So you guys are saying that on a motor the heads can and will actually see greater the 28" of derpession vacuum? I wouldn't thought this was the case based on the formula (Cubic Inches)(RPM)(VE/3456)=CFM Requirement. Vizard suggested that if you think about the flow rating of a carb at 28" you must think about it at lower ratings. Obviously a port will flow less air with less depression/vacuum on it. The end result of this way of thinking was that if you have a motor that ingests 1000cfm of air given the above formula a 1000cfm @ 28" of depression carb may leave some hp on the table. This is because a vacuum of 28" of depression is not actually seen by the carb, therefore the carb will not trully flow that kind (flow rating at 28" of depression) of cfm on your motor.

Now, I think this can be applied to a intake port as well. How could the full flow potential of a port be used in most n/a street motor applications as measured at 28" or depression. If you multiply 300cfm by 8 you are talking about 2400cfm of total flow and none of the motors, even n/a race motors, are comming close to using that kind of air.

Just some food for thought, what are your thoughts?

 

...dont say that.

 

Just some examples.

Say you have a 10000rpm 400 cubic inch sbc with 435 cfm sb2 heads on it. Assume a ve of 130% (1.3). 10000rpm (400 cubic inches) (1.3)/3456 = 1504 cfm

now take 435 cfm and multiply it by 8 and you get 3480 cfm

but then again this is the peak number of flow. lets say the average flow that is seen in the intake port is 300 cfm (average for which the intake valve is actually commanded to see by the cam lobe). multiply 300 by 8 and you still get 2400 cfm

obviously 2400 and or 3480 cfm are way higher then the value required by the motor

 

First off Carbs and TB are rated at 25" not 28"

What I'm describing is what is going on in the port during the wave tuning effects. You get spikes in the port pressure that can be much higher than 28", but when they all meet back up under the carb they equal out to a lower pressure. There are a lot of things going on dynamically guy normally don't test for.

Bret

 

http://www.accufabracing.com/Flow%20Data.htm

OK, so I assume that when you are getting a 750 demon carb that it flows 750 cfm at 25" then right. I just posted the above link so people can see that flow dimishes with less vacuum.

I think I kind of see what you are saying. There are some many different things going on at different times that it is hard to make an assertion like i have. But to me it kind of makes sense but kind of doesn't.

What I'm attempting to find is that if a head backs up at .700" when flowed at 28" of depression is it really actually backing up on my motor? I understand that during the overlap cycle a very large vacuum can be created at the intake valve at low lifts base off of the suction induced by the pressure wave tuning of the exhaust. Now that being said, is there another pressure wave that may be induced at higher valve lifts which may create the same spike in vacuum? If so I assume this would be the intake tuning that people speak of and I'm not real sure about how it works. If not though the port may see much less vacuum at these higher lifts and not actually stall out like indicated on a flow bench.

Can some one touch on the intake resonance tuning I keep hearing about. I assume it is a separate event from the exhaust induced vacuum on the intake valve at lower lifts caused by overlap.

 

I think this thread has now gotten WAY over my head.

 

I can begin the ball rolling on the wave tuning discussion; but your post is
a touch confusiing.

Let's iron out the ambiguity first. At the intake valve:

Low Vacuum = High pressure

High Vacuum = Low pressure

Agree?

 

Understood, agreed.

 

If we're good on those concepts, then we can make sense of this:

If less/low vacuum = high pressure, and fluids/gasses move from high pressure
to low pressure areas, then it's easy to understand that flow will diminish.

If pressure in the intake manifold/plenum is relatively high compared to outside
atmospheric pressure, then the quantity of air moving through the carburetor
will be less.


I wont even pretend that I'm an expert on physics, so I'll point out some basics
and let others elaborate.

Intake tuning is similar to exhaust tuning, except the engine wants high pressue
events on the intake side of the intake valve, and low pressure events on the
exhaust side of the exhaust valve. Keeping in mind that fluids/gas move from
high to low pressure areas, the train of motion will commence.

Anyone with background in audio tuning can relate some of the concepts to
engine tuning. I believe the two main characteristics of tuning involve pressure waves
and acoustic energy.

Pressure waves moving at different rates and lengths over the engine's operating
range.

Acoutic waves moving at different rates and lengths over the engine's
operating range.

At any point in time there are negative and positive pressure bumping into
each other as the pistons are flying and valves are opening/closing.

Depending on the length of intake runner and exhaust runner, the pressure
waves will be in and out of phase. The acoustic energy is also in and out
of phase.

At a certain RPM, the parts will become resonant at some harmonic
length.

This might mean that a high pressure wave is heading toward the intake valve as the intake valve is open; and there is a negative region at the exhaust
port at the very same time (during overlap).

Even while the piston is at TDC, the sucking effect created by the tuned
exhaust pulls in intake charge without mechanical aid.

As for the acoustic energy, you may have heard the exhaut note sound
somewhat louder, or different at certain points. When a tube matches
a sound frequency's wavelength (or harmonic lengths), the tube is said to
be resonant.

It will begin to vibrate and create standing waves in phase to help move
the air.

Just imagine all of this happening at 50-60 times per second, or faster if you
have a top fuel funny car motor laying around!

Anyone with an acoustic guitar can try humming tones into the sound hole.
When you sing the right note, you'll notice certain strings begin to vibrate.

 

Adrenaline, you couldn't have broken that down any better. Can you say tech genius? Thanks again.

 

Thats a good description of what is going on in there.

As for Anytime you see flow stall at any depresson in a head at any lift it's a bad thing and describes that something is going wrong in the port. To me it says a better head porter needs to get a hold of it.

Bret

 

Thanks Adrenaline, that was very helpful.

Now, moving onto the head port and my cam lift question. First off is it possible that a port when flowed on a 4.100" bore will act differently then when flowed on a 4.155" in regards to stall. I know the flow numbers will traditionally go up on the bigger bore but will it influence stall at all? The original owner of the items I have purchased had a cam setup for .807" lift on the intake on a motor with 4.100" bore, that's a good bit past the stall point of my head according to the flow numbers I saw from HPE on a 4.155" bore. Is it possible that the head did not stall out, but simply returnd slightly lower numbers passed .700".

Edit: Flow (4.155" bore, no pipe) numbers inserted.

Intake:

.100 67
.200 150
.300 222
.400 295
.500 341
.600 368
.700 367

Exhaust:

.100 43
.200 111
.300 168
.400 207
.500 222
.600 227
.700 231

 

If the numbers go lower then it's stalling...

Either way this is something Eric can handle I would expect.

Bret

 

Thanks again for the help