Hello everybody,

I've been spending some time on lurking around forums and poking around old hotrod magazine articles and I think I might have caught some kind of nasty bug. I keep having visions of tire smoke and empty wallets. So in an effort to minimize damage to my bank account i'm looking at doing a whole bunch of porting on my 318 magnum and finishing off with a cam, headers and a tuner. I'm having some trouble understanding airflow and was wondering if somebody with some more knowledge could help me out. Borrowing numbers from the hughes website, if the stock intake flows 219 cfm and ported it flows 248 is there really any point to opening up the throttle body with a stock 635cfm?

 

248 × 8 = 1984
so what do you want to known?

635 can flow enough 4, 1984 CFM.

 

So, there is never any point to upgrading a throttle body?
How is 635 enough for 1984?

 

Yes replace the parts including the manifold, I was being sarcastic. Your motor is noting more than an a air pump. If it has easy in and out it will make more power! So cam and headers also.

 

With any upgrade it's good to know your end use for the engine and the maximum RPM desired. All these things go hand in hand with cam selection, ports, valve size and manifold. One to small or too big component in the package can kill it.

 

The engine is in a pickup, not looking to send it down any drag strips, just a little fun on the street. This will be my first time making performance modifications to an engine and I am trying to avoid shooting myself in the foot with some of my decisions. I was trying to avoid just asking if x part works for y application and making somebody else do the research for me. I just got hung up on those cfm numbers.

Stock magnum Heads


LIFT----AS CAST

---------IN/EX

.100"--67.3/N/A

.200"--131.3/97.3

.300"--185.6/132.5

.400"--212.5/144.9

.450"--213.9/146.3

.500"--212.2/146.3

.550"--212.5/146.3

Ported

LIFT------IN/EX

.100"--67.3/N/A

.200"--132.8/115.6

.300"--196.0/156.6

.400"--225.0/172.2

.450"--238.1/174.3

.500"--250.5/176.6

.550"--257.1/178.0

So, the ported keg is supposed to get 248 cfm, if I go with this cam HUG SER0814ALN-10 the modifed stock intake becomes a restriction. Would there still be usable power up to say, 4000rpm?

 

For sure 4000+. A bit more cam than I would use in a Truck with a 5.2. If your not planning on towing, why not, it will be fun if you get the right computer tune.

Be sure to check your valve retainer to seal clearance with that lift, may need to cut the guides down.

 

OK.... I'm lost..... Or asleep at the controls again.....

 

Thanks for the reply's so far, i've got another question about the exhaust side of our air pump. The smaller exhaust valve obviously flows less than the intake valve. Is this offset by the longer duration and additional lift of the cam for the exhaust? Is this where exhaust scavenging comes into play clear the chamber?

 

Without getting deep into theory most heads have smaller exhaust valves than the intakes, a camshaft can have the same lift and duration for both, "Single Pattern". Most modern camshaft design are "Dual Pattern" and usually will have more lift/duration on the exhaust side.

Exhaust scavenging is a result of exhaust gas momentum aided by exhaust manifold design, camshaft overlap with incoming charge can also aid at upper rpm's.

 

One simple thing to consider is the exhaust valve works under pressure and the intake works under vacuum.
Hence the difference between valve size, it's easier to push air than pull.

 

This is probably where I was getting hung up trying to understand the air flow numbers.

Thanks for taking the time to answer my questions and explain some things to me. I appreciate the help.