Thread: Choose A Cam By Head Flow Rates.

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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

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

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.

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.

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... 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.

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

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

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

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

Sorry,I thought you where posting that for me.

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... 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.

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.

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