I'm sure this is a loaded question, but how much perf gain have some of you seen having a set of cylinder heads CNC ported that were already run as cast from the manufacturer? Think of having my engine builder run mine while engine is in for service.

Honestly CNC is fundamentally just a way for the manufacture to save labor and assure all runners are the same size….it doesn’t mean the heads will perform better. I recently sold an engine that had a set of heads “hand ported” roughly 20 years ago, this engine has proven to make more power than the same castings that are sold today with the CNC option.

If you know the CNC program was digitized from a proven port design that is compatible with your combination, that’s different.

quote:

Originally posted by Jerry Kathe:
Honestly CNC is fundamentally just a way for the manufacture to save labor and assure all runners are the same size….it doesn’t mean the heads will perform better. I recently sold an engine that had a set of heads “hand ported” roughly 20 years ago, this engine has proven to make more power than the same castings that are sold today with the CNC option.

If you know the CNC program was digitized from a proven port design that is compatible with your combination, that’s different.

I'd like to see more people chime in, as an as-cast head doesn't necessarily always need to be CNC'd if the port is already properly shaped and the casting is good, typically you can see lot's of gains from bowl work and blending in the valve job from what I have seen.. But curious if there's anyone who has experience with this..

Depends on the CNC porting program and the castings they used. Some castings have more material in the right places allowing for a better porting program and some programs are just better than others. The thing about the CNC is they are very consistent but only as good as the program.

There is one small block Chevy head that flows about 320 as cast and one CNC program it flows 370 with same head and another program 395 same head.

Plus there are some CNC SBC heads that flow under 300 and that is nothing. But they are CNC. Big Deal. You would be better off with better heads as cast than those CNC heads.

Isn't it true you can also "over-port" the head and make it so large that yea it "flows" a lot of air, but the velocity at which it moves through the port is so slow you actually lose power?

quote:

Originally posted by NC3x58:
Isn't it true you can also "over-port" the head and make it so large that yea it "flows" a lot of air, but the velocity at which it moves through the port is so slow you actually lose power?

Chad S. Doesn't call it V factor for nothing...lol. I'm not a head expert. Few are. But yes flow without velocity can hurt.

NC3x58,

Exactly right. Head has to be ,matched to rest of combo. A cam that make max power at 4500 and a head that flows great at at high lifts are both good parts but should not be bolted together. Like putting a huge cam in low compression motor.

Absolutely. Essentially the point I was getting at. I guess I should have disclosed both the CNC version and the hand ported version was on the same CI/combo.

Port volume, size, shape, taper, corner radius, window size, pocket/guide shaping, short side R, etc….all play a role in the end result. There is no one size fits all when it comes to cylinder heads if your goal is chasing peak power for that combo.

In most cases the CNC version is perfectly fine making the finite differences irrelevant in bracket racing. As you suspect, the further you get into bigger CFM numbers, the power band will certainly move up on the same displacement engine and so must the operating range, if not the velocity will suffer.

I’ve seen more than one case of where a former Pro Stock engine was purchased and put it in a bracket car, ultimately to be disappointed with the results – pretty much that reason is why….

ooops....looks like I was "treed on that"....carry on - lol

Yes it's a loaded question without specific details but normally( 95% plus) you will pick up performance from and as cast to a ported head whether it's CNC'ed or hand ported. If porting the head from your typical CNC deal from Dart, Brodix etc makes the head too big, it was too big as cast! Too many people hear this and that without really knowing what's right vs what racer X told them... Air speed/velocity is a big buzz word that's not understood by a large percentage of the racers out there.

Some as cast heads are better than others out of the box so the amount of increased HP can vary from this alone in addition to your engine combo which will also contribute to increased HP.

From scratch, determine the HP level you want, pick the CID which will then determine the RPM needed to develop the goal HP and everything follows from there!

quote:

Originally posted by Jerry Kathe:
Absolutely. Essentially the point I was getting at. I guess I should have disclosed both the CNC version and the hand ported version was on the same CI/combo.

Port volume, size, shape, taper, corner radius, window size, pocket/guide shaping, short side R, etc….all play a role in the end result. There is no one size fits all when it comes to cylinder heads if your goal is chasing peak power for that combo.

In most cases the CNC version is perfectly fine making the finite differences irrelevant in bracket racing. As you suspect, the further you get into bigger CFM numbers, the power band will certainly move up on the same displacement engine and so must the operating range, if not the velocity will suffer.

I’ve seen more than one case of where a former Pro Stock engine was purchased and put it in a bracket car, ultimately to be disappointed with the results – pretty much that reason is why….

jerry

you seam to have a good idea of what's going on but a few words of caution...

Port volumes - need to use this with the same style heads, and it don't tell nearly enough info

Porting a head doesn't mean the runners are made larger and therefore air speed slows down and in fact it can create air speeds that are too high which weren't un-ported and hurt HP, everyone thinks porting is making a runner bigger,,, and if you just consider port volume then yes, but this is not always true with regards to cross sectional area through out the entire length of the port! Some areas are just cleaned up to give it the PORTED/CNC'ed look, if it was left alone everyone who purchased a head like this would scream bloody murder! They see with there eyes, air and fuel have no eyes!

TOP38…KNOWS!

I’ve stated numerous times here in the past that porting your heads and manifolds is mostly a waste of $, all you can be sure of is your ports will be larger, look shiny snd shiny doesn’t typically result in quicker ETs and faster mph. Been there, done that.

quote:

Originally posted by BG7X77:

quote:

Originally posted by NC3x58:
Isn't it true you can also "over-port" the head and make it so large that yea it "flows" a lot of air, but the velocity at which it moves through the port is so slow you actually lose power?

Chad S. Doesn't call it V factor for nothing...lol. I'm not a head expert. Few are. But yes flow without velocity can hurt.

