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Discussion Starter #1
hi,
there seems to be a lot of questions about how to select a cylinder head for a given application. I'll give you an idea of how/why i choose a given head for a given application. One thing to keep in mind is that nothing is ever perfect. People who have dedicated there entire lives to cylinder head developement still change there mind daily about what is best and most important. The information presented is mostly gathered from peers in the industry and can be used as a general guideline. As a whole these numbers are generally agreed upon, but these are not rules written in stone.
First things i want to know are these-
budget
cubic inch
budget
rpm
budget
intended application
and did i mention how much you have to spend?
It will never cease to amaze me how many people want an 8000 rpm small block and have an old set of "camel humps" and $250 to spend. Some things just aren't doable.
When i have that information there are some formula's that i use to help me determine where i want to start. I have a pretty good data base that allows me to choose a head based on the cross sectional area available. This minimum cross section will determine when the head will "shut off" or fail to make decent power past a certain rpm level. I'll use a typical engine for an example. We'll build a hypothetical 500hp 350 engine. We'll use 7200 rpm as our target rpm. We will also use a standard 4.00x3.48 bore/stroke. Using this rpm level and bore/stroke this is the formula i would use to get a baseline minimum cross section needed-

(bore x bore x stroke x rpm x .00353)/614

Using this formula and our numbers we can determine that we need a min. cross-sec of about 2.30 square inches. This will allow our motor to turn 7200 rpm without exceding 614fps or .55 mach (the same thing). That number is generally considered to be the point at which most "conventional" type cylinder heads will reach a point of choke. A modern pro-stock style head moves that number up a bit, because of port efficency. A flat head wouldn't even come close to that number. If you put numbers in for a typical 400" small block you would see that it would take about 2.64"sq to make power to the same rpm level.Quite a bit larger. If you go the other direction, that same head with 2.3" on a 302" engine would put your rpm level up around 8300 rpm! Makes it easy to see why a smaller motor will "rpm", huh? On a conventional aftermarket small block chev head, the pushrod pinch area represents the smallest cross section. Normally 1.050" is about all you will safely get at the width of the port. This means we need about 2.2" of height in the same spot. This would probably put you into the 210-220cc volume for a typical head. Now it starts to get a bit tougher. When you are porting a cylinder head localized velocities are what will make or break a head. When i say localized velocities, i mean the actual measured air speeds when flowing it on a bench. These are not to be confused with the air speeds generated by rpm/cubic inch/cross-sec. that we just talked about. These are numbers that can be figured using a formula with measured airflow and cross-sec or by using a pitot. A pitot is a steel tube with a small hole in it that will measure pressure differential when put into the airstream. They look like this-
http://www.superflow.com/flowbenches/index_1017.cfm
We use these to figure out how fast the air is moving through various parts of the port. I'll take and stick the pitot in the port and measure speed at the top of the short turn, across it in three or four different spots and three or four different levels. I'll take other measurements in the same manner at different spots including the pushrod pinch, and opening of the port. I prefer to use this method of measurement to the "calculated" method because it is hard to get an accurate measurement of the cross section at the short turn, and in other spots, without making a port mold, cutting it and measuring it. Even then using the calculated method you will only get an average, not the true localized velocities. In most conventional heads, you don't want these velocities to excede 350fps, as measured. So if i probe the port and find that at .500 lift my short turn has speeds that are 450fps, i need to do something to that port to change that. Anything that is over my 350fps limit is going to create turbulence and separation of the air in the port. This is going to limit my ability to fill the cylinder as well as i can. This is where the time and energy spent by a good head porter separates themselves from the fluff and buff crowd. Knowing what to do in these instances is what makes a good head. The formula used to figure out flow bench air speed based on flow in cfm @ 28"h2o and cross sec. is this-

(cfm/cross sec) x 2.4
this will give you air speed at that point. If we use our example of 2.3" at our minimum cross sec you'll see that we would require about 335cfm @ 28"h20 before our head would have a flow problem at that point, at least based on our numbers anyway. This is why going in there and wacking that area larger isn't always the best thing to do and why having a huge opening at the gasket doesn't do anything but slow that air speed down. We want to approach that 350fps as best we can, to get maximum filling, without stepping over it.
I just "fixed" a set of heads on an engine. It had a problem with short turn airspeeds. The head flow well for what it was, but the short turn needed some work. I spent about 10 minutes in each port fixing the area. The head gained about 2 cfm of airflow. On the dyno it picked up 27hp and extended the usable power range of the head by about 400rpm. All with 1 1/2 hours worth of work. Pretty good gain for just paying attention to one small thing, huh?
If you pay attention to air speeds, localized velocities, and cross sectional areas you will end up with a head that is driveable and makes good power. I don't even flow a head for a "curve" until i have satisfied these needs. You need to be realistic with power expectations and rpm levels when you set out on your project. An 8000 rpm n/a 327" motor probably isn't going to be very streetable. Almost allways error to the side of small and you normally won't go wrong.
shawn
 

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Shawn, I want to use my combo as an example to see if I am calculating this correctly.

It's a 383, 4.030 bore, 3.75 stroke, opperates at 7000 RPM.

