subscribing. have wondered the same thing. also the intake being used would have a big part in all this correct?

 

The average velocity must match your intended RPM.

In that thread, I mentioned that would be a good head for a low engine speed, which was exactly what it was going on.

Now I’m a big believer that the velocity profile vs. the port volume vs. the air moved, is key to a efficient port that will perform as it should. CFM isn’t the first thing to use in choosing a cylinder head, and it’s not tops on the list, but it is critical. When it comes to cylinder heads, port shape, localized velocity, average velocity and then CFM are in the order of importance.

 

The average velocity must match your intended RPM.

In that thread, I mentioned that would be a good head for a low engine speed, which was exactly what it was going on.

Now I’m a big believer that the velocity profile vs. the port volume vs. the air moved, is key to a efficient port that will perform as it should. CFM isn’t the first thing to use in choosing a cylinder head, and it’s not tops on the list, but it is critical. When it comes to cylinder heads, port shape, localized velocity, average velocity and then CFM are in the order of importance.

Okay, thanks.

 

After buying a set of heads last year and doing some clean up on them, having them flowed and lurking on this site I have a few questions. Recently in the cylinder head show and tell thread Scott Foxwell showed some work on some heads he was working for a blown pump gas 588. When asked about the runner size he originally guessed about 320-325cc, Chad Speier commented that the runners seemed way small to him. Later Scott replied they actually poured 335cc.

Now this makes me question cfm and velocity. I assume and have heard that you can actually have too much velocity and get a dry center with the fuel tending to go to the outside which causes a non uniform air/fuel mixture in the cylinder is this true?

For example say we have something like a fully cnc’d AFR 290cc head and say a larger 320cc runner head both flowing identical numbers in the cam lift range to be used. And they were going on something like a 572 and the owner wanted to turn at least 6500rpm but stay with a hydraulic roller cam. Would the smaller higher velocity head be better or could it possibly have too much velocity in an application like this?

Just asking a question and I guess make the assumption that the cam would be tailored for each head.

Do both heads have the same port center line length?

Stan

 

Do both heads have the same port center line length?

Stan

Hypothetical just trying to increase my understanding.

 

The cylinder head guy Johnny, goes by CFMCNC on here told me an interesting thing when I was talking to him and I believe it. He said the cfm number is not the thing that is most important, port shape, velocities, runner taper, ST shape, and VJ is more important then the cfm number, BUT when all of those factors are correct the cfm number is usually there.

This made so much sense to me and I think alot of the cylinder head guys on here will agree. Chad is the last person to discuss cfm numbers and always like to compare his heads based on cross section and design on the port, but his cfm numbers are still up there with the best of the heads. It sucks that the industry has dumbed cylinder head rating down to a simple number when it is much more complex then that.

 

The cylinder head guy Johnny, goes by CFMCNC on here told me an interesting thing when I was talking to him and I believe it. He said the cfm number is not the thing that is most important, port shape, velocities, runner taper, ST shape, and VJ is more important then the cfm number, BUT when all of those factors are correct the cfm number is usually there.

This made so much sense to me and I think alot of the cylinder head guys on here will agree. Chad is the last person to discuss cfm numbers and always like to compare his heads based on cross section and design on the port, but his cfm numbers are still up there with the best of the heads. It sucks that the industry has dumbed cylinder head rating down to a simple number when it is much more complex then that.

It does suck...It's a reason why most people pick a cylinder head based soley on flow also. To beginners, an engine is an air pump so they see "more" CFMs and pick that one. Sucks that marketing did that...

 

I think more people are seduced by cc's than CFM these days. Have a look at how many heads are marketed by the cc size, such as one brand I will not mention for fear of the "ruin your head thread" gang dropping in. How many time do you see the phrase "you cant put a BIG enough head on that engine"
I would have thought that shape, cfm and airspeed were higher on the list than runner size? Wouldnt a badly designed big runner perform worse than a well designed small runner?

 

When Ive used those words in the past.... Its been with cross section in mind.

Imo, when the port is designed right.... Theres a direct corelation between the mcsa of the port and how many cc's it pours.

