rking

iTrader: (3)

OK, I dont have my degree in all of those fun sciences that allow some of you to answer these questions, so forgive me if I am missing something in this concept!

As we watch the heads for these engines reach close to 400cfm, full race, and with sheetmetal intakes and all of the super high flowing components we can get now, I was wondering to what extent the engine can actually use it. I mean, the engine itself, whether its the displacement or some other determinate, has a maximum flow capability at given given rpms, right? I mean, in an ideal world, could a 4 inch bore engine use a 4 inch intake valve and a 4 inch exhaust valve. To make things a little easier, what, in cfm, could a 4 inch bore, 4 inch stroke cylinder flow?

If the flow of the cylinder is much better than that of the heads and intake, then do you get insufficient cylinder filling during operation. Do the bigger heads and intake help increase that efficiency??

TIA
Just some fun thoughts that I have been kicking around

SStrokerAce

iTrader: (2)

First think of the concept of CFM as the more CFM you have the less flow restriction you have on filling the cylinder.

The engine demand for CFM at a certain pressure drop (which is determined by the venturi area of the port) depends on the displacement and speed at which it is happening. So more cubes or more RPM means it demands more air, so the less restrictive you have that the better, AS LONG AS IT'S SIZED CORRECTLY FOR THE RPM RANGE YOU ARE RUNING IN.

Bret

rking

iTrader: (3)

So, in theory, we could never develop a set of heads that will outflow the capabilities of a given engine?

ringram

You can, but thats what Im pretty sure Bret is saying, if it flowed more than the engine needed, velocity would be low and the cylinder would not get the ram effect from optimal intake port sizing and hence power/torque would be less that with the smaller, but still good flowing ports.

rking

iTrader: (3)

With the L92 and LS7 heads, reaching close to 400cfm fully ported, how close can we guess that they are to reaching the full potential of say a 402 stroker. Can we still go bigger, with positive results, or as these heads getting too big at this cfm and should be reserved for even larger displacements?

TrendSetter

look up pro stock truck motors. they are less than 400 inches and make enough hp to run 7.50s.

mrdragster1970

There will be a point where more flow will not increase HP. As in putting a head that flows 500+ cfm on a stock 396" with a whimpy little cam. The cylinder will only fill as much air as the combo will allow.

I made a test a few years ago. We put the same head on a 477" & a 557". The car ran 9.31 best on one and 9.32 on the other. Air flow is air flow, there is always a weak link somewhere. Bore size, cam, heads, intake ect, something can always be improved. The only problem is rules, material, technology something that keeps a 350" from making 5000 HP.

As everyone knows, the most important item in any engine is the total combination.

SStrokerAce

iTrader: (2)

Yes, and as odd as it may sound you don't always want the smallest head possible. The L92 proves that now. As John said "the most important item in any engine is the total combination"

Bret

rons 00z

iTrader: (10)

it anger's me that NHRA pulled that class

Old SStroker

In spite of PS Truck 950+ hp 10k+ 358 inch engines, which made BMEPs in the range of ProStock engines, and put on a neat show, if there is not enough sponsor money for the teams AND for NHRA, it's not a viable class. It's a business, as is Cup, F1 and World Rallye, major league baseball and NFL.

Gearheads love Pro Stock for the cutting edge technology, but many regular fans only go to the races for the 330 mph fuel cars. I suggest if it weren't for the OEM $upport, PS would fold up.

mrdragster1970

.

I don't watch or care about P/S or P/S truck. The judge felt that NHRA cheated those guys and that there was a following in specatators and sponsors. From what I remember, there was plenty of big name sponsors in the class. Around here plenty of people loved the class. I for one don't understand most of the decisions NHRA makes. If they weren't happy with the class, they should have been more honest with teams.

I'm glad that Pro-Mod is doing well, and I'm still hoping that it will be a full time class some day. Hopefully the big check they had to write will make them wake up and be more honest with everyone. As for the new assine points system, let's not even go there again. There are some very stupid people out there in glendora.

rons 00z

iTrader: (10)

thats what i remeber as well. now i was only a kid back then but i clearly remember my dad and *** his racer buddies all pissed from the class getting the axe. i wonder what happend to all those old chassis???

mrdragster1970

Top Sportsman sucked up a bunch of them, I see a bunch in comp also.



Does anyone know what the amount they won was when they won the lawsuit???

