All Small Six Increasing Valve Size

[Cesarvi17]

Hey everyone,

Just got curious. I’ve been reading that the large log heads have restrictions that don’t allow much of an increase in valve size. I’ve been reading the tech page and see that the aluminum head they tested had 1.84/1.5” valves. They cylinder head I have is a large log that has 1.75/1.5” valves. I’m curious, after porting, has anyone been able to increase the intake valve to 1.84” or even 2.02”? From what I’m reading, larger ports sacrifice some torque at low rpm’s and increases it at higher rpm’s, so also I would like to know if that sacrifice is worth having bigger ports for a 1/4 mile application. Thanks guys for taking the time.

 

[xctasy]

Anything out of context is a pretext.

The small bore engines are the 144 and 170 cubic inch 3.500" bore engines.

The 200 and 250 are +180 thou big bore engines, with 3.680" bores.

Generally, via a calculative process called the "discharge co-efficent method", a wedge head can have an intake valve about 0.48 of the total bore size, as the biggest valve size.

An example is the Big Log C9 170 head, which had a small 52 cc chamber, yet Ak Miller in 1970 , using a SuperFlow Flow bench, found it out flowed the more common 60 cc chamber head by 15%. The technically closed chamber head should have had more shrouding and less air flow, but the bigger 200/250 chamber for that year flowed less than the smaller head. Everything is quite inter-related, so hard and fast rules depend on application. Many of our head flow posts end up 12 pages long here at Ford Six.

In some overseas engines with a 18 degree wedge chamber, you can open up the tops of the cylinder bores for relief cuts, like was done in the 1.65 inch valve 202 Holden sixes with a 3.625 inch bore. In other instillations, like the 253 Holden V8 engine, a 1.78 inch valve can fit in a 3.625 inch bore.

In Australia, 200 or 250 wedge chamber engines, both the iron log and 2V engines, they could take 1,74 inch Holden 202 Yella Terra intake valves. The limit to how big you go is governed by valve shrouding, the distance or proximity from the rim of the intake or exhaust valve from the upright wall of the cylinder heads chamber. Any time valve size is increased, the shrouding measurements get tighter and tighter, which reduces gains in air flow at peak lift, and sometimes reduces the gains at lower lifts.

Air flow management by re-profiling the cylinder head chamber with a die grinder and linishing paper can ensure all the bigger valve changes or increases, are always positive ones.

Ford generally put 1.75 inch intake valves in the D6 heads from 1976 model years, depending on what valve seats were used. Compared to the 1.65 inch valve heads, the later big log, bigger intake heads flowed better, but on a stock small log older head, the 1.75 gains drop off. There is a gain in flow in up-sizing valves, but without chamber porting and valve seat work, but not a great gain.

The Alloy Vintage Inlines head is the 2006-2015 Classic Inlines head, and that was designed by Elmo, an ex GM engineer via a mixture motion program, and if the top of the bores are clearances, you can go over the idealised 1.75" valve size to 1.84 inches if you do the chamber modifications right.



Have a great day.

 

[Cesarvi17]

Thank you xctasy for the info.