Did they use the same valve size for the stage III cnc????


As for hand vs. computer porting or cnc, I have seen in many cars very good results from both......As for air flow as stated earlier by a wise man.....it does not matter as much as how much your bottomend can move and velocity of the air and a bunch of other crap...... Any way I will get my physics book back out and try to figure out static friction..... :skep:

At least they look cool :thumbs:

 

CNC has a very bad name because there is a lot of complete JUNK out there that is "CNC'd"

Here's the way a CNC is SUPPOSED to work. Somebody takes a lot of time and carefully hand ports a head. They might get one port on that head to flow really well, they might get them all to do it. In any case you take the good flowing ports and chambers and digitize them. Once you have a good flowing chamber / port for the entire head you make this into a CNC program.

After you have the CNC program you can make 1000 heads EXACTLY the same. Now does the CNC flow as well as the hand ported head? Probably not, but that doesn't mean it's a bad thing. For certain heads there is a lot of power to be had by reshaping the chambers and runners, not just polishing them.

Now what would you rather have? All of the major material removal and reshaping done by computer so it is exactly the same, and then have somebody come in behind it and just smooth it up. Or would you rather have somebody spend days working on one set of heads hoping they don't gouge just a little too deep here or sneeze over there?

A set of heads straight off the CNC is probably going to be junk. A set of CNC'd heads that have been cleaned up by hand will flow just as well as the original hand ported head and can be done in a fraction of the time with a whole lot less margin for error.

BTW, the CNC marks aren't always ALL bad. I have personally flowed a head straight of the CNC vs a head that has been polished to a mirror shine and had the CNC head outflow it, and I don't mean by just a little bit. The CNC also had a smaller port volume so it maintained better velocity.

Short and sweet, if used properly, CNC can produce a product 100% equal to a hand ported head and can do it in a fraction of the time. It can also be much more consistent in the process. Is the CNC "better" than hand porting. No, because it's program is made from a hand port job. You must digitize something, you can't digitize a stock port and expect to get good results by trying to tweak it on screen. Is CNC any worse than hand porting? NOT IF DONE PROPERLY. CNC is a time saver, not a finished product maker. CNC removes the bulk of the material so that the person doing the hand porting doesn't have to. The CNC does this faster and more accurately and allows the person doing it by hand to concentrate on the final details, not the tedious bulk work.

 

Very true.

Maybe in the local dirt track or circle track motors, but not on higher end racing heads. Most local engine builder types cannot afford a good enough digitizer to take enough data points on the master to get a good mesh for the program, thus you get something that doesn't exactly match the master. If you're talking about an engine builder that can afford the good equipment (laser digitizer head that takes 30,000 data points per snap shot), you're going to have a dead repeatable and accurate CNC head that needs no finish work done to it.

Someone mentioned that they had flow numbers but wouldn't post them because they felt that flow isn't everything in a head. Assuming are getting additional flow without drastically increasing the port volume, flow (especially mid range) is EVERYTHING in a head. The whole point of the head is to flow as much air into the cylinder on an intake stroke and as much air out of the cylinder on an exhaust stroke as possible. That's it. It's that simple. Flow IS everything (again assuming that it's not at the cost of port volume). If flow wasn't everything, why are racing teams spending so much time and money trying to find a better flowing porting job. Why do hobbyists spend so much money buying AFR or Dart or Brodix heads for their stroker motors?

 

$250 extra for a spring upgrade? I can understand $25 but no way $250 :bs you can buy three sets of springs for that much! I just priced a three sets of Comp Cams springs that would work on 99.9% of most street roller cams up to .590 lift and none was over $85! These are Comp part numbers 978-16, 977-16, and 987-16.

As for CNC heads not flowing as well as hand ported heads. Well that depends on the CNC profile and the hand ported heads. CNC machining can replicate any hand port job and get the repeatability that hand porting can't do and it don't matter how good your hand porter is, he can't replicate his port to port or chamber to chamber demensions as a machine can.

Cost of CNC machining? Well I for one would not send my heads in to have them done. My preference is to buy a new set of either Dart or AFR CNC heads. They both provide flow sheets and the heads are built to your specs. AFR for one will custom build they heads for you based on your engine. If you run a flat top 396 or 383 and desire a 11.8 compression ratio, they will finish off the chamber volume to give you exactly that based on your engine specs. Cost you may ask, cost is just a bit more than having someone CNC your heads and you can offset that cost by selling your stock heads. And what you get is a better designed head over the stock GM units, well worth the few extra bucks.

Another member of this forum had a set of heads done by Pete at CNC heads. He is very happy with them. CNC Heads also sold him a custom ground cam, which was ground by Comp Cams, its pretty similar to a Hot Cam grind. I helped him with the PCM stuff and he fine tuned it on a Dyno Jet and many miles of street datalogging In the end he was very happy with his results.

Just my 2 cents worth.





