TheSamba.com :: View topic

Fri Apr 11, 2008 9:05 am

I am rebuilding my 1600 motor and I want to make sure to get the compression right where I want it. With the crap gas available these days, I consider this to be one of the most important parts of the rebuild process. Before the rebuild it was running .040" shims under the cylinders to lower the compression, but this is not really the best way to do it. I plan to instead remove ~2.7cc from each combustion chamber:

in centimeters:
bore x Pi x shim thickness = displaced volume
8.55 x 3.14 x .1 = 2.68

The reason for doing it this way is that it retains the original function of the squish band area around the perimeter of the combustion chamber. Shimming the cylinders up only increases the clearance between the piston crowns and the head, which leaves pockets of combustible fuel charge where it shouldn't be, far away from the spark. This can cause pinging and/or detonation, and can actually negate the desired cooling effects of lower compression. The ideal clearance is .75-1.0mm (.030"-.040").

Another advantage of doing this is that I can (theoretically at least) get compression equal in all four cylinders, as I will be measuring the volume in each chamber by filling it with a light oil measured from a syringe. If the casting isn't accurate, I can compensate for this with my Dremel tool. In other words, I may need to remove 2.3cc from one chamber and 3.2 from another to get equal volumes.

Of course I will also be measuring individual cylinder heights very carefully, and measuring piston to head clearance at TDC by mocking up the motor without rings on the pistons, turning the crank over, and squishing some modeling clay between the piston and head. I will then measure the thickness of the clay and compare it between cylinders. If necessary I can either have a cylinder shortened or run a shim underneath it. Hopefully I won't find that two side by side cylinders are equal in height but with drastically different squish tolerances. I'd rather not get into re-cutting squish bands into the head, and I certainly don't want to modify piston crowns and risk messing up the balance factors between the pistons.

Has anyone here done anything like this? I welcome any tips or experiences you might have.

Fri Apr 11, 2008 9:29 am

vincebodie wrote: I am rebuilding my 1600 motor and I want to make sure to get the compression right where I want it. With the crap gas available these days, I consider this to be one of the most important parts of the rebuild process. Before the rebuild it was running .040" shims under the cylinders to lower the compression, but this is not really the best way to do it. I plan to instead remove ~2.7cc from each combustion chamber:

in centimeters:
bore x Pi x shim thickness = displaced volume
8.55 x 3.14 x .1 = 2.68

The reason for doing it this way is that it retains the original function of the squish band area around the perimeter of the combustion chamber. Shimming the cylinders up only increases the clearance between the piston crowns and the head, which leaves pockets of combustible fuel charge where it shouldn't be, far away from the spark. This can cause pinging and/or detonation, and can actually negate the desired cooling effects of lower compression. The ideal clearance is .75-1.0mm (.030"-.040").

Another advantage of doing this is that I can (theoretically at least) get compression equal in all four cylinders, as I will be measuring the volume in each chamber by filling it with a light oil measured from a syringe. If the casting isn't accurate, I can compensate for this with my Dremel tool. In other words, I may need to remove 2.3cc from one chamber and 3.2 from another to get equal volumes.

Of course I will also be measuring individual cylinder heights very carefully, and measuring piston to head clearance at TDC by mocking up the motor without rings on the pistons, turning the crank over, and squishing some modeling clay between the piston and head. I will then measure the thickness of the clay and compare it between cylinders. If necessary I can either have a cylinder shortened or run a shim underneath it. Hopefully I won't find that two side by side cylinders are equal in height but with drastically different squish tolerances. I'd rather not get into re-cutting squish bands into the head, and I certainly don't want to modify piston crowns and risk messing up the balance factors between the pistons.

Has anyone here done anything like this? I welcome any tips or experiences you might have.

Yes. For the 1911 cc, VW 17/1800 Type 2 & 4, modified hybrid engine (66 mm stroke x 96 mm bore), intended for transplantatiom in my 1973 VW 1600 Type 2, I measured the combustion chamber volumes of my 1974~75 VW 1800 Type 2 cylinder-head combustion chambers, to circa ± 0·1 cc, using a normal laboratory, 50 cc burette and a transparent, perspex disc, with two small holes drilled in it; one for the burette nozzle and the other to dissipate air bubbles.

I made a cardboard template of the combustion chamber outline, so that I could maintain reasonable consistency of shape, for all four combustion chambers, which were reshaped using a set of rotary files, on the supplementary flexible drive, for my electric drill.

The final combustion-chamber volumes are 60·0 ± 0·1 cc, giving a compression ratio of 8·0 : 1, with 1·20 ± 0·05 mm deck-height clearance.

Fri Apr 11, 2008 9:43 am

yes, i have done this many times.

first, at accidental high RPMs crank flex can be as muh as .040", so i would leave that much between piston and the head flats.

Quote: Shimming the cylinders up only increases the clearance between the piston crowns and the head, which leaves pockets of combustible fuel charge where it shouldn't be, far away from the spark.

yes thats right, unburned fuel is the drawback. ideally, one would use dished pistons. they are out there if you look on ebay and here. you will find it difficult to remove ~3cc from the chamber without removing the flat spot. hemi cutters remove the whole flat spot and take only 4-5cc. you dont want to unshroud too much around the valves as it affects air flow. and if you hog out the center, then you create a sharp outside corner btween the flat and the hogged out area- not so good for flame propagation

Quote: I will be measuring the volume in each chamber by filling it with a light oil measured from a syringe.

you can use isopropyl alcohol, burrete and a CC plate to measure chambers, and forget the clay. you can find a burrette on ebay. under "business and industrial" there is a lab sciences section there. about $50 for a glass 100ml burrette. you can get the CC plate from gene berg for $15 or so. the valve contacts & the edge of the CC plate must be greased and the plate must be clamped down with an old cylinder that has been hacksawed off just under the first fin. that fin gives you a place to bear against with bolts- 5/16" with wing nuts. dispose of any illusions that you will get accurate readings any other way. to measure the cylinder volume at TDC, use a vernier depth guage- again ebay.

sometimes, you can juggle the rods to match deck volumes to the head chamber that needs more of less, as rods are not the same length unless they are bored before honing. but i do advocate boring before honing to square up. more power, longer bushing life, more ring life. if everything in an engine is perfectly aligned and square it will make insane power. i once had an engine like this. a 1600SP single carb bone stock that created severe repetitive wheel hop on a hard take off. enough craziness that a cop pulled me over!