenance or repair every six weeks? (the upper echelon (180 – 200 HP) of performance VW engines typically need this kind of attention)
Power and Performance
- How much power do I want/need? Engine power is a function of engine displacement and RPMs, and will be the basis of many of your engine component decisions.
- What is my gearing and tire size? This determines the operating RPM range of the engine. Am I prepared to change my gearing and tire size?
- MUST you have those chrome wheels you saw on another car? If they aren’t the same size as stock tires, they will change your final drive ratio and engine RPMs.
- How important is engine life/longevity to me?
- What is more important – driveability or performance?
- Am I trying to achieve a certain MPG?
Requirements & Limitations
- Must my engine meet any emissions requirements?
- Does my vehicle type have any inherent performance limitations?
- Where do you live? (Climate and/or elevation can affect some of your choices)
Once you have wrestled with and quantified the above demons, you can match your goals to the appropriate engine size and components. The importance assigned to the above factors will dictate ALL of your engine building choices. You must understand that focusing on any one particular feature, in most cases will force you to sacrifice in the other areas in order to achieve it. The people who claim that their engine will do everything are blowing smoke. When it comes to performance engines, in general, “Jack of all trades – master of none” is the axiom that applies. You must be honest with yourself about your intentions and your budget, and hopefully you will be able to find a happy medium between the two.
As performance increases, reliability decreases. Add to this a budget restraining effect, and you can see the balancing act that must occur. Many years ago, I too was ‘a hopeful believer’ in the possibility of building a do-it-all engine…*sigh*. But through the school of hard knocks, I learned that I was wrong – and now I have 4 cars – each with a specific purpose – drag race, street performance, daily driver, and early vintage stock.
Engine size is a function of piston diameter and the stroke of the crankshaft. Bigger is almost always ‘better’ (bigger engines will generally have more power at lower RPMs, therefore the engine will last a lot longer). But the bigger you go, the more expensive your engine will be to build. Also the larger the engine you build, the more complicated and intricate the engine building process.
Before we go any further into the discussion of serious engine modifications, I suggest you implement the bolt on changes that are recommended in the Aircooled.Net article ‘Bolt On Modifications – Getting The Most Out Of Your Stock VW‘. Once you have familiarized yourself with and upgraded your ride with these, you will be ready to apply more advanced and extensive performance oriented engine modifications.
Pistons, Cylinders & Bore Size
The aircooled VW engine uses cylinder sets that are removable; they are not cast into the block like most engines. This makes them fairly easy to replace. I will refer to “machine in” and “slip in” piston/cylinder sets. “Machine in” sets require engine case and cylinder head machining before they can be used, since they are significantly larger than the original sets. “Slip in” sets do not require any machining, since they use the stock VW case and head hole sizes and increase piston size by “thinning” the walls of the cylinder.
36-40 HP engines are small, and you should not attempt to build a performance engine from these starting points. Prior to 1966, Beetles and Ghias came equipped with these engines. The 1300/1500/1600 engine should be the basis for all performance modifications. The cylinder hole in the case is the same for all 3-engine sizes! Both the 1500 and 1600 engine use the same bore size in cylinder head, but the 1300 cylinder head is smaller. This means that a 1500cc engine can be upgraded to a 1600cc engine by simply installing the 1600cc piston and cylinder set! The 1300cc engine can also be upgraded if you bore the 1300 cylinder heads out to the 1600cc size, or replace them with the 1500 or 1600 cylinder head.
Almost all piston and cylinder sets are available in two versions: short stroke (these are considered “A” pistons), and long stroke (these are considered “B” pistons). A pistons are used on up to 76mm stroke engines and B are used on 78 and longer stroke engines. The difference between the two versions is the location of the wrist pin hole in the piston. Be sure to get the matching set for your crank and rod combo.
77mm, 83mm, 85.5 mm: Stock VW engines (aircooled type 1 1300/1500/1600 cc) came stock with these piston sizes, respectively. Slip in piston and cylinder sets are available in ‘size upgrade’ of 87mm and 88mm, but we do NOT recommend them EVER, PERIOD. There are NO special cases or exceptions to this recommendation. Aircooled.Net doesn’t sell or support applications that use the ‘size upgrade’ slip in sets on 1300/1500/1600 cc engines. When slip ins are made, the cylinder walls are thinned so that the larger piston will fit. The cylinder walls become too thin to maintain their integrity as the engine gets hot, and the piston, cylinder, and piston ring seal breaks down. Overheating and loss of power are the result.
