de-shroud in or ex

[bentvalves]

4 valves per cylinder, modern pentroof chamber.

if you had to pick a side to de-shroud which one takes priority in a turbocharged engine and why?

in my minds eye the exhaust valve side needs the help as the intake side is already being helped by the turbo....

but I'm still green

[ptuomov]

4 valves per cylinder, modern pentroof chamber.

if you had to pick a side to de-shroud which one takes priority in a turbocharged engine and why?

in my minds eye the exhaust valve side needs the help as the intake side is already being helped by the turbo....

but I'm still green

Also interested in the answer.

My logic would say that if the intake manifold pressure and exhaust manifold pressure are about equal, doing whatever one does for a normally aspirated version of the engine would be the way to go.

[DCal]

I would chose the exhaust because the exhaust is what's drive the whole setup. The intake's under pressure-it don't have a choice in the matter.

[Newold1]

Please qualify and quantify what you mean be de-shrouding the valves. In today's specialty valve, port rework and preparation de-shrouding is a very broad term.

[Keith Morganstein]

Post a photo of that “ modern 4 valve pentroof chamber” , there isn’t usually a shrouding issue. Not much of any valve is close to the edge and deep in a pocket of the chamber. There are a few that have a clover leaf shape to them, But are they shrouded?

[mag2555]

If your thinking in terms of the need to unshroud the Exh valve like in a 2 valve wedge type chamber, that logic changes in a 4 valve set up as the bulk of the flow leaving the chamber uses the perhipery of the 2 Exh valves different!
In short I would not waste any time with unshrouding them!

[ptuomov]

I'm thinking that there should be three different flow regimes in that kind of four valve head. The first is when both intake and exhaust valves are open. The second is when only the intake valves are open. The third is when only the exhaust valves are open. By my logic (not experience), you want the valves to flow as much as possible on the flow squish-pad sides in the first regime and want to prevent the cross-flow directly from intake to exhaust or, worse, from exhaust to intake. If this logic is solid, then one should port the combustion chamber and the piston valve pockets to flow well in the back side (squish pad side) at low lift. How to accomplish this is a separate question.

By the way, why is it either-or intake or exhaust deshrouding? Why not both? Are you trying to lower the compression and just asking where it's best to take off a certain amount of material?

[englertracing]

are you trying to gain some cc's?
there are square 4v chambers
round 4v chambers
and clover 4v chambers
I think in some instances square 4v chambers can use some work, especially with bore increaces
the same for clovers, if they are over clovered, they could use some work to make them more square

but really this is all dependent on the specific cylinder head and ports

[panic]

the "modern 4 valve pentroof chamber" looks pretty much like the Offenhauser of 70 years ago.
Unless the turbo package is well developed the exhaust pressure may be twice the intake pressure, only the best establish parity. For ordinary apps the exhaust needs more work.

JM2¢?
I'm not aware of any test, comparo, metric, SSR, CFM, X-area, etc., that begins to quantify what happens in an intake port at high boost. The successful 2JZ-GTE (factory Supra, Soarer, and Aristo twin turbo) head can produce 1,000 hp without larger valves, porting, etc. and 2,000 with (what we would think of as) minor mods (no larger runner inserts, chamber shape revision, in/ex bias revision, plug relocation, downdraft angle change), while its extremely similar predecessor 7M head fails miserably.
The GTE (turbo only) head and th GE (non turbo) are almost identical (except for more water volume to the GTE), and yet the GE is perfectly docile at 3 liters = 225 hp and suitable for your Mom's station wagon with automatic transmission. The opposite of "hot".

[ptuomov]

How about a photo and a detailed explanation of what the objective is?

[n2xlr8n]

The posts I've read here over the years regarding turbo engines (from the gifted among us) state that one would develop the cylinder heads / camshaft much like a NA engine, with the exception of squish (doesn't matter with boost, iirc).

That being said, I personally would develop the exhaust to the max in any turbo engine.

[mag2555]

If with the average engine in good shape we can add 6 psi of boost and gain 125 to 150 hp then why if your not building for all out race power would you want to go nuts with Exh flow trying to achieve a 90% or better Exh to int ratio?

The bigger residual bang made by boost or nos will push more Exh out all on its own !
If your motor is a part , then by all means do some more head work !

[ptuomov]

If with the average engine in good shape we can add 6 psi of boost and gain 125 to 150 hp then why if your not building for all out race power would you want to go nuts with Exh flow trying to achieve a 90% or better Exh to int ratio?

The bigger residual bang made by boost or nos will push more Exh out all on its own !
If your motor is a part , then by all means do some more head work !

Logically, nitrous should be very different from turbo. Even a small turbo should be very different from a large turbo. It’s the intake and exhaust flow and pressures that matter.

The good news is that as long as the turbo is anywhere close to reasonably matched to a reasonably running medium to large displacement engine, it’s going to have so much mass air flow that with a street car and pump gas little tweaks don’t seem to matter too much.

[ptuomov]

Any update on this "deshrouding" question?

[bentvalves]

NEGATIVE. Once the head leaves my door, these guys are usually pretty tight lipped about whats going on.

dyno graphs posted online and 1/4 mile timeslips lead me to believe I was headed in the right direction.