Of course the kicker to that whole thing.... Is the port being designed right.

 

The correct way to label head sizes should be by MCSA,only Chad does that in his marketing,It is easier to label heads by port volume but not the ultimate criteria.All of our heads are designed with the manifold crossections and runner length included.Maybe Johnny will log on later and comment.Bill C.

 

You can only get so much air through a X.XX" size hole. It's how well it get's there for the intended use.

 

what we have learned in the superstock world, is..................................how quickly you can get that air in there and the shape of the port. But we are regulated to a certain runner volume. I've seen a car run faster with the same runner volumes but different port shapes..........

 

what we have learned in the superstock world, is..................................how quickly you can get that air in there and the shape of the port. But we are regulated to a certain runner volume. I've seen a car run faster with the same runner volumes but different port shapes..........

Very true statement right there. You can have equal volume and two entirely different ports. The architecture of the head has a lot to do with how a port is designed.

 

I work strictly off velocity, then check the flow. I know by the flow if the port is working well. I also know by the sound.

 

After buying a set of heads last year and doing some clean up on them, having them flowed and lurking on this site I have a few questions. Recently in the cylinder head show and tell thread Scott Foxwell showed some work on some heads he was working for a blown pump gas 588. When asked about the runner size he originally guessed about 320-325cc, Chad Speier commented that the runners seemed way small to him. Later Scott replied they actually poured 335cc.

Now this makes me question cfm and velocity. I assume and have heard that you can actually have too much velocity and get a dry center with the fuel tending to go to the outside which causes a non uniform air/fuel mixture in the cylinder is this true?

For example say we have something like a fully cnc’d AFR 290cc head and say a larger 320cc runner head both flowing identical numbers in the cam lift range to be used. And they were going on something like a 572 and the owner wanted to turn at least 6500rpm but stay with a hydraulic roller cam. Would the smaller higher velocity head be better or could it possibly have too much velocity in an application like this?

Just asking a question and I guess make the assumption that the cam would be tailored for each head.

Chad made that statement without knowing what the application was. If that head was going on a 427, it would be huge x large. For a 420cfm port it's close to an AFR 335cnc, but with a smaller area at the p/r.

 

I have a good example of why shape and velocity are KING. The Ford Brodix v2.50/SBF head. Moves 325cfm, 2.45in at the pushrod, and 225cc. However, IMO, the velocity profile is as good or better than any 20 degree Ford head I've had on my bench. It peaked at 7100 rpm on a 427 cid and made 680hp with a 260ish cam and pump gas, with a port matched manifold.

 

how do they compare to the your SBC V2.50/23DEG ?? if both have the same measurement at the pinch but the port shape and valve angle are different

 

I didnt read the replies but if it hasnt been said. Anytime you can make your runners smaller and not lose flow it will make more power.

 

I don't agree with this..

 

A little off topic, but if you have a head with a 2.3 valve, can you go back to a 2.25? Or are you just stuck with the larger diameter becasue the head was cut out for it?

 

i,m sure one could install new seats if it,s worth the work


on a another subject while surfing for heads i noticed some post cfm ##,s from two different flow benches with a difference in ##,s

 

Not true.
Correct min CS for the CID @ RPM is key. Then try and make that CS as efficient as possible.(most air flow for the desired CS) JMO.

Absolutely !!!!! That's the name of the game. We know going into designing or "correcting" something for a given CID...give it the area if you can fit it... then try to make it move the most air possible. I'm a big fan of AREA...not port volume because it's just a relative number . But...I'm also not proponent of the smaller higher D/C valve theory...up to a point yes....but at some point in RPM , you simply need more valve...regardless of the flow number. It's CID and RPM based...just like John said.

Shawn...you listened to something I said...what's wrong with you?

Cheers , Johnny

 

True dat! I also base everything off my pitot tube rather than a formula. A formula will get you a big port. The localized velocity with a balanced pitot tube, isn't lying to you.

 

Everybody seems to have their own favorite area that they believe shows how good the port is. I believe that a great port is like a perfect storm all of the factors must come together.

Stan

 

Everybody seems to have their own favorite area that they believe shows how good the port is. I believe that a great port is like a perfect storm all of the factors must come together.