88mm: The next bore size up from stock is the machine in 88mm set. These work VERY well, but you are looking at a lot of machining (this costs approx. $100) for a very small displacement increase! If you’re going to go to that work or expense, you should opt for the larger piston set! However, 88s are very good for busses and type 3 engines, since these engines run hotter than beetles, ghias, or buggies/rails.
90.5mm: This is a very common bore size. You are finally beginning to get a substantial displacement increase for your $. These sets have the same cylinder wall thickness that a stock 1600 cylinder has, so they are VERY reliable. These are an excellent choice, and routinely last 100K miles or more.
92mm: For 30 years, this piston was essentially a “slip in” set for the 90.5 bore size. It works ‘okay’ for low mileage race applications, but it’s prone to the same problems as the 87/88mm slip in combination. However, now the 92mm P&Cs are available in the old version (which we recommend against), but also in “Thick Wall” versions, with a 94mm register at the top, and EITHER a 94mm register at the bottom, or a 90.5/92mm register at the bottom. These 92s are ultra reliable, and are an excellent choice for convertibles, busses, and type 3s.
94mm: NOW we’re talkin’! 94mm cylinders have the same cylinder wall thickness as the stock 85.5mm set (1600cc). A slight drawback, however is that since the fin area is the same, and the engine is now larger, these do run slightly hotter than stock or 90.5 piston sets. When these were first introduced, I did not trust them! 92s had problems, so how were 94s going to be better? Well, the jury is in and 94s work. Reliability is very good; you can expect about 50K miles before a tear down and replacement is needed. Some sets have gone over 100K miles when low compression and sane driving are exercised. The machining for 94s costs slightly more than 90.5s — in addition to “boring”, you must have the case “decked”, so they do cost a little more to build (even though the piston set is about the same price as the 90.5mm). The additional displacement is definitely worth it!
Once you’ve got all your engine components selected, you will almost always need cylinder spacersand/or head gaskets to set the engine’s deck height. Some guys will do a bunch of fancy math to try and tell you what spacers to get, but our experience is that the best way to determine what spacers you need is to mock your engine up and then measure! Real world beats theory every time!
The engine case is what holds everything together, so selection and preparation are critical. Use anew case. Simple. NO LINE BORED CASES. Line boring will most likely result in overheating and low oil pressure problems soon after the engine is built. Don’t compromise, and use a new case. The used ones out there are worn out, and not up to the task, so just factor a new case and machining into your budget from the beginning. You will also need an engine case installation kit, which includes all the miscellaneous hardware for assembly.
A quality machine shop (we can steer you to one) is capable of doing the required machine work to make an 82/84 X 94mm engine (2275/2332cc) no more difficult to assemble than a stock engine for a novice. You heard that right. I field questions all the time from people who want more performance, but who are hesitant or simply won’t consider something larger than a 74mm stroke because they are afraid of “clearancing”. When the parts arrive from the machine shop, or us, just clean ‘em up, assemble the engine, make sure everything clears, and that’s it. The mystery and concern regarding parts clearancing for large engines is overrated and a highly misunderstood part of performance engine assembly. 86 mm and longer strokes, however, do require more attention to assembly detail.
Use 8mm head studs. New cases have case savers (Helicoils) built in, and these are far stronger than head studs threaded directly into the case. Use factory VW 8mm head studs (used are fine), since they expand and contract at the correct rate, and keep head torque constant. 10mm studs don’t do this, and that’s why 10mm stud engines have a problem with pulled studs. The VW factory changed to case savers and 8mm head studs in 1972 to solve this problem.
Adding a full flow filter is a good idea. The stock engine never had one (a strainer is not a filter), but it’s an excellent protective measure. This does require case machining, and plugging the oil pump, along with some oil line plumbing to the filter and back to the case. Plan on $150 for this modification, but it’s highly recommended, especially if you have $5K+ worth of parts that can get damaged by dirty oil! You’ll also need an oil pump, which is an easy item to choose – if the car is turbo charged, you need a 30mm pump, if not, then you need a 26mm pump. Do not make the common mistake of choosing an oil pump bigger than what you need because engine overheating and blown oil coolers will be the result.
