Choose A Cam By Head Flow Rates.

If we know the given flow rates for a set of heads.How do we pick a cam then?I understand the exhaust port flowing say 75% or 80% of the intake port is great but what if we want to run N20. or a blower.If the exhaust does not flow where we want it to then can we try and hold the exhaust valve open a few more degree's at .050 to compensate for this .I guess my question is how much longer can a person hold the valve open without blowing it all out the exhaust side.Is there a formula out there to determin this.I do know some of the cam makers ask for the flow rates in picking out the cam.Yes I know dyno time would tell me this. But how do the cam makers make their guess ?

lobe centers get open up for nos and a blower i looke at dur and cl more than lift for a car or truck with some cr i like to see a cam on 106 or 110 ls if the car or truck has low cr i look at 110 to112 sL but it has alot to do with the application i look at cam i have used in the past and look at five cam books and think about useable rpm spread good flowing BIG PORTS can take more cam lift and dur but evey thing gose up to CR RPM cfm and bigger headers i like smaller ports heads with good are speed :whacked:

Found this on the web.

The section on 'cylinder flow' addresses cam duration parameter and the resulting actual head flow.

Flow vs Velocity

Bert

air speed = velocity i tryed to say that with out the math:whacked:

Thank's guy's.I have come to the same conclusion about heads as I do about cams.If in doubt go with the next size smaller.

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Originally posted by erik erikson
Thank's guy's.I have come to the same conclusion about heads as I do about cams.If in doubt go with the next size smaller.

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Perfect :)

yes a dyno would be nice and a killer flow bench i have done a lot of work on heads and intakes and headers and my stuff work very good and holds to gether. not to much math on the flow of a port but just good sense .not tring to brag i do not talke about it to much . my engine i have buit for customer they seam to be happy with what they get.on heads i end up using some dart stuff and some of the gm 861 and the zz 502 heads and world heads and go from there and i go to swap meats and look at head port work. and i knew someone that made heads and look at alot of his stuff and i do not turn down any time to get a hold of a set of heads and get a good look at them .but i work on alot of gm stock heads i spend a lot of time in front of the valve and in the bowl and spend a lot of time on valve seats .we have done harley heads to and had good luck there to .but where i am from not to many guys can afford the nice stuff so i do my best on there stuff . all i can say is a lot of guys get it wrong all they think about is a big cc intake runner and not think at all about headers or ex. big CCheads small hyd cam somtimes flat .not a roller. no CR in a heavy car or truck and top if off with a 900 cfm carb: :whack

Those of you that have installed a hot cam and THEN found that your car has turned into an unmanagable mongrel raise your hands...:CRY: Fact is that the cams and other performance goodies are big money makers and so they don't tell unsuspecting kids that the parts are only worth a nickel if you are doing 3000rpm and higher. And that simply isn't practical unless it is a race car. Cam selection involves dozens of varibles of which about 70% or more of street rods don't come close to being a good matching combination. Sometimes, operating range (RPM) should be what we make our selection of parts around. Just ask the auto-makers about that. And I've made alot of these mistakes of building em bakards.:3dSMILE:

here it is tires size how tall they are. and the gear size at 55 what is the rpms. sometime cam book will tell you but i have the formula and the weigh of the car . if light like a 30 ford with a small block in it you need more stall on the trans converter than you think. and to much rear gear will not help. you will have both feet on the brakes and it will chug thru stop lights most of the time a good rv cam is all you need. but who builds stuff like that :whacked:

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Originally posted by pat mccarthy
here it is tires size how tall they are. and the gear size at 55 what is the rpms. sometime cam book will tell you but i have the formula and the weigh of the car . if light like a 30 ford with a small block in it you need more stall on the trans converter than you think. and to much rear gear will not help. you will have both feet on the brakes and it will chug thru stop lights most of the time a good rv cam is all you need. but who builds stuff like that :whacked:

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I was refering to when they ask me what the flow numbers are on my heads.

i know what you ask and i may of not help you but ?? i did try.was tring to tell the last guy shevey not heme how to go about finding a cam for a mild street rod

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Originally posted by pat mccarthy
i know what you ask and i may of not help you but ?? i did try.was tring to tell the last guy shevey not heme how to go about finding a cam for a mild street rod

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Sorry,I thought you where posting that for me.

