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Alright bear with me, I'm 19 and don't know a whole lot bout this. This 400 has 383 heads put on but why? Were they trying to compensate for the 400's low compression? And wht is the result as it is now?
A very nice head for moderate performance. Upgrading to the 1.74 exhaust valve is simple and inexpensive. A small amount of pocket port porting produces big power increases. It's a great head for a mid level performance motor. I have done numerous of these with excellent results.
The intake port of the 915 has a different shape which is like the large valve 915.The 915 is pretty much like the 906.
The 516 has a flatter intake floor, much like the later B heads. The 516's may not be the best, but you can them really good for a moderate motor.