J-Rod

BTW, if you are going to post, make sure you post any formulas you are using in making certain determinations so folks can see how you arrived at a certain determination.

Iv_z28

I believe I posted on a similar subject a while ago. While pressure drop is obviously important, it's impossible and meaningless to talk about flow requirements without taking a cam into account.
To make it very easy - IF your valves are only open 1/100 of time, then you will need thousands of CFM to fill the chamber. Therefore any calculations of actual CFM requirements would require:

1) Plotting CFM as a function of lift and pressure.
2) Plotting lift and pressure as a function of cam position (angle)
3) Using (2) to plot CFM as a function of cam position (angle)
4) Integration of CFM, found in (3) over the cam position (the angle).
5) Result found in (4), may be compared to the requirements, calculated from engine geometry and RPM.

...then you may start to think about dynamic effects

soundengineer

iTrader: (16)

Take 1/2 of the total cubic inches and divide it by 1728 cubic inches (the amount of cubic inches in a cubic yard). This gives you a multiplier that is air displaced per crank revolution.
Multiply this by your max rpm.
Multiply that by your volumetric efficiency. (90% is reasonable).
This is the amount of cfm required at your given max rpm. Remember that just because a carb is rated at a certain cfm, doesn't mean it will flow that much air since you have to take frictional losses into consideration as well.

DenzSS

Step away slowly from the crack pipe Scott....slowly....put it down...

Bink

Yeah...I'm in. 4 stroke ..only one intake event per 4 strokes.

I was thinking about this in the shower yesterday in relation to cam Duration. At Baro what would be the maximum flow needed by the combustion chambers ( vol. of the chambers) and what minimum intake Valve event/flow (Duration). Of course max would be at redline RPM.
Anyway SPOOKY that this thread is here.........


joel


OooPs - my bad....I didn't see the 2nd page!!


You guys are getting lost in the "inefficiecies". I think DenzSS is going to relate it to valve events duration etc......????

QuietTahoe

Seems to me that many people are trying to pinpoint an elusive target here. The limits to flow of most intake manifolds (even highly ported manifolds) for LS1's have a huge affect on flow through the inlet port. Primary tube length and collector volume together with LSA all have a huge affect on the pressure drop through both the intake and exhaust ports. These all cloud the comparison of flow numbers with real world performance. In the real world (not chassis dyno racing) the airflow past the exhaust tips at speeds above 70 mph has a measureable affect on power and, by extrapolation, probably port flow. In over 34 years in the performance industry, I have had the opportunity of making hundreds of cam and head changes, thousands of airbox flow tests (both wet and dry) and literally thousands of dyno pulls(both engine and chassis) I have never been able to completely predict how a combo that was tested on a dyno would perform in competition. The number of permutations and factors that cannot be simulated in a test cell is mind boggling. Theory is great, but when the test results don't jive with the theory, believe the results and change the theory!

Bink

Welcome!!! I might be wrong (as I often am) but I think they're looking at absolute max?? Nice 1st post.

DenzSS

Actually, we're just talking about one aspect of camshaft design. Just one small starting point.

The point that will eventually be made is that many of the camshafts being marketed to the LS1 community are very poor designs. Several of us on this forum have been saying this for some time. I don't totally fault the vendors selling these cams, they are only catering to public perception. People want to say they have a big cam, even when it may do absolutely no good for them.

As far as test results go, I've seen a whole lot in person both in the Gen III world and elsewhere. We need a serious change of perception in this community and I , for one, am going to do whatever I can to bring that perception back around to a more reasonable center.

I will fully agree that camshaft design is an incredibly complex topic and can only be successfully accomplished by professionals. We do, however, have the ability to educate the population and get them really thinking about these topics.

I also fully agree that the only way to really know how a combination is going to work is to try it. Sometimes you get what you expect, sometimes you REALLY don't. Theory only goes so far, eventually you have to get of your *** and take the combo to the track. Jere Stahl had a very good example of this. He had a bright idea about a different oil pan design. He spent a lot of time, effort, and money putting one together. Once he had finished, he slapped it on the engine and put it on the dyno. He lost 20hp. To quote Jere, "I learned I don't know how to design an oil pan."

We have been setting the bar very low as far as our design standards go. I'd like to see that bar raised a bit.

/soapbox

DenzSS

And to run along with Blink...welcome and good post.

critter

iTrader: (9)

It is always dangerous to disagree with experts, but I come up with 558 CFM at 82% VE. While there are 4 stokes per cycle, there are only two revolutions per cycle. I can't see where the other factor of 2 comes from.

