Comparing 2 heads...port size vs flow

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travis
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Comparing 2 heads...port size vs flow

Post by travis »

Sorry... theoretical question...

Say you have 2 sets of heads, 1 set is a good flowing 170-180 cc intake runner while the other is a mediocre flowing 200-210cc runner. Both peak around say 250-260 cfm with similar flow numbers in the .300-.550 lift range. Would you expect the bigger port to make more top end power, with soggier low end response especially at part throttle? And would this be because the smaller port will start choking at higher rpm's (too much velocity)?

Now if you do a little porting/blending on the bigger head and pick up say 30-40 cfm with a minimal CSA increase, this will increase the velocity through the port. Would this actually improve the lower end throttle response? Or is this another one of those flow bench vs reality situations?
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Re: Comparing 2 heads...port size vs flow

Post by digger »

travis wrote: Thu Sep 23, 2021 3:49 pm Sorry... theoretical question...

Say you have 2 sets of heads, 1 set is a good flowing 170-180 cc intake runner while the other is a mediocre flowing 200-210cc runner. Both peak around say 250-260 cfm with similar flow numbers in the .300-.550 lift range. Would you expect the bigger port to make more top end power, with soggier low end response especially at part throttle? And would this be because the smaller port will start choking at higher rpm's (too much velocity)?

Now if you do a little porting/blending on the bigger head and pick up say 30-40 cfm with a minimal CSA increase, this will increase the velocity through the port. Would this actually improve the lower end throttle response? Or is this another one of those flow bench vs reality situations?
On the first part the missing piece is how much is the engine demanding? if the smaller head has plenty of CSA already to feed the engines demand, then more isn't particularly beneficial in itself
1980RS
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Re: Comparing 2 heads...port size vs flow

Post by 1980RS »

Given both heads have about the same air flow the heads with the smallest ports will make the best power and will be quicker at the track IMO. This year I ran my 358 with the 200cc Sportsman II heads. Next year I will do a set of 170cc Vortec heads and will make them flow about the same 260ish range. Car has run 11.30, so it will be a real good test if the smaller Vortec's are better.
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Re: Comparing 2 heads...port size vs flow

Post by mt-engines »

1980RS wrote: Thu Sep 23, 2021 10:40 pm Given both heads have about the same air flow the heads with the smallest ports will make the best power and will be quicker at the track IMO. This year I ran my 358 with the 200cc Sportsman II heads. Next year I will do a set of 170cc Vortec heads and will make them flow about the same 260ish range. Car has run 11.30, so it will be a real good test if the smaller Vortec's are better.
Why go backwards?
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Re: Comparing 2 heads...port size vs flow

Post by steve cowan »

travis wrote: Thu Sep 23, 2021 3:49 pm Sorry... theoretical question...

Say you have 2 sets of heads, 1 set is a good flowing 170-180 cc intake runner while the other is a mediocre flowing 200-210cc runner. Both peak around say 250-260 cfm with similar flow numbers in the .300-.550 lift range. Would you expect the bigger port to make more top end power, with soggier low end response especially at part throttle? And would this be because the smaller port will start choking at higher rpm's (too much velocity)?

Now if you do a little porting/blending on the bigger head and pick up say 30-40 cfm with a minimal CSA increase, this will increase the velocity through the port. Would this actually improve the lower end throttle response? Or is this another one of those flow bench vs reality situations?
Just recently on my cast iron dart heads,I took the pinch and took it out completely and reshaped with Belzona 1111 super metal.
On the bench- went from 266cfm at 0.700" @ 28 inches and 360 ft/sec avg velocity at the pinch.
After modifying the pinch was 1.95" up to 2.08-2.10" CSA.
at 28" went 280 cfm at 0.700" and speed down to 320ft/sec .
As far as piston demand - a 383 sbc at 6500 rpm needs approx 304 cfm.
These heads at 182cc with a ported victor jnr went 295cfm at 40" depression at 0.700" lift, How much depression at that lift and rpm occur in a running engine ???
Things to consider in my opinion-
What is the size of Induction when fuel introduced as in min CSA ,avg CSA and velocity profile??
What rpm will engine operate in.
Smaller faster port better suited to a sub -6500 rpm street engine.
Bigger port help carry power past peak this is where Mach index plays a part.
Does a bigger induction tract fill the cylinder earlier pumping up torque??
I am hoping MT will give us more bread crumbs to chew on :D
steve c
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Re: Comparing 2 heads...port size vs flow