Get a lightened flywheel, drilled for 8 dowels. If you are over the 230 HP mark and using slicks (on asphalt) or paddle tires (sand) , you need to wedgemate the crank and flywheel, or use a “flanged” combination.
Use the stock disc for all applications up to 120 HP. From 120 HP and up, you have the option of using our dual friction clutch disc. This disc uses metal woven pucks on one side (for grip), and organic material on the other (for smooth engagement). I have never found another clutch disc this versatile! Another excellent disc is the Daikin. The dual friction disc requires a pressure plate one step lower than the Daikin. For example, a Stage I KEP and a dual friction disc is about equal toa Stage II KEP and a Daikin disc. I use the Daikin disc and a Stage II KEP on my 230 HP engine.
Use the stock bus pressure plate for applications up to about 90 HP. After this point you need to switch to the Kennedy Stage 1 pressure plate. This Kennedy pressure plate is fantastic, and is good up to about 220 HP. Higher HP than that and you need the Kennedy Stage II pressure plate. Drag race applications (including sand) must use the Stage II so the clutch doesn’t get destroyed! All these pressure plates are 200 mm.
If you have a 6V car, or have a pre-63′ bus, you likely have a 180mm clutch. Normally I would recommend that you use the 200mm clutch size, since it has much more grip. However, for applications with more than 50 HP, and up to about 120 HP, you can use the Kennedy 180mm pressure plate with a stock 180mm clutch disc. This is an economical and EASY change that should prevent the destruction of your stock 180mm clutch (which would otherwise need replacement every 10K miles). Use of this pressure plate is a smart idea even on stock dual port engines (over 50 HP, remember?) installed on these early cars. The use of this Kennedy pressure plate eliminates the grinding and fitting of the engine and starter bushing adapters for the 12V/200mm flywheel and clutches. Simple, huh?
Crank and Connecting Rods
These two items need to be treated and selected as a set, not as individual components. The stock crankshaft stroke on the 1300/1500/1600 engine is 69mm. The first consideration is that you MUST have a forged, counterweighted crankshaft. Some companies are selling cast crankshafts. A performance engine will put out more power than stock, and cast cranks will break or flex, it’s just a matter of when. Even if the crank doesn’t break, the flexing stresses other engine components and can cause their failure. Some people get mislead by this situation since the crankshaft didn’t actually fail, and they assume that the crank is fine and attribute the failure to the broken part when in fact, the flexing crankshaft is the cause. (Two examples that spring to mind (pun intended :0) are: pounded out cases and popped out wrist pin clips). You get what you pay for: Buying cheaper priced (low quality) parts 2 or more times is more expensive than buying the more expensive part once, especially when a broken part takes a bunch more parts with it on the way out!
The first crank upgrade above the counterweighted 69mm crank is the Stroker crank. “Stroker” refers to any crankshaft with a stroke longer than stock, (which in the case of Type 1 engines is 69mm). The driveability and torque of a stroker engine has to be experienced to be believed! A stroked crankshaft allows the engine to make power without higher RPMs. At AC.N we refer to RPMs as “Ruins Peoples’ Motors” (LOL!) Remember this: Higher RPMs wear out parts MUCH faster. Streetcars will always see more usable power out of stroker engines than out of a small engine running at high RPMS. This is true unless you have a buggy or sandrail, which are both very light.
Here comes your first decision point, and as I warned, it’s dependent on your budget and goals. All sizes of crankshafts are comparably priced (around $400) – the cost difference is in what additional parts they require for reliable operation. I never recommend reusing or rebuilding old rods. Other companies may suggest the use of reworked stock connecting rods (which will cost around $120 for a good set) – but when you can get new 4340 I-beam rods for $150 a set, it doesn’t make any sense. If your rods fail, you get to START OVER FROM SCRATCH since all your expensive parts are junk – new everything – so why risk it?
