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The 273 heads will have less velocity because they have a slightly larger port volume. Not that they would be a bad choice for that reason, they would make less pumping losses. That is also a reason for me to consider the magnums, I have a set of them and the 273 heads but no 302s...

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I called my buddy that owns the flow bench. He recalls the velocity was greater on the 273 head vs the swirl port. He's going to pull it up on the computer and I'll post the numbers. Also remember that with the magnums you'll need the corresponding valve train and intake in order for the heads to work. There will be a lot of port mismatch with a Magnum/273.

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The 273 heads will have less velocity because they have a slightly larger port volume. Not that they would be a bad choice for that reason, they would make less pumping losses. That is also a reason for me to consider the magnums, I have a set of them and the 273 heads but no 302s...

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I called my buddy that owns the flow bench. He recalls the velocity was greater on the 273 head vs the swirl port. He's going to pull it up on the computer and I'll post the numbers. Also remember that with the magnums you'll need the corresponding valve train and intake in order for the heads to work. There will be a lot of port mismatch with a Magnum/273.

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Here are the numbers off my head. i tried to scan the test data but it wouldn't show in color and was impossible to read. I'll try and explain what is depicted on the sheet.

Used a 302 Swirl port vs the stock 273.
both had the same modifacations except we stayed with the stock intake size on the 302 and only modified with a stainless valve.
The 273 used a cut 1.88 stainless valve down to 1.84. We found this size ideal for the 273 to prevent shrouding.

First test

273 with a 1.84 intake, 1.56 exhaust with alittle bowl work Same on the 302 except larger intake and exhaust valve.
113% max velocity on both heads

2nd test

273 head with same mod with the addition of more port work, 70 degree bowl cut and 3 angle valve job. 302 the same, both had Ferra valves

Gained 116% on the 273 and 115% on the 302

1st test

The flow numbers showed a big differance with the 273 showing max flow of 166.9 cfm at .400 of lift with the flow curve remaining steading and not falling off until .475 of lift.

The 302 however showed a max of 161.8 and started to fall off at around .500 lift

2nd test

273 showed 169 cfm at .450 lift where the 302 showed 163 cfm at .450 then dropping off.

273 and 302 had the same modifacations.

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At what point does good swirl/tumble/mix become more important than just velocity? If we're talking a couple % faster in a 44-year old head vs. a 24-year old head, I choose the newer one every time.

Clair

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Absolute Velocity(cfm) is a very small factor when attempting to judge combustion efficiency.

I did some cylinder head development with Cummins in college and they actually measure "SWIRL TORQUE" in the cylinder to judge comb eff. This quantifies exactly how fast the air is swirling around once in the cylinder. An increase "swirl torque" had a significant impact on comb eff, but was somewhat independent of absolute port velocity ei: more HP, more mass air flow and less HC emissions from the same CFM.

In the last 20years steady state CFM has become an increasingly less significant design aspect of new OEM cylinder heads.

Bottom line: When in doubt always go with the newer designed head.

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when heads are flowed on a flow bench, they're allways bolted down to a bore plate simulating the cylinder sidewall, and yes, it will make a difference in flow, as the valve gets closer to the cylinder wall, it will be shrouded more, and recorded flow will drop. IIRC Ryan J did a flow test of a set of heads on like a 4.25" bore plate and the same heads on a 4" bore plate, and he saw over a 10% decrease in flow between the tests, IIRC.

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I think he was just pointing out that with the same valve size b/w the 273 heads and 318 #302 heads, even if the flow is affected (which it is), it would affect both heads the same, so the comparison is valid.

However, I agree, anytime flow is tested, I like to know the bore size

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This is an interesting thread. I have 273 heads on a 318 Duster. Has identical mods as yours and I ran a 15.9 (at 5000ft) with it. My next project is a 318 Volare that I'm going to put 273 heads on. I'm going to use the 2 barrel single plane intake that goes with the 273 heads and see what kind of mileage I can sqeeze out of it. I just like those 273 heads! They are lighter too and they don't seem to crack or warp like the 302's

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The 273 heads will have less velocity because they have a slightly larger port volume. Not that they would be a bad choice for that reason, they would make less pumping losses. That is also a reason for me to consider the magnums, I have a set of them and the 273 heads but no 302s...

