exhaust valve size...is smaller more efficient?

[Guest tejas74260]

hey guys. i was reding my popular hotrodding mag and i was interested in the article about top fuel dragsters and how they make 8000hp from 500CID. although it might not seem relevent to L6 heads, i have heard from this article as well as others that a smaller exhaust valve creates more exhaust velocity out of the head. now, the rest of the system doesnt have to be irregularly small, but i was just curious and wondered what you guys thought about that. thanks

Mark

[Phantom]

The L28 had years of development by both Mercedes-Benz and Nissan so I would expect the valve sizing to be pretty well optimized for that head and their expected uses for it.

[Guest Marshall]

The L28 had years of development by both Mercedes-Benz and Nissan so I would expect the valve sizing to be pretty well optimized for that head and their expected uses for it.

 

But that development also took place in the post-gas crisis 1970's. (For instance, the Camaro had over 350 hp in 1970, while by 1975 the most power enginen was 155 hp). With more modern technology, computers, and better understanding of the physics involved, I'm sure someone could make better heads now for an old engine than they could back then.

[Phantom]

So now we've gone from a discussion on intake valves to the entire head? If that is the case then I heartily agree the head can be improved. I do not, however, think that you would see any more than possibly 1-2 HP improvement by optimizing the intake valve diameter and it could easily cost $thousands$, especially if it is smaller as the original post suggested. It would be much easier and cheaper to just replace the cam with a more aggressive grind. THe intake valve diameter is kind of like swatting at gnats IMO.

[Guest Marshall]

So now we've gone from a discussion on intake valves to the entire head? If that is the case then I heartily agree the head can be improved. I do not, however, think that you would see any more than possibly 1-2 HP improvement by optimizing the intake valve diameter and it could easily cost $thousands$, especially if it is smaller as the original post suggested. It would be much easier and cheaper to just replace the cam with a more aggressive grind. THe intake valve diameter is kind of like swatting at gnats IMO.

 

You're probably right about that. If you just focus on the valves, you won't get much more out of it, and it'll still cost good money.

[JMortensen]

There is a general rule of thumb that the exhaust should flow 75% of what the intake flows on a bench. You can get that number by porting the head correctly. I would think that porting the head would be cheaper than installing different valves until you hit the magic number, but if you wanted it exact you would need to find a shop with a flow bench.

 

The other benefit is that you can make the whole head flow much better and still hit the 75%. I know the 75% is for NA, not sure if that goes out the window when forced induction comes into play.