If a head is said to flow 170 CFM, does this mean that theoretically this is the maximum airflow at sea level per intake runner? So that a head that flowed 170 CFM would mean an 8 cylinder engine is capable of flowing a maximum of 170 x 8 = 1360 CFM? And if so, under what conditions?

Thanks

It means that a specific test vacuum measured usually between 24 to 28 inches of water pressure drop between atmospheric and the inside of the test cylinder that this is what the head will flow in air or mixture in cubic feet per minute. CFM is a computation from a formula not an actual measurement.

While in general terms this test does point to what a particular heads or port and valve combinations flow compared to others, it doesn't really have a lot in common with what the actual flow is in the real world. Air or mixture flow is a victim of many forces in an operating engine in pretty simplistic terms flow varies with RPM and cam profile selection, and is subject to many dynamic vagrancy's of pressure waves that move thru the engine.

So CFM from what essentially is a large vacuum cleaner while meaningful one head to another at test conditions really doesn't tell much about any given moment in the real world. If you put a high flowing head on a small cammed engine turning at slow RPMs you will be quite disappointed. Conversely if you put a low flowing head on a large cammed engine turning high RPMs you will also be disappointed. So a message here is that the head needs to have a flow that is reasonably compatible with the expectations of its use. The size of the engine also is a major player. A larger engine can use a higher flowing head, even when more mildly cammed than a smaller engine.

Bogie

 

...subject to many dynamic vagrancy's of pressure waves...Bogie

vagrancy = the crime of wandering about without employment or identifiable means of support

vagary = an unpredictable or erratic action, occurrence, course, or instance

(Sorry Bogie, couldn't let you get away with that one! :nono: )

 

Along with what Bogie said, you can tweak certain combinations with smaller or larger heads to get to a certain special result. As a for instance, if you wanted to build a 350 street motor with a very strong bottom end (tons of torque), you could use a cylinder head with intake ports that would flow a little less than a head you might use on the same motor to make max power in a, for instance, street/strip motor.

If you install a set of 230 CFM heads on a 350 street motor, the velocity of the air going through the intake ports will be so slow that cylinder filling with the intake valve open will suffer significantly. It has been determined that a 180/190 size intake runner is about right on a 350/355. But like I said, if you wanted to make a motor for a specific purpose, like a high torque street motor, you might choose heads with a 170 CFM intake runner such as this head....
http://www.summitracing.com/parts/RHS-12410-01/
These little jewels will make 435 ft/lbs of torque @3500 on a 9.0:1 350 with a VERY MILD cam (258/264), high-rise dual-plane intake manifold mounting a 600/650 carb and long tube headers. You could use a stock converter and stock gear and such a motor would run on cat-pee pump gas all day long. There is always a down-side though. In this case it is that the smaller head will "stall" at higher rpm's, reaching a point where the port will not pass any more air or make any more hp, whereas a head with larger runners will still be passing air and making power. This is one of the important questions that you want to ask yourself when you begin a motor build. Do I want a nice little street motor that will run on 87 octane and skin the rear tires down to the rims and make good power to 5000 or do I want to give up some bottom end, run more expensive gas and make power higher up in the rpm range?

 

You got to measure them somehow.

The intake runner, adds to, and effects the charge velocity.

 

You may be confusing head flow with intake volume. Aftermarket heads are usually sold in different port volumes. For example small block chevys typically have anywhere from 160 to 230 depending on the desired rpm, displacement and horsepower requirements.

 

Dougie is asking total flow

How I read the question is "what is the flow of a given engine based on the Cyl head flow calculations?". At some point
we all fell into the trap of CIxRPM= CFM and went for the big carb...knowing if the heads support the flow is critical just like exhaust.
To do that we need to understand the potential flow of the motor at a given rpm. Cyl heads are reported to flow X cfm at a given lift. Great information although we need to know if that number is per Cyl or for the combined (cfm x 4 cyl on that head) Cyl head and I believe it is for the head... Here is why.

