Head flow number vs cam shaft profile

[SappySE107]

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[The Dark Side of Will]

I've had quite a few thoughts on this... but no time right now. I'll be back.

[S8n]

I don't know if this will help, it's taken from a Race Flow Development at LS1tech.com:
"The exhaust ports, .. we tend to look at velocity maps through the port, ..
and at the valve seat area as more important then just a cfm number
from the flow bench. In exhaust ports, cfm numbers compared to
the intake just doesn't work the same.

Two things to remember with an exhaust port, .. when the valve cracks
off the seat the air is over 600 - 700 lbs of pressure and very hot and expanding.
Also the design of the exhaust valve seat area has to help the port flow go sonic.
There's MANY more dynamics to exhaust port flow to look at."

Here's the link to the entire post:
http://www.ls1tech.com/forums/showthrea ... ge=1&pp=20

[Aaron]

A good rule of thumb is that normally aspirated engines whose heads have exhaust-to-intake flow-ratios under 70 percent (i.e., the exhaust flow is 70 percent that of the intake) like a dual-pattern cam to crutch the weak exhaust port. Over 80 percent, you're in single-pattern territory. Blower, turbo, and nitrous motors generally prefer dual-pattern grinds because of the larger-than-normal exhaust volume they generate.

This sounds like it will be a good discussion *clicks watch this thread*

Anyways, I think you made a typing error here, or I'm just confused...Yah, that's probably it...

So boosted motors tend to like dual pattern grinds because they make more exhaust volume? Yet N/A motors with under 70% exhaust flow with respect to intake prefer dual pattern as well? So both motors, one with a lot of exhaust volume, the other with very little (in comparison), both like dual grinds?

[teamlseep13]

Do you have the flow sheets for the head your working with now? For both the exhaust and intake? Post that information and I can help ya out much better.

If this is a nitrous motor, then yes exhuast volume will be much larger when on the bottle. Having extra exhaust flow will help get all of that exhuast out as fast as possible to allow for the next fresh charge.

Now, some people think its ok to have medicore heads and just crutch them with a larger cam. I say that you should put your time and money into getting the intake and exhaust ports flowing as well as they can. After that, then worry about your E/I flow ratio and choose a cam that will work for your application and heads. Don't do it the other way.

If you get the heads flowing as well as they can, then a single pattern cam should work just fine, but to get the most power out of the motor while it is being bottle fed, you will want to have a little more lift and duration on the exh.

Post those numbers and I'll get back to ya.

[SappySE107]

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[SappySE107]

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[Aaron]

aaron. I didnt write that, its a copy/paste from a magazine article.

Gotcha, my mistake.

[SappySE107]

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[Shaun41178(2)]

What heads are you testing?

[SappySE107]

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[Aaron]

I am working on a set of 3400 heads now

:thumbleft:

[SappySE107]

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[The Dark Side of Will]

Two things to remember with an exhaust port, .. when the valve cracks
off the seat the air is over 600 - 700 lbs of pressure and very hot and expanding.
Also the design of the exhaust valve seat area has to help the port flow go sonic.
There's MANY more dynamics to exhaust port flow to look at."

Here's the link to the entire post:
http://www.ls1tech.com/forums/showthrea ... ge=1&pp=20

I haven't read the post on the original forum, but I agree with most of this, except...
600-700 psi of residual pressure? Even if the dynamic expansion ratio is down around 8:1, that's still 4800-5600 psi of peak pressure. With cylinder pressures like that, you're basic 350 will be making about 9,000 ftlbs of torque. Obviously, the cylinder pressures (and residual pressures) are less than this.

Intake/exhaust flow ratio and cam selection is all about equalizing the flow on either side of the cylinder... making sure that everything that got in can get out just as easily. If the exhaust flows better when temperature and residual pressure are factored in, then the intake is the restriction and vice versa.

As the intake valve opens and the piston moves down, the pressure difference across the port in a naturally aspirated engine will never exceed 14.7 psi (at least not by much). Assuming that the engine is well sorted and acheives 100% VE, then the mass of air in the cylinder will be the displacement of the cylinder times the density of air at STP.

The air is compressed by the dynamic compression ratio, and the mixture lighted. The mass of fuel included will be 1/12 or so the mass of air. Using the known thermal content of gasoline, the energy released can be calculated. The pressure/temperature rise can be approximated, as can the residual pressure and temperature, via dynamic expansion ratio. The dynamic expansion ratio is basically the same thing as the dynamic compression ratio, except figured using exhaust valve opening angle.