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I called my buddy that owns the flow bench. He recalls the velocity was greater on the 273 head vs the swirl port. He's going to pull it up on the computer and I'll post the numbers. Also remember that with the magnums you'll need the corresponding valve train and intake in order for the heads to work. There will be a lot of port mismatch with a Magnum/273.

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Here are the numbers off my head. i tried to scan the test data but it wouldn't show in color and was impossible to read. I'll try and explain what is depicted on the sheet.

Used a 302 Swirl port vs the stock 273.
both had the same modifacations except we stayed with the stock intake size on the 302 and only modified with a stainless valve.
The 273 used a cut 1.88 stainless valve down to 1.84. We found this size ideal for the 273 to prevent shrouding.

First test

273 with a 1.84 intake, 1.56 exhaust with alittle bowl work Same on the 302 except larger intake and exhaust valve.
113% max velocity on both heads

2nd test

273 head with same mod with the addition of more port work, 70 degree bowl cut and 3 angle valve job. 302 the same, both had Ferra valves

Gained 116% on the 273 and 115% on the 302

1st test

The flow numbers showed a big differance with the 273 showing max flow of 166.9 cfm at .400 of lift with the flow curve remaining steading and not falling off until .475 of lift.

The 302 however showed a max of 161.8 and started to fall off at around .500 lift

2nd test

273 showed 169 cfm at .450 lift where the 302 showed 163 cfm at .450 then dropping off.

273 and 302 had the same modifacations.

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what bore was that tested on, a 273 bore, or a 318 bore, or a 4" (360) bore?

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I think he's going to raise himself up a crop of dental floss..........

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I hope he saves the wax...

BTW, I hate to ask a pretty basic question this late in the thread, but what exactly is this "115% max velocity" stuff referring to? I guess I haven't seen this terminology before...

Clair

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I don't think a lighter rotating assembly is going to save you any noticeable mpg. Acceleration will be better, but cruising, you are already spinning the rotating assembly. It sounds like you are building more or less how the 318s came from the factory in the 80s, w/ a tiny solid cam.

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The problem those cars had with intown miledge was gettin everything moveing, they got good miledge on the highway. The light weight stuff should help a bunch in town. I will be removeing at least 1600 grams from the rotateing assy plus removeing weight from the crank to balance it roughly 1200 more grams for a total of 2800 grams Now consider that weight in first gear has to accelerate 7.5 times as fast as the car it will be like removeing 21,000 grams from the vehicle, or almost 50 lbs everytime the engine speeds up or down. I also remember reading the weight actually goes up exponetialy not lineraly so it would rev even easier. And also my car is already 500lbs lighter than those dippys and 5th aves.

Compresion will be at least 1.5 higher ratio and should be good by itself for %4 more power from the same fuel or %4 more miledge. Also the tighter quench will cause a lot more turbulence resulting in a faster and more complete burn so I can run less advance to get les negative work on the piston before TDC. Even if the headers don't hit there tuned RPM the engine will do less work forcing the gasses through them than the 3 cats and very restrictive single exhaust the mid 80s cars had. I will have no EGR valve to slow the part throttle burn rate either.

The cam is a couple degrees smaller so a tad less overlap and solids have slightly less friction than a hydro mabey a tad more than a roller but I can tune the valve lash for a little more efficency.

If I can find a dual port intake it should also help build mixture velocity at low speeds around town.

I can run it a little leaner (14.7 is the goal)without worying about burning up the cats or NOx emmisions.

I am expecting to beat my current best MPG by at least 5 mpg while I am really hopeing for 10mpg better. My current best ever was 23mpg mostly highway driving. Strictly in town I would usually get 17mpg.

I think it should be a drastic improvement from the 80s