Hiya, I,m looking at supercharging my a15, it has an H89 head on it. I,m unsure from the wiki if this is high or low compression, open or closed chamber? Or do most s/c a15 run a shim....Any help...thanks.

for a supercharger u do not need to decompress the engine
but for the head i do not kno which is high or low comp.

Realy, can run standard, I thought s/c engines have lower compression ratio.....

u can decompress ur engine like a turbo but really it all depends on how big boost u wanna run but generally there is no need pm aidaniel he just did a s/c a15 for his 1200 he did use a low comp head there is a project build on it too it was talked about recently
have a browse through project builds

H89 head is closed chamber.

Actually with A-series engines, the pistons control the compression. A14s come with dished pistons which makes low compression (regardless of the head used). Use flattops or domed pistons for high compression.

H89 head features: Oval Port, Water Ports, New bolt pattern. Was fitted to most A15s.

H89 Head

Thats helpful, I,ll check the pistons tonight.

That is not entirely true DD. In Australia we have the H75 head which is an open chamber and we got the H89 which is closed chamber. Depending on the bottom end, if it has a slight dish or flat tops and you intend to run over 7 psi of pressure then I would source a H75. Using that you should not have to run a decompression shim. Unless you plan to run much higher boost. Then you should consult a professional engine builder.

What is not entirely true? In america we got both the heads you mentioned plus a couple more. I didn't mean to imply that if you found an A15 it would have the H89. It may have one of several heads.

What I mean DD is that if you have 2 or 3 or more different heads with different volume chambers then the compression won't be dictated by the dish on the piston regardless of head choice. It will be a combination of both the dish of the piston and the relevant chamber of the head. If you had an A15 with standard dished pistons that had a H89 then bolted on a H75 you would have an increased volume of compression space giving you lower compression.

Unless the open chamber head and closed chamber head have exactly the same CC then I am wrong.

Yes, you are right. It isn't entirely true that the head has no effect on compression ratio.

What I meant to say is that it has a small effect compared to the piston dish. Some open chambers have similar CCs to some closed chambers, but there are several variations and we haven't measured them all.