ZDriver96

Ok my friend and I are figuring out some head porting techniques. He's been porting for awhile but its learning more all the time.. Anyways he bought some pedo tubes to measure velocity and a flow bench to measure cfm.

Here are what my moderately ported heads looked like from a kick *** porter on here. Has 2.20/1.6 SI valves , 3 angle job.

0.1000 89.0
0.2000 139.6
0.3000 193.5
0.4000 236.4
0.5000 261.9
0.5500 -
0.6000 255.1

Here is what my friend Bob was able to bring out in my heads after doing a full port job.
Number 2
0.1000 84.1
0.2000 143.3
0.3000 198.1
0.4000 244.4
0.5000 275.8
0.5500 282.3
0.6000 269.0

Intake radius guide used on both sets for these numbers.
Exhaust numbers on the fully ported heads max at about 200cfms @.550. at the 70-75% sweet range. The moderatly ported heads were in the 70-75% sweet range too.

He was not able to test velocity until after my set were finished. He believes that my heads have a velocity of 190 fps. Stock heads have a velocity of 150fps. He recently tested his next set of heads and they were at 220 fps.

Ive been searching and asking around but nobody seems to know much about velocity numbers. I dont have a good reference as to how to interpret 190 fps. Better than stock but not as good as it can be. Where the CFMs are awesome.

So help me interpert how i should look at these velocity numbers in respect to my CFMs. Do I want max velocity at the highest CFM available or do i want to get Max CFM and try and get max velocity as a secondary goal?
How much does velocity matter with a larger cam?
I have a xe230/236 .544/.555 112lsa. A larger cam should drive the air through the heads better so u might not have to worry about velocity as much?

Hopefully u guys can shed some light on this.

Injuneer

I don't know the answer... but I altered the "Topic" to correct it to "CFM". And are you using a "pitot" tube to measure velocity?

Not busting on you.... just trying to make sure everyone understands the topic and how you are measuring.

ZDriver96

Yeah i just spelt how it was pronounced.. still really new to all the head porting theory stuff.

Come on fred u have to know it.. if u dont know then who does?

Mindgame

I have a textbook understanding of the subject so maybe I can help... or at least we can get to more specific questions.

One clarification... when you say peak flow velocity, where along the port are you measuring this? A small block chevy head typically flows strongest along the top of the port, so are you giving figures from that area or from mid-port? And at what lift are these velocity measurements?

Numbers I've seen for Vortec heads seem to suggest a velocity of ~260 ft/sec @.500 lift and in the highest flow portion of the port. So you really need to strive for a degree of consistency in making comparisons.

In respect to CFM, velocity is simply a product of port efficiency. Take identical cylinder heads A and B. Head A flows 15% more cfm with a port volume increase of 5% and head B flows 18% more cfm with the same 5% increase in volume. Stands to reason that head B will also have higher average port velocity.

Now if that were all there was to making race winning heads, everyone would be doing it well.

At low valve lifts, the port flow is slow, seat area flow is high. So seat area flow is king where street engines are concerned. That comes down to good seat work (correct angles) and bowl work. The more efficient the seat, the better. If my job were porting heads... I'd spend alot of time experimenting in this area. That, and taking advice from guys who have.

Velocity is important regardless of cam selection. More velocity means better cylinder filling (ramming). Even though pressure wave effects can boost peak torque range volumetric efficiency... ramming does it's thing throughout a greater portion of the rpm band.

There is a "limiting port velocity". The smallest area along the flow path is the area that dictates when limiting port velocity is reached. Prime variables in the equation would probably be... piston diameter, piston velocity, minimum cross section area and upstream pressure. Same as figuring flow through anything... need a pressure upstream, pressure downstream, minimum flow area and maybe a ratio of specific heats.

I believe Vizard states that limiting port velocity is somewhere around .55 Mach. That's the speed at which airflow is "choked" and no more power is realized. I've talked to some people recently who say peak Mach can be higher (like .65) if the port form is highly efficient. But that's usually only the case with different cylinder head designs like the canted valve, sb2's and such. So it all depends.

My 2 cents.

-Mindgame

Mindgame

Think this goes along real well with the discussion I've been having with myself so figured I'd post it up.

Good luck.

-Mindgame

ZDriver96

Sorry mindgame. Spyware has been beating the crap out of my anti-spyware so its been hard to do stuff..
Im going to give my friend a call tomorrow and ask him at what lift the velocity was measured. I know he said that he did not bother with the ones below .300 lift. Im guessing 190fps were the max on mine probably at .550. I'll find out for sure though.

And good read also.

I know he experimented with clay to find which way of porting yielded the best flow results with the least amount of material taken off.
I'll get some more info on it soon though.

SStrokerAce

"There is a relationship between the port area, port volume, and the power produced at various speeds by the engine."

Good statment there! Remember that rule!

Now this one takes some digestion.....

"But the transient pulsation in the running engine, the inertia supercharge, and the tuning effects, are proportional to the velocity of the air going down the port. If it is too small, the flow will drop off. That’s not good. "

Well we can tell he knows what he's talking about, but putting that into English for most people to understand is not his strong point there.

Basically what he is saying is that the intake tuning pressure (I have reffered to this a "faux boost") needs to have enough room to work properly. The best example of this is a LS1 vs. a LT1. A LT1 usually like a small port and small cross section because the intake has a short runner length with the LT1 intake manifold, therefore giving lower intake tuning pressures. Not as much is trying to be crammed down the port, since the tuning pressures are low. Where on a LS1 you have about 4 times the runner length and these motors like port volume and cross sectional area. The longer intake manifold runners create a bunch more intake tuning pressure. Now...........

