Thoughts on Valve size?

Valvebounce said

What real world experiences have you guys had with upgraded valve sizes on a SBM with a 3.940 bore and small port heads?
"Conventional wisdom" says that 2.02's suffer from shrouding. Yet plenty of guys have slapped J heads onto 318's. (Biggest hurdle there is getting over the miserable compression ratio that combo typically yeilds)
I've got some 1.88's/1.6 which I had planned to fit to some 318 heads. But I'm wondering if it's worthwhile to take the next step. (2.02) Cam will be an XE 274.
Since I'll likely be getting the guides done too, I might even go to 5/16 stems if I go bigger than the 1.88's I've already got.

What thoughts have you?

Click to expand...

I have real world experience with 2.02" / 1.6" ferrea 5000 series valves in a pair of "915" J heads fitted to a 318 with a 3.940" bore.

I cc'd the chambers of the stock 318 heads with 1.78" / 1.5" valves, they came to 66.5cc and had never been removed since they left the factory until that point.

My J heads with the ferrea valves also came to 66.5cc. deck machining history unknown, but looked minimal and the valve job looked factory spec.

At an estimate, I'd say the ferrea valves reduce the chamber by about 4cc compared to stock 1.88" / 1.6" valves.

With dome top pistons bringing it to 10.3:1 and a comp thumpr cam (227/241 on 107) she made 352hp corrected at the fly on a water-brake engine dyno with an exceptionally flat torque curve. I've since ported the heads and put in solid flat tappet cam, but it was a lot of fun in that configuration.

Personally, if you're looking to make more than about 300hp I'd start with something with more port cross-sectional area than an old school 318 head.