[Modified by tjwong, 6:35 PM 5/24/2003]

 

This forum member would be me. I am very happy with my CNC Cylinder Heads.

The way Pete at CNC described their machining was that they were more concerned on port velocity than total flow. I m no expert on port velocity or total flow, but Pete really seemed to have the heads/cam combination that suited my needs. I know that my engine does not have the highest dyno numbers on this forum, but my numbers are respectable and my engines performance is everything that Pete promised and more.

Also, thank you Tom "tjwong" for all your tuning help.

 

There is a difference between CNC'd heads and CNC Heads. One is a process done by many shops and operators, while the other is a company down here in Southern Florida. I had my heads ported by Pete at CNC Heads, and then had them shipped to MORE Performance where Mike Osucha installed the heavy duty valve-train and double springs. Like others here, I have no idea what they flow but I am happy with them. It would be nice to have some numbers though...

Mike

 

Here's another VERY satisfied customer! CNC Heads did a great job
on my LT1 heads. Here's what I remember:
Larger valves, mult angle valves, Brass valve seals, Hardened valve guides, larger studs, double (heaver) springs and seats, swirls on the valves, CNC'ed port and Polished.
I've had these heads for well over 40K miles, not a single issue.
Began with just a top-end job and later on (spun bearing) did the bottom end.
I had the opportunity to visit his shop in 1997. I remember because he was in the process of developing the C5 heads. I would buy another set in a heart beat. The top-end performance was just awesome. Flow sheets are not everything and I'm sure if asked he would provide them.
My .02
Hank

 

It'll be nice to see how they flow for you.

The intake ports look pretty smooth until you get down past the valve stem. I don't like seeing ridges like that - especially the pronounced ones in the cc. That's either tool chatter or the shop had their feed rate cranked way up and the tool path optimized for minimum machine time.

That's maximum economy for the shop. I know they would have had to use a long, small-diameter ball-end mill to do that work, which always gives you tool flex and chatter at depth...

My finger's'd be itching to get in there with a fine burr or polishing point and make it as slick as an aluminum casting can get. Slicker without increasing the port volume also means higher velocity. Maybe at the expense of some turbulence, but hey...

Still a magical art, all told.

 

If CNCed heads are not as good as a hand port and polish, why do reputable tuners like Lingenfelter and Callaway Cars CNC machine their own heads?

I'd personally go with a set of CNCed heads. I don't care what anybody says, a hand port and polish job will never be as consistant from port-to-port as is possible with today's CNC equipment. Flow numbers or not.


[Modified by TheCorvetteKid, 6:02 PM 5/24/2003]


[Modified by TheCorvetteKid, 6:03 PM 5/24/2003]

 

Yep, I am happy with them so far.. they are great on my 355.. not sure if they would work well on larger cubed L98s though

 

CNCd heads are still very good, and are dead consistent from port to port. The three main reasons the CNC process is used are 1) repeatability from port to port, chamber to chamber, and head to head; 2) FAR less labor intensive...a GOOD set of hand ported heads should take ~2-3 days to do. A good set of CNCd heads can be done in a couple of hours, depending on how much porting needs to be done. 3) because of the reduced labor, the company has a much higher profit margin per set of heads once the initial cost of the equipment is depreciated.

The quality of a CNC machined set of heads is largely a function of the accuracy of the digitizer and the number of points digitized from the master. As Matt Black mentioned, it's also a function of the parameters programmed into the machine (speeds, feeds, and mill selection).

 

As people have hit on here there are plenty of "CNC" heads out there that are crap. People buy the equipment and don't know how to use it properly but they can sell a product because "CNC" is high tech.

Then in the long run CNC devoloped a bad name and now everybody is afraid of it. If done correctly the CNC process can produce a very good product. The problem is that there are a lot more ways to do it poorly than there are to do it correctly.

 

Thank you Nathan CorvetteZ51 Racer for making a couple more points about CNC vs. hand porting!

Really, it all comes down to the skill of the person doing the porting and the CNC machine is just a tool. Just like a surgeon, the right tool in the wrong hands can be misused.

Head porting is not about removing lots of metal. It is all about shaping the ports. The majority of the gains are realized by correctly shaping the port bowls just below the valve seats and carefully smoothing the port floor short side radius. You want the airflow to follow the short side radius past the valve seat and into the chamber. If this part of the port is not done correctly, the air separates from the short side radius and tumbles before the valve seat. The greatly reduces maximum port flow.

Whether the heads are CNC or hand ported, you still have to do this area correctly or you have ruined the heads. I am sure there are excellent CNC ported heads, but my point is CNC is no guarantee.
Below is a photo of my LT4 combustion chamber after hand porting. The CNC has grooves in the combustion chamber in contrast to my smooth chambers and there is a debate which is better. But if you study the area just around the valves, you should see a slight depression or radius. What this does, is promote increased airflow as the valves start to come off there seats.




[Modified by Mez, 4:56 AM 5/25/2003]