When you stroke an engine, I feel there is no point in using a crankshaft smaller than 78mm. Stroker crankshafts are all the same price from 74mm to 84mm. The only reason you would ever use a stroker smaller than 78mm would be if you are racing in a class that limits your displacement. 78mm is the largest stroke you can reliably use with reworked stock connecting rods (but remember I don’t recommend that!) With the 78mm crank you can get away with using a $140 set of reworked connecting rods (or our 4340 I-beam connecting rod set, which is a real bargain!).
82mm crankshafts require the use of I-beam or H-beam connecting rods. H-beams are lighter and stronger but cost about $150 more than I-beams! If you are on a tight budget, and want to maximize your displacement you should go with the 82mm with I-beams.
84mm strokers require a different connecting rod, which costs more. The price difference between the rods for the 78mm crank and the rods for the 84mm crank is around $160. The decision between I-beam or H-beam connecting rods depends on the maximum RPM you plan on operating the engine at. Our 4340 Chromoly I-beam rods are good to 6500 RPMs, and if you plan on going higher than that, (or want peace-of-mind) you need the 4340 Chromoly H-beam rod (about $300-320 a set of 4, depending on length), which is good to 9000 RPM or 500 HP.
Next increment is the 86/88/90mm crankshaft. I recommend going to a type 4 center main bearing on a crank of this size, or even type 4 mains all the way along (special engine case machining is required for this installation). The larger main bearing makes the crankshaft stronger, and you need it if you have a stroke of this size! Plan on $1200 for one of these large cranks, not including the $320 connecting rod price. If you get one of these cranks, you also need a SPECIAL set of long cylinders which run about $300 a set of 4, and do NOT include pistons or rings. One more thing: things start getting pretty tight in that engine with a crank of this size, so you had better know what you are doing if you get one!
Connecting Rod Length
“Rod Ratio” is the length of the connecting rod divided by the crankshaft stroke. The small end of a connecting rod is affixed to the center of the bore of the piston, and the big end is attached to the crankshaft. If you increase the crankshaft stroke, and do NOT also increase your rod length, the rod angle is increased. 78mm is the maximum that the stock rod can withstand; beyond that the bolts will fail. Note that I said stock rod, not stock rod LENGTH. The stock rod length is fine as long as you use a stronger than stock rod. Longer crankshaft strokes require a longer connecting rod, and/or a stronger or a better-designed rod connection system. This is exactly what 4340 Chromoly rods accomplish.
69-82mm crankshafts can use the VW (5.394″) or Porsche (5.354″) length rod, IF it is made of Chromoly. Once you start using an 84mm crank, you are required to use a 5.5″ or longer connecting rod so you don’t overstress the rod and rod bolts! I recommend a 5.7″ or longer rod for strokes longer than 88mm.
Another thing to consider when you increase connecting rod length is that the longer the connecting rod, the further out from the crankshaft the piston is, and could potentially stick out the end of the cylinder. Since the cylinder is attached to the crankcase, you need to use cylinder spacers to adjust the compression ratio to where you want it to be.
The longer the connecting rods are, the wider your engine will be. This does not matter for an engine installed in a sandrail, dune buggy, or racecar. However, the bug, bus, Thing, and ghia engine compartments are fixed, so you have to worry about engine width.
The engine compartment of your vehicle is going to dictate some of your engine choices because of the relationship between rod length, crankshaft stroke and engine width. You can’t fit an 86mm or longer stroker engine in an Oval beetle engine compartment without a good amount of work to “narrow” the sheetmetal — unless you use overly short connecting rods. But these short rod engines will have a VERY premature engine life, and a very limited RPM range (I don’t recommend them). The extra work involved with 86 mm or larger stroke engines in small compartments is not worth the performance gain. In addition to the engine work, the longer strokes require that you also address the issues of the exhaust system and tin being too narrow, etc. Putting an oversized engine in a small engine compartment is a very involved “step” with many ramifications.
This is where most engines (including yours) are really restricted, and where money spent will really make a difference. Find the most expensive set you can afford, then get the ones 1-2 steps higher than that! Even an 1800cc engine can make 160hp if it has good heads (although it has to spin high RPMs, which wears things out quickly).
