Swirl Port Design for High Compression Engine

[dreamspeed]

I've been told over the years that a swirl port design is not necessarily a good thing for high compression engines...i.e. 15.5:1 to 16.3:1 compression ratio. However, I have bought a set of SB Chevy cylinder heads from a reputable company for a high end drag race engine and am struggling to get it to make good power in the upper RPM ranges. The engine is 358 CID with 15 degree heads with 2.200 intake valve diameter and 1.610 exhaust diameter. Theoretical valve lift ranges are .927 intake and .850 exhaust. After disassembling the engine, I find that the carbon around the gas ports in the piston are swirled and looking at the intake ports of the heads, it is definitely a swirl design. The heads have a swirl wing from the valve guide to the seat that turns the air as it exits the throat into the chamber. We flow tested the heads @ 28" on a SF600 bench and here are the inital numbers:

.200 131
.300 194
.400 251
.500 307
.550 323
.600 341
.650 358
.700 375
.750 387
.800 398
.850 406
.900 411
.950 417

We noticed that the "mid-lift numbers seem to be a little weak so we tried some small changes to the valve package to try to bring these numbers up and here are the results:

.200 137
.300 206
.400 259
.500 309
.550 329
.600 347
.650 362
.700 379
.750 391
.800 402
.850 turbulent rest of way

As you can see, the mid numbers were improved but the swirl port went turbulent over .800 lift.

My main question is whether or not the port is going into a turbulent flow when the engine is operating due to the swirl design and would it be best to remove or reduce this swirl? If this swirl flow is so touchy at 28" that a very minute valve back angle change would cause it to go turbulent, what does it do in an engine when it sees 60" or so? Let's face it, a lot of head porters sell heads based on flow numbers and these heads flow big numbers at .800 and above but are they real numbers that the engine would appreciate or did my head porter just win the flow bench competition?

I apologize for the lengthy and possibly difficult to read post. Any comments or suggestions would be greatly appreciated.

Thank you,

Brian

 

[Lou Scannon]

If you work out the Reynolds number I'd hazard a guess that the flow is turbulent the whole time, i.e. Re > 4000.
What criterion are you using to determine whether or not the flow is "turbulent"?

 

[dreamspeed]

I'm definitely not an expert at flow but I'm just calling turbulent when the flow sound changes dramatically (gurgling sound sort of) and flow goes backward.

 

[richdubbya]

Are you sure it is a swirl design? Swirl designs are very restrictive and usually reserved for low rpm or emission reduction.

I have seen ports like you describe except these were designed to direct the airflow away from the backside of the bowl to promote airflow around all of the valves diameter, even that area close to the intake side of the head.

A call to the mfg may shed some light on what is happening and they may have info on piston shape and cams needed for their design.

 

[dreamspeed]

richdubbya,

Thanks for the response. You are correct, it is a port designed to direct flow all around the valve's diameter but it does create some swirl in the combustion chamber. My problem is that the engine simply doesn't make the power it should and the mfg only can say that the heads are good and want more money to work on them to "maybe" make them better. I've seen several sets of heads for these types of engines without this design and they flow much better throughout the lift range but not as much up high. I'm just concerned if this design was created on the flow bench to show big numbers or as a result of horsepower gains in an engine.

 

[AboveRedline]

Here is an example of a head with more mid-lift flow as you were discussing.

http://www.chapmanracingheads.com/p_cylinder.asp?ID=54

I always understood that we are going for Tumble instead of swirl, as there are no flow penalties with tumble and its more suited to high piston speed activities.
If you can't pick up more mid-lift flow easily can the Cam Manufacturer recommend an appropriate grind for what you have?
Is your exhaust flow >260 cfm?
Sorry I can't be more helpful, it's amazing when SB ports flow over 400 cfm, and we still don't get what we expect.

 

[dreamspeed]

The exhaust flow is above 260...numbers are:

.200 112
.300 181
.400 229
.500 257
.600 271
.700 278
.800 281
.900 284

These numbers are flowed without a pipe extension.

The real intesting part is that my engine builder has built an engine for another customer with supposedly identical heads (according to the manufacturer) but this other engine makes 30 more horsepower and carries it 400 rpm higher and makes 6 lb-ft peak torque. The two engines have the same bore, stroke, deck height, compression ratio, camshaft, rocker system, etc. We've swapped intake manifolds, carbs, etc. with no change and have made approximately 50 dyno pulls trying everything under the sun. The heads are the only stone we haven't turned yet. On the flow bench, these other heads have more intake flow everywhere throughout the range...average of 8-10 cfm...exhaust is the same. I'm just looking for advise on how to deal with this type of port. It just seems very tempermental and unstable the way it's designed. Wondering if a more conventional intake port design would be a better way to go. Maybe someone like Larry Meaux could point me in the right direction.