Port volume vs flow vs velocity

[travis]

I’m trying to better understand the relationship between the items in the subject line and the overall effect on power output and rpm range. This is related more to street performance builds rather than racing applications although I’m sure there is some crossover there.

On a small port OEM head, if you concentrate on blending and smoothing but try to limit actually increasing the port size, you can see some pretty sizable gains on the flow bench with very minimal increases in port size…but it comes at a cost of much higher port velocity. My question is…does the increased flow increase power at the bottom end but hurt top end more because of the much increased velocity? An example head that I have memorized…

Stock D0OE head w/1.84” intake valve. 183cfm peak at .400” lift, 145cc port

Bowl hogged only D0OE with 1.94”, 193 cfm at .400, 320 fps at peak flow, 148cc port

Bowl hogged and blended/smoothed D0OE w/1.94”, 229 cfm@.500”, 152cc port, 415 cfm at peak

Another question…in this situation of better cylinder head flow but velocity being too high, would it be a good move to try to increase the port volume a bit even if no more flow is gained?

[steve cowan]

I’m trying to better understand the relationship between the items in the subject line and the overall effect on power output and rpm range. This is related more to street performance builds rather than racing applications although I’m sure there is some crossover there.

On a small port OEM head, if you concentrate on blending and smoothing but try to limit actually increasing the port size, you can see some pretty sizable gains on the flow bench with very minimal increases in port size…but it comes at a cost of much higher port velocity. My question is…does the increased flow increase power at the bottom end but hurt top end more because of the much increased velocity? An example head that I have memorized…

Stock D0OE head w/1.84” intake valve. 183cfm peak at .400” lift, 145cc port

Bowl hogged only D0OE with 1.94”, 193 cfm at .400, 320 fps at peak flow, 148cc port

Bowl hogged and blended/smoothed D0OE w/1.94”, 229 cfm@.500”, 152cc port, 415 cfm at peak

Another question…in this situation of better cylinder head flow but velocity being too high, would it be a good move to try to increase the port volume a bit even if no more flow is gained?

I am not going into a long rant about the subject because there are so many What IFS,
Depending on what MACH index you use - 0.55,0.58 mach or whatever that's what stops you in your tracks,but its the entire induction size including headers and cam events plus other aspects that limit you,
I think you have answered some of your own questions as well.
MIN CSA,AVG CSA , RPM main players, CFM will be there if sizing is correct.
The last few years I have smaller heads,intake manifolds, cams,headers for the cubic inches, my stuff not world beaters but fun on the street with no reversion issues at all,and rpm no more than 6000 6500 rpm.
And yes we are talking OEM type heads,not raised runner,stock eliminator type deals where FPS is very fast and very efficient

[PRH]

On a small port OEM head, if you concentrate on blending and smoothing but try to limit actually increasing the port size, you can see some pretty sizable gains on the flow bench with very minimal increases in port size…but it comes at a cost of much higher port velocity.

Within the realm of “milder street performance builds”, say....... under 1.2hp/ci........ unless you’re using heads that are clearly unsuitable for the application, I don’t think of the increase in velocity, even when the numbers are text book “too high”, is a bad thing at all.
Most of these builds will never use up the head to the point where those velocities are achieved....... especially at lower engine speeds where the higher velocities are actually beneficial.
Just think of what the OEM’s are trying to achieve with the variable sized intake runners.

I’ve built and/or tested many motors that exceeded the theoretical cfm based hp potential........ using heads that are “too small”.

For this type of build, I like to take a pretty conservative common sense approach and try to taylor the combo to the application.
I don’t overthink it.

[Steve.k]

Im under the understanding if you see a increase in velocity through out the lift range you will gain all through rpm range over non ported head.

[1980RS]

Last year I had the chance to run some small CC runner SBC Vortec heads. While these heads are not the race type they did teach me a few things that really worked well. One was you don't need a huge valve to make power or a giant camshaft. Never in my life would I have thought something like this could push a car into the ten sec. range. Sometimes the small head can be made to make "big" power, just takes skill. For me I seem to find this stuff by accident.

[Steve.k]

I’ve primarily worked on Clevelands and bbf for the most part. Lately some big 500”+ chevy. Basically big port style engines. My son bought a 77 m2 mustang so we bolted a engine together for him. We put flat tops in and a set of ported estreet heads. 2.02 valve and mild flat tappet cam. Dinky headers to fit chassis and 670 brawler carb. The ports to me seem unbelievable small i said if this thing makes power it will surprise me. He wanted a a higher gear for cruising so installed a 3.55 gear with c4. To my surprise the little 306 in street trim 2800lbs without driver ran 12:48 in 1/4. From the takeoff it will shred tires through 1st and part way into 2nd. After building this small port engine gave me a new respect for small port configuration.