Flow bench numbers don't seem to corelate well with this IMHO. An intake port may flow 330 CFM, but that is full time flow rather than bursts of 1/3 cycle, which is non-linear since the valve can't open instantly. And, the moving air can't stop instantly.

Colonel

Here here! Welcome!

I like your "Test first, explain later" way of thinking. That's how I feel myself. Sooooooooo many times (in fact, more often than not) I've seen calculations fail to pan out in the real world. The best we can do, IMO, is take our best guesses (derived not just from pen and paper but also from what we've seen take place), test and then make changes based on the results....and then repeat, repeat, repeat.

DenzSS

In general, I agree Colonel. As I notice, you're a proponent of the stealth cams that you've worked on for so long. In my mind, those are reasonable camshafts and have had good results. They also fit right in with theory and observation. They are well within the parameters of acceptable design.

Many of the other camshafts on here are not, speaking from both experience and theory. Theory will tell you if someone is way off, or kind of close but you can't nail it perfectly. To pull off a good design takes putting theory to practice. Sometimes you're right, sometimes you're wrong.

I'm positive that the direction people are headed with higher and higher duration camshafts for little 346s is incorrect in theory and practice. We can do a lot better.

Bink

So what is theoretical max intake/exhaust duration?

Bink

Colonel - off topic, that avatar of yours puts a grin on my face everytime I see it. Too damn funny. All due respects of course.

joel

DenzSS

I just got to looking at the formula we used. I can't really post the thing. To get to the CFM requirement by rpm requires several integrations into a big wad of calculus. If someone is really interested, I can get that portion split off of our application and give that to you. Just shoot me a PM if your really serious and I'll work on getting that split off when we get some slack time. Currently it is mixed in with our calculations descibing rotating assembly motion, position, velocity, and acceleration.

QuietTahoe

Thanks for making a new comer feel welcome. I've watched this site for months and have been reluctant to contribute, but I have seen some young men in my area read some of the claims in these threads and believe that they can put a relatively large cam in a 2002 Camaro and that it's going to be easy to tune, run like "Jack the Bear" and be totally reliable. After they have spent a small fortune on parts, installation and "tuning" they become discouraged about modifying their cars. Performance has always been fun for me. I like to see people enjoy their toys. I sure enjoy mine! Maybe a few comments based on my real world experience might be helpful to some. I'll try not to be boring or intrusive. Again, thanks for the welcome!

Bink

HeHeHe.......I bought a big cam - for a challenge (my 1st tune!) ...took me, and a bud, 9 months to tune it. Without NoGo's coaching it would still be sitting.

FASTONE

I also do not totally understand the question DenzSS, I pretty much follow Steve Bryant's calculation on engine air intake needs based on 100% VE,also I know you have seen or been to probably dyno test sessions were they measure airflow into an engine under a load on the dyno.A 350 at say around 6000 uses about 600 cfm but that does't mean it makes the most hp with a 600 cfm carb although it might feel better to drive with that same 600 carb.I had a mildly modified 350 and it made more power above 5000 with a 780 vacuum secondary Holley, but was a lot more fun to hot rod around in with a modded 600 vacuum secondary carb.Steve's calculations on required CFM were pretty much dead on to me.Maybe you guys will teach me some thing I don't know.Oh by the way techguy at Barry Grant told me my 408 was overcarbed with a 750 Mighty Demon I about laughed in his face THING DRIVES GREAT AND PULLS LIKE A BAT OUT OF HELL!!OK now I'm goiing to sit back and learn something!!

DenzSS

Big welcome Tahoe, you hit my gripe right on the head. Thanks.

I keep seeing young guys do that and 6 months later they are selling their car.

Don't worry about being boring or intrusive, I am all the time.

DenzSS

Good to see you again FAST.

I think I see where some of the confusion is. We're talking CFM/cylinder, not all 8. Carb's are measured an entirely different way.

This is how much air flow is required to fill just that ONE cylinder. Not the same ball of wax. At this point, we're leaving out the real world. We're just stating this is how much air flow a single cylinder needs to completely fill itself at the stated RPM. After we get this nailed down, we'll start adding the actual cylinder head to the mix, then the intake manifold, and eventually we'll get to the camshaft. Make sense?

This is just a way to see how much we require and then we move on to how we're going to get it actually in there. If we can.