Something to ponder when you consider exhaust pipe diameter...

[Hohn]

Gary, modern cylinder heads don't rely on that bump much for turbulence, though that IS why it's there on the older heads. What year Chevy heads are you discussing? The original small block 23° heads?



Modern heads use quench for turbulence generation, as the close near-impact of the piston with the head causes a shock wave through the chamber. Thus, small, closed chambers and dished or flattop pistons allow more complete combustion with less ignition lead. It also all but eliminates detonation, even at "high" compression ratios (10:1 and up). In fact, you can take a quench style piston/chamber configuration and lower the compression (by increasing piston/head clearance) and end up with MORE detonation at a lower compression ratio.



The cutting edge of modern 2V combustion chambers looks more like this:



#ad




The ridgeline between the two valves is not so much for turbulence: it's more the inevitable result of the chamber blending from the valve to the chamber. Smoothing the transition from valve to chamber is VERY important to preventing flow separation.

 

[Gary - K7GLD]

Hohn, those heads you show are lots later than the dark-ages SB Chevy stuff from 70's and 80's I played with in my hotrod/custom days - and the intake setups I was using at that point were usually along the lines of the "Performer" type used with a big Carter 4-barrel, with or without a B&M supercharger... And in those days, the focus was upon as wide and clean an intake/exhaust path as could be managed - and any "bumps" - intentional or otherwise, were removed in the name of increased performance.



But times and technology change - and those heads you display sure are purty!

 

[2-ND TIME]

Air!

For you people in the know I would like to ask the area ( sq ft or meters )for one pound of air please. For some of the turbines I read about they mention pounds of air so what size is this?



Thank you to those in the know.

 

[BushWakr - RIP]

"..... Surface texture is important, as a turbulent boundary layer helps to reduce drag..... "



Hohn, if I am understanding your statement can you tell us if the effects of "port 'N polish" reduce or destroying that turbulence, thus increasing drag. . ??

Wouldn't it be of *some* benefit to maintain a certain level of "texture" when performing the port/polish process... say for example the exit runners of a manifold, or, even the area leading to the exhaust outlets of the cylinder head.

How about the boundary layer at the compressor wheel edge... wouldn't there be a benefit to some "texturing" at that immediate intersection of the pinwheel's outer edge and ID of the compressor housing???



or... did I miss something here... ??



bob... .

 

[Forrest Nearing]

intake side should not be polished...



chambers and exhaust ports should be polished to prevent buildup and to reflect heat



Forrest

 

[swankmetal]

something i've been wondering about for a while, is if i could make a 5" to dual 4" adapter. hohn, would you care to comment on the effects of doing something like that? thanks



dave

 

[Hohn]

What Forrest said. Porting is good, but surface texture should NEVER be mirror-smooth on the intake side. Now, this is primarily for gassers, but the principle is the same.



Leaving a 60 grit finish or so on the intake helps to prevent fuel wet-out on gassers. You want to keep all the mixture suspended, so ANY condensation is a bad thing. The turbulent boundary layer that a coarse surface causes helps to make sure this happens. It has the effect of setting up tiny vortices near the surface, which help to prevent the flowstream from actually contacting the port walls, reducing the chance of wet-out (condensation).



I've even seen some high-end race heads that had what appeared to be little rings machined into the intake ports, so that the resembled the scaly appearance of a snake's belly. These concentric rings are machined for the same reasons I mentioned above. The High-End CNC ported heads that are available now have an almost perfect finish just as the come out of the CNC machine. The finish left by the carbide/diamond cutters requires no grinding or polishing-- it's already perfect.



On the exhaust side, I don't think that it matters much because everything eventually gets covered with soot (gasser or diesel), so the reflective aspect of a polished exhaust port becomes pretty insignificant.







As for a 5" to dual 4" adapter, knock yourself out if you want to. There's no real performance pro or con-- it just costs more and takes up more space while adding weight. I'd stick with a single exhaust.



JLH

 

[Forrest Nearing]

polishing chambers and exhaust ports also serves to reduce surface area...



*I* don't polish mine beyond a fine sanding roll, too much effort for not enough gain, but for an "nth degree" job?

 

[BushWakr - RIP]

Ok, Hohn that makes it a little more clear for me... I can see the need to maintain the texture and that is exactly the thing I was wondering about... . thanks bud. .



pb... .

 

[AKAMAC]

Doesn't the problem of wetting get thrown out the window in the direct injection applications (gas and diesel)?