Compression adjustment

[andreic]

Hello,

I am in the last stages of rebuilding a 1.8L motor for my 1975 914. I am rebuilding it with the slightly larger 96mm pistons and cylinders (new), but with the original crankshaft, bringing it up to 1911cc.

I have decided to go with an 8:1 compression ratio. I have measured the burn chamber volume, did the calculations, and that means I need a deck height of about 3mm. (The distance between the top of the piston and the top of the cylinder at TDC.) Yesterday I did a test fit of one cylinder onto the short block, and measured this deck height to be about 1.6mm.

I have read that to adjust the deck height one uses extra spacer rings. I have some rings that have come with my complete gasket kit (Victor Reinz), but I don't know where exactly these rings get installed. From checking their sizes, they seem to fit perfectly on top of the cylinder wall, inside the burn chamber in the head. (If I put the heads with the burn chamber facing up, the rings seem to go perfectly inside the big circle around the valves, fitting exactly inside.) Is this where the spacer rings are supposed to go? I am a bit confused, because my initial impression was that the spacers were supposed to go around the bases of the cylinders, where they mate with the crankcase. (But for that I only have a very thin aluminum gasket.) I am worried that the spacer rings, which seem to be made of a soft metal (aluminum?) may not hold well to the temperatures inside the burn chamber.

A second question: where do the shim rings that adjust the crankshaft end-play go? I assume they are supposed to go flush with the crankshaft, behind the main crankshaft seal. Is that correct?

Thanks for any answers you may be able to provide.

Andrei.

[Dave at Pelican Parts]

The rings you reference are head gaskets, or what passes for head gaskets in the 914 engine. Many people advise not using them, and instead lapping the cylinder tops to the heads. You make that up by putting spacers underneath the cylinder, between the cylinder and the crankcase.

The shims that set crank end-float go behind the crank seal, yes.

--DD

[andreic]

As I said before, I am in the last stages of rebuilding a 1.8L motor. I shall be using a 9550 cam from Jake Raby and 96mm pistons and cylinders, everything else is stock. I plan on using it with a stock L-Jet fuel injection. The engine is aimed to be installed in a daily driver.

I am trying to decide what to do about the compression ratio. From computing it over and over it appears that if I don't put any spacer rings, nor head gaskets I end up with a CR of about 8.5:1, which is a bit higher than what I originally aimed for (I wanted 8:1 to stay on the safe side). However, if I put the VW recommended spacer rings (1.6mm) I end up with a CR of 7.2:1 which is way too low.

Two questions:

a) Would a 8.5:1 with the 9550 cam (fairly mild, only slightly more aggressive than stock) work well on 91 octane gas?

b) If not, and I want to get down to 8:1, where can I find spacer rings of thickness of about 0.6mm?

Thanks,
Andrei.

[Dave at Pelican Parts]

I believe that you should be OK running 91 with the cam and compression you want. If you want to be sure, contact Jake Raby (who designed the cam) or Charles Navarro (who now runs the Type IV store).

I would bet that the Type IV store will sell you shims (spacer rings) in lots of thicknesses...

--DD