Stan

:-D

 

If I am getting this right I realize we are working around head bolts and push rods ect and confines of an engine compartment. But if you have a 2.02" valve and a 2.15 valve their ideal cross section you would have 2 different starting points and finishing points right? Do you go from 90-91% of the valve for the throat and taper up 2-5% from there? With the length and size factored for size of motor and RPM range? seems like the bowl is larger is that to straighten up the air flow for the turn and making up for valve stem in the way? I see minimum cross section size and see intake opening cross section size and interesting to see correlation.

 

if you think about it this way, volume is length x width x height, the only way cylinder heads are apples to apples is with a fixed straigth length & adjusting the hieht and width. I am certainly no cylinder head guy, but I have been a piping engineer for over 20 years. I can get you the same volume of 1' of 12" pipe with a shit ton of 2" pipe, but adding the 2" pipe to get the volume back is taking away flow. Or, I could give you 1' of 12" pipe and a length of 14" pipe that would look huge by volume and minimum cross section, but be a bigger restriction due to its legnth. I would be interested in what criteria is used to say x cfm is good for x HP at x rpm. I would think it has to different for each type of cylinder head and manufacturer. In the SBF world, you have people that would put a 245 high port on anything fighting with people claiming 8's with a turbo & an AFR 185. As soon as you go BBC, a 290 is a small head, but may only be going on a 396.

 

In my experience almost all heads I have seen that are the same size for the family of heads and are shaped correctly flow almost the same. In other words most of the correct shapped 335cc heads with a 45degree valve job flows about the same. Their are some exceptions when the chambers are messed up.

I also think the chambers are the most overlooked thing. All these forums talk about the runner then ignore the chamber. I have a head in here right now that has an excellent port shape and sizing is good then has one of the worst chambers I have seen and does not move any air.

 

In my experience almost all heads I have seen that are the same size for the family of heads and are shaped correctly flow almost the same. In other words most of the correct shapped 335cc heads with a 45degree valve job flows about the same. Their are some exceptions when the chambers are messed up.

I also think the chambers are the most overlooked thing. All these forums talk about the runner then ignore the chamber. I have a head in here right now that has an excellent port shape and sizing is good then has one of the worst chambers I have seen and does not move any air.

Just because they're not talked about on the internet doesn't mean they're overlooked. Certainly not the case in my shop. Maybe that's why they're not talked about.

 

The longer the port, the less average CSA, the more average velocity, with the same amount of air moved, the same poured cc's.

 

Everyone wants to know the answer to these simple questions about air speed. Although the question itself is simple, the answer is not. Head porters will try and explain this complex subject matter in layman’s terms or with quick concise to the point answers formulated to give insight into, but not quite answer the whole question. Why is this, because its not cut and dried and it FARRRRR from easy to explain. To explain questions on velocity, mixture homogeneous, average air speed, instantaneous air speeds and air speed profiles would take two days or more and if I had to write it out, it would be at least 200 pages and probably 300 pages. I love it when people are interested in this and I love to talk about it but its far from easy and that is why it gets bantered around so much in the threads. The engines induction system air speed not only depends on how much the heads flow or how much the engine is asking for but what the engine is used for and the power band the engine will be operated in. Plus about 20 other things I can think of. Air speeds are induction system design and engine specific. There is not one situation that works in all cases. That’s why I spend an hour or two on the phone with customers who wants and induction system. I must ask them a LOT of questions to hone in on what they need. (NOT what they want) Questions like

1. Engine cid
2. Type of vehicle and vehicle weight
3. Engines intended purpose ( type of class racing) (VERY important because this has profound effects on over all head and manifold design)
4. Intended /expected operational rpm range
5. Rules regarding intake manifold design
6. Rules regarding cylinder head design
7. Expected TBO
8. Type of block “Bore limitations”
9. Rules regarding compression, carb size, restrictions, trans, gearing, and many other things,,,
10. Type of fuel mandated or fuel customer wants to use
11. Power adder? Type, boost levels, inter-cooled, amount of nitrous. Long list here:
12. Engines operational temperature, under hood temps,,,, ( can have big effects on over all design)
13. Carbureted, EFI, EFI injector placement, Mechanically injected, (then refer to fuel type which changes things again)!
14. And a couple of other things I am forgetting

At times I can go around and round with these questions before I can hone in on what the customer needs. They forget things, omit things or most of the time, just don’t know. Many times they are stuck with a combination that for some reason ( usually financial) they cant change a whole lot and in those cases you have try and manipulate a series of bad choices on the part of the customer into a half decent running engine. I find this to be the case about 50% of the time and in those cases, the air speeds can be wildly different than what you would expect in a well thought out combination..

