ATTN: Ben Wight (& Others)

Frederick Stienheld · petrol head Joined: 05 Sep 2002 Posts: 2719

Mate, are there many gains to be made by porting the exhaust side of a gemini head? I was looking in one last night and there didnt seem to be a lot that could be done to improve it.
Also, is it the same story for the inlet side?

hypaTG · backyard mechanic Joined: 15 Oct 2002 Posts: 524

yes, the bigger the ports, the more mixture u can pump into the engine, but if u have big inlet ports and small exhaust ports, what's the point? have a look at ur head and put the gaskets on and have a look how much room there is between the edge of the port and the edge of the gasket, this is usually the amount that gets taken out when the ports are done.

Ben Wight · backyard mechanic Joined: 04 Oct 2001 Posts: 946

Yes there are gains to be had. Especially if you increase the air flow capability of the engine, by doing mods such as bigger carby, cam and exhaust. The ports will become a restriction, (like a bottle neck). they will be the weakest link, and you will have to say GOODBYE!.

I have never bench flowed a gem head, but when I do my heads, I unshroud the valves in the combustion chamber. This means removing the machined step where the valve meets the aluminium of the combustion chamber.

I also smooth out the transition of valve seat to port wall, as sometimes the factory effort is crap.

I also try and smooth out the outer radius of any bends/turns in the ports themselves.

the ports are already quite large, I usually port match to the gaskets, but you dont just want to go huge, as you loose air velocity, meaning you loose a lot of low down power, but gain a bit of high rpm power. that seems to be the trend.

its the same principle as a carb. bigger is better, but you can go too big.

BTW, its exactly the same principle for inlet as exhaust.

Ben