What exh valve size?
03-27-2013, 05:59 PM
#1
What exh valve size? Building a 5.7 based on an LS6 block and a pair of LS2 243 heads. Just a good all around motor (stock crank, eagle rods, forged pistons) with a "TR TRuTorq" Level 3 - 223/236 .612/.610 113 LSA.
In the "old days" (10 years ago), everyone used the 2.02/1.60 combo. Now I see everyone using 2.02/1.57. Why the change?
Chris
In the "old days" (10 years ago), everyone used the 2.02/1.60 combo. Now I see everyone using 2.02/1.57. Why the change?
Chris
03-27-2013, 09:51 PM
#2
My theory is solely my theory.
But, I'd say exhaust port velocity is improved with a smaller valve. The 1.60" valve and 1-3/4" headers worked well together, but a 1.57" and 1-7/8" combination may work even better.
I had asked that question a few years back with the TFS heads, and no one knew for sure, but they seemed to respond well to a larger header. The speculation was because the port was efficient at evacuating the gases at a higher velocity, you just needed to have enough exhaust/header piping behind it to support the overall flow.
That and a 1.57" valve weighs less and provides more PtV clearance.
But, I'd say exhaust port velocity is improved with a smaller valve. The 1.60" valve and 1-3/4" headers worked well together, but a 1.57" and 1-7/8" combination may work even better.
I had asked that question a few years back with the TFS heads, and no one knew for sure, but they seemed to respond well to a larger header. The speculation was because the port was efficient at evacuating the gases at a higher velocity, you just needed to have enough exhaust/header piping behind it to support the overall flow.
That and a 1.57" valve weighs less and provides more PtV clearance.
03-28-2013, 10:04 AM
#6
My theory is solely my theory.
But, I'd say exhaust port velocity is improved with a smaller valve. The 1.60" valve and 1-3/4" headers worked well together, but a 1.57" and 1-7/8" combination may work even better.
I had asked that question a few years back with the TFS heads, and no one knew for sure, but they seemed to respond well to a larger header. The speculation was because the port was efficient at evacuating the gases at a higher velocity, you just needed to have enough exhaust/header piping behind it to support the overall flow.
That and a 1.57" valve weighs less and provides more PtV clearance.
But, I'd say exhaust port velocity is improved with a smaller valve. The 1.60" valve and 1-3/4" headers worked well together, but a 1.57" and 1-7/8" combination may work even better.
I had asked that question a few years back with the TFS heads, and no one knew for sure, but they seemed to respond well to a larger header. The speculation was because the port was efficient at evacuating the gases at a higher velocity, you just needed to have enough exhaust/header piping behind it to support the overall flow.
That and a 1.57" valve weighs less and provides more PtV clearance.
http://en.wikipedia.org/wiki/Venturi_effect
Also relevant:
http://en.wikipedia.org/wiki/Choked_flow
Last edited by ckpitt55; 03-28-2013 at 10:10 AM.
03-28-2013, 11:47 AM
#7
Well it's a theory based partly on what I know of physics and partly what I've seen. I haven't done the math on it, but it seems like a reasonable conjecture.
Also, I went with bigger valves in my head, but did hollow-stem. Because, well, unless you have Z06 243s, you have solid stem 2/1.55" valves. And the aftermarket stainless valves are only a few grams heavier.
Also, I went with bigger valves in my head, but did hollow-stem. Because, well, unless you have Z06 243s, you have solid stem 2/1.55" valves. And the aftermarket stainless valves are only a few grams heavier.