When using racing gas, i 100% agree with this statement. But switch to alky and it will usually fly.

quote:

jerry

you seam to have a good idea of what's going on but a few words of caution...

Port volumes - need to use this with the same style heads, and it don't tell nearly enough info

Porting a head doesn't mean the runners are made larger and therefore air speed slows down and in fact it can create air speeds that are too high which weren't un-ported and hurt HP, everyone thinks porting is making a runner bigger,,, and if you just consider port volume then yes, but this is not always true with regards to cross sectional area through out the entire length of the port! Some areas are just cleaned up to give it the PORTED/CNC'ed look, if it was left alone everyone who purchased a head like this would scream bloody murder! They see with there eyes, air and fuel have no eyes!

Thanks Top, appreciate the input, but for me the definition of porting is shaping. Just to put it out there, you could have two different runners on the same casting, of the same CC, but end up with two totally different shapes and results. Kind of what I was eluding to on the characteristics of the finished port features (taper, window size, radius etc) and largely the difference between the example I mentioned with hand porting and CNC on the same casting.

329L - You are correct on the Alky comment for sure, largely a product of 2x volume of fuel….obviously it needs the space for the proportioned ratio increase. The features of the finished configuration can still make a difference in power from one concept to anther.

quote:

Originally posted by Jerry Kathe:

quote:

jerry

you seam to have a good idea of what's going on but a few words of caution...

Port volumes - need to use this with the same style heads, and it don't tell nearly enough info

Porting a head doesn't mean the runners are made larger and therefore air speed slows down and in fact it can create air speeds that are too high which weren't un-ported and hurt HP, everyone thinks porting is making a runner bigger,,, and if you just consider port volume then yes, but this is not always true with regards to cross sectional area through out the entire length of the port! Some areas are just cleaned up to give it the PORTED/CNC'ed look, if it was left alone everyone who purchased a head like this would scream bloody murder! They see with there eyes, air and fuel have no eyes!

Thanks Top, appreciate the input, but for me the definition of porting is shaping. Just to put it out there, you could have two different runners on the same casting, of the same CC, but end up with two totally different shapes and results. Kind of what I was eluding to on the characteristics of the finished port features (taper, window size, radius etc) and largely the difference between the example I mentioned with hand porting and CNC on the same casting.

329L - You are correct on the Alky comment for sure, largely a product of 2x volume of fuel….obviously it needs the space for the proportioned ratio increase. The features of the finished configuration can still make a difference in power from one concept to anther.

Jerry

Exactly why I put port volume on the bottom of the list! (Same thing for IC locations on a 4 link setup!) What your looking for is a port shape the creates an air velocity profile that is evenly distributed as possible meaning air speed in the middle is equal to, or as close as possible to that on the top and bottom. With regards to air moving from the plenum to the valve, you don't want large changes in velocity in a short distance due to shape or size, air can speed up and slow down faster than fuel, so this issue will cause fuel to drop out of suspension.

As you stated, CNC is just a machine porting a head using a digitize hand ported runner. Some associate CNC with making more power than hand ported which is not necessarily the case.

One other point that is often overlook, everyone seams to focus on heads and flow numbers, or a cams or carb's etc,,, what make a good running motor is the entire combination. When you match all the parts to work together, you got a great running motor.

What you get with CNC is speed and consistency. Of the final product, head in this case, not meaning the car. I have a room full of them running every workday.

/\
Biblical!

Your 4 link comment intrigues me though!

lol

Cylinder Heads are all about average velocity. TOP38 is dead on. You need goals before you do anything. First question I ask is what CID? How hard do you want to turn it? And how much power?

A good as-cast head will be well balanced. Like flows proper for available area. Like a tight velocity graidiant front to back.

You can do far more with much less if the shape can handle the localized velocity. So at the end of the day, shape is King.

Look at what your typical Super Stock engines make. A typical 358 with 172cc heads make 650+.

Conventional style heads now days are making crazy power and running crazy ET's. Lots has happened in our hobby over the years.

quote:

Cylinder Heads are all about average velocity. TOP38 is dead on. You need goals before you do anything. First question I ask is what CID? How hard do you want to turn it? And how much power?

Yes he is. (hence, "biblical")

Kinda the way I see it, the topic inputs in general, by all, have been dead on.

Each comment seems to get into the “technical weeds” a little deeper each time, not sure if we have centered on the general question of the OP though.

Core to that story is; CNC doesn’t mean better.

Pretty sure if you digest all of those prior comments for what was said, and not the details left out, it kind of went down the potato – patato path….

To ad lib to the Chad Speier comment, you have to tie CID and mean RPM into volume required to achieve the correct mean velocity for the given CFM….its not that he did or didn’t say it, but it also wasn’t stated that way but it is extremely relevant when you travel down to the level of detail. We may “assume” that its understood a given "average velocity" will change with the volume change required during engine acceleration, but it doesn't. Acceleration is "king" - the SS analogy used by Chad is a prime examples of that.

I think I said it prior: “you cant paint a clear picture with a few technical insights on a chat room post, if you could it wouldn’t take the class room hours to teach or learn”.


Disclaimer: I don’t do cylinder heads, I do package engines for myself and a few buddies and want them to perform their best when needed….so I do get into the weeds on the physics side to attempt the best choices possible, after all – who doesn’t want results that perform better than the “big box” stores on a “apple to apple” basis……kinda gratifying isn’t it?

This message has been edited. Last edited by: Jerry Kathe,

Most Valve jobs are CRAP out of the box from most Mfr. I would take a properly sized cast port with a proper valve job over a CNC port with a factory valve job.

Then again most mfr. standard CNC ports are MEh as well.

I would rather buy my heads from a talented/experienced/proven specialists and get exactly what I need and done right.

Jason G.