All Big Six ProMAXX CNC ported big valve heads are now available.

Relates to all big sixes

pmuller9

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I believe that after i told jason about SI offering 4.81 valves he is using them. I am pretty sure my head is the first to have them zc though i have not measured them.
I talked with SI three weeks ago and they are not making 1.94/1.60 valves in 4.75” or 4.81” lengths. Please measure your valves just to be sure.
 

Imagemineye

Well-known member
I talked with SI three weeks ago and they are not making 1.94/1.60 valves in 4.75” or 4.81” lengths. Please measure your valves just to be sure.
Thank you. I thought I had read somewhere that someone got a head with 4.81s, maybe they had some old stock? I dunno... I suppose that the 4.91s will just make installing springs that much more interesting. Y'all seem to have figured the ins and out of the springs they come with so maybe the will do for my project, but I am thinking not.
 

pmuller9

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Supporter 2018
Supporter 2021
Y'all seem to have figured the ins and out of the springs they come with so maybe the will do for my project, but I am thinking not.
The springs that come with the head have a light seat pressure and a heavy open valve pressure.
For turbocharging it is better if the seat pressure is heavy and light open valve.

The Isky 235D valve spring should be a direct replacement size wise and has a 350 lb/in spring rate. If installed at 1.800" the seat pressure will be 112 lbs and 287 lbs @ .500" valve lift.
 
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