A cylinder head houses the valves and spark plugs and functions as a ‘cap’ on the cylinder to complete the closed system. Each head covers two cylinders. The heads control engine air and exhaust flow via the valves and the ports. The intake valves let the air-fuel mixture into the cylinder, and the exhaust valves let combustion by-products (exhaust) out of the cylinder.
Which cylinder heads you buy is mostly dependent upon your budget. The more money you spend, the bigger ports and valves you will get. Larger valves and larger ports mean more airflow. The more air that can flow in and out of your engine, the more potential you have for power. You can purchase racing heads which have HUGE valves and ports, but have diddly for cooling fins, so they may not meet your application needs if you regularly drive your car on the street. The racecars that these heads are designed for only run for minutes at a time, so engine cooling is not a priority. For any other application, cooling is a MAJOR priority. So, pay close attention to cooling fins!
There are two basic types of cylinder heads that can be used on an upright engine and the number of intake ports differentiates them: Single Port or Dual Port. Single Port heads have one intake port that serves 2 cylinders. Dual Port heads have an intake port for each cylinder. Obviously on Single Port heads, the shared intake port is more restrictive and will limit power potential. For stock engines, I strongly recommend the use of the dual port head if you are going the performance route. Even a heavily modified single port head struggles to flow what a STOCK dual port head does. But, on the other hand, if the engine overheats, a dual port head is more likely to crack than a single port head is – so this is where the factor of engine life comes in. Don’t get me wrong though, a dual port head can easily last over 100K miles without cracking, if the engine isn’t abused.
Aircooled.Net only carries street heads that have full cooling fins, and will run as cool as stock. I am not familiar with any aftermarket head (Superflo, Competition Eliminator, “Street” Eliminator, etc) that has adequate cooling fins for long term street use. Just because someone drives their vehicle on the street doesn’t mean they are running a head that is optimized or appropriate for street use! These aftermarket heads run much hotter than heads based off of the 040 castings (a.k.a., the stock dual port casting). We prefer to use the 043 VW casting which is superior to the 040 and 041. It’s a little more difficult to work with than the 044 (which most shops consider to be the standard of VW performance heads currently), but we feel the end result with the 043 is superior. The most recent 044 heads are an excellent basis for port sizes that would normally require welding on the cylinder head. Be wary of “great deals” on when buying cylinder heads, because many stores cheap out on the components (like retainers, guides, valves, & springs) in order to keep the price down. The end result are heads that require ‘freshening up’ by a good head porter in as little as 5000 miles, and right around that time, you get to buy retainers, guides, valves and springs all over again!
Many other heads seem impressive to potential customers because they have big valves. Most of these “big valve” heads have stock size ports. Chalk it up to another nonsensical design flaw, but these DO NOT WORK. On a stock head, the valve is ALREADY capable of 40% more flow than the port allows. The PORT is the limiting factor. Think about it — if the head port only flows a certain amount, the installation of a large valve does not help, since the valve is not the restriction! A perfect example of pure marketing hype. Most stores (including Aircooled.Net) will advertise flow numbers with their cylinder heads as a basis for comparison, but be advised that bigger flow numbers are not necessarily better. Air speed through the port is far more important, but this measurement is difficult to quantify. Rest assured that heads from AC.N are the finest that you can buy for across the board performance.
In the line of performance cylinder heads that we carry, the Series 3 head is a great starting point, and is the bare minimum I would recommend if you want to increase performance. This cylinder head is capable of 90-100 HP.
The best all around performance head for street cars that we sell is our Series 5. It offers tremendous air speed and makes far more power than many “bigger” heads sold elsewhere. This cylinder head is so versatile that it even works on busses and Type 3s. A test 2.0L engine with these heads made 186 HP with Dual 44 Webers.
The Series 3 and 5 heads are also fully driveable. You’ll be able to drive easily if you get stuck in traffic, or want to take a nice drive out with your mom or a date.
When you move up to the Series 7 heads, you will experience a decrease in driveability with the increase in power. They are more of the off-or-on type; they don’t do too well below 3000 RPMs, but at higher RPMS – hold onto your hat! These heads are more difficult to drive easy, but are the leader in horsepower. This head puts your engine in the 220 HP range assuming the rest of the engine is set up to make use of it. An equivalent head to this model from other companies is about $2200 a pair! Use the product descriptions in our catalog to help you choose the right set of heads for your engine.
