Small exhaust valves?

[broncr]

We've had quite a warm up. To the point I could finally get through the snowdrift to where my old SBC FI heads were stored. I've got a couple pairs of the "camel" or "double hump" early 60's 1.94 fuel injection heads. They used to perform quite well on 8,000 rpm+ flame throwers.

The thing I found interesting was that the exhaust valves appear to be in the vicinity of 1.45". Definitely under 1.5". Those heads were great for small block (283 - 327) engines built for hive rev HP. Does the difference have to do with the short stroke nature of SBC's?

'Seems to support Iskederian's stance on the "modern myth" of dual pattern cams (being unnecessary for most) and raises an even more relevant concern for some of us. As DP cams are intended to increase relative exhaust duration, and larger (exhaust) valves/longer exhaust duration have a "low end torque" stealing effect, just how big of an exhaust valve is really needed for the I-6? I know - 'depends on your application"...

I realize it's a balancing game, but when I held a 1.94 up to the (smaller)stock 300 intake valve, and then did the same with the exhausts (and found the stock 300 exhaust valve to be the larger), it made me stop and ponder... Might a smaller exhaust valve be beneficial to at least some of us?

broncr

 

[SuperMag]

I would think not. A 300 I6 dsplaces 50 ci per cylinder, while the 283 displaces only 35.4 ci.

 

[Anonymous]

Backwards: larger exhaust valve allows LESS exhaust duration w/o power loss.

 

[broncr]

Thanks, that makes sense.

I wasn't arguing for smaller exhaust valves, just wondering why a 1.94 sbc head could use such a small one. I see that the equivalent displacement /size matched exhaust valve for the 300 would be 8/6 X 1.45 (whatever) - which is almost 1.94!

Similarly, the 1.94 sbc is ''seeing " the (rough) equvalent of a 2.59 intake on a 300 - both assuming same duration/displacement events. It's no wonder a cammed 283 could wind up like a turbine.

I know it's a crude analogy, but my two remaining live brain cells both reside in the right lobe. No need to beat me up with the math - unless you feel the need...

broncr

 

[Anonymous]

Big exhaust valves were the choice when all motors had low compression - they allow the cylinder to blow down to ATM efficiently without deducting from the power stroke or adding overlap.
Now that chamber size is limited by compression and engine speeds are up, intake valve size is more important and the exhaust valve is frequently limited to what's left.
Chevy guessed wrong on the size - as evidenced by the frequent use of dual pattern cams (i.e. exhaust valve isn't working as-is, and needs more duration to balance intake flow).