[steve cowan]

People have to ask themselves where is there right foot 90% of the time,
In a racing application its a different story to some degree but in a 6000 rpm streeter smaller wins in my opinion.
There is so much choice in cam lobes,cylinder heads etc compared to before.
I think people underestimate a genuine 500hp on the street especially in a old school jalopy

[Steve.k]

Yes i second that.

[mag2555]

It’s always about the best average numbers in the average rpm band your motor is running in.

In a pro stock motor that rpm band may be only 1000 rpm or so wide, in a street strip motor that needs a 3500 rpm wide power band things will be very different!

With normal flow testing done at 28” port air speed is hair over 350 FPS on average if your port your dealing with is not already overly large.

Ford did there development flow testing at 60” which would be about 710 FPS, and to me in terms of there non flat out race motors is why some of there motors have ports so overly large!

A lot of the rice rocket motor tinker’s are finding more average power with port speeds over 500 FPS.

One thing is for sure as you run port air speeds above 350 FPS @ 28” in your motors power band the port shape needs to retain stability even when tested at 38 FPS or some 400 FPS.

Also of certaincy is that the smaller your motors power band is and the higher up it is in rpm in terms of a flat out race motor the more you need to keep port air speeds below 400 FPS with like 350 FPS being the ticket!

When you keep in mind that at only 5000 rpm the intake cycle is only 180 degrees long out of 720 and intake valve is only open for .006 second!
This should easily convince you that moving air into the motor needs to be done in a efficient manor .

[GARY C]

Here is an old thread from Larry with a lot of info on this subject.
viewtopic.php?f=1&t=840

[steve cowan]

Here is an old thread from Larry with a lot of info on this subject.
viewtopic.php?f=1&t=840

Larry has put up amazing information over the years,we need to remember application as well rpm etc
PRH made a good point about airspeed.I know a couple of guys on this forum whose 23 deg head stuff has apex speeds near 500 FPS and they make great power around that 6500rpm mark.

[GARY C]

Here is an old thread from Larry with a lot of info on this subject.
viewtopic.php?f=1&t=840

Larry has put up amazing information over the years,we need to remember application as well rpm etc
PRH made a good point about airspeed.I know a couple of guys on this forum whose 23 deg head stuff has apex speeds near 500 FPS and they make great power around that 6500rpm mark.

Whats your thoughts on the OEM like GM's LS engine's going to big ports and big valves for low rpm truck engines and being up 50 hp and tq over the traditional Vortec truck engines?

[BLSTIC]

My question is…does the increased flow increase power at the bottom end but hurt top end more because of the much increased velocity?

Another question…in this situation of better cylinder head flow but velocity being too high, would it be a good move to try to increase the port volume a bit even if no more flow is gained?

No. If you increase flow in a flow restricted engine you are going to increase power. You may find that you have more of a power increase in the lower end than the top end if your port sizing is actually too small, but at high rpm you are still going to fit more air through the hole than before. Flow doesn't drop to zero at supersonic speeds, it just doesn't increase any further. At higher port flow with the same size port it just means that you hit that flow limit earlier in the induction cycle, there will still be more air in the cylinder.

Port volume isn't a goal that you should be concerned with too much. Its the minimum cross section area that defines when your port goes supersonic and that doesn't usually occur for much of the port. Do NOT go hogging out the port entrance to match the gasket, it basically never has too high airspeed (unless you are trying to move the ports a little for some other reason)

[BLSTIC]

Whats your thoughts on the OEM like GM's LS engine's going to big ports and big valves for low rpm truck engines and being up 50 hp and tq over the traditional Vortec truck engines?

In my mental model, big valves and big ports allow you to run smaller cam timing to get a broader spread of torque while still allowing decent top end. The problem is that with massive ports you can never get decent inertia ramming.

95% VE everywhere vs 110% somewhere makes for a broader torque spread. The extra peak torque is probably down to non-port factors like manifolds, combustion, exhaust tech, and far superior knock sensing

[GARY C]

Here is another thread where Larry shows dyno results from small production heads and the tq and hp gains by reducing localized velocity.
viewtopic.php?f=1&t=860