If the Cyl head flows 350 cfm at a given lift and it was for the total then I presume that a V8 motor would have a maximum combined flow of 700cfm based on the limit from the heads... Now looking at it the other way 350cfm x 8 Cyl =2800cfm and that seems excessive for a naturally aspirated motor. This seems closer to reality since a 350ci va at 5000 rpm at 100% volumetric efficancy is 506cfm. The same motor at 7000 rpm needs 708cfm carb.
As mentioned earlier the manufacturer rates the cfm flow at 24-28 inches of water drop... That is a flow bench and you may flow more or less than that standard so this is a starting point although you should not be going for the Holley dominator when your calculated flow is 708 cfm.
Look... I'm just trying to be analytical but there may be something I am overlooking.
Cheers

 

Port air speed never approaches the speed of sound in any N/A engine, where do you get this data from?

 

My point is, airflow never exceeds the speed of sound in any engine.

0.5-0.6 Mach at sea level is the maximum I have ever heard quoted and that was on extremely developed race engines beyond the average guys ability to finance.

Also, what do you mean by this statement?

An engine pulls a hell of a lot harder on the carb than 28 inches of mercury

While it is true during engine braking from high rpm with closed throttle plates you can exceed 32 in/hg manifold depression , however an engine at wide open throttle never approaches that depression...mainly because the ports are open to atmosphere through the wide open throttle plates!

Manifold depression and airflow don't correlate this way, as a matter of fact a manifold depression of zero is required to flow the highest amount of air since that reflects no restriction to atmosphere.

I don't understand what you are trying to convey, its very confusing and makes no sense?

 

it tells you the potential an engine can make in power.

 

Where have you witnessed these airspeeds?

 

Where did this statement come from...."An engine pulls a hell of a lot harder on the carb than 28 inches of mercury"...???

Ports are tested at 28 inches of water depression which is a super long way from a depression of 28 inches of mercury.

Anybody, Bueller, anybody?

Bogie

 

So what your saying is calculated airspeed through a large displacement small port engine at some stratospheric rpm point will exceed the speed of sound?

 

The speed of sound is relative as well so explain the actual air speed please.

 

Exactly Bogie, advanced engines with nearly straight ports can approach .6 mach in the inlet ports for short periods of time, any disturbance to flow such as harmonics and entrained fluids can disrupt this transient flow causing choke.

Airflow in engine inlet ports do not approach the speed of sound. There are a thousand reasons why it cannot and entire industries are built on the handling of gasses that exploit this such as oxygen production etc.

Manipulating data from calculations to prove it happens is misleading since it does not happen in real life because enclosed ports choke at supersonic speeds without perfect laminar flow...its not happening in your hotrod, racecar or grandmas grocery getter with tiny valves.

There are many research papers available that show from analysis of models and actual engines designed that .6 mach is the very limit of what can be achieved and that exceeding .5 mach is detrimental.

Mach 1 or .8 or even .75 of the speed of sound isn't happening in your engine except for small transient peaks in localized areas near valve openings on highly developed engines beyond the average rodders reach.

 

.74 is mighty good and takes a lot of rocket science to get, this ain't gonna happen on anything like a set SBC production or even similar aftermarket heads. A set of Barnes maybe but try and find 'em or the Ford Indy engine's heads if you're whittling on an SBF. Both examples of an almost straight intake. The bends and twists of something that has to fit under a hood is much more limited. I think a corollary can be found in WW-II aircraft when you see the P38 having issues of local sonic shock development at speeds well below the total aircraft speed becoming anything close to the speed of sound leading to a loss of control and structural breakup or crashes finally being withdrawn from European service for those reasons. Then you see the P51 entering service to replace it. The P51 is certainly a faster plane with a cleaner design that delays the onset of local sonic flows to much higher speeds than the total aircraft could achieve. The control loss and structural break up problems just don't happen with its design.


This is also a place where the math breaks down, the cylinder doesn't get what the math says it can. The torque peak RPM is that point.


Bogie

 

Show us your math, you have made a mistake somewhere HC.

Do your math for a 1"X1.5" port (289 SBF) @7500 rpm, you can assume a 621cfm carb which is the calculated airflow requirement for that ci engine at that rpm. Assume a 1.78" intake valve which is stock and a cam of 270 duration which is 3/4 of 360 degrees making calcs easy.

This would be a good example of a high rpm stock engine such as the early Shelby's which made about 306HP back in the day, they were port limited so velocity should be high.

 

You guys are off in outer space.