"But if it is not as small as possible, there will be a reduced inertia supercharge effect. The engine can lose 10% or 15% of your power, particularly near peak torque, instead of peak power. "

and

"Oversized ports, if they are too large, will reduce the inertia supercharge effect, which rams the air in just as the intake valve is closing. Inertia supercharge helps provide high volumetric efficiency."

We all get that though!

Now this is where TOO BIG is TOO MUCH. You will still make the same HP because at higher RPM that is where the larger port is less restrictive so it makes up for it's TOO BIG size. Where at TQ peak where the tuing pressure is high the RPM does not warrant that big of a cylinder head.

Remember the first quote! RPM is also part of the equation. A 1500-5500rpm motor will need a smaller cylinder head than a 4000-8000rpm motor OF THE SAME DISPLACEMENT! That's why at higher RPM you will not feel the loss of that is caused by the BIGGER port.

Then he goes on to talk about how efficient a port is if you can find dead spots on the bench, then you have a inefficient port. It can also be looked at in simple terms as how much average flow per cc (or per sq/in if you have min cross sectional area) you have. The stock OEM Vortec head is pretty dam good in that department. The dead spots also relate to port shape and cross sectional tapper, which he also touches on.

That's a good post MG

Bret

Mindgame

I interpreted Mr. Bettes' statements a little differently Bret.

I think the point he is trying to get across is simple. He is making a case for velocity in stating that "transient pulsation", "inertia supercharge" and "tuning effects" are all related/proportional to air speed.
In other words, the best cylinder head for a given situation will be one that produces the volumetric flow rate necessary to support an engine of x displacement at y rpm with a small high-efficiency port shape and cross sectional area that doesn't go critical before the engine reaches max rpm.

From what I've observed, based on the number of exceptional street engines built these days, they all seem to have the capability of making between 1.9-2.0 hp per cfm of port flow.
Might seem like an oversimplistic rule of thumb but I don't think it's too far off the mark. I could list a number of examples, but that will make this too long.

Taking that one step further, these well developed 260-270cfm (~190-200cc) cylinder heads have the potential of producing 520-540 hp at the crank. If the whole package is well thought out, it should reach that mark. Back to velocity.... these same engines will have strong, wide torque curves thanks to the variables Bettes mentions.

If an engine falls short, then you have some work to do... cause the power IMO, is hiding there somewhere. Could be airflow restrictions prior to the cylinder head or just out-of-phase tuning elements.... maybe a combination of both. Or it could be losses do to port area variations and things that aren't so simple. But I like this oversimplified look at it.

Everyone wants a 300 cfm head these days, but I don't think many of them realize what they really need one way or the other.

-Mindgame

marshall93z

hell, i wouldnt have a problem with a head that flowed 260-270 cfm's if i could get the 1.9-2.0 hp/cfm out of it!

1racerdude

To back up Mindgames theory, a 270CFM head,using my rule of thumb values, is capable of 540- 555FWHP if the rest of the combo is correct.

Mindgame

Absolutely.

Here's a Ford example.

Some people here would say the heads are lousy cause they only peak at 265cfm. The 391 stroker still made 504hp and 515 lbs-ft of torque with a mild cam. Not too shabby.

edit... sorry, broken link. I'll try to find the correct URL and post it later...

-Mindgame

1racerdude

The bad thing about that example is it run's out of wind under 6000RPM's.Good number's,but IMO it would make a good towing engine V something to race with.Dyno Don Nicholson told me YEARS ago "To win races ya gotta keep the ROPM's UP" That's where I've been ever since.
I suppose you could put a 3.30 gear in it and have fun with the torque.
I feel that a 270CFM head would be better suited to a 383CID or less.Set up right it would make plenty HP.It shows what can be done with a good combo and small head,though.

Mindgame

Take that same engine (let's say it's a chevy 383) and throw a ported LT1 intake on it and you're probably going to see another 3-500 rpm before she peaks. ~6500rpm and shifting a little beyond that... isn't too far off from what most of these guys shoot for in their street builds with 3.73-4.10 gearing and full weight. So it's not a bad comparison. Car could still put some lightly modified Z06's to shame.

Not your or my cup of tea LR but not everyone here is into the straight line competitions.

I agree on the rpms but must say I prefer cubes and moderate rpms for hot street cars. Easier on parts.

-Mindgame

1racerdude

Raced dirt track sprint's for 11 yr's in Penn and Ma and a lot of other places in between, and did it for a living for 8 of those,Just like to hear one get UP on the motor.It's a spine tingling sound to me.
Don't see 6500-7000RPM's as a lot even for a street engine.

ZDriver96

Good read..
Here is some more info that i got from my friend.

"The velocity readings I took were highest at max CFM, so typically .550. I believe yours were similar. "

"As for the location of max velocity, I am getting max velocity on the BOTTOM of the port - opposite of what they said most people get. Perhaps thats because I am reshaping the port floors to get the max velocity. There is some variation from top to bottom, side to side. But I am getting most at the seat area, which according to them is the best place for it. "

Man im dying to find out what these 282cfm heads do (190fps at the seat .550 lift.??..?..) I wish we had time to measure the velocity on the LE stage 2 heads but we both have time issues and calibrating the equipment was alittle time consuming..