JBugs Video Series
VW Super Beetle Clearancing Pistons & Setting Compression Ratio:
Video Overview:With the 1800cc stroker short block engine completed for our 1971 Super Beetle, we can start to mock up the top end with pistons and cylinders along with new cylinder heads. We'll clearance the pistons for the stroker crank, check the deck height, CC our heads and most importantly, set up the compression ratio for our engine.
Video Tips:The tools you will need are:
Snap Ring Pliers
Deck Height Measuring Tool
Head CC Measuring Kit
Air Powered Angle Grinder With Sanding & Scour Pads
Angle Grinder With Cut Off Wheel
The chemicals you will need are:
Hi, Sam here with JBugs.com. In our last video, we finished the assembly of a short block for our 1800cc stroker engine build for our 1971 Euro Look Super Beetle. With our engine taking shape, well get started on the top end by test fitting our pistons and cylinders setting the deck height and most importantly setting the compression ratio for our engine. This step is often overlooked but crucial to the power and performance of an engine. Higher compression produces more output, but the byproduct of that is heat. On the other hand, if the compression is too low the engine won't be efficient and actually ends up wasting fuel.
Typically in VW engines somewhere between 7 and 8.5 to 1 is the norm. We're going to set our engine at about 8.25 to 1 towards the performance side of the spectrum. The top end assembly begins by coating the ends of threads of our head studs with aviation sealant and threading them into the engine case.
Our 1971 engine has a deep stud at the number three cylinder. The top back threads at the number three and the eight lower threads all use the longest studs. The top inner studs are the shortest and the other three remaining top outer studs are the middle length. The studs are threaded in by hand whenever possible a pair of vice grips can be used gently otherwise.
Next, we'll get our thick walled 88mm pistons and cylinders pulled apart and cleaned for test fitting. The cylinders are somewhat unique. At the top, they are extremely thick and require the use of heads board out for 90.5 cylinders. At the case though, they're very thin. Since most of the load, pressure, and heat are at the head not at the case, the pistons are actually a good alternative for those who are looking for more displacement and performance out of a stock 1600cc block.
The pistons are pushed out of the cylinders, the rings are all pulled off, and the pistons and cylinders are cleaned. At the engine stand, we rotate the engine so that the number 3/4 side is at the top and lube up and test fit a wrist pin into the number three connecting rod. It's removed, then a piston is placed over the connecting rod and a lubed cylinder is slid onto the head studs, over the piston and down into place on the engine.
With the crankshaft rotated to top dead center for cylinder three, we can see that the piston sticks out past the edge of the cylinder because of our longer stroke crankshaft. Cylinder shims will be used to space the cylinder away from the case. We rotate the crank to make sure the piston clears the crank and the crank stops. We pull the cylinder up and see that the crankshaft is hitting the skirt of the piston. This isn't out of the ordinary when using a stroker crankshaft, and there are a couple ways of clearancing the piston.
We pull off the cylinder, the wrist pin, and the piston. One way of clearancing would be to notch the skirt to allow the piston to clear the crankshaft. The other way, which we go with, is to cut the skirt clear. We measure the distance from the edge of the crankshaft bottom dead center point to the bottom of the hole of the connecting rod and note the measurement. We measure that distance from the bottom center-line of the wrist pin hole in our piston and mark the skirts to be cut there.
What you do to one side of the piston ideally you'd do to the other to keep the piston balanced. What you do to one piston, do to the remaining pistons to keep the engine and rotating assembly as balanced as possible. The skirts on both sides of the pistons are cut as close to equally as possible noting the measurement. A sanding pad and scour pad, on an air powered angle grinder, are used to even and smooth out the skirts and any rough edges on the piston.
The pistons are cleaned up and we reinstall a piston to the connecting rod to check that the engine rotates without hitting the crank. With the pistons now clear of the crank, we can get on with setting our compression ratio and deck height. At our workbench, we have an EMPI GTV2 cylinder head that is machined for 90.5 or in our case thick walled 88mm cylinders. The head has %20 larger intake and exhaust ports than stock heads
so it will flow better and make more power than a stock head.
We chose a head with 40x35.5 stainless steel valves and dual valve springs. EMPI lists the cc's of the compression chamber at 58, which we want to confirm, and at the same time, we'll show you how to cc your heads using a head cc measuring kit. We thread a 12mm long reach spark plug into the head which is sitting level at our bench. Then we apply a ring of assembly lube around the outer edge of the appropriately sized disc, three discs are included to fit 85.5, 90.5 and 94mm heads, to provide a seal between the disc and the head.
Next, we fill up a large syringe with water, up to the 60cc line. We fill up the combustion chamber with water until there's no air remaining. We know there is 2cc's left in the syringe, 60-2 gives us 58cc's, confirming EMPI's measurement. Now we use those known measurements in our online engine calculator at JBugs.com. We enter the following factors. Our bore is 88mm. The stroke is 74mm. The deck height will be our variable so we leave it blank. Our cylinder head volume is 58 cubic centimeters. We enter the compression ratio we want, 8.25. Clicking the deck height button below tells us that a deck height of 0.026 inches will give us the compression ratio we're looking for.
Now knowing the deck height we want, at the engine, we set our number three cylinder to top dead center. We do all our measurements and set the compression at number three as it runs hotter than the other cylinders. So we make sure that if nothing else, it is at our compression limit. The other three cylinders should be nearly identical to number three but there's no harm in verifying the rest of the cylinders if you'd like to be certain.
A deck height measuring tool is set on top of the piston which is sticking out past the end of the cylinder. With a set of feeler gauges on the outer edge, we measure the gap between the top of the piston and the cylinder at 0.050 of an inch. We set two 0.040 shims on top of the cylinder and reinstall the deck height tool. A feeler gauge is used on the inside edge of the tool and we get 0.030 of clearance which is closest to what we're looking for.
As cylinder shims are only made in multiples of 0.010. We'll use a copper shim between the cylinder and the head which will compress a little bit and most likely get us very close to the 0.026 thick heights that we want. In prepping for our next step and knowing that our 0.080 shims will be used we mock up the number three and four cylinders and pistons on our engine and install each of the cylinders with two 0.040 steel shims between the case and each cylinder.
We set our cylinder head in place and install washers and nuts at the inner four studs
and snug down the nuts in preparation for our next step. We've thrown around a bunch of numbers today while setting our compression ratio and next we'll be setting up the rocker geometry for our 1.4:1 ratio rockers. That is going to involve much more number crunching, so we'll pause for now. Stay tuned for our next video. In the meantime check out our other how-to videos and when it's time to order parts for your vintage VW, stop by JBugs.com
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