OldSStroker

Great post, Lloyd.

I can't imagine how many times Lloyd has taken the time on the phone or emails to explain this to folks. Not only is he an excellent head porter, he goes out of his way to help/educate us on heads.

Moderators, this one should be a sticky!

jkipp84

I second the motion.

jerrysta

I feel so much better after reading this now because my engine builder pretty much said the exact same thing you said in the original post.

Thanks for the very informative post.

NightTrain66

I offer $25 flow tests and I get to see TONS of heads that people bring by. This is more of a customer service than a money maker since you take 30 minutes out of your day to meet the people, disassemble the head, clean the head enough to go on the flow bench, set up valve fixture and then spend about 1 minute flow testing the head, LOL. In doing so, I get to see ALOT of heads. I spend a good bit of time looking over heads that come from cars that are REALLY flying and make good power. I measure/inspect everything I can and after a while you see they have alot in common.

They are getting certain measurements at the minimum cross sectional area, venturi diameter, valve job angles/widths, chamber shape, cross section over the short side, how far the short side is laid back, etc, etc, etc. The heads that really WORK have a different sound on the flow bench like the air is moving effortlessly through the port and no matter what depression you raise the bench to, it never goes turbulent.

Obviously you CAN get a port too big but I doubt if you could ever get an LT1 casting too big for a 6200 RPM 350 and definitely not too big for 6500-6800 RPM 350 or ANY 383. You can have it shaped wrong or the wronmg measurements in key areas that allow the head to not perform well but I doubt if you could ever get it too big.

If these are the two heads to chose from . . . .

180 cc
1.700" venturi diameter
1.800" cross sectional area

200 cc
1.800" venturi diameter
2.100" sross sectional area

If they flow the same at 28", just crank the depression up to 48" and see what happens. Use a velocity probe at the pushrod pinch, shortside and venturi/valve seat. Once you install the smaller head, the air will be moving TOO FAST to make stick to all of the walls and make the turn at the short side to go through the valve seat and into the cylinders and stay stuck to the walls.

There is ALOT more to it than that but at least this explains things a lil better to the "average" person on the board.

Obviously a 6200 RPM 350 is a mild set up and I would probably shape things a lil different than I would for the "average" customer running 230+ duration and turning 6500-6600 RPM or more cubes.

Lloyd

MyShibbyZ28

This should really be a sticky! A ton of informative stuff here.

SStrokerAce

FWIW, Pro Stock guys are working in a situation already where the heads are already pretty dam close to perfect. So if they can get a reduction in area and the same flow it MIGHT help them make more average power.

A Pro Stock head in comparison to the ones on your car, are highly developed out of a casting that most times doesn't come with valve seats, valve guides or even spark plug holes. These guys place the valves at the angle they want, the spark plug where they want and can put the head port in where they want with what shape they want.

This all allows them to get a head with a relatively small area move air very smoothly yet still mover the same volume of air as a bigger area/volume head. As Lloyd mentioned velocity can only be so much before it makes the port turbulent and not fill the cylinder as efficently. Street motors as he said are normally in the 85-90% (Volumetric Efficentcy) range while a Pro Stock motor is in the 125% VE range. A good street motor can do 100-115% VE, but that means everything is working together to do this. The higher the VE the more air that's filling the cylinders and higher velocities overall in the port. The specially designed Pro Stock heads can handle higher velocities with less problems than a LT1 or LS1 head casting can. People will talk about the MACH level of the port, which is relative to the speed of sound. A typical street head casting if it gets velocities over .50-.55 MACH then it's going to loose power, but a Pro Stock head can work in the .60 MACH range. Most likely what the Pro Stock guys are seeing is a port with the same cross sectional area as what works now but it can handle higher velocities and produce more power. That's the hard part to get the more velocity to make more power, a lot of times it doesn't.

Bret

stonebreaker

What Lloyd is talking about, velocity getting too high and causing turbulence, is the Reynolds number of the port.

Warren Johnson's statement can be better understood if you look at the parameters for calculating the Reynolds number in the above link. It's somewhat misleading on its face since most folks measure port flow by the volume of airflow, whereas Warren appears to be talking more about mass. If all the variables in the Reynolds number calculation stay constant (fluid density in particular) except for port velocity (which is pretty much what he said), then the higher velocity of the smaller port means the port has a higher critical reynolds number and is therefore more efficient. Taking a WAG here, I'd say that a higher Reynolds number probably means the port is less likely to develop areas of turbulence under actual running conditions.

Reynolds number calculator

Most of this is just a SWAG, since I don't have access to Warren Johnson's head porter. But it makes sense.

SStrokerAce

Good post!

Mass flow, would be dependant on temp, which in any situation a lower intake port temp would increase TQ. A denser charge would have more O2 and therefore more mass.

Add on Injuneers quote on this...

To really read into what Warren is saying, I think you need to look at little deeper. What they are doing is working in a situation where if they flow the same volume at a smaller cross section they need to be able to do it at the same Reynolds number to get the same quality of flow. What really counts is the dynamic condition, which is what Lloyd is talking about here. How the airflow works thru the heads while it's running on the motor.

Compare what you said to this portion of your link....