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Originally posted by shevy not heme
Those of you that have installed a hot cam and THEN found that your car has turned into an unmanagable mongrel raise your hands...:CRY: Fact is that the cams and other performance goodies are big money makers and so they don't tell unsuspecting kids that the parts are only worth a nickel if you are doing 3000rpm and higher. And that simply isn't practical unless it is a race car. Cam selection involves dozens of varibles of which about 70% or more of street rods don't come close to being a good matching combination. Sometimes, operating range (RPM) should be what we make our selection of parts around. Just ask the auto-makers about that. And I've made alot of these mistakes of building em bakards.:3dSMILE:

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Yep, so have I. That's why in a recent post I stated that the cam is the final item that should be purchased for a motor. Younguns don't understand that the cam should be used to compliment the build. They think that the wilder the grind, the faster their car will be.

I recall back to the days when my youngest son was in high school auto shop. I used to go up when I had the time and help Mr. Painter (shop teacher). I'd take a dozen or so of the youngsters into an adjacent room and field their questions about motors. They used to call me "Mr. No-cam". I remember one youngster in particular who had a '69 Chevelle 350/350 with a 2.73 open rear and stock converter. He had chosen a cam with over 300 degrees duration (advertised) and couldn't understand why the car wouldn't pull the hat off his head. :LOL:

Well I got most of my info from Tech1 so he has covered it already but so far no one has mentioned the use of an OD transmission. That means as said above that tire size and rear ratio may put the rpm range in the OD gear down at 2000 rpm or even lower so head flow needs to be good at low rpm for cruising. Also no one added my usual knee-jerk reference on the bench flow of common heads:

http://chevyhiperformance.com/techarticles/41598/

Don Shillady
Retired Scientist/teen rodder

i think a good engine bulider shoud know that i do and this thread was more on air flow numers on the intake and the differs on the ex side of the head and how it can be help out with the events of the cam pro file:LOL:

Pat, with your experience you have a feel for the way to improve flow while I would have to go back to the good articles SBC linked to except that I know you really can't solve problems in fluid dynamics without a computer, although charts help. Since I am going to use the lowly 882 heads at first on my SBC350 for economy and low rpm torque, maybe you or someone else can explain to me how the 882 heads flow as well as they do with the smallest runner volume in the list? Maybe that is a case where the smaller runners help velocity which helps flow up to some limiting rpm. Clearly the 882 heads are limited for high rpm but maybe the GM engineers did a good job designing these heads as a way to detune the earlier heads and still get good low rpm torque back in the day of the '70s gas crunch. So far all I have had done is to have the exhaust ports smoothed and shaved the heads down to a 73 cc volume.

Don Shillady
Retired SCientist/teen rodder

Don, I have an idea your motor is gonna be a little sweetheart and that you'll be very happy with it.

i have been lucky?? i have looked at heads that my friend made and he spent good money to get it flow good numbers. i have had a good look at a lot of ports and have talk to one of the best porters and read some about flow. and i talk about this with all my engine builder friends all the time and get down to small port hi speed = velocity i can not talk about you head. but for what you do with it will work i do not have a flo bench so if you are looking for number i can not help but i can tell you flo bench is a tool it will not help with what is really go on inside your engine .the exhaust is so very important header tub size leigh of primary. your carb cfm and intake and cam all this has to work to gether the port that i think about is the exhaust. when the exhaust leaves the the head that is not it. better this works the better the heads works .on your head i would back cut the ex valves up to the 45 and work in the bowl the valve angles 30 45 60 70 hand blend the blowl to 75 and to 60 tring to make a radii and make the valve hit the 45 at the upper part of the 45 on the ex valve if not work on the seat till you get it rigth and i do not like wide seats on the ex 0.60 and clean the floor of the chamber so the vavles works good at low lift:whacked:

that why i did not say cut the head out the short side radii as were it at:LOL:

that ok i am a little up tight work on a set dart heads all day setting them up for 800 lift roller cam:whacked:

Pat, Tech1 and DennyW, thanks for the comments. I am getting impatient to get the engine running but I still have several details to finish on the chassis first. I forgot to mention that I had the valves on the 882 ground at 30/45/60 so I guess I have a pretty good "poor man's" econo-head.

Don Shillady
Retired Scientist/teen rodder