.

FieroZ34

iTrader: (1)

On most 4 stroke internal combustion automobile engines (Did I cover all the bases there?), dare I say all, the practical limit to power is the cylinder head. Fact is, it is an inherit bad design, that cannot get better (At least hasn't yet).

Here is the formula for engine flow, in CFM:

Displacement (L) X RPM X VE X Pressure Ratio / 5660 = Flow in CFM

We'll use a stock LS1 for example, I believe 346 cu.in., which translates to 5.6699L of displacement. As for RPM, we'll compare at 3000, 4000, 5000, and 6000. For Volumetric Efficiency, we'll use 100%, however realize that a stock LS1 doesn't break 85%, at any RPM. The pressure ratio will be 1.0, since we are normally aspirated.

This gives an LS1 a maximum flow of CFM - RPM:

300.5 - 3,000
400.7 - 4,000
500.9 - 5,000
601.0 - 6,000

So on a factory displacement LS1, running at 100% VE, you'd be flowing around 600cfm max.

Also remember, to run at 100% VE, you need some pretty kickass head/cam/intake combo. I'd go so far as to say there is no average built LS1 running over 95% VE. However, as RPM rises, so does VE. So it is possible that one of those really exotic builds, that turn 8,000rpm, is capable of 95, maybe even 100%. And with acoustically tuning the induction cycle, cracking 100% is possible (Just highly improbable with a 2 valve configuration).

Volumetric Efficiency = Flow = Power.

rking

iTrader: (3)

Nice Write Up!

Old SStroker

Hmmmm. There a quite a few folks building 2-valve NA engines that would disagree. Highest VE generally occurs near torque peak, or perhaps VE determines torque peak rpm. Some really nice 2-valve engines achieve over 100% VE @ power peak rpm (PPRPM). Look at engines with power peak rpm BMEP over 13.5 Bar (about 200 psi ) for some examples. ~110% is doable @ PPRPM. Not easy, but doable.

Steve Bryant

iTrader: (1)

Since cylinder head intake and exhaust flow rates are in CFM per cylinder, you would divide the four figures above by 8 yielding 40 to 80 CFM per cylinder (between 3,000 and 6,000 RPM's) with your assumptions.

Steve

RednGold86Z

But, the intake valve is not 100% open 100% of the time, so it's not a direct comparison. Maybe if you integrate the flow (at an unrelated pressure drop, though) over the cam profile and crank angle, it could be a little closer, but then again, you'd have to fudge or improve the model.
VE *IS* (pretty much) the important number from head flow and displacement, as far as intake valving (flow @ lifts, timing, etc). Torque from that VE is the main objective, and comparing torque and VE will show combination efficiency.

1320FEVER

iTrader: (9)

I'm glad you guys brought the up, because I've been trying to justify why I need a 90/90 setup. Basically, I have a 408 that spins at 6500 rpm. Your formula says that I need 408 x 6500 x 1.0 x 1.0 / 5660 = 767 cfm. I used 100% volumetric efficiency for arguments sake.

I've read somewhere that the stock throttle body flows at least 800 cfm. It might be a good assumption that the LS6 intake flows comparable to the stock throttle body.

I keep hearing all these gains by aftermarket intakes, yet since my 408 only requires 767 cfm (at 100% volumetric efficiency, which we know is unrealistic), then how am I going to gain power from anything that flows better than the LS6/stock tb with the current setup that I have?

FieroZ34

iTrader: (1)

You misused the units in the equation a little bit.

First, you need Liters, and 408 = 6.6859L. Next, the VE need sto be in percent, so use 100 and not 1.

So for your motor, you'd need 6.6859 x 6500 x 100 x 1 / 5660, which yields 767.8cfm.

Remember that intakes play hard into your VE. Sure some may flow more, some may not, but flow isn't everything. Runner length, runner diameter, port shape, plenum volume, all of these affect a certain target RPM. The stock LS1/6 intake is tuned to match with about 5200rpm IIRC, but more importantly it needs to give a "stock" driving feel. Thus it needs great throttle response, needs to be quiet, emissions complient, etc. And while I cannot tell you how exactly it was designed to do this, it was. So a FAST 90 changes a lot more, and I think with such a large displacement motor, and with such a high redline (6500 is high....I guess....lol j/k, high for the SBC), I think a FAST 90 would be very beneficial to your setup.