The Reynolds number is the ratio of inertial forces (vsρ) to viscous forces (μ/L) and is used for determining whether a flow will be laminar or turbulent. Laminar flow occurs at low Reynolds numbers, where viscous forces are dominant, and is characterized by smooth, constant fluid motion, while turbulent flow, on the other hand, occurs at high Reynolds numbers and is dominated by inertial forces, producing random eddies, vortices and other flow fluctuations.

What would be nice is to know the mean fluid velocity of air. Or air/fuel but then again that ratio is all dependant on how well the mixture stays atomized so we would need to know the BSFC to get it exactly right with a particular example.

I would imagine that the lower the Reynolds number the better since what we are really looking for is the most Laminar flow in the ports.

Bret

1racerdude

It looks like what I have posted in the past about Mach numbers and "ya can't put to much head on it" is true.
That being said I don't mean put a 400+CFM head on a 283 and put a stupid cam in it then complain "it's got no bottom end". Well that engines bottom in STARTS at 5000RPM's and the top is 10,000+RPM's
Most on this board don't have the money for 400CFM heads anyway and there are a couple of castings that will do it.

Given the common available castings and the sizes they can go to and the flow they can get from them "Ya can't put to much head on it" But ya can put a stupid size cam in it and wind up in the same boat as all the tales that are said to be caused by "to big"of a head.
IMO WJ is talking about the smallest port that will put the air in to make the HP he is looking for. That is an efficient port that flows xxx rather than a large port that flows XXX amount of air. I don't think he was talking anything like a 200CC port as most of these guys think.
Another thing most of the board compares heads with intake runner CC and ya can't do that because of different lengths. A high port head has more CC's than a stock LT1 head because of the longer length. The high port's and a LT1 with the same cross section will flow close to each other but the high port has more CC's. Thus everybody wants to say that head has to big of a port for my "350". Wrong answer,It has longer but not to big.

12SCNDZ

Lloyd,
What's your opinion on valve size? My heads are "race ported", and flow 271 CFM @ .550" with 2.0" - 1.56" Manley "Pro-Flos". The heads worked great on my stock bottom end /Cc305 combination. The new combo is a 6" rod 12:1 355 with an 847 cam. Are the 2.0" - 1.56" valves adequate? I've had several people tell me not to go to 2.02" - 1.60" valves in the LT1 castings because the valves will be shrouded because of the small combustion chambers.


Thanks,
Frank

TurboSS

If you haven't actually flowed heads that go turbulant let me tell you it is a very noticable event on the flow bench. The sound of the air entering and exiting the head changes dramatically.

Q. How does positive pressure above atmospheric effect laminar air flow vs air flow at or below atmospheric?

AKA. turbo or blown motor vs NA.

Seems to me that pressure would tend to keep things a little more inline.

TAD

96capricemgr

I definetely respect Lloyds knowledge and he is someone I have pointed a lot of guys to when asking about head porting BUT you all have to at the same time consider he makes his living at thins and has his opinions on what works well. Others will get good results going another direction with their theories on what is best that does not make either shop wrong just means they go about a given goal in a differnt manner.

I has already been established that flow is just a tool not a real measurement of how a head will perform let us take that a step further and realize a valve is open for a fraction of a second at a time not held steadily open like a flow bench tests them. Which is just one more reason flow doesn't tell the whole story. It is actually the open and close along with exhaust blowdown which allows the VE over 100% on well built COMBINATIONS.

SStrokerAce

12SCNDZ,

In terms of valve size. #1 no point of going to a 2.02 valve on a stock seat, and if you have to put in new seats might as well go to a 2.055 valve. On a 355 the 2.00" valve will work well enough if you have enough lift for the RPM you want to run. As far as the chamber is concerned I've heard it from a few people that the LT style small chamber is the way to go for a 23° head.


TurboSS,

Actually some of the big turbo or blown guys like a nice lazy head port, either way the port should behave in a boosted motor, but yes it makes less difference since the motor has other means of getting air to the cylinder other than the low pressure in the cylinder.

96capricemgr,

Actually I would say about any shop would go the direction Lloyd is talking about. He's not alone in this "theory", which BTW is not just a thought on how it works, it is how it works.

BTW high VE's are achived thru the use or intake and exhaust tuning in concert with the camshaft timing to get the motor to work its best in a given RPM range. It's the combination but the tuining effects of the intake and exhaust are what makes the combination make more power.

Bret

1racerdude

If ya have a 350CFM head on a 350CID engine ya don't need all that pull through from the exhaust to get the intake charge in. The exhaust is going to "blowdown" from the nature of the internal combustion engine. A farm tractor get's it done and so does my lawn mower and they are both "stock".Can't tell ya what the VE is but they run good.
Just like I have been saying big heads=smaller cam for a given HP level.

IllusionalTA

I spent about a hour on the phone w/ dennis and airflow development in regard's to my RR 245 All pro's that are going on my 396... he stressed the port shape is going to be critical to get me what i want out of my head.. I am by no mean's building a budget head and probably would have been better off to go w/ a 15 deg setup.... I like post's like this that talk about the dynamic's of head porting.. good info..

STICKY THIS!!!