1320FEVER

iTrader: (9)

Oh yeah, I assumed his unit factor was the same as what I used.

I used [408 x 6500 x VE x PR] / [2*(12^3)] which I thought was the same basic equation that he used. That is why I got the correct answer with the misuse of units.

antivenom370ci

iTrader: (6)

with a flow bench with your intake mounted to the head you can test both Find out where peak fow at what valve liftis then have a cam ground for that valve lift keep the duration high without killing bottom end (street driver). The Cylendar is going to fill it to the best way that it can due to atmoshereic conditions that is what a MAF and MAP sensors are for depending on running speed density or not. As with intake designs long runner=more torque, Short runner less torquehigher hp, fords factory intakes (Dual Runner) As is with my experience the more air you can get to a cylendar the better off or easier it will be to make more power. Then you throw in humidity and that is a whole different ball park. It is possible to fill more than a cylendar will allow, over 100%, ie nascar restricter plate racing 130%ve I have done these things to a Lq9 and got 567rwhp on a 370cuin 8.5:1 engine and will rev to 7500 easy. It is all about airflow.

Bink

greysteel_M6

iTrader: (5)

I agree. Just visualizing this with an integral, as the potential is approached then achieving the maximum flow of an engine would approach infinity.

greysteel_M6

iTrader: (5)

I think that has to do with the exhaust pulse and vacuums created by the valve events.

Ls2Sonoma

iTrader: (4)

yes, but dont forget that the air moving down the intake runner has momentum and mass, which can also help overfill the chamber

P Mack

iTrader: (6)

The maximum flow of an engine wouldn't approach infinity if you kept improving the heads but not changing the displacement or rpm range of an engine. You could approach a theoretical "optimum VE" but that VE would not be infinity.

The theoretical optimum VE I came up with is 158% for an NA engine. Keep in mind this would be the unreachable ideal, like 100% thermodynamic efficiency for a heat engine.

P Mack

iTrader: (6)

These flow numbers are always misunderstood. That throttlebody flows a certain cfm at a certain pressure drop. At 0 cfm there will be no pressure drop across the throttle body. At 100 cfm there will be a small pressure drop, at 800 cfm there will be a larger pressure drop. What pressure differential did they have to put across the throttlebody to suck 800 cfm of air through it? If you used a throttlebody rated at 1000 cfm (same pressure drop) it would lose less pressure on the way to the intake manifold when it's only flowing 800cfm instead of 1000. At some point diminishing returns kicks in but it's not like there's a definate cutoff at 800 cfm.

Wnts2Go10O

iTrader: (12)

it isnt all about airflow. you could have the best breathing setup in the world. the thing is if you cant get the air where it needs to be at the velocity it needs to be, youre motor will be a dog.

i think the reason why l92's changed things a little is the long runner. it seems that its able to speed air up a bit in that high volume of a runner. a short runner head of the same volume would be horrible for anything less than high winding or big inch motor. this kind of brings me to a question. do you use the same charecteristics of an intake to design a runner? ie-runner length, height, width, etc, or is there a set of properties (besides the valve at the end of the road) that dictate how to design each?

treyZ28

The simple answer is that there is not a simple answer.

Wnts2Go10O

iTrader: (12)

its not a simple question:shrugs:

1320FEVER

iTrader: (9)

Yes, I realize that. I can take a quick look at Bernoulli's equation and see that. You can a stock tb and flow it alot more than its rating, but it will experience a greater loss due to drag than a larger tb will.

But at some point, the flow WILL become turbulent, and the flow WILL cut off. Take a look at various flow charts for a given set of heads and you will see this.

At what point turbulence occurs on a stock throttle body is, I'm not sure.

Wnts2Go10O

iTrader: (12)

i take it this is part of the advantage of itb's. higer flow, less turbulence?

P Mack

iTrader: (6)

Bernoulli isn't really relevant to finding pressure loss due to friction because it assumes frictionless flow. Also, the flow only "cuts off" when some part of it is going mach 1, that's the point where increasing the pressure difference will not get you any more flow. Turbulence does not "cut off" flow unless the flow is going fast enough that the turbulent boundary layer chokes the rest of the flow to mach 1. 800 CFM through a 90mm throttle body is about Mach 0.2, not even close to being choked (mach 1).

Really the benefit of a larger throttlebody comes from the fact that the air moves slower through it, and slower moving air loses less total pressure due to friction. So you end up with 100kpa in the intake manifold instead of 98 for example.

1320FEVER

iTrader: (9)

That is not correct. Benoullis equation is very relevant in determining pressure loss in a system such as an intake system. Read this and you will see exactly what I mean:

http://www.princeton.edu/~asmits/Bic...Bernoulli.html

As far as "cut off" flow maybe I should rephrase that to say that it "chokes" flow so that the increase in flow is negligible at some point. Take a look at any head flow chart and you will see what I mean...

P Mack

iTrader: (6)

If you assume friction plays a negligible role, you could get your cfm rating from bernoulli's equation. You can't find the pressure drop across the throttlebody from bernoulli's though.

Louie83

iTrader: (3)

I love phsyics but sometimes you have to prioritize real world results above it. Well, physics is real world but there could always be factors not taken into account.

Numerous results in the dyno section have shown that the 90/90 adds power to even the 346's, with zero loss down low. And I have yet to see a 346 that did not gain even more by having the 90/90 ported.

Fastvette94

Engine CFM:
CFM = CID * RPM * VE / 3456
Example: 383 * 6700 * .90 / 3456 = 668 CFM
This is the engine's volumetric intake (swept volume if you will, not a pressure drop calculation). Imagine the piston moving from top to bottom and resting on the bottom until the cylinder is completely full then multiplying the result by a volumetric efficiency factor.

Head CFM:
Example: 298 CFM @ .600" lift 28" H20 pressure drop.
This is a steady state flow measurement. Open the valve .6 inches off of it's seat and then turn on the vacuum pump. Keep turning up the vacuum until a column of water 28 inches tall is sucked up a horseshoe shaped tube. Once the pressure drop is stable at 28 inches, measure how many much cubic feet per minute of air is passing through the head. You cannot compare this flow number to the Engine CFM flow number (flow per given pressure drop verses a calculated swept volume over time.)

Throttle Body CFM:
Example: 750 CFM
Typically TB's and carbs are measured at 1.5 in HG pressure drop which is different than the commonly used 28 inches of H2O for heads. 1.5 in HG converts to 20.4 in H20 pressure drop. So you cannot compare TB or carb flow numbers against head flow numbers directly. A rough comparison can be made by taking the carb flow and multiplying it by 1.17.

Another way to think about TB flow is to remember that 1.5 in HG pressure drop is 0.73 PSI. So if 750 CFM of air is passing through a TB rated at 750 then at that flow rate the pressure of air after the TB is 0.72 psi less than the
atmosphere. In other words you are running 0.72 pounds of negative boost.