Port flow vs Max power @ RPM

[Garys1969RR]

Was trying to figure out how intake port airflow limits engine RPM. In say a 500 C/I engine, the intake port airflow averages 215 CFM, throughout its lift range. A 500 inch engine is 3.45 cubic feet (approx). That is, 1728 cubic inches equals 1 cubic foot. Dividing 3.45 cubic feet by 8 equals .43 cubic feet per cylinder. .43 times 5000 RPM would mean the intake port would need to flow an average of 215 CFM. A Stealth head flowing 290 CFM at .500" lift, would be doing well to flow an average of 215 CFM from .050 to .500" lift. So the only way to increase the RPM at which the engine develops max power, would be to increase airflow, duration, max valve lift, or all of the above. Am I doing the math on this correctly? My question is: what cam would be required in a 451 to make max horsepower at 6000 RPM? Using Eddy Performer RPM heads flowing 290 CFM at .500" lift? Or is there even a way to answer that. Just trying to learn about relationship of airflow vs power at RPM. Thanks.

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Oops, is 500 cubic inches .345 cubic feet? I don't have my calculator here. Some one please shed some light on how to figure this out.

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Might be easier to just call Comp cams, give them the details and have the computer make a suggestion. Lol

 

[67Satty]

Try this:

http://www.camquest.com/

It's from the Comp Cams website. You will have to fill out some registration info but it's free.

 

[rev.ronnie]

I don't use average flow numbers. The pressure differential is much different at max piston speed than at BDC. Flow at 70°-80° ATDC is very important where this occurs.
Also, flow at BDC and the lift in that range is of equal importance. Finally, intake valve closing is the final, critical figure. It will determine cranking compression for one, and also has to be correct to prevent "backing up" the intake tract.

You would be surprised how mild a cam really needs to be if the head flows really well.

 

[Meep-Meep]

I'm certainly no head flow specialist, but from everything I have understood is you need a port that will wet flow like mad. It's all about getting a properly atomized mixture to stay that way down the port and arrive into the cylinder with minimal loss.

Ronnie's last comment is spot on to everything I have observed. If I told you that a 3.31" bore by 2.77" stroke hemi engine had the valve sizes of a 360 MoPar (1.88" I x 1.60" E) we all may think that was insane. But in the case of a 2.4 liter Porsche 911 engine it works very well. 8.0:1 compression; 150 PSI of cranking pressure; .400" lift cam and 262 duration. And all this revs freely to 6000 RPM..

 

[rev.ronnie]

The supporting data that engines with good heads don't need huge cams is actually proven by the fact that engines with forced induction need relatively mild cams compared to the rpm they will run up to.

A port that wet flows tremendously, will not always make good power though. As Alex says, it's atomization that leads to efficiency. The things we look for in wet flow is poor distribution, excessive swirl, "fallout" and other factors.

For most practical applications, a nice flowing port and correct intake to exhaust percentages are most important. We're not trying to win a nascar race or 24 hours at LeMans. We just want nice running, efficient engines that make good power, get reasonable mileage and don't wash the rings out. Let the cam do it's job with the available cylinder head.

I wouldn't get to worked up about big or "target" flow numbers. Making a port overachieve for sake of a few cfm often times messes up the dynamic of the port. Sacrificing velocity for flow, in many cases, is a irreversible step backwards.

Case in point. I have a 565” engine with a 323cc intake runner with a 2.25” valve that flows 405cfm at .700 lift. The engine makes 930hp at 6400 rpm and 780#/ft at 5200. The head has tremendous mid lift flow, more than the similar 360 cc runner from the same mfg.
The relevant figures on this engine don't even make sense if you only look at RPM, HP and max flow. Oh, the valve lift is .730" intake and .680” exhaust.

It's almost comical that the car runs high 8s like a tractor, while similar cars need to turn 8500 to do the same job. I drive it around at the Car Craft Nationals once in a while.

 

[Herr_Poopschitz]

correct intake to exhaust percentages are most important.

I agree w/ most of what you've said, but not sure about this, nor the comparison to forced inducton as justifying big head/small cam physics. The manner in which differential pressures are generated for cylinder fill are completely different.

So, what exactly is a 'correct' percentage? What's the reasoning behind this? Going back to your forced ind example, wouldn't that effectively skew the intake #s from calculating conventionally accepted (ie, 'correct') %s?

 

[rev.ronnie]

I agree w/ most of what you've said, but not sure about this, nor the comparison to forced inducton as justifying big head/small cam physics. The manner in which differential pressures are generated for cylinder fill are completely different. it was more of a similarity than a mechanical parallel

So, what exactly is a 'correct' percentage? What's the reasoning behind this? Going back to your forced ind example, wouldn't that effectively skew the intake #s from calculating conventionally accepted (ie, 'correct') %s?

It's generally accepted that the exhaust flow 70-75% of the intake. This needs to be higher on forced induction because of the extra intake charge that is forced in. Obviously, the exhaust needs to be able to accommodate that. Boosted engines and nitrous, can have exhaust to intake ratios in the 80% range. At some point over scavenging becomes an issue.

 

[Herr_Poopschitz]

Here's a lengthy discussion on in/ex ratios and what they mean, or don't mean...

http://www.yellowbullet.com/forum/showthread.php?t=451038&page=5

 

[rev.ronnie]

I'm not really into YB. A lot of internet gurus on there. It's too much "I read this, I read that"...

I like to back my research up with dyno testing and practical application.

 

[dvw]

Use this http://www.wallaceracing.com/calcafhp.php
Doug

 

[Herr_Poopschitz]

I'm not really into YB. A lot of internet gurus on there. It's too much "I read this, I read that"...

I like to back my research up with dyno testing and practical application.

'Internet gurus' and you infer there is no dyno time? Umm, the first page of the thread I linked to every contributor is a business owner...oh well, your loss.

 

[rev.ronnie]

'Internet gurus' and you infer there is no dyno time? Umm, the first page of the thread I linked to every contributor is a business owner...oh well, your loss.

Honestly, I didn't go back through 25 pages of banter and conjecture to even see the original post. I'm not at any loss either way.

I'm just stating, YB is not my source for tech info. I like "Speed Talk". It has many industry professionals who offer a lot of real life data. Sure there are the hobbiests on there too, but the tone of it all is different. It is much more technical and a lot less chest pounding than many sites.

For me, having been in the engine and machine business for over 25 years (as a shop owner), I have many great rescources on my Rolodex. I like to think that my name is in quite a few of other people's as well. An answer to a serious question for me, usually involves hearing another person's voice.

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'Internet gurus' and you infer there is no dyno time? Umm, the first page of the thread I linked to every contributor is a business owner...oh well, your loss.

Ha! That is funny. I did re-look at the post.. George would probably be one of those people I would call too...

Ok, you got me on this one.

 

[Herr_Poopschitz]

Ha! That is funny. I did re-look at the post.. George would probably be one of those people I would call too...

Ok, you got me on this one.

I considered bringing out pics of Curtis Boggs or John Marcella's stuff...haha.

Chris' naturally aspirated sub-forum is great, don't know about the others.... I have a low threshold for BS myself.

 

[451Mopar]

I have the Dynomation5 sim software
http://www.youtube.com/watch?v=JqFqa4L6--o