This will be just a brief summary, and not a full discussion of carburetor selection since I have covered this topic in depth in the article ‘Carbs 101: Selecting the Right Carburetor System‘.
Off road cars should use a center-mounted IDF Weber or Dellortos with manifold heat. Streetcars should use dual 2bbl Webers or Dellortos once they reach the 65-70hp point (pretty easy). Racecars should use 48 IDA Webers. IDA Webers really require modifications if they are going to be used on the street. These modifications make them easier to drive by improving the progression circuit. Out of the box, IDAs are pretty much either off or on. Imagine driving a car with two power settings; idle and full throttle! That is what the IDAs are like if they aren’t modified! If you have a set of IDAs you would like to use, check out our article ‘Carbs 104: Rebuilding Weber IDAs’.
I generally recommend that people size their carburetors like this: the venturi sizes should be about 4-5mm smaller than the intake valve on mild engines, and 0-3 mm smaller than the intake valve on very high performance or race engines. Now, this is only a GENERAL guideline to get you close. Heavy cars need to be more conservative (smaller) on carb sizing. Light cars can be more radical.
The valvetrain is term that describes the system of parts that work together to actuate the valves in the cylinder heads. The crankshaft turns the camshaft. The camshaft lobes push the lifter outward. The lifter pushes the pushrod outward. The pushrod pushes out on the rocker arm, which pivots like a seesaw and opens the valve directly.
What is Lift? Valve Lift is the measure of how far the valve is opened at it’s maximum. More lift is better, when trying to increase engine power because more valve lift increases airflow and thereby increases power potential. To achieve more valve lift you can change the camshaft or the rocker arms.
There are basically two extremes of lift you can achieve. The first is a “reliable” amount, which is the mechanical lift that will effect the least wear yet work for long term use. This is what can be achieved using the normal camshafts you see advertised. There are also “high lift” cam designs, that give maximum power, at the expense of noise (they are louder), and accelerated wear to the valvetrain. Normal high performance camshafts have valve lifts in the 0.400″ to 0.450″ range and high lift cams have greater lifts that 0.450″. Do not expect a high lift cam to live longer than 40-50K miles on any engine!
What is Duration? Camshafts are rated/compared by crankshaft duration. Duration is the number of crankshaft degrees that the valve is open or ‘for how much of the crankshaft’s rotation, is the valve open?’. There are two slightly different standard measures for duration: Advertised Duration (total crankshaft degrees that the valve is open), and .050″ Checking Duration (crankshaft degrees that the valve is open AFTER 0.050″ of lift). The longer the valve is open above 0.050″ lift, the more airflow you will achieve at high RPMs, but this is at the expense of low end power. In camshaft selection you trade off low RPM for high RPM power or vice versa.
This may come as a surprise, but basically, the advertised camshaft duration is a meaningless standard of measure. The .050″ number is what really tells you how the cam will behave in a certain engine. This is because airflow through the heads is not significant until the valve is open .050″ or so. In general, the more duration a cam has the larger the engine needs to be to run it, AND the higher up the engine’s RPM band will be. Cam selection should really be done last, after choosing heads, carburetion, and exhaust. Consult an expert. (We are more than happy to help you with your cam selection when the time comes – email email@example.com with engine specs and compression). When in doubt, go smaller. I am a proponent of huge engines and mild cams. This provides INSTANT power, without having to rev the engine a lot, (which only wears out parts fast).
If your engine will be revved over 4500 RPMs or if you use a camshaft bigger than stock, you MUST have heavy duty (HD) valve springs. When you upgrade to HD valve springs, you must also upgrade your pushrods and rocker arms. If your engine revs over 6000 RPM, you MUST have dual valves springs. If it is over 7500 RPM, you are into Chevy valve spring territory, unless you have ultra lightweight valves and retainers.
