Its not so much the 5 mm difference as it would be the immediate air flow difference between the 2 sizes. On my 4.6 I had a BBK 75mm TB going into a stock upper plenum. Stock plenum is about 72mm. Doesnt seem like much,.. but I hand ported the stock upper and port matched it to the TB then blended it so the air flow didnt hit any hard edges going down. Picked up 8hp on the dyno from the hand porting alone. So there is hp to be found.

You can use a cordless drill or a dremel. Lows and Home Depot both sell bits to do it. I found that the stone bits dont work so well on aluminum. They become clogged and covered in aluminum shreds. The sanding wheels and roller bits with like 180 grit work best to take out large material,.. then move to 400 grit and then a polishing bit to finish up.

Dont have any picks of my plenum finished,.. but here are some of my heads that I did.






Doesnt seem like much. But I made 272 RWHP and 295 trq on a 4.6 with mild hand ported heads and bolt ons. Its really easy to do. I just took the gasket, used a sharpie to show me how much material I had to play with to match the gasket. And Viola. Took me 3 days to do both heads, intake and exhaust ports.


Hurst