3.2 or 3.4 airflow: galons per minute or cubic feet per minute?

[gliding_serpent]

In this post I was trying to wrap my head around the flow required for a 3.4L conversion using common online calculators. These told me I would need 342CFM for my 3.4L, which I interpreted as 57CFM per plenum/cylinder. This just seemed too low however.

Now, this calculator for carb sizing makes more sense, suggesting a 206 cubic inch engine at 6760rpm needs ~342CFM per carb (all calculations assuming 85% volumetric efficiency), or 2052CFM for the entire engine. Given that my stock plenum flows about 260CFM per plenum, it would make sense to extrude hone

Now, Sal, who knows a lot more than I do, suggests a 3.2 needs about 12,000 grams per minute of air. He estimates a 3.4 needs around 15,000g/m.

2052CFM above calculates out to 15,350g/min. right in line with Sal's prediction.

So my questions are:

1. are the CFM calculators all calculating the need per cylinder, and some are just not telling me?

2. Are the numbers/calculations above in line with what others understand the engine requires for intake?

3. An finally, how can folks like Steve Wong get 3.2 cars that are stock aside from exhaust and race fuel (i.e. intake is stock), to get ~280hp with the stock intake/plenum? What I am getting at is, is there really any real-world value to bench flow numbers of your intake plenum if you can get that much power out of it. Or, is it that race gas increases power without the need for more air, the combustion process is just more efficient?

Basically, I am trying to figure out the need/benefits of extrude hone for my 3.4 conversion (and my flow to each cylinder is pretty balanced at ~260CFM per plenum, only 8% difference between best and worst).

[mytoy]

gliding_serpent;

Sal's calculations are spot on.

Your assumption of 57 cf of air per cylinder is correct except that you are forgetting that is for every cycle of each cylinder. At 6760 rpm each cylinder will cycle 6 times per minute therefore you will need 57*6=342 cubic feet of air per minute to feed each cylinder enough air.

Your intake only flows 260 cfm on average which for your engine will limit the rpm to 5410 rpm before starting to take a nose dive.

Your total intake system must be able to flow 342 cfm of air. That includes the throttle body, air tubing and air filter system.

The question I have is do you know what your heads will flow? If your heads only flow 260 cfm then it's no use worrying about your intake flowing more.

You have to look at the whole picture to get the most out of your build.

You ask how Steve Wong can get 280 hosepower out of a stock 3.2 intake. It is simple in that 260 cfm can sustain approximately290 horsepower if every thing is working together as a system.

Now 342 cfm will be able to sustain approximately 375 horsepower if all components ie: cam, compression, cylinder head flow, intake flow, and exhaust flow are all in sync.

I do not know if extrude hone can increase the flow of your manifold by 82 cfm or not. You would have to talk to them to see what they can do.

[gliding_serpent]

Thanks, I knew I was missing something when I looked at Sal's numbers.

Factors far beyond my understanding at this time are in play. But you confirmed my suspicion if you are correct, in that increasing flow of the intake plenums will not necessarily help.

Steve always said the limit in my system will be the SSI's. No use bringing in more air, if you can not get that air out at an equal rate. Thus his skepticism of extrude hone, and his recommendation for larger primaries (stock works just fine).

I do not know the flow of my heads, but I do remember that it is good for over 300hp. ~340 range according to Steve Werner posts. My understanding is the Porsche overbuilt the heads in the 3.2, so no need to go in there.

Now, I am going by a tried and true build of 3.4L max moritz (9.8:1), 964 cams, 91 octane, SSIs. also adding to that 66mm Throttle body, open airbox. I expect 260hp. Steve said i would get 10-15hp more if I put the stock headers back on. Tough decision given the extra weight...

[scarceller]

Let's keep this simple by simply just mathematically calculating max air flow in Liter/min and assuming a Volumetric Efficiency of 1.0

First we'll talk about VE and the 911 engine and some history, the 3.2L engine has a tuned intake system, the runners are tuned for increased VE at about 5800RPMs and this is the sweet spot for peak torque in these motors. Amazing is the fact that 5800RPMs these engines run at a VE of about 1.1 this means the engine is actually ingesting 10% more air than the displacement of the cyls! At the time the 3.2L was developed it was the first OEM (non-turbo) motor to exceed a VE of 1.0 and Porsche was recognized for the achievement. This is why all those online calculators don't work! Then the 911 engine internals on a per HP base are far lighter than any Muscle Car V8 on a pound per HP base. One needs to understand the that spinning 30-60lbs of metal on a crankshaft at 6000RPMs is no easy task and can easily consume well over 100HP internally alone! This is simply wasted HP that never makes it out to the wheels. The 911 engine really reduces these losses a lot compared to big American iron V8s.

So now for the math:
A 3.2L 4 stroke ingests 1/2 it's displacement in air per rev, so for each rev we ingest 3.2/2 = 1.6Liters of air per rev. Let's save that value.

1.6L / Rev

Now let's assume a red line of 7000 RPMs, 7000 revs per minute we are ingesting 11,200 Liters per minute.

So if we assume a VE of 100% we can ingest at most 11,200 L/min at 7000 RPMs it really is that simple to calc in volume.

