has anyone ever heard of putting a chevy 1.94 intake valve on the I6 head? anyone ever done it, and was it worth the work and money? I cant remember what web site I saw it at.

 

The 1.94/1.60 C***y valve swap has been discussed at length here and over at www.fordsix.com. It's not a straight drop-in, the C***y stems are slightly longer and they alter the rocker geometry. If you're willing to do the required surgery on the rest of the head and intake, there are some significant gains to be had above 4500 RPM; no big gains in the "normal" operating range of the engine though

 

I don't see why you would want to use a chevy valve don't they make oversized valves for the I6?

 

Phrogman- the chebby valves are the oversized valves for the 300-6. The nice part is that you drill the head for 7/16" screw-in rocker studs, and use BBC roller rockers too. I think longer pushrods are required. I will know for sure before too long.

 

I have been thinking about larger valves in my truck for a while. One of the sources of my indecision is the shrouding of the stock valves. In stock form the valves, especially the exhaust, are heavily shrouded by the combustion chamber. The exhaust valve only has about 1/8" between it and the chamber wall. Adding larger valves would move the valve even closer to the wall and probably negate any flow improvement. You could modify the chamber for better flow, but you would probably have to take 15-20 cc out of it to make it worth while, which would cost you just over a point of compression. This could be made up for as well, but how many compromises is it worth making? Don't forget that installing larger valves might also mean installing hardened valve inserts if the stockers are thick enough for the larger valves. Larger inserts might hit water during installation and then you have a boat anchor with oversized valves.

What are the stock sizes? 1.78/1.45? 351W valves might be a viable alternative at 1.84/1.54. I compared a carb 300 valve to a 302 valve I had laying around and they were the same length. I don't know if the fuelly 300 valves are the same length or not.

 

There is enough meat to the 300-6 head to fit hardened valve seats without hitting water. On the EFI engine, you just trim back the edge of the combustion chamber to keep the same gap as stock. You only loose about 5cc's. If you took out 15-20cc's, you would be halfway to the next cylinder. The whole combustion chamber is 73cc. Even if you fully unshroud the valves, it is only about 8-10 cc's of material.
I didn't know the 302 valves are the same length, I thought they were longer. I may have to look into that option.

 

According to some back of a napkin calculations I did, just unshrouding the valves to a stock gap would be 5-7 cc, like you said. The exhaust is shrouded by the chamber about 1/3 of the way around it, and the intake valve is only about a 1/16"-1/8" from the chamber 1/2 way around. (Now that I am writing this I think I previously said that the exhaust was more heavily shrouded, my bad) At any valve lift over about 0.100 you are loosing half the valve to shrouding. Removing another 0.10 from the chamber would help a bunch, but would still leave the valves significantly shrouded and would remove about another 7 cc. At that point the valve could flow up to about 0.250 before the shrouding started to hurt it. If you only open the chamber enough to keep the same gap as stock I think the larger valves are a waste. You would probably be better off just unshrouding the valves and leaving them the stock size. To get the full benefit of larer valves I still believe the chamber would be opened up between 15 and 20 cc.

 

On EFI engines, if you fully unshroud, you have to re-curve the ignition timing. The EFI chambers have a very fast burn rate and flame front travel speed, compared to the old carb head. If you ran that head unshrouded with the stock timing, you would have a major loss of power.