Post by Elroy »

steve cowan wrote: Fri Sep 24, 2021 6:08 am Does a bigger induction tract fill the cylinder earlier pumping up torque??
Do you feel it does? If so, are we talking about below peak torque or at peak torque?
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Re: Comparing 2 heads...port size vs flow

Post by 1980RS »

mt-engines wrote: Thu Sep 23, 2021 11:20 pm
1980RS wrote: Thu Sep 23, 2021 10:40 pm Given both heads have about the same air flow the heads with the smallest ports will make the best power and will be quicker at the track IMO. This year I ran my 358 with the 200cc Sportsman II heads. Next year I will do a set of 170cc Vortec heads and will make them flow about the same 260ish range. Car has run 11.30, so it will be a real good test if the smaller Vortec's are better.
Why go backwards?
Bigger is not always better.
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Re: Comparing 2 heads...port size vs flow

Post by PRH »

Or is this another one of those flow bench vs reality situations?
It’s some of that, with a big part of the equation being what they’re going on, the rest of the combo(what the associated parts are), and what the intended use is.

There’s a reason they didn’t put rectangle port heads on 366’s that went in dump trucks.
Somewhat handy with a die grinder.
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Re: Comparing 2 heads...port size vs flow

Post by PRH »

Here’s an example I use pretty often when trying to explain how the flow number of a head doesn’t really tell the tale.......

I work mostly on BB Mopars, and the heads can vary quite a bit in size and flow.

A totally untouched ootb Indy 440-1 will flow a little more than 300cfm.
310cc runner, MW sized port opening, raised port, 2.19” valve.

A ported Edelbrock RPM head can flow at least that much...... with an under 250cc, non-raised runner, std port opening port with a 2.14” valve.

Take the two different heads and install them onto a 9:1 383 with a 268* hyd cam with .450 lift....... and then a 15:1 580” motor with a .750 lift roller cam.

You’d see that there can be vastly different outcomes from heads that flow the same........ and that bigger def isn’t always better......... but........sometimes it is way better.

And....... if that 383 was destined for a plow truck, a stock 516 head that flows 220cfm would be a better choice than even the ported rpm head.
Somewhat handy with a die grinder.
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Re: Comparing 2 heads...port size vs flow

Post by Geoff2 »

Yep, bigger is not always better....
Germans & Japs figured it out decades ago, rest are playing catch up...
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Re: Comparing 2 heads...port size vs flow

Post by cab0154 »

are we assuming everything other than cross section and average airflow are the same and ideal conditions? because if not, the only way you would be able to know would be to test them. the bigger head could very well run better if say the airspeed gradient in the small head was all over the place, along with having airspeed that could be too high (and the bigger head had a nice velocity gradient and the ave cfm per sq inch wasnt too low). i have seen stuff run good with an average of 115cfm per sq inch of cross section, i have seen stuff run good with over 130 cfm average and everything in between. so I wouldnt go so far as to say the smaller the better. its going to depend on the port design, as well as the rest of the combination (cam, intake, engine, converter, gear...)
Last edited by cab0154 on Fri Oct 01, 2021 1:08 pm, edited 1 time in total.
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Re: Comparing 2 heads...port size vs flow

Post by lefty o »

in general, if they flow the same up top, and the smaller one flows more down low, in most applications the smaller head will make better avg power, and pretty much the same up top, making it the better choice. on a larger engine thats going to spend the bulk of its time wide open, well then the bigger head might be better. lots of what ifs in a theoretical discussion.
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