But volume is not the same a mass of air so we can adjust for mas assuming sea level conditions at room temp of 68F then the we simply multiply by aprox. 1.2 so at sea level 68F and 7000RPMs we can take 11,200 * 1.2 = 13,440 grams/minute

So here's air flow at 7000RPMs at sea level in volume and mass:
11,200 l/min (volume)
13,440 g/min (mass)

Of course these are not exact because we assume a VE = 1.0 and that's not the case at 7000RPMs but for quick calcs to size injectors, fuel pumps and such this is a very good way to mathematically do the calcs.

In my modified 3.2L it puts down about 230HP at the wheels (265HP at the crank) and I monitor air flow very precisely during a dyno pull. The car has a lab grade MAF installed so I know exactly what the air flow in g/min is at any given time during a run. I also very accurately calculate VE in my monitoring system and log everything. I'll post more details on real runs soon, with the graphs showing what/how the VE behaves across the RPM range. It's very interesting to see this data from a real run.

[scarceller]

Not gallons /min it's grams per minute. And yes about 12,000 g/min for the 3.2L at 6800RPMs is a good round estimate. But follow my math above to do your own calcs

[scarceller]

Keep in mind most air flow benches work in volume not mass and some use Cubic Feet per min not Liters per minute but you can easily convert l/min to cf/min by dividing by 28.3 so:

cf/min = l/min / 28.3

So our 11,200 / 28.3 = 396cfm at 7000RPMs assuming VE=1

But at 7000RPMs VE is not 1.0 it's quickly dropping and most likely it's only 0.8 so
396 * .8 = 317cfm we can round up slightly and assume a max cfm of 320 or so.

[scarceller]

Now for the problem areas in the intake, in order:

1 - the AFM is restricted above 13,000 g/min
2 - the throttle body should be bored as large as possible.
3 - extrude hone the intake, most important is equal flow between runners.

[scarceller]

Also worth noting at 7000RPMs the cyl is filled 3500 times per minute
or 58times/second! Or filled every 17 milliseconds (0.017seconds) It's amazing to think about what's going on in the engine at hi-RPMs.

[gliding_serpent]

Thanks sal!

I will digest this all tonight.

Does race gas increase volumetric efficiency? I.e does it allow more power without increased air need (thinking about stock 3.2 afm cars with race fuel and headers making ~280hp).

My plenums are 8% variable in flow from best to worst plenum. Trying to understand if i should extrude hone. May do so. Tb being bored to 66mm.

I will revise my comments on restricted intake/exhaust flows. There is no cliff, but rather a point at which more and more work is required to move air, resulting in an efficiency loss that becomes more extreme as you push the limits.

[scarceller]

Race gas does not effect VE. It only allows you to avoid detonation and push timing a tad more in detonation prone areas. It only helps if you have a detonation prone condition. If for example you can optimize timing with 93 octane fuel and detonation is not an issue for the given condition then upping the octane is a waist of money.

The very best strategy is to eliminate detonation using other methods first, like dual plugs per cyl.

Here's a myth you can disprove every time, you can NOT increase torque without increasing air flow for a given engine condition (say WOT at 5000RPMs) assuming you have properly tuned the motor in the first place. It's simple theory: for every gram of fuel you need 12-13 grams of air at WOT. This is what we mean by AFR 12.0-13.0 for best power. You can't throw extra fuel in and expect more power without the air to match the fuel.

Then you also need to consider at what RPM you wish to increase torque, it's a holy grain to always increase torque at higher RPMs since that's where you make the most HP, you need to think about this point carefully. This is why race engines run at 7000-8000RPMs and achieve very good air flows at these high RPMs. It's also why our little 3.2L motors feel so strong, they simply pull and pile on HP well into the 6000RPM range, try that with American muscle V8s.

Originally Posted by gliding_serpent

Thanks sal!

I will digest this all tonight.

Does race gas increase volumetric efficiency? I.e does it allow more power without increased air need (thinking about stock 3.2 afm cars with race fuel and headers making ~280hp).

My plenums are 8% variable in flow from best to worst plenum. Trying to understand if i should extrude hone. May do so. Tb being bored to 66mm.

I will revise my comments on restricted intake/exhaust flows. There is no cliff, but rather a point at which more and more work is required to move air, resulting in an efficiency loss that becomes more extreme as you push the limits.

[scarceller]

Again, the AFM is the first issue, I've had a AFM flow benched and it has a lot of pressure drop across it at above 10,000 L/min enough so that it results in close to -1.8PSI intake vacuum. This is simply free power to be tapped. The AFM was well designed for the 3.2L in stock config but once you increase air flow 15% or more it becomes a problem. The next component is the TB. Then the plenum runners being out of balance is more the issue than what they flow, the reason to hone them is to make them flow the same not really make them bigger. You just need to find the one that flows the most, polish it first then hone and polish the others to match within 2% and your good.

I've seen reports that the Air Box is an issue but my testing shows otherwise, we've run with and without the air box, with cone filters and such and this makes no difference. At least the tests I've done show the air box is not the issue.

[scarceller]

This is a log from a modified 3.2L running with the factory AFM here the blue line is showing you volumetric efficiency during a 2nd gear WOT pull done on the street under normal conditions. Notice how these motors really have a sweet spot for VE at 5000-6000 RPMs and below or above this sweet spot the VE drops off considerably. Porsche did this on purpose, and these motors exceed VE=1.0 in the sweet spot. This graph has a lot of useful data to help better understand what's going on with air flow.

Sorry the blue line is hard to see but that's the VE line, download the pic if you need to see it better.



The green line is actual air flow in Liters / minute