+2 -2 Head

[Dan H]

So you never got to find out if the bigger intake valves worked better?


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I ran it, but too many other things were changed at the same time. One of the biggest mistakes, not knowing better at the time, was lowering the compression with no understanding of squish area.

I still have the head, but at some point I worked with a caveman (BB Chevy guy ) who wanted to port a head for me and this was the one I gave him. I've thought about going back to it with +1 exhaust valves but not sure I want to take the time not knowing if his porting was even sound for these engines.

[Dan H]

I found one of the messages from Ken- 5Digiits (RIP)

Earlier in this thread I saw you post and I had wanted to ask you about something. Now the discussion has developed further and I figure it is late to bring it up.
I wanted to pick your brain on the +2, -2 head............. Back around '01, I decided to try the concept on a swirl head. I used the +4mm MP intake valves with the stock exhaust valves. I don't believe any other serious work was done to it. The ports might have been smoothed a little. .....
When I used the head I had made some other significant changes at the same time. The setup overall was a mistake, but I never knew what part this head played.
Would you believe that there would have been any benefits to this staggering of valve sizes in either of the factory head castings?

Dan

Hi Dan.
Your approach of shifting the sizes up (+2/-2) to (+4/stk) was a sound approach.
Although, its not just the relation between the two valve sizes that makes it work - It's the valve size with relation to the valve seat.

Exhaust:
With the above indicated, nothing would be gained on the exhaust side until a larger seat ring is installed based on your approach.
The original intent allowed a stock valve diameter to be ground down which produces a wider valve margin which aids flow.
Additionally, the smaller valve head area for the same sized seat means there's less of an obstacle to interfere with exiting gases.
Finally, it allows the valve to sink within the seat which provides an improved taper which also improves flow as soon as the valve begins to open.

Intake:
Placing the larger valve on the stock intake seat allows the seat to be tapered and cut to the furthest edge of the seat ring.
If the valve size is being stepped up even further then a proper relationship between the valve diameter, seat ID&OD must be maintained.
This provides a larger effective area and (like the exhaust) flow improves due to the added taper that can be achieved within the intake seat ring ID.

In short, the +4/stk approach does work but numerous supporting items beyond the relation of intake to exhaust size must be completed to insure the proportions and concept isn't lost.

I hope this helps.
-Ken