So know a known mass of exhaust gas with approximately known temp and pressure needs to get out through the exhaust port. The lower the compression/expansion ratio, the higher the residual pressure and the greater the pressure difference across the exhaust port. This means that low compression engines can make do with lower exhaust port flow numbers.

High compression engines (traditional high performance engines) have less residual energy in the exhaust and therefore less pressure difference across the exhaust port when the valve opens... This means that higher compression engines need higher exhaust flow numbers.

Supercharged and N2O engines have very large exhaust volume compared to intake volume. In the case of the supercharged engine, the intake is pressurized while the exhaust is not, and in the case of N2O engines, the extra oxidizer enters as a liquid and leaves as a gas.

Turbocharged engines are more complicated because both intake and exhaust sides are pressurized. The exhaust has, in almost all cases, higher pressure than the intake, so the residual energy at a certain boost level needs to be compared to the back pressure created by the turbine to come up with the pressure difference across the port, which can then be used to calculate ideal exhaust flow ratio.

Anyway... the "flow integral" over the entire valve event needs to be taken into account, which is how the cam comes into the equation. A dual pattern cam is a bandaid for cylinder heads that are not ideal for the application. The smaller side of the cam is running at less than its potential.

[S8n]

[quote="The Dark Side of Will]
I haven't read the post on the original forum, but I agree with most of this, except...
600-700 psi of residual pressure? Even if the dynamic expansion ratio is down around 8:1, that's still 4800-5600 psi of peak pressure. With cylinder pressures like that, you're basic 350 will be making about 9,000 ftlbs of torque. Obviously, the cylinder pressures (and residual pressures) are less than this.

[/quote]

I agree to a point. Combustion pressures are around ten times that of compression pressures. If you have 100 psi compression and 1000 psi combustion, and the exhaust valve starts to open halfway down the exhaust stoke, 500 psi in a gas wouldn't be to far off. The person that I took that quote from builds racing engines which could see 140-150 psi compression pressures, which would make a very high pressure when the exhaust valve opens, of course depending on cam timing.

[The Dark Side of Will]

I agree to a point. Combustion pressures are around ten times that of compression pressures. If you have 100 psi compression and 1000 psi combustion, and the exhaust valve starts to open halfway down the exhaust stoke, 500 psi in a gas wouldn't be to far off. The person that I took that quote from builds racing engines which could see 140-150 psi compression pressures, which would make a very high pressure when the exhaust valve opens, of course depending on cam timing.

You're working the math wrong.
If the engine has 11:1 static compression and 1K psi combustion pressure, then the volume with the piston half way down the bore will be 6 times the chamber volume. It will have ~166 psi of residual pressure at that point.

[S8n]

You're working the math wrong.
If the engine has 11:1 static compression and 1K psi combustion pressure, then the volume with the piston half way down the bore will be 6 times the chamber volume. It will have ~166 psi of residual pressure at that point.

Doh, yes, well sort of. Remember, combustion is a timed event, not a point in time. Combustion can continue for 20-25% of the piston's stroke down the bore. I agree, the highest point is around 1k psi, but is doesn't drop off that much right away. * But after further research, it would take a hell of a combustion cycle to have 600-700 psi when the exhaust valve opens.* Maybe half that, I don't have an exact number.
But, back on subject, the point of the quote in my first post was to show that the exhaust ports have alot of help from the exhaust stoke in evacuating gases. CFM is not the only thing that should be looked at, as velocity through the port should also be a factor.

[The Dark Side of Will]

Yeah... I was approximating for the sake of simplicity. LPP (location of peak pressure) is typically 14* ATDC, AIUI, and combustion continues afterward to some extent, but the production of higher pressures is overcome by the increasing volume the gases have to fill.

The difference in delta P across the ports was the point of my post as well... the greater the delta P, the fewer STP CFM you need in your exhaust port.

Velocity is important as it affects inertial scavenging/supercharging effects. A useful approximation of port velocity relative to other cylinder heads can be had by comparing flow numbers and port cc's. The smallest port that flows a certain amount of air will have the highest velocity. AFR LSX heads supposedly have the port volume of stock LS1 heads, but the flow of well ported LS6 castings... meaning high velocity and good streetability while having the potential for VERY high HP.

Read a short article once that mentioned that the resdual pressure in the cylinder of a Top Fuel Dragster when the exhaust valve opened was greater than the peak combustion pressure of most street engines.

[The Dark Side of Will]

Bump.

Was looking for "chamber volume" specs recently and stumbled on this thread.

Dam I sound smart.

I wonder what happened since then... :scratch:

:thumbleft:

[Aaron]

I wonder what happened since then... :scratch:

We are too.










j/k Will :thumbleft: