I read using a 2.02 or larger valve actually cuts the airflow into the cylinder. As apparently nonsensical as that reads I have actually come across the statement several times.

Working up to new heads for 2015. Still using the LS6 intake and so ask this one last time before I decide on heads.

What say yee?

 

The fastest cathedral heads run larger valves.

A larger valve equals more curtain area at any lift and allows a larger throat, so it should equal more airflow unless something is very wrong. Most people will just argue that a 2.02" valve is such a marginal increase that it's not worth it.

 

Ok. I thought it was the stock bore of the LS1 would have a shrouding effect at the cylinder wall area with larger ones.

Looking at Trick Flow 215s and notice they are 2.04" or Lingenfelter which are 2.02"
others keep 2.00"

 

depends on your bore size. A 6.2L block will flow more air through big valve heads than a 5.7 L block because the valves are further from the cylinder wall with bigger bores

 

regular 346cid

 

The OEM heads have a 2.02" diameter seat insert, so a 2.02" is about as big as you would want to go in a stock head unless you change out the seat inserts. Aftermarket heads usually have a larger seat from the get go for even larger valves.

You can go a lot bigger than 2.02" on a 3.898" bore LS1 so it's not really going to be all that shrouded. Look at the 4.8L engines with 799 heads. Straight from the factory with a 2" valve in a 3.780" bore. That's like a 2.060" valve in a 3.898" bore.

 

Ever seen a set of stock heads on block from the bottom side? The Exhaust valve is much closer than the intake valve. Open it far enough and it will actually hit the cylinder wall.

Attached is junk set of 317's on a stock bore 5.3L with no head gasket that is being used for mock up in a project. Since it was up on a lift I found this view interesting...

That's the exhaust valve hanging down hitting the cylinder wall.

 

I have ported patriots with bigger valves and they were originally going on a 5.3 until my friend noticed the valves wouldn't even clear the cylinder walls.... Got them on my ls2 now no problems

 

MOST of this depends on the bore size (and shape) of the combustion chamber.

If the larger valve is shrouded by the wall you may not see a benefit....if the cross section of the head wont feed the curtain area available at the valve you may not see a benefit.

If you do have a proper shaped port and chamber a larger valve will usually pay dividends.....throat percentage and the valvejob also play a large role.

There is no steadfast rule but if you know how to properly port a cylinder head and you don't over do it based on the actual valve size relative to the port and cylinder bore, bigger is usually better but its one of this diminishing returns type of situations.

The right combination is really the answer (like most things engine related)....lots of things done properly will help it achieve good results and the opposite holds true as well.

Hope this helps

-Tony

 

What performance application did they use the 4.8l in?

Point being OEMs do things for cost reasons more than about any other reason so I see no validity to the 4.8l argument.

 

What does shrouding hurt? Wet flow.

What does wet flow hurt? Mostly fuel economy.

Why would one buy a 4.8L? Fuel economy.

Which is cheaper, an 862 casting with the 1.94" valves or the 799 head with the 2" valves? From a consumer stand point, the 799 head, but maybe there was a financial gain in minimizing the variety of castings GM had.