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Manifold To Head Matching - Or Not


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#1 unburntfuelinthemorning

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Posted 04 September 2019 - 05:49 PM

Thought I'd see how the head and manifold lined up.  Was suprised to find a big step down into the head.  The inlet ports in the head are up from standard small bore of 15/32" to 13/16" as supplied by Mr Calver.  However the C-AHT770 single SU manifold from Minispares has ports of 13/8".  Even the standard manifold ports are chamfered to 13/8". 

 

Vizard talks about chamfering the head ports to make them slightly bigger than the manifold ports which helps to reintroduce puddling fuel into the air stream and says that a step in this direction causes virtually no loss in flow.  The way mine are I'll have a few cc's of fuel hanging around at the step into the head and a loss of flow.  Thass orl a bit strange.

 

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#2 mk1leg

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Posted 04 September 2019 - 07:53 PM

you need to decide what manifold you are going to use also most heads that are modified have this already done inlet manifolds will need work which you can do just use the correct manifold gasket supplied with head if just std its nor worth the effort



#3 Turbo Phil

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Posted 04 September 2019 - 09:37 PM

The Minispares manifold is/was available in two different internal sizes, you want the one with the smaller internal porting for a small bore head.

Phil.

#4 unburntfuelinthemorning

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Posted 04 September 2019 - 09:40 PM

Seems to be a one size fits all deal now:

 

"C-AHT770 takes 1.5" AND 1.75"carbs. Medium size port runners suit all small bore engines, and large bore engines up to fast road spec (dependent on head intake port size)."



#5 Turbo Phil

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Posted 04 September 2019 - 09:44 PM

That’s certainly a hell of a mismatch and will most certainly limit flow.

Phil.

#6 unburntfuelinthemorning

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Posted 05 September 2019 - 06:57 AM

And yet that's the manifold they also sell with the stage one kit for 998s.  I bought one in the nineties and it was certainly a different design then. 



#7 Turbo Phil

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Posted 05 September 2019 - 07:19 AM

I have a Minispares small bore manifold in the workshop. I’ll have a look at it tonight and get some measurements.

Phil.

#8 unburntfuelinthemorning

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Posted 05 September 2019 - 07:27 AM

Thanks Phil. 

 

I've been looking into it a little and Minispares used to sell a C-AHT770A manifold which appears to have been of a larger bore than the C-AHT770.  They now only do the 770 not the 770A but it is definately different, at least externally, to the one they sold years ago.






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