Only chromoly or heavy duty aluminum pushrods are strong enough for performance engine applications. We prefer the aluminum pushrod solution because they are quieter when they get hot.Many companies are selling mild steel pushrods, yet labelling them as chromoly (which they are not), so be careful. Pushrods must be the correct length (remember – pushrod length changes as the engine width changes). Proper length ensures that at “half lift” the rocker arm is pushing the valve straight in. If “rocker geometry” is off, the valve will be pushed up or down, and this results in valve guide wear. Get it right! This is one of the most intricate parts of engine building. Cutting and assembling your pushrods is a real pain (I hate it). For assembly purposes, you need an adjustable pushrod to have any hope of achieving optimal rocker geometry. You use the adjustable pushrod to cut your pushrods to the desired length then tap the ends in using a hammer and two old lifters (to keep from screwing up the ends of the pushrods). Sometimes, depending on the pushrod brand, they have to be drilled out to match the pushrod ends! Sound arduous? Well it is, but guess what!? Aircooled.Net offers the service of cutting and assembling your pushrods, saving you the do-it-yourself grief.
Stock VW rocker arms are reliable for stock use, but are frail for applications that exceed the conservative stock design parameters. The stock rocker ratio was 1 to 1 on early 40hp engines, and this ratio changed to 1.1 to 1 on late 40hp engines and all 13/15/1600cc engines. This ratio is the relationship between pushrod movement and valve movement. Simply, a camshaft with 0.300″ lift will open the valve 0.300″ with 1 to 1 rockers and 0.330 with 1.1 to 1 rockers.
Once you decide to run at higher than stock RPMs (4500 redline), or install HD valve springs, or a high performance camshaft, the stock rocker assemblies WILL fail, it’s just a matter of when. The solution is to either strengthen the existing rocker assembly or install the alternative: Ratio Rockers.
To strengthen a stock rocker assembly, you simply shim the rocker assemblies to remove the “wavy washers” (the washer with a concave shape). Replace them with solid washers. The washers come in varying thicknesses (.015, .030, and .060″), and you swap them in and out until you find the combination that gives you about .005″ per rocker side clearance (side play that the rocker has). It’s a pain at first, but after a bit you get the hang of it. Rocker shim kits are around $10, and take an hour to put together.
If you upgrade to dual valve springs, you also need to upgrade to solid rocker shafts, with bolt on ends. Plan on $60 for a GOOD kit. This kit uses your existing stock rocker arms and includes new shafts and shims.
The alternative to stock rockers and associated modifications, are ratio rockers. Ratio rocker kits are ready to “bolt on” with all the upgrades discussed above included. Ratio rockers have a 1.25 through 1.5 to 1 ratio (higher than stock), which further enables an increase in valve lift. It’s important to note that the camshaft has a limitation on how fast it can accelerate the lifter and the valvetrain. This means they can only open the valve so far before they have to start closing the valve again. The ratio rocker arm overcomes this limitation since it multiplies cam lift to result in additional valve lift. It’s one of the few parts that let’s you have your cake and eat it too! No trade-off!
The stock camshaft is compatible with ratio rocker upgrade due to its conservative design. But there are also camshafts that are specially designed for use with ratio rockers. The result of using a ratio rocker cam with ratio rockers is that more valve lift can be achieved without sacrificing durability or engine life. The downside is you can expect to pay $2-300 more for a ratio rocker arm equipped engine, since you have to buy the ratio rocker set. If you have an engine equipped with a ratio rocker camshaft, you could use a stock rocker assembly, but you’ll sacrifice performance, the cam will last forever since it’s so under-stressed, but you’ll be defeating the purpose of a high performance engine component! Some people build their engine with the ratio rocker camshaft and use stock ratio rockers temporarily, with the intention to upgrade rockers later (for budgetary reasons. But if you do this, don’t forget that pushrod lengths are different for stock rockers and ratio rockers, so that is another consideration).
Performance engine building is, by no means, a simple endeavor. Many of you may have noticed that this information has been difficult, if not impossible to find, online or otherwise. Well, I’m committed to sharing this kind of advice and information, in fact, I established this company on a common sense philosophy that emphasizes accurate information and technical assistance in addition to our parts sales and service – Aircooled.Net will help you determine and meet your specific goals and needs, so we can sell you the RIGHT part, ONCE, saving you headaches and hassles!
article original post date: Feb 21, 2000
last updated: June 19, 2013