Inliners International Forums General Discussion Engines 261 chevy 848 porting

This may or may not be a fair comparison being it's a list of Chevy V-8 heads and it's hard to compare flow testing from different sources and different engine designs.That and actual wet flow when using carbs is different than dry flow with port fuel injection.The two lowest flowing SBC chevy heads are the so called smog heads found on pedestrian 350 engines.Generally,the unmodified smog head can support about 280-300 hp at 5000 rpm on a 350 Chevy that's quite streetable.Both a 261 and 350 V-8 have about 44 cubic inch cylinders.The higher flowing heads make a lots more power at 6000 rpm and higher.The V-8 heads were tested at 28 inches of vacuum.
Chevy V-8 head flow

28 inches is the recognized industry standard for flow comparison on cylinder heads, much like cam companies using the "@.050 lift" measuring to compare one companies camshaft against anothers. The previous question was asking what effects scavenging has on engines with siamese ports.And since SBC's dont have siamese ports, your post doesn't seem to follow any of the recent queries, maybe you can elaborate on what you are trying to say more clearly....

CNC Dude - you have a PM regarding hopping up these heads

Cheers

Mark

Thanks Mark, the more input the better! Welcome to the discussion.

PM'd you again

Hey its been to long. Whats happening???? 1 OL REDNECK I.I.#113

Having to make a dial indicator stand to place the dial indicator over the valves to open them. Since it is a rocker shaft engine, there is no way too place the indicator over the valves as in an individual rocker stud type head. Almost done!

Sorry for being impatient. I don't know how you do all you do and still have time for this, BUT I'm loving these imformative forums Thanks DARRELL

Since heads like this aren't the run of the mill cookie cutter design, sometimes you have to think outside the box to "gitter dun". Luckily,i've got access to a state of the art CNC machine shop, and can make anything I need with little effort, just time. Im anxious too, its getting closer every day!

This is cool. I volunteer the "test mule" if we need one for head and cam selection etc. after all this is done on paper.

Ok,great! That would be a good starting point....

Here are the details Frank McGurk published regarding modifying this head which could represent a benchmark:

“This latter operation was done on the intake ports by using a 1 1/2 inch diameter shell reamer piloted into the valve guide bores and enlarging the valve pockets to the point where the ports change direction. Next, a 1 9/16 inch diameter piloted shell reamer was inserted halfway into the valve pockets and the ridges between the two reamed diameters were removed by blending the surfaces together with a hand grinder. A 70 degree piloted hand reamer was then used to increase the minor diameter of the valve seat to 1 ¾ inches. The 1 ½ inch reamer was also used to enlarge the port openings on the side of the head to the point where the ports change direction. At the juncture, the hand grinder was again used to blend the surfaces together. The exhaust valve pockets were enlarged by using a 1 3/8 inch diameter piloted shell reamer. The exhaust port openings were not enlarged appreciably, but were merely cleaned up with the grinder. The 70 degree hand reamer was used to enlarge the minor diameters of the exhaust seats to a consistent 1 3/8 inches. Next the “barnacles” were removed from the surface of the combustion chambers which were then ground to a smooth finish, the hand grinder used for both operations. Finally the intake valve seats were ground to 30 degrees and were narrowed from the top to a width of 1/16 of an inch by using a 10 degree valve seat grinding stone. Similarly the exhaust valve seats were ground to an angle of 45 degrees and were also narrowed to a width of 1/16 inch.”

He goes on to say .030” was milled from the head and the same amount was removed from the top of the valve stems. These improvements were documented as providing a power increase of 8.9 percent and a torque increase of 4 percent. Compression improvement was 16.9 percent due to head milling. I would be curious to know if those values would be repeated based on the difference between the stock port configuration and what Mr. McGurk did and how much further performance improvement beyond that is possible with the tools and methods CNC Dude will use.

I intend to do this work on a basic 3 axis vertical CNC mill and plan on posting the toolpath so that other Inliners can get this done in most average machine shops and not just the few with 5 axis capability. That’s the beauty of CNC in that one tool can be made to behave like a whole collection of shell reamers and generate smooth transitions.

Digitizing determined that the intake valves are set in the head at an angle of 5 degrees so I needed to come up with a way to hold the head so that the valve guides would be parallel to the machine spindle. The valve cover bolt holes measured 12.766” front to back and 5.562” from side to side and I decided to use them to mount sine blocks. I drilled and counterbored a pair of 1” keystocks and milled them at 5 degrees with a difference of .450” in height. Now I’m ready when CNC Dude has data on where and how much to cut. Here’s a couple pic’s of the blocks:


http://s176.photobucket.com/albums/w186/curt160/848%20head/?action=view¤t=000_2420.jpg

http://s176.photobucket.com/albums/w186/curt160/848%20head/?action=view¤t=000_2421.jpg

Hey Curt, most valve bowl cutting today is done with a 85-90% size of the valve that is used. So if you are using a 1.94 intake valve, you would cut the bowl from 1.649"-1.746" . I would use a 3/4" ball mill to just touch where it meets the roof, and let that rad blend the pocket walls and roof. Heres a G13 to do it. G13 Z-x.xxx Dxx I.873 F5. You just need to add the Z depth to the actual amount of depth you need, and put the appropriate D# in it as well to match your tool #. Your X and Y would be the valve centerline. You can kinda' see how the thinking has changed since the 50's as far as bowl cutting goes. But the 85-90% rule of thumb is what all race stuff is today. That is the direction im heading, if you want to go ahead and do a test run with your head,it might reveal something. Also, I had planned to replace the guides, since a G13 starts at the XY centerline, adjust your feed so you don't crash your end mill into it.