So, there is no one size fits all answer only generally accepted maximum and minimum values to throw out there for people to digest and that’s what we head guys generally do when asked about air speed.

NOW, Lets get to your question and see if I can adequately answer it without writing a book.

“When asked about the runner size he originally guessed about 320-325cc, Chad Speier commented that the runners seemed way small to him. Later Scott replied they actually poured 335cc.”

First off, CCs or port volume measurements are “Close guesses” to a ports average air speed only. It gives no insight what so ever into the ports velocity profile. The reason for this is simple. There is 100 different ways to achieve the same over all volume and yet, have completely different peak velocity measurements, air speed recovery measurements and port shape. CCs came about in the 60s when the heads where to small so the larger the port, the more power one could make. Now days, its useless with one exception and I wont get off into that now because its an entirely different subject matter. Head porters still use CCs as a quick dirty guess as the average air speed in the port, that’s all.

“Now this makes me question cfm and velocity. I assume and have heard that you can actually have too much velocity and get a dry center with the fuel tending to go to the outside which causes a non uniform air/fuel mixture in the cylinder is this true?”

No, the parts about the fuel is not only untrue but totally false on its face. I don’t know where you heard this but its junk science and does not happen. If the air speed is low the fuel falls out of suspension and usually hit the floor or back walls of the port itself. To put it simply, When air speed drops so does the fuel.

Now I will say this. If the air speed is to fast the engine simply stops making more power and will hang for a while then drop off rapidly while showing huge increase in BSFC numbers while this is going on. This is referred to as a Sub-Sonic choke and is nothing more than the pumping losses taking over. There are limits to everything. As you increase air speed the cylinder fill (Volumetric Efficiency) will increase somewhat proportionally up to the point where more energy is used to accelerate the air than you get in cylinder fill and at that point power will hang for a while then fall off showing a huge increase in BSFC numbers in a short time which is an indicator of decreased in engine efficiency.

I hope this answers your question.

 

Darin have you written any books on porting ??

 

He offers a class at RM a couple times a year.Bill C.

 

Great Topic fellas!

 

Great topic Loren,

In the next few months I plan on starting two engine builds:

1st will be a 488" ( 4.42 x 3.98 ) 14:1, A-headed engine power to 7,400 ( have most of the parts except cam and pistons...untouched 340cc A-460's are sitting on the shelf)

2nd will be a 600" 11:1, pump gas, A-headed engine power to 7,000.

It will be interesting as to the difference in the heads for the two engines.

I'm going to be looking for a Cylinder head guy........anyone have any ideas ...you can send me a pm if you have interest.

G

 

Great topic Loren,

In the next few months I plan on starting two engine builds:

1st will be a 488" ( 4.42 x 3.98 ) 14:1, A-headed engine power to 7,400 ( have most of the parts except cam and pistons...untouched 340cc A-460's are sitting on the shelf)

2nd will be a 600" 11:1, pump gas, A-headed engine power to 7,000.

It will be interesting as to the difference in the heads for the two engines.

I'm going to be looking for a Cylinder head guy........anyone have any ideas ...you can send me a pm if you have interest.

G

Thanks, it's a getting bit deep for me at this point and I find myself rereading and researching even more. lol

 

Good read and same here Loren!!!

MikeyfromBC, They have some items you can purchase over on speedtalk.com.. I personally haven't but would like to here in the near future..

 

I always think of the chamber as part of the port itself, the even pressure recovery in the chamber is a very important part of the port and has a lot to do with the other thread on valve seat angles.