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gas flowing the head


Well Known and Active Website User
VOC Member
Me and a couple of lads at work have been discussing this with regards to older bikes and I'm curious to know what the general opinion is. So then, does gas flowing and porting the head make much difference on a Vin' and also does having a double spark plug conversion make much difference?:confused::confused:


Comet Rider

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VOC Member
A big can of worms

Hi John,
You do like to open big cans of worms don't you:D:D

1/ Mild gas flowing can and will help a Vin's performance. More so by matching the inlet port to both the manifold and carb sizes. If necessary taper from the carb size to a slightly smaller size behind the vavle to increase gas velocity.

2/ Twin plugging will improve the flame front on ignition, but with a totally standard setup you will not see much in the way of an increase in performance. Modern unleaded works best when you have a squish band and twin plugs (most modern superbikes). Another point to bear in mind is that with twin plugs the flame front is much wider, and as a result the fuel load will burn much quicker, so therefore you may need to pull the advance back by as much as 10 degrees:eek:


Tnecniv Edipar

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Non-VOC Member
Depends on who does the 'gas flowing' !
If done correctly it will increase power. To make the best of it the compression ratio should be increased and a larger carb installed. It's all about volumetric efficiency and BMEP. Twin plugging helps with older combustion chamber designs.


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Non-VOC Member
As it was very succinctly explained to me recently by a Vincent racer, there are two kinds of porting, the "open up the ports a little" kind, to take a larger carb, and match the port to the manifold , and full gas flowing. The former can be done by almost anybody with a high speed grinder and a decent sense of caution, the latter takes an expert with a flow bench and much experience, and involves welding up and completely reconfiguring the ports , piston head, and combustion chamber. This can easily cost as much as $2500 per head, and, it is rumored, sometimes much more.


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VOC Member
With dual plugging a hemi head ,you will definitely need to retard the timing.The flame front is so improved.
In the 80's this became a big issue with airhead BMWs which are also hemis.The timing was so retarded that at idle,it went AFTER TDC!

Puddle jumper

Well Known and Active Website User
Non-VOC Member

Hi Guys,
I am running 32mm carbs, and have matched the inlet manifolds and inlet tracks in the heads, I then added the second plug to each head, as for the timing I use a Pazon unit which is designed for Vincent’s with twin plug heads. The initial use seems very good, she pulls like a train.

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