so is it possilbe to use a12 extractors on a a15 head? would i have to change the cam cause of the larger head? or can i use a larger cam thats suited to an a15 n use that on a a12( basicly will larger cams fit smaller motor )

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That's a big fat NO. Not true at all. If you want 15 extra horses you need to do put a better head on and a better cam as well as some twin carbs (weber or SU) in and perhaps electronic ignition. You need to spend a lot of money on the A-series to make good naturally aspirated power out of it.

The most powerful factory A-series was the A15E at 92hp. So your options (to get bang for buck) are EFI or forced induction.

Spend some time doing some research, most of the answers to your questions can be found with just a little research.

Forced Induction

A-Series Cylinder Heads

Your exhaust manifolds are all square ports, they can all be used on each head but some flow better than others. Better flow helps with higher powered engines. Most people go for extractors.

Exhaust Manifold

Porting the Cylinder head will be the next step in chasing more horsies (cam, carbs, compression, ignition, head all tend to come before this sort of work)

Head Porting

Here is the index page for Engine Mechanical

Here is the index page for Engine Modifications

Here are some search results when I searched "Power Upgrade":

Cam & Head Suggestions

A14 Make the biggest bang?

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A14 Suggestions

There were literally 100's of posts where people have asked the same thing you have, you might want to give bike carbs a go? they're fun.

so I'm gonna have to change exhaust manifold or I can still use round shape manifold? and can I still use the a12 rocker cover on the h89 head or is it different bolt patterns or size?

I read somewhere that this head will increase by 15hp on the overall hp is this true?

thanks guys

Yes, fits, bolts on, and works fine.

It will increase HP if you keep it over 4000 RPM. Under that it probably won't.

The main differences between A12 head and H89 head is oval ports and slightly larger valves.

hey guys as title says h89 head on a late model a12,will it fit and bolt up with out any mods? and will horse power increase ??

any advise would be great
cheers