A local engine shop told a friend of mine not to worry about 10-15 cfm here or there on ported LT1 heads. Their heads flow about 260cfm@.550 lift.

They said it doesn't really matter how well the heads flow because when you put the intake on the motor it reduces the intake flow to approx 240cfm anyways and that the intake is the restricting factor. They want something with longer runners for the intake and offered to make one for about $2000.

Is this true?

I'm curious cuz he's getting his motor done from them.

 

longer runners would only be more restrictive - think TPI (okay, extreme example, but)

if they've already ported his heads, why don't they port-match and port the intake as well? Should cost a heck of a lot less than a new custom intake.
Heck, just do it at home - port match the intake to the ported heads, and then grind the rest of the intake runner out to the same size, maybe smooth the plenum-runner transition a bit, and port some behind the TB...

 

The intake has been port matched.

They want to fabricate an intake with twice the runner length of the stocker.

 

I have always heard that the stock intake is pretty good, flow wise. I am in process of doing a little p/p work on a spare one of mine, just to see what gains I can get out of it.
I definately would NOT get a custom one made for that kind of $, but would instead do what Blueyes suggested (and I am in process of).

Jeff

 

Dumb question, but how exactly do they plan on doubling the runner length and still clearing the cowl?

 

Increasing the runner length decreases the resonation frequency, which in turn provides a "mini-supercharger" effect at a lower RPM.

For example, I believe our cars peak (torque) at around 2800RPM (Stock). That is very likely where the stock runners resonate. If you make them longer, it will drop the torque peak to a lower RPM. Obviously the math is intense, so I cannot spit out an equation.

Consider this though:
1. HP = Torque * Constant * RPM
2. If your max torque occurs at a lower RPM, then it also hurts your HP.
3. Boat engines (Think High RPM) have very short, straight runners.

I don't know where the person gets off saying the intake is more restrictive than the heads, if I'm not mistaken the stock throttle body is good for ~650 CFM.

 

Do not let that person do your headwork .

That's an excuse b/c their heads aren't doing what others are.. well.. very few others, most aftermarket places like to claim theyre getting 280ish, but most of them are only 265-270 peak.

Either way.. port matching.. bah.. who cares. Considering how much power we make (and some others make) on untouched intakes, I wouldn't bother with it. If that guy honestly thinks a better set of heads on a stock intake wont make a difference b/c of the intake... well... like i said.. might wanna find someone else to do your stuff.

 

The intake manifold above the injector is smaller than the intake port opening and cylinder head.

It doesn't have to be as big at that point because it only flows air and no fuel.

 

Don't waste your money...longer runners will only be more restrictive.

Why do you think TPIS developed the Mini-Ram with 3" runners vs stock TPI 17" runners? MORE POWER!

 

The LT1 intake is very flow efficent. the only thing bad about it is it abosrbs a lot of heat unlike the LS1 intake.

 

He has an aftermarket Predator hood.

Thanx for the replies guys.

He's most likely not going to do the intake.

He was just curious as to how much of a difference power wise it would make to have heads that flowed 280cfm instead of his 260cfm.

He has verified from another shop that they do in fact flow 260 cfm.

 

After seeing Steve Quinn's new intake this weekend, I plan on adding plenum-volume, not runner length, to my intake this winter.

 

aren't the runners like 3" long or thereabouts? I doubt the resonant RPM for that short a length is 2800rpm. To tune an intake for low rpm's, you're looking for something more like TPI...

I believe GM's thinking with the LT1 was more that the hyd roller cam and good flowing heads would make all the lowend torque you need, thus make the intake with short runners to allow for more topend power.

 

Don't forget, they were also thinking how are we ever gonna fit an intake in the F-Body??? As everything else, it was a compromise, but not a bad one.

 

George, for us with the room, it would be interesting to find out, knowing either one, longer runners or larger plenum, would improve torque output, which one wold make a bigger impact over a broader range.