Thanks Scott. On the subject of the guides have you investigated what the optimum geometry would be on the ends of them? Will a chamfer or radius be beneficial? While I’m there they could be shaped too.

Curt, I am going to taper the guides as it protrudes from the roof on both the intake and exhaust.

Also creating a vein on the leading edge of the guide on the roof of the port to help steer the airflow around the guide helps also. You might experiment with Bondo first to see about size and shape. But I would have the leading edge more blunted than sharp. After flow testing with Bondo, I would build-up with some brass brazing rods to make something more permanent, and do the same for the "lumps" on the floor of the port.

The more I look at those valve guides the bigger interference they seem to be. Once the ultimate flow geometry around the guide is determined why not make guides that incorporate those aerodynamic features and just orient and press them in?

You probably could,but most guides only have an OD of about .500, and whether you could create that significant of a change that close to the valve centerline or not, I don't know.Most race heads have a vein that starts about .750"-1.000" away from the valve centerline to give it a little lead-in transition and blending before it gets to the guide. You might take a new guide and stand it up in a vice and use your CAM software to create an ellipse shape around the circumferance of the guide that protrudes thru the roof. This should create less of a frontal area to obstruct the airflow around the guide, and im sure it will help the flow as well to some degree by creating a surface similar to an airplane wing leading edge with less turbulence than just a round obstruction.Something to try....have you tried a G13 for your bowl cutting yet!

I guess we’re not on the same page. I’m talking about completely new high tech guides installed from the bowl side with an integral vein, taper, slight helix, and shouldering up against the roof. The vein could start just slightly less than the opening behind the valve which was mentioned to be somewhere between 1 5/8” and 1 3/4”. This would allow for a vein of up to 7/8” from the valve centerline. As far as using G13 goes it will only generate a circle starting and ending at the center point and I plan to take full advantage of my software by generating point to point surface contours. The tool I have allows me to roll part way around the corner as shown here:

http://www.iscar.com/Ecat/familyHDR.asp/fnum/1614/app/71/mapp/ML/GFSTYP/I/type/1/lang/EN

Creating a helix will help promote swirl into the bowl area. Are you thinking of under cutting the valve seat with that cutter and creating a venturi shape above the seat in the bowl.

I cant wait to see the chips fly. And then get one of these on the flow bench! Who are these premade "aeroguides" made by?

I think that Curt is contemplating making them for his head to test....

It is this thread in particular that led me to this forum and made me join it. I have a strong interest in the subject that was being discussed. I appear to have started reading a great book. Not necessarily looking for the ending, but would like more to read. I am currently in the teardown stage of my 235 with an 848 head. Whats the next chapter?

The thing that this thread does for me is give me hope. I've got a 235 with an 848 in my 56 Chevy and although it will likely never be rebuilt to more than a stock specification, whatever results come from this thread will provide tantalizing "what ifs" for me to dwell on. I've always been a fan of applying new thinking and new technology to old stuff.

Thanks for all the R&D time you've spent on this topic.

holy moly its been so long ago that I started this thread I forgot that I started it lol, anyhow guys I still have an already bare head that I can donate to the project and research if needed, its not an 848 head but an 850-261 chevy head pretty close tho minus the combustion chamber being a tad larger. I already had my 848 head ported and polished but due to the strange port design I think that there is rom for improvement tho, let me know if ya guys want it

The 848 appears to be the best subject for development, because:
1. small chamber = best static CR in stock form
2. pretty common
3. fits all 1954-63 motors

Also: I'll bet that the compression bump on Ross etc. pistons will clear the exhaust pocket on the 848 better (viz. won't crash anything or require big chunks carved off) than a casting chosen at random.

Good afternoon, I just finished reading the entire thread and the more I read the more I think I understand about this fascinating topic and the more questions I have about Stovebolt engines.

I have a '54 850-261 that I want to start on this winter. I spent all of my son's youth working and didn't share time with him. Now we're going to re-work a 51 pickup to take to Reno next year. I want to put one of Tom Langdons Bulldog cams in it, dual Rochester BCs and headers.

If you guys were building this for the street what would the build be comprised of? Porting the head, domed pistons, cam
with solids, and??? This would be my second engine build and I have access to good machine work but don't know what to ask for or be concerned about.

I live outside Salt Lake City, Utah. Do any of you have a similar project going on that I could chat/visit with you about as the project goes along?

Thanks,

Ted #4730