4.030 x 4.030 x 3.75 x 7000 x .00353/614 = 2.45 minimum cross section.

Correct so far?

Now, how do I find the cross section my AFR Race Ready 210cc heads? Do I have to measure the port, is it advertised in the specifications?

I'm trying to piece this information together, so bear with me.
 

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Discussion Starter #3 (Edited)
It looks like your math is correct. Your 210's are probably around 2.2". The thing to keep in mind is that most engines are only going to have a "usable" rpm band of about 1500 rpm. This means if you build an engine that you want your peeks in the 7000 rpm area you are going to put your torque peek around 5500 rpm. In a drag race application this would need a 5500 converter and cam/intake/compression/gear to go along with this combo. Depending on how tight the converter is, you would probably shift this combo around 7500 rpm. A good ported set of 210's on your 383, some compression a cam in the 270 @ ..050 area, good intake, should get you about 620-630hp. Just make sure you have plenty of converter.
Shawn
 

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Shawn is the RPM you mention above in the formula is that the MAX RPM you wanna spin the motor at or where you want peak power? Sorry if this is a stupid question:eek: :p
 

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Discussion Starter #5
This is a estimation of where the head will go into a choke situation. At this point it gets difficult to move more are through the engine. If you don't have the ability to effectively fill the cylinders at a higher rpm level, you stop making power. You can move that mark around a bit with intake/exhaust/cam tuning but in a n/a engine your curve will fall pretty fast after that point.
Shawn
 

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Discussion Starter #7
It looks pretty good to me. I skimmed through there and it looks like you have a converter in the 4500 rpm area? With one that tight putting larger head on the engine might make better dyno numbers but with the weight of the car and the conveter/gear it would probably be slower at the track. The head is allowing you to recover from the gear change better. If you want to go faster I would call a good converter company like A-1,Coan, or the like and have them spec you out one. It will be expensive, but I wouldn't be surprised if you picked up .40 or so.
Shawn
 

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I'm currently running a TCI group 2 competition converter and the stall range is 4400-4800 RPM.

Thanks for all of the great info Shawn!
 

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We'll use 7200 rpm as our target rpm. We will also use a standard 4.00x3.48 bore/stroke. Using this rpm level and bore/stroke this is the formula i would use to get a baseline minimum cross section needed-

(bore x bore x stroke x rpm x .00353)/614

Using this formula and our numbers we can determine that we need a min. cross-sec of about 2.30 square inches. This will allow our motor to turn 7200 rpm without exceding 614fps or .55 mach (the same thing).
Shawn, Can you tell me where you got this formula and how it works?

The bore x bore x stroke is part of the equation for CID but what's the .00353 number?

The 614 number your 614 fps max?

How do you convert cubic inches into square inches and how is revolutions per minute resolved with feet per second?

I'm just curious. I played around with it and say a 283 at 5,000 rpm needs 1 square inch of cross section? That's pretty small. Even at 6,000 rpm it results 1.22 square inches. I think an unported 283 head has 1.5 square inches?

That means by your logic that the cross section makes it rev happy, then a 283 with a stock 1.5" cross section port, stock 1.72" intake valves, would or could make power (under load) to 7375 rpm.



Again, can you explain the formula?
 

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I can attest to 6900 with stock power packs and a big cam in a 283.

Kev
 

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OK, back in the late 60's I was given a Duntov cam that was lifted from the GM tech center in Warren. I used this in a stock 283, powerpac heads, solid lifters, and 2 barrel carb. I used to shift at 6500, 3.55 gear, go thru the lights at 6500 in second gear(three speed) ran 16 flat and 85 mph. Full size biscayne.
Frank(the terror of PP/S):D
 

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Discussion Starter #15
Shawn, Can you tell me where you got this formula and how it works?

The bore x bore x stroke is part of the equation for CID but what's the .00353 number?

The 614 number your 614 fps max?

How do you convert cubic inches into square inches and how is revolutions per minute resolved with feet per second?

I'm just curious. I played around with it and say a 283 at 5,000 rpm needs 1 square inch of cross section? That's pretty small. Even at 6,000 rpm it results 1.22 square inches. I think an unported 283 head has 1.5 square inches?

That means by your logic that the cross section makes it rev happy, then a 283 with a stock 1.5" cross section port, stock 1.72" intake valves, would or could make power (under load) to 7375 rpm.



Again, can you explain the formula?
Hi,
you sure you want know? Just kidding.