The rule of thumb I use is that the N/A HP potential of a head is 2X the intake port's max CFM. So 200 CFM head can support roughly 400 HP. I get the impression that the rule of thumb starts to skew over 800 HP, as I know of N/A engines over 1000 HP but not many 400+ CFM heads.

 

The SBC 2.2 in NASCAR cup trim produced about 800-850 horses at about 8500-9000 RPM these flow around 400-450 cfm, seems to support your theory.

Bogie

 

I always used rule of thumb .257 ( potential cylinder head intake flow at max cam lift time X .257) , But when air bench flowing heads, intake manifolds and headers there is a problem in efficiency when bolted to the engines the flow bench does not show mixture separations or other factors caused by heat etc.

250 CFM through the intake port has the potential to produce over 500 hp. Even though the amount of intake airflow provides the potential for over 2 horsepower per CFM, the engine must have the best compression ratio, the right camshaft profile and a properly tuned high flow exhaust system (everything must be close to perfect) to reach these HP numbers !

Restrictions in the intake system -(Even an air cleaner, or choke system, under hood tyemp and air turbulence, ETC)must also be minimized as air flow through the entire air intake system is what determines total power . When the intake manifold is bolted to the cylinder heads the intake airflow can be reduced drastically compared to the intake manifolds flow bench figures, flow losses to well over 25% depending on the efficiency of the intake manifold..This makes the flow efficiency of the intake manifold very critical for engine performance which is why performing porting modifications to improve intake manifold airflow is done !! And Bad header or exhaust choice can really decrease velocity !! There is also a give and take scenario involved in port size and max flow you must decide on what you sacrifice in your low and mid range RPMs torque, Max flow numbers do not tell you what its characteristics will be in the real world when bolted to your engine !! Two intakes that max flow exactly the same or 2 heads that max flow exactly the same on a flow bench may perform very differently when combined on an engine in the real world in their efficiency one may increase HP and torque while the other may cause drastic losses!! A specific velocity is a must for a maximum air and fuel charge to reach the cylinders. Port velocity and efficiency relies on intake manifold , head port' and header velocity, and has a dramatic effect on flow. The velocity of the mixture will be greater when traveling through a small port,but a small port cannot carry a large amount of mixture needed to fulfill the engine's requirements at high RPM levels. Large ports will flow greater volume, but the larger size cuts down the velocity, killing low and mid range RPM torque levels.

Over the yrs. I relize Max flow numbers are not written in gold or really the best in head choices!! In the real world things that determine the performance of a cylinder head are complicated. A head that is ported without thinking about air speed, the size of the engine, the rpm range, the location of the valves, and many other things isn’t going to be the best head, regardless of its peak airflow. And yet I see people who are influenced by big cfm numbers and bolt a new and custom flow bench ported head on that flows for a 10,000 rpm screamer and their engine is a 6,000 RPM street cruiser and then come to my shop wondering why it lost power??? And want it tuned LOL

This is a great thread and I don't agree that its " off in outer space" LOL

Jester (Chris)

 

We just made 1300HP with a head that flows 550 cfm. It does have 4.5" arm so it will some good RAM effect. It is also 17 to 1 and 611 CID

 

Does it have supersonic ports?

Imagine the marketing opportunities!

 

If you just want to list your supersonic port flow engine specs to save time and effort is fine.

Funny you mention FZ Yamaha engines as this was one of the first motorcycles I modified extensively, my 5 valve made just under 140hp on 750cc's...spent a lot of time on that engine...port velocity at 11700 rpm was just under 350ft/s (240mph) which was pretty much the limit on a stock head. Built basically an Eddie Lawson replica for myself, held its own against 1000cc bikes back in the day. A bit peaky to ride and was a light switch powerwise, had to be awake and ready to go or it could catch you if you weren't ready for it.

 

My neighbor has an RD400 Daytona that makes me sweat every time he fires it up. Tuned pipes, 2stroke and big CC's makes Nate a happy boy LOL

He says the same thing about my 93 Arctic Cat 700 efi with Dynaport pipes LOL

 

Eddie Lawsons 1986 "FZ 750".

http://www.fz750.it/content/fzfamous/fz_lawson/index.php?lng=ENG

Not an FZR, those didn't come until later.

 

I will just leave this here, bang on accurate from my calcs...best money I ever spent.

Speed-Wiz port velocity calculation

 

Any update on the near supersonic port calcs?