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Bolt-On ratio Rockers

 

 

 

So you’ve fitted some twin carbs and a performance exhaust but still want more power without having to strip the motor down? A higher-ratio rocker arm gives greater total valve lift, which will essentially lead to improved volumetric efficiency.

 

High ratio rockers are a great way to increase the flow trough your VW engine without splitting the case. Coupled with a few other simple bolt-on-mods-carbs, exhaust, etc.. You could free another 5-7hp simply by fitting these on. It’s all about improving the volumetric efficiency of your motor- the more fuel mixture you can pump into the cylinders on each stroke., the more power the motor will make. Racers aim for 100 per cent volumetric efficiency with giant valves and very high lifts- allowing as much mixture into and out of the cylinder as possible- while the average street VW will be lucky to find 80 or 85 per cent. High lift cams work on a simple principal (disregarding increased duration for a moment) – more lift = more fuel mixture. It’s all about airflow. When you consider your stock late-model VW camshaft has around 0.290-inch lift, the Eagle W110 suddenly looks huge with 0.392-inch lift. Add a set of 1.25s to your stock cam and you’ll be somewhere around the 0.363-inch mark, but add some 1.4:1 rockers and you could make 0.406-inch! The 1.4:1 rockers are often of the slipper or roller-tip design, however, and will almost certainly require shorter pushrods. They are probably best left for a camshaft designed to be used with them, like the Eagle FK7 or FK8 and a more serious engine build. Smaller 1.25:1 rockers like the ones we have here. Will work with milder cams like the Eagle 100, 110 and 120, but you really need to start watching your valve train geometry carefully and doing some careful measuring. Consider that a stock VW head will start to suffer from coil bind with lifts of around 0.470-inch and above. Even with trimmed-down valve guides, etc, you will be limited to around 0.550-inch. Also, without decent valve sizes and some good porting, you’ll find that your heads literally won’t flow enough cfm at high rpm to make the most of crazy lift figures. Anyway, we’re getting well ahead of ourselves here- if you’re into these kind of problems, buy a copy of Keith Seume’s Engine Interchange Manual, which covers these factors in much greater detail and also helps explain how to adjust your valve train geometry correctly. So, back to our 1641cc motor with 1.25s. Once we’re shimming and ready to go, we need to check our geometry out. With the swivel-feet adjusters, we needed to use the supplied shim under the rocker stand to keep the angle of the adjuster in line with the valve. Almost as if it was a continuation of the valve stem itself-at halve valve lift. Our stock pushrods were of acceptable length, but we will soon replace them with heavier duty items. Another good idea with faster valve acceleration speeds on a modified engine would be uprated valve springs. Although the higher ratio rockers effectively operate with an increased valve spring rate- spring rate x rocker ratio, i.e. if a spring was rated at 200lb at open load, the effective rate would be 250lb with a 1.25:1 rocker arm-the increase in RPM through the use of twin carbs and a performance exhaust means we could do with some heavier springs and chromoly pushrods to maintain accurate lifter/cam profile tracking and to prevent harmful valve float.

 

Once we’re happy that everything is working smoothly and our valvetrain isn’t going to bind up, it’s time to strip it all down for thorough oiling and final assembly. Use Loctite on the end bolts and submerge the complete assemblies in oil overnight before finally fitting. Now go and enjoy some extra grunt!

 

 

Here is our kit out of the packets and ready to start fitting up. On to our stock VW 040 cylinder heads we’re going to fit EMPI bolt-up rocker shafts with 1.25:1 ratio rocker arms. Note the many shims that come with the shafts.

Fitting might sound confusing from the description but, if you’re fitting them to a fairly stock engine (stock cam), it’s basically a straightforward swap over. Even the bolt-up shafts aren’t technically necessary-just highly recommended!

 

In the figure above we have a close-up of the rocker arms themselves. There are the heavier-duty versions with the ‘Rhino’-sized adjusters. This means they have 9mm adjuster screws instead of the usual 8mm, so it should be impossible to break one! In order to make the valve train as strong as possible, we also chose to use Porsche-style swivel-feet adjuster screws instead of the usual solid adjusters (see pic. 5). The rockers themselves are a good quality piece of kit with factory-style oiling holes to provide a long life even in high-hp applications

 

Although we’ve got new shafts, arms and adjusters, we need to use the original spacer/mounting blocks from the original rocker shafts. The 1.25s can be fitted to stock shafts but bolt-ons prevent clip failure and engine damage.

 

At the top are our new 1.25:1 ratio rocker arms; while at the bottom are the oily original 1.1:1 arms. Note that they are visually very similar. However, the new arms will lift the valves a bit further, thus increasing airflow.

Here’s a good comparison between the stock style 8mm solid adjuster screws and our Rhino 9mm swivel-feet adjuster. As well as the thicker profile, we get 15mm adjuster nuts. The swivel foot will reduce valve-tip wear and follow the valve better.

 

 

There are several bolt-up rocker shaft kits on the market and we chose to use the EMPI steel kit with permanent centre spacers. Some kits feature a floating centre spacer, but they can prove to be trickier to set up and the aluminium spacer is not as hardwearing. Note the various shim thicknesses.

 

Slide one rocker on to the shaft, then a freshly cleaned mounting block, followed by the second rocker arm and the locking washer and bolt. You need to work out the side clearance so you’ll know which shims to use. We used feeler gauges to measure the total gap on this side.

 

We’re looking for 0.005 to 0.007 inch side clearance on each rocker, so deduct this figure from your total clearance to work out which shims you’ll need. Trial assemble the rocker with the closest shim you have – you may need some of the old washer/spacers that you stripped earlier.

 

Once you’re happy with the inlet valve rocker arm clearance (0.005 – 0.007 inch), move on to the exhaust valve. If you have trouble getting it just right, you might want to shave a small amount off.

 

 

If your measuring was precise, you should be close, but you often need a few attempts with various combinations. The shims can be used to position the swivel foot on the valve-tip too.

 

 

1.25 rockers with stock cam are usually OK with a stock length pushrod, but you may need to space a swivel-feet-equipped rocker shaft away from the head with a shim (sometimes supplied with the swivel-feet). With a 1.4:1 rocker or a wilder cam, you may well need shorter pushrods.

 

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Last modified: 01/09/06