(Pi/4) = (3.141592654 / 4) = .785398163

360 * .785398163 = 282.7433388 or .003536777


the .00353 is just the 1/x reciprocal of 283.286119

its a combination of conversion constants into 1 constant = 282.286119
it converts inches into area and inches into feet
The 614 is the limiting port velocity for most applications. The number is actually 613.9758744, but that creates just a bit more math than I want to do, and 614 will get you close enough.
Yes, i would start there with a cross section of that size, for that application. For years the corps built cylinder heads that were waaaaaayyy to big for the application. GM wasn't as big a culprit as Ford. Have you ever seen a 302 BOSS head? Perfect example. Chev did have some like it too, though. The 283's were a bit oversized for their application, like you noted. The worst ones were the square port 396 engine. We fill those intake runners in any motor smaller than 500" and 7500rpm. Way to big.Like i said above, we unfortunatley don't work in a world of perfects. There will always be exceptions to the rule. I'm not certain that this works with some 4-valve engines or formula 1 type stuff. I don't have experience with them. But i would use it to start, if i did.
There are TONS of other things that i take into consideration when doing a cylinder head. These are some other ones-
Curtain area
Curtain area cfm/in2
valve cfm/in2
throat velocity
primary choke velocity
runner opening velocity
curtain area velocity
discharge coefficient
While i truthfully wouldn't spend a lot of time figuring these things out for a 600hp small block, it does show some of the things that are considered when doing an unlimited type engine.
shawn
 

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That's a pretty neat math trick, at least to the outside observer:)

The rods let loose in my 283 going down the highway at 2500 rpm lol. Sometimes you wear em down instead of going out revved to the moon.

Kev
 

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Hi,

Very cool info.. I really do not know anything about the math. If it wasn't for the mention of the 283 port vs rpm under power I would just be quiet and read.

I have seen many "stock" port 283 (really 292-294) cid super stock engines that exceed the 9000 rpm level. The ports are generally modified but the basic port is still generally the same. Years ago there were no mods on the ports at all and the rpms were comparable but the power was not equivalent to today's standards. This is usually with a 4gc carb, an aftermarket intake and a "huge" camshaft, high dollar valve train etc., but a very small port- 1.72 valve.

These cars really do amaze me in how quick they are. The same with the stockers.

I don't really have any opinion here, I guess I am just commenting.



Thanks
Jeff

Thanks

Jeff
 

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Discussion Starter #18
I have seen many "stock" port 283 (really 292-294) cid super stock engines that exceed the 9000 rpm level. The ports are generally modified but the basic port is still generally the same. Years ago there were no mods on the ports at all and the rpms were comparable but the power was not equivalent to today's standards. This is usually with a 4gc carb, an aftermarket intake and a "huge" camshaft, high dollar valve train etc., but a very small port- 1.72 valve.
Stock and Superstock heads are interesting creatures. A true "stock" eliminator head doesn't allow you to change the port at all, other than the valve job. A superstock head you can port all you want, as long as the finished runner volume meets the NHRA standard. I'll give you an example. An '041x chev head in superstock is limited to 165cc's. You can do just about anything you want in that port, as long as it is 165cc's when finished. A good '041x head will allow a 350ci engine to make peek power around 7200rpm. Here's the specs on a good running superstock, 041x engine-
280-286 @.050,.726-.726 on 106 installed 103
victor e intake
these engines have to maintain the "stock" compression ratio of about 11-1
1.94-1.50 valves
this engine would produce around 575hp and will run low 10's in a superstock car. It utilizes a 5500+ converter and is shifted at 8000+rpm. What we are doing is using cam and horsepower to accelerate the car. If we could find a way to move the torque peek higher, we would. The biggest single thing holding back the combo is the intake port. It simply cannot be made large enough to do what we are trying to accomplish. A smaller engine would allow you to do this, with the same cylinder head. This motor does quit making power at 7200rpm and coincides fairly well with the math format layed out before. The reason it's turned higher is to get it to accelerate off of the bottom of the gear change. With the cam, the torque curve is moved high, so in order to hit that point on the gear change, the engine is reved high, but it doesn't really make power up there. It just makes the car faster.
shawn
 

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Hi Shawn,

I basically agree with you. My only observation is that you still have a 165cc runner 1.94 valve on 360-362 cid engine. The compression at "only" 10.5-11.0 may or may not help the limited port application- I dont really know.

The 520t head on the 283 is really, I think a better example of what I was getting at. Its a very small port, small valve, and generally a very high rpm application. (super stock applications)

Most of the 520t head stockers I have been involved with dont see many Rpms (about 7000-7500 I guess) but they have only about .380-420 valve lift (the 67 has .390-.410).

The 302s and 350s with 041 style head (with 2.02 -1.6-ususally a 186-492 casting) and 455 lift cams will usually run in 8000-8500 rpm range with a stick- and thats a "non modified port" of 165 cc.

I guess thats my observation, a pretty small port can still produce an amazing amount of power at a pretty high rpm. Maybe a very narrow power band and very specific tuning, but still an amazing amount of power.

I dont doubt that the motor you were referring to was not making peak power at the max rpm but for a "low" compression engine its still pretty impressive to me. The 7200 figure seems pretty low to me, but Im not sure. When the porting first became legal most of the improvements I noticed were not in "size", but location. Raising the runners showed great gains, but the porting was very expensive and usually involved serious welding or furnace brazing.

This is very interesting. I enjoy the discussions here.
Thanks Shawn

Jeff
 

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Discussion Starter #20
They are pretty neat motors. A good friend of mine holds the A/SA national record, so I still get my fingers in some stock/superstock stuff on occasion, but I don't do much of that stuff anymore. I'm amazed that we are making 600+ with our non-ported vortec head circle track motors. Seems like just yesterday a good one was about 530hp, now that would get you lapped.
shawn
 
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