How does the intake manifold restrict head airflow?
 

Let's say that the intake manifold flows only 280 cfm and you got two identical pair of heads except that one flows 20-30 cfm more than the other. Obviously the heads with more flow should make more rwhp.

My question is, how come that the intake manifold does not restrict the additional flow? How could those additional 20-30cfm pass to the cylinder if the intake manifold max flow is way beyond that?

Its all about intake runner diameter & length. Turbulence also plays a bit into the formula. Better heads flow better due to intake runner volume as well as valve size.

Look at Tony Mammo's port matching that he is doing with AFRs and Fast intakes. By porting and matching the intake runner to the head, he is freeing up additional HP. I guess you "could say" that the stock FAST 90mm is theoretically restricting flow :)

Intake flow tested thru a radius plate and then tested thru an intake usually show very different results....especially at higher lifts where the intake has a harder time keeping up. While the FAST is arguably the current manifold of choice ("bolt-on" style), it still has it's limitations and in my opinion simply can't keep up with a 300+ CFM head. Its one of the reasons I think the AFR 205's have shined....velocity coupled with a strong "flow curve" and about all the flow a FAST intake will support equals good cylinder fill and impressive power and torque numbers....in fact, the FAST intake (even ported) won't even fully support an AFR 205's intake port.....It knocks the flow to somewhere in the mid-high 260's (stock FAST) and low-mid 270's after I modify it (The modifications I make are also more than a "port match" implies).

Bottom line, this intake was meant to be bolted on a stock (or slightly modified) LS engine and provide power gains. Out of the box all my data and time spent on the dyno proves it certainly does, and with the right modifications can be made to flow and work even a little better, but it IS a compromise "bolt-on" style intake meant to fit under the hood of your C5 or F-Body.....make no bones about that. I'm quite sure FAST would have designed us an even better breathing and more efficient piece if the designers didnt have their hands tied regarding point of entrance, point of exit, and the fact it had to be packaged to fit a production car.

I'm hoping someone comes out with an even better intake for an LS1 that will still fit under thr hood of a production car....That will take the performance level of these engines to an even higher level IMHO as I think the current intake arrangement still presents a significant restriction when compared to an all out race piece with the right runner length, plenum area, and approach to the port. I have some "ideas" and might experiment with a FAST at a later date during some future dyno testing....If I find anything of interest I'm sure word will get out.

Anyway...hope this info helps

Tony M.

Thanks Tony, that explains alot. I hope that GM did a great job with their LS7 intake manifold

How much better is the open plenum Edelbrock and GM carberated intake manifolds? Will these keep up with a 300+ cfm head?

Thanks for sharing the good info Tony M.

how about the LS2 intake ?

Is it true in a FI application the intake plays less of a role compared to the heads?

I put Tony's comment back to the top so more people could see and understand that we are limited by our current plastic LSX intake designs (excluding the 427 LS7 intake).

So our naturally asperated engines are limited by the restrictive intake. What are the largest recommended cam specs using the AFR 205's, ported FAST intake, 90mm throttle body, and large MAF or MAFless system in the "restricted by intake" street 346ci engine. Tony :confused:

So if the FAST intake knocks the AFR 205 flow numbers down to mid-high 260's or with ported FAST intake low-mid 270's. Wouldn't cam lifts higher than .550 be a waste? Anything higher would be wasted energy.

here's something to add. all i did was add VHP ported heads. kept all else the same. even the stock LS1 intake. i gained 17 rwhp untuned. 34 tuned. hmmmm why?? are the stock heads that restrictive compared to the intake? if that was the case, then why would you gain power with the LS6 intake over the LS1 with stock heads?

I think the restrictive plastic intake of the Naturally Asperated LSX engine could be reason enough why some big camshaft designs are reverse split. (232/230, 234/228, 240/232 etc.)

I like these basic cam specs: 230/230@.050, 112lsa, .580 lift

They are much better if used correclty. With the runner design on both Intakes you're going to lose power down low, although you will gain it on the big end. It's a trade off IMHO I'd say almost all street cars will gain more useable power with a FAST setup vs the Single Planes, but I'd put one on a race car with a narrow powerband in a heart beat.

Something to consider with a Single Plane Carb intake manifold is matching things like cam lobes, and headers to each of the different length intake runners.

To the top.

Good logic, any good answers if what Tony stated is true?

Nice post, appreaciate all the info. :)

Not really. The shape of the lobe determines how long the valve stays open
at each lift point.

If you throw more lift at the valve, the effective duration at 0.550" and
anything below that lift point will change.

IE: If a head flows 300 CFM @ 0.520" you would benefit from the additional
lift because the valve stays open longer above 0.520".

IOW, the longer period of time the valve can stay open in the head's best
flow area, the more air will theoretically enter the chamber.

I wonder if switching to an ITB setup would help free up airflow on the intake side of things?

Robert, I'm surprised you would even say something like this.

As Tony Mamo mentioned above, better heads will give you more airflow potential. Even if you have a restricted entry, better heads will give you more power. For example, NASCAR restrictor plates choke down the power by making the entry of the intake manifold and carburetor very small. Since they have this huge choke in front of the heads, would it still make sense to put killer flowing heads on the car? The answer is yes! When the teams put better and better heads on their restrictor plate motors, the cars went faster and faster.

Let's say a stock LS1 head flows 230 cfm (bare) and 215 cfm with the stock LS1 intake installed. Now you add ported heads that flow 280 cfm (bare). The stock LS1 intake might knock the flow down to 245 cfm, but it's still 30 cfm better (at the head) than running stock heads and stock intake (245 cfm vs. 215 cfm).

Take this exercise further, add an LS6 intake and now you get 255 cfm at the head. Your motor feels like you just added heads that flow 10 cfm more, but the power increase comes from eliminating much of the intake manifold's restriction.

Add a FAST 90 intake manifold with a 90mm throttle body and you'll probably see another 15 cfm to the head. Now the head is getting 270 cfm of flow and the motor feels like it just got a head that flowed 15 cfm more. Basically, 280 cfm heads are flowing within 10 cfm of their potential. The motor makes more power because you've eliminated more of the restriction before the cylinder heads. Make sense?

i put it up so people would have something to think about. give something to add to the post.

i guess my other real question would be is where does the LS1 intake become such the restriction you need a better one?

i think i'll be trying to beat the 400 rwhp mark in an auto with LS1 intake. i'm at 366 rwhp now. i'm also contemplating maybe doing 400 rwhp with stock cam. just don't know which way i want to try. unless they have already been done.

let's see finish bolt ons (pulley, LS6 intake, etc..) and do 400 rwhp or
add cam with stock LS1 intake and do 400 rwhp

pick one for me.

Tony, if you are still following this thread.....

What are your thoughts about possibly "Extrude Honing" a FAST manifold ??? The manifold is supposedly ~meaty~ enough for porting. Why not gain the benifits of the Extruding process that balances out individual port flow to within 5cfm and increases plenum volume, that the process alone is capable of ??? Not to forget to mention the other advantages of the finished product, that would help a dry manifold.......???