I have a 12g940 head with 29.2mm exhaust valves and 33.33mm inlets. I know some heads had larger 35.6mm inlets - can this head be machined to take larger valves, and if so, would it be a noticeable improvement? Engine is 1275 +20 thou A+ with 266 cam and hif44 carb, RC40 exhaust.
Bigger Inlet Valves In A 12G940 Head?
Started by
mister bridger
, Dec 08 2017 11:51 AM
3 replies to this topic
#1
mister bridger
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Posted 08 December 2017 - 11:51 AM
#2
Cooperman
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Posted 08 December 2017 - 12:03 PM
With a 266 cam and a gas-flowed head the optimum size of inlet valve is a 35.6 mm as this will allow good inlet flow at high revs and still give the great mid-range torque for which the 266 is so well-liked.
However, that assumes that the valve guide bosses have been mostly ground out in both inlet & exhaust tracts, the short-side radii improved and the inlet valves de-shrouded in the combustion chambers with the CR then corrected to suit.
It is easy to get the head machined out for the bigger inlets and, in fact, the head will still take unleaded seats, although only exhausts really need the unleaded seats. Most people fit unleaded seats to inlet & exhaust.
#3
Turbo Phil
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Posted 08 December 2017 - 12:55 PM
Yes the smaller 33.3mm valved heads can be fitted with the larger 35.6mm inlet. Though there are a few things to do to get a decent gain in flow from it.
Some people simply cut the seat in the head to accept the bigger valve, but this leaves a stupidly wide seat and no improvement in flow.
If you're using the road spec valves (as opposed to the stainless race valves)
The first thing to do is put a 30degree back cut on the valve and narrow the seat to around 0.060" , they're excessively wide in standard form. The seat in the head then needs to be cut to match the valve seat width. Wiith a 60 degree bottom cut added and blended into the port throat.
The chamber should be cut back around the inlet valve to deshroud the larger valve.
From flowbench testing I found the bigger valve almost pointless without the above modifications, after the work the flow improved quite dramatically.
Phil.
Some people simply cut the seat in the head to accept the bigger valve, but this leaves a stupidly wide seat and no improvement in flow.
If you're using the road spec valves (as opposed to the stainless race valves)
The first thing to do is put a 30degree back cut on the valve and narrow the seat to around 0.060" , they're excessively wide in standard form. The seat in the head then needs to be cut to match the valve seat width. Wiith a 60 degree bottom cut added and blended into the port throat.
The chamber should be cut back around the inlet valve to deshroud the larger valve.
From flowbench testing I found the bigger valve almost pointless without the above modifications, after the work the flow improved quite dramatically.
Phil.
#4
mister bridger
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Posted 08 December 2017 - 01:42 PM
Thanks for that. Having spent a lot of money on the bottom half of the engine I think I'll refurb the existing head and fit as it is, and see if I'm happy with it. If I feel the need for more grunt I can always find another and get that sorted for a quick swap.
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