Simple chamber mod that can get overlooked

[Tuner]

In the '80s when Chevrolet progressed from the 292 Turbo head to the first iron Bow Tie head the porting instructions cautioned against relieving the chamber wall away from the valve because that would allow the flow to separate from the port floor at short side radius.

The 292 head was a "line of sight" port with a low floor and flow was improved by moving the chamber wall away from the valve. The line-of-sight flow in the Turbo head did not utilize the full diameter of the valve to seat flow window.

The iron bowtie head was an evolution with a high port floor and a large radius short side and depended on some valve margin to chamber wall restriction to keep the flow organized around the entire valve circumference.

[HDBD]

Looks like a candidate for non traditional top angle(s) above the seat.

[Tuner]

In the case of that early iron bowtie head it hurt the flow to relieve the chamber wall around the valve, so it isn't actually a similar situation to the Ford head in the OP but the fact that in the Ford the chamber mod reduced the swirl would likely require more spark advance, so the minimal increase in flow could result in more pumping loss (negative work from more combustion pressure before TDC) and the power increase be lost in the wash, the baby thrown out with the bathwater.

[Mummert]

More port swirl usually requires more timing. Higher rates of swirl usually indicate more fuel seperation and less inward rush intensity of the air column.

[steve cowan]

More port swirl usually requires more timing. Higher rates of swirl usually indicate more fuel seperation and less inward rush intensity of the air column.

That's interesting,
Is that a normal situation or only for this example on the chamber you modified?
I can't see how your chamber/ cylinder-head aren't better after the modification you shown,the flow curve was smooth and more consistent than before regardless of the 10cfm.
I would not be surprised if it even sounded better on the bench.
When a port is choppy regardless of intake or exhaust they sound terrible. Turn the bench to 40" and have listen to a turbulent port.

[juuhanaa]

Good or better depends on what the engine might want more timing for.



-juhana

[Mummert]

More port swirl usually requires more timing. Higher rates of swirl usually indicate more fuel seperation and less inward rush intensity of the air column.

That's interesting,
Is that a normal situation or only for this example on the chamber you modified?
I can't see how your chamber/ cylinder-head aren't better after the modification you shown,the flow curve was smooth and more consistent than before regardless of the 10cfm.
I would not be surprised if it even sounded better on the bench.
When a port is choppy regardless of intake or exhaust they sound terrible. Turn the bench to 40" and have listen to a turbulent port.

???
I feel its better too, what about the quote is contradictory?

" More port swirl usually requires more ignition timing." It swirled less.

Higher rates of swirl usually indicate more fuel seperation and less inward rush intensity of the air column.
I don't feel these things would be as pronounced.

[steve cowan]

More port swirl usually requires more timing. Higher rates of swirl usually indicate more fuel seperation and less inward rush intensity of the air column.

That's interesting,
Is that a normal situation or only for this example on the chamber you modified?
I can't see how your chamber/ cylinder-head aren't better after the modification you shown,the flow curve was smooth and more consistent than before regardless of the 10cfm.
I would not be surprised if it even sounded better on the bench.
When a port is choppy regardless of intake or exhaust they sound terrible. Turn the bench to 40" and have listen to a turbulent port.

???
I feel its better too, what about the quote is contradictory?

" More port swirl usually requires more ignition timing." It swirled less.

Higher rates of swirl usually indicate more fuel seperation and less inward rush intensity of the air column.
I don't feel these things would be as pronounced.

Exactly,
Less swirl,better flow curve.
Smarter people than me have said swirl is a form of turbulence and not something wanted in a performance based engine.

[juuhanaa]

That's interesting,
Is that a normal situation or only for this example on the chamber you modified?
I can't see how your chamber/ cylinder-head aren't better after the modification you shown,the flow curve was smooth and more consistent than before regardless of the 10cfm.
I would not be surprised if it even sounded better on the bench.
When a port is choppy regardless of intake or exhaust they sound terrible. Turn the bench to 40" and have listen to a turbulent port.

???
I feel its better too, what about the quote is contradictory?

" More port swirl usually requires more ignition timing." It swirled less.

Higher rates of swirl usually indicate more fuel seperation and less inward rush intensity of the air column.
I don't feel these things would be as pronounced.

Exactly,
Less swirl,better flow curve.
Smarter people than me have said swirl is a form of turbulence and not something wanted in a performance based engine.

Swirl and turbulence are nasty and difficult things to understand. Pretty dont make more power



-juhana

[Tuner]

More port swirl usually requires more timing. Higher rates of swirl usually indicate more fuel seperation and less inward rush intensity of the air column.

I consider the above quote regarding timing to be exactly backwards.

The importance of induced combustion chamber turbulence, whether squish, swirl, tumble, or all combined, has been the subject of immense research effort since 100 years ago when Sir Harry Ricardo's research in combustion proved the principle of increased turbulence improves fuel/air mixing and the turbulent mixture requires much less spark advance to target peak pressure to the desired crank angle ATDC for best torque and power. This had been understood as fact for 100 years.

In all research results I have ever studied, never has mixture turbulence required more spark advance. In fact in all cases less advance is the result and one of the goals of the research. Engine over-all efficiency is improved by anything that results in requiring less spark advance to achieve best torque/power. The obvious reason for this is any heat release and subsequent pressure increase occurring before TDC is negative work.


This Chevy (and similar Mopar) head with the corkscrew intake port is the epitome of intentionally induced swirl and both the GM and Mopar only require 25 degrees of advance to make best power.



This MotorTrend article about heads describes the effort to improve swirl as head design evolved. We all know current cylinder heads require less advance than the old-school designs.
https://www.motortrend.com/how-to/cylin ... nd-tumble/

The OP's flow improvement of 5% is significant and probably will result in a power increase but that would need to be confirmed with dyno or track testing.

My point of suggesting reducing swirl may require more spark lead was to point out a possibility for discussion, not state an absolute, because when two things change at the same time the outcome can be conflicting influences.

[Mummert]

That head needs more timing than 25 degrees when run above 5000rpm like its later counterparts. What I'm trying to say is that as the vorticie get more vertical like a tumbling state the chance for fuel seperation in the cylinder goes down. An SBF TFS twisted wedge heads swirls "0" that doesn't mean it doesn't have an active running vorticie its just more vertical. BSFC and timing requirement are more optimal in high performance engines.

[1980RS]

I cut the swirl ramp out of my L-29 heads when I ported them on some good advice from our local head porter I know. Must have worked, that car ran pretty well last fall with those heads, better than expected.

[Tuner]

That head needs more timing than 25 degrees when run above 5000rpm like its later counterparts. What I'm trying to say is that as the vorticie get more vertical like a tumbling state the chance for fuel seperation in the cylinder goes down. An SBF TFS twisted wedge heads swirls "0" that doesn't mean it doesn't have an active running vorticie its just more vertical. BSFC and timing requirement are more optimal in high performance engines.

What head? If you are referring to the Chevy screw port head it is obvious you have never tuned one. Stock or "improved" with higher compression ratio and a radical cam such as used for 7000+ on a fast 3/8 mile dirt track, it does not matter how high octane fuel is use with that head, power goes in the tank with any more timing than 25 degrees. It will tolerate 26 but it does not make any more power. At 27 power starts to drop. I have tuned these heads since they came out in the late '80s and guarantee you, take it to the bank in CAPITAL letters, 25 is all they need for best power at any RPM above 2000. The reason people think that head is no good for power is because they try to make it run with too much timing, such as 36 degrees like an old school Chevy head.

[Tuner]

I cut the swirl ramp out of my L-29 heads when I ported them on some good advice from our local head porter I know. Must have worked, that car ran pretty well last fall with those heads, better than expected.

The small block screw port head gets bad press but they run good when the timing is not beyond 25 degrees.

L-29 is a 454 head. What does that have to do with this? I don't know what the L-29 head likes for timing. A pal's Chevy PU that has those heads never advances more than 30 when under load, sometimes less from knock retard I think, at least what you see using OBD scan tool.

How much timing did you use with yours? Did you ever try to find the minimum timing it needed?

[Mummert]

Yep done little with that head. What kind of power do they make compared to late vortec head. Reason I ask is my 4.3 had those. I messed with them wasn't happy